Download Pollen and seed desiccation tolerance in relation

Journal of Experimental Botany, Vol. 62, No. 15, pp. 5267–5281, 2011
doi:10.1093/jxb/err154 Advance Access publication 9 August, 2011
REVIEW PAPER
Pollen and seed desiccation tolerance in relation to degree of
developmental arrest, dispersal, and survival
G. G. Franchi1, B. Piotto2, M. Nepi3, C. C. Baskin4,5, J. M. Baskin4 and E. Pacini3,*
1
Department of Neurological, Neurosurgical and Behavioral Sciences, Section of Pharmacology ‘G. Segre’, University of Siena, Strada
delle Scotte 6, 53100 Siena, Italy
2
Department of Nature Protection, ISPRA-Istituto Superiore per la Protezione e la Ricerca Ambientale, Via Curtatone 3, 00185 Rome, Italy
3
Department of Environmental Sciences ‘G. Sarfatti’, University of Siena, Via P. A. Mattioli 4, 53100 Siena, Italy
4
Department of Biology, University of Kentucky, Lexington, KY 40506, USA
5
Department of Plant and Soil Sciences, University of Kentucky, Lexington, KY 40546, USA
* To whom correspondence should be addressed. E-mail: [email protected]
Received 23 December 2010; Revised 08 April 2011; Accepted 15 April 2011
Abstract
In most species, arrest of growth and a decrease in water content occur in seeds and pollen before they are
dispersed. However, in a few cases, pollen and seeds may continue to develop (germinate). Examples are
cleistogamy and vivipary. In all other cases, seeds and pollen are dispersed with a variable water content (2–70%),
and consequently they respond differently to environmental relative humidity that affects dispersal and maintenance
of viability in time. Seeds with low moisture content shed by the parent plant after maturation drying can generally
desiccate further to moisture contents in the range of 1–5% without damage and have been termed ‘orthodox’.
Pollen that can withstand dehydration also was recently termed orthodox. Seeds and pollen that do not undergo
maturation drying and are shed at relatively high moisture contents (30–70%) are termed ‘recalcitrant’. Since
recalcitrant seeds and pollen are highly susceptible to desiccation damage, they cannot be stored under conditions
suitable for orthodox seeds and pollen. Hence, there are four types of plants with regard to tolerance of pollen and
seeds to desiccation. Orthodoxy allows for dispersal over greater distances, longer survival, and greater resistance
to low relative humidity. The advantage of recalcitrance is fast germination. Orthodoxy and recalcitrance are often
related to environment rather than to systematics. It has been postulated that certain types of genes are involved
during presentation and dispersal of pollen and seeds, since molecules (sucrose, polyalcohols, late embryogenic
abundant proteins, antioxidants, etc.) that protect different cell compartments during biologically programmed
drying have been detected in both.
Key words: Desiccation tolerance/sensitivity, orthodox and recalcitrant pollen, orthodox and recalcitrant seeds, pollen
germination, pollen presentation, pollen water content, pollination, seed dispersal, seed germination, seed water content.
Introduction
The term ‘developmental arrest’ is used to mean cessation
of plant or animal growth that makes survival possible
during an adverse environmental phase (Footitt and Cohn,
2001). In addition to developmental arrest in plants, it also
occurs in some animals and algae growing in habitats with
temporary moisture and in those subject to complete
drying, as in deserts (Gray et al., 2007), or complete
seasonal freezing. Many examples of developmental arrest
in plants as well as biological, physiological, biochemical,
and evolutionary aspects of this phenomenon are described
in ‘Desiccation and survival in plants. Drying without
dying’ edited by Michael Black and Hugh Pritchard (2002).
Two types of developmental arrest can be recognized in
land plants: (i) the whole plant and (ii) reproductive
structures, namely spores, pollen, and seeds (Alpert and
Oliver, 2002).
ª The Author [2011]. Published by Oxford University Press [on behalf of the Society for Experimental Biology]. All rights reserved.
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5268 | Franchi et al.
From an evolutionary viewpoint, developmental arrest is
a mechanism that allows survival in vegetative and/or
reproductive structures when liquid water is only temporarily present because of water-level fluctuations, freezing, or
dryness. Mechanisms to survive and remain in a vigorous
condition when water is lacking became important as life
moved onto the land (Footitt and Cohn, 2001) and were
indispensable for the evolution of land plants (Hoekstra,
2002). From an evolutionary point of view, it is very
difficult to say whether desiccation of vegetative or of
reproductive structures originated first. Both mechanisms
are nevertheless present in all of the groups of land plants
even if at different levels and with different strategies, and
desiccation tolerance of seeds, in particular, has received
considerable research attention (Pammenter and Berjak,
1999; Berjak and Pammenter, 2008). However, although
there are important issues related to desiccation tolerance of
pollen (Nepi et al., 2001; Franchi et al., 2002), pollen has
not received as much attention as seeds. Also, most
biologists have not considered the similarities/differences in
desiccation tolerance of pollen and seeds. Thus, the
objective of this review is to compare pollen and seeds with
respect to water content and desiccation tolerance.
Most cases of developmental arrest in plants involve loss
of water to a certain level, which varies with plant part and
species (Gaff et al., 2009), and then the moisture content
remains relatively constant as the plant part acquires
resistance to drying (Ingram and Bartels, 1996; Alpert and
Oliver, 2002; Moore et al., 2009). The expression ‘desiccation tolerance’ applies to plants or plant parts that
withstand drying (Bartels, 2005, and references therein).
Desiccation tolerance may last for different periods of time
and involve structures of different complexity, from single
(spores of fungi, mosses, and ferns) to a few (pollen),
thousands, or hundreds of thousands of cells, as in seeds
(Pammenter and Berjak, 1999) and whole organisms such as
in resurrection plants (Proctor and Pence, 2002).
In primitive land plants such as mosses, gametophyte
(the haploid perennial vegetative part of these small
organisms) development often is arrested during dry
periods in environments with marked seasonal differences
in rainfall and/or temperature (Mayaba et al., 2001;
Proctor and Pence, 2002). In contrast, moss spores do not
always show developmental arrest. Those containing
chloroplasts are metabolically active even during dispersal,
which must be fast (and to a moist substrate) and probably
only to distances rarely exceeding a few metres if the spore
is to survive during dispersal (Hoekstra, 2005). In other
cases, moss spores contain amyloplasts that become
chloroplasts only after the spore germinates, as in Dawsonia superba Grev., a moss reaching 40–50cm in height
(Stetler and DeMaggio, 1976). The same is true of ferns
having spores with chloroplasts (Lebkuecher, 1997; Hoekstra, 2005). Some fern spores also have chloroplasts and
are subject to the same limitations of dispersal as those of
mosses with chloroplasts. However, some moss and fern
spores have proplastids instead of chloroplasts and are
tolerant of drying (Hoekstra, 2002). In rare cases, fern
gametophyte development may be arrested (Warkins et al.,
2007).
The mechanisms explaining desiccation tolerance have
been well studied in seeds, especially since Roberts (1973)
coined the terms recalcitrant and orthodox to indicate the
difficulty and ease, respectively, of being able to maintain
seeds in a viable state during dry storage. Specifically, seeds
with low moisture content shed by a parent plant after
maturation drying can generally withstand further drying to
moisture contents in the range 1–5% and were termed
‘orthodox’ (Roberts, 1973). Pollen that withstands dehydration was also termed orthodox (Pacini et al., 2006). On the
other hand, seeds and pollen that do not undergo maturation drying and are shed with relatively high moisture
contents are highly susceptible to desiccation damage and
cannot be stored under the same conditions as orthodox
seeds, tending to germinate immediately if moisture is
available. Such pollen and seeds are termed ‘recalcitrant’.
Since orthodoxy and recalcitrance are not strict categories
but part of a continuum of seed and pollen behaviour,
today the term desiccation sensitivity is preferred to recalcitrance and desiccation tolerance is preferred to orthodoxy (Dickie and Pritchard, 2002; Hoekstra, 2002). Another
storage behaviour that has been identified in seeds is called
‘intermediate’, meaning that seeds tolerate/survive drying
and storage at low temperatures (but >0C) better than
recalcitrant seeds but not to the same extent as orthodox
seeds (Ellis et al., 1990)
Since pollen and seeds of different species show various
degrees of desiccation tolerance and can be located on
a continuum ranging from desiccation tolerance to desiccation sensitivity, a better understanding of the physiological
and evolutionary significance of the different combinations
of desiccation tolerance and/or sensitivity in pollen and
seeds of the same taxon is needed. Thus, the specific
objective of this review is to show how certain ecological
and physiological strategies of a species may be conditioned
by seed and pollen water content at dispersal. In this
context, developmental arrest of dispersal units and their
tolerance to dehydration, as well as environmental conditions at dispersal, are also considered.
Hydration status
In the evolutionary history of plants, pollen and seeds are
consequences of heterospory (directly in the case of pollen
and indirectly for seeds), and they appeared first in the now
extinct Pteridospermae, and then in all gymnosperms and
angiosperms. Pollen is the direct consequence of heterospory, and each pollen grain is an immature male gametophyte derived from a microspore division and encased by
two walls. The seed is an indirect consequence of heterospory, being due to fertilization of the egg formed by the
female gametophyte that is held on the plant and encapsulated by new tissue layers derived from the mother plant
(ovule integuments) (DiMichele et al., 1989; Friedman and
Carmichael, 1998). Both pollen and seeds may show
Pollen and seed desiccation tolerance | 5269
developmental arrest involving programmed dehydration
(Hoekstra, 2002; Kermode and Finch-Savage, 2002). The
main similarities and differences in the various stages of
development of pollen and seeds are shown in Fig. 1.
Pollen develops in the anther while seeds develop in the
ovary/fruit. However, before exposure to dispersal agents,
both pollen and seeds lose water by evaporation and/or
reabsorption (Bianchini and Pacini, 1996a; Kermode and
Finch-Savage, 2002; Pacini and Hesse, 2004). This dehydration before and/or in the initial phase of dispersal
occurs in most species of angiosperms and gymnosperms
(Kermode and Finch-Savage, 2002; Pacini et al., 2006).
Seeds of some species do not tolerate water loss below
a certain species-specific level and thus die when water
content falls below this level (Fig. 2). It has long been
known that seeds of some species, especially those of tree
species, are desiccation sensitive. In most cases (70%), the
recalcitrant seeds are from tropical species (Engelmann and
Engels, 2002), including some of economic (cocoa, rubber,
mango, and mahogany) and environmental (mangroves)
importance (Baskin and Baskin, 1998; Berjak and
Pammenter, 2008). It was recently shown that certain kinds
of pollen also die rapidly after opening of the anther or
after pollen dispersal because they do not have homeostatic
mechanisms for maintaining a constant water content.
Pollen of this type, previously described as ‘partially
hydrated’ (Nepi et al., 2001; Franchi et al., 2002), has been
named recalcitrant (Pacini et al., 2006; Franchi et al., 2007)
or, more precisely, desiccation sensitive, by analogy with
the terminology used for seeds (Figs 3, 4). Again, this
characteristic occurs in some economic plants such as
sugarbeet, avocado, cotton, hemp, poppy, walnut, certain
Poaceae (Hoekstra et al., 1988), and many others.
The first clue to recognizing recalcitrant seeds or pollen is
their relatively large dimensions. Pollen grains >100lm in
diameter and seeds with a mass of >10g are often
recalcitrant (Nepi et al., 2001; Dickie and Pritchard, 2002).
Also, pollen without furrows (structural elements enabling
variations in shape and volume with loss or uptake of
water) invariably has low desiccation tolerance (Fig. 4). For
seeds, desiccation sensitivity is characterized by relatively
high volumes and masses, and sometimes by green cotyledons, indicating the possibility of a quick start of photosynthesis upon full rehydration. However, a large size does not
always indicate desiccation sensitivity. For example, the
desiccation-sensitive pollen of spinach, Parietaria (Franchi
et al., 2007), and Daphne sericea Vahl is small (Fig. 4). With
regard to seeds, >8000 spermatophytes have been studied,
but only ;8% of them have desiccation-sensitive seeds
(Tweddle et al., 2002). There is an experimental protocol to
determine seed storage behaviour (Hong and Ellis, 1996),
and methods have been published explaining how to
determine seed storage behaviour when only small quantities of seeds are available (see Pritchard et al., 2004).
According to some authors, a relationship exists between
the ecology and the storage behaviour of seeds of a given
species (Roberts and King, 1980). In general, seeds of
species from environments with dry seasons or occasional
droughts are orthodox, but there are exceptions. In
orthodox seeds, resistance to desiccation is an essential
Fig. 1. Similarities and differences between pollen and seeds. (1) Reserves are used partly to produce molecules to resist desiccation
and partly to sustain the first phase of germination. (2) Pollen desiccation may occur in the anther before dehiscence, during
presentation, or during dispersal. (3) Pollen and seeds lose water, depending on relative humidity, to a level typical of the species. The
water content reached and the possibility of maintaining it distinguish the categories of orthodox and recalcitrant. (4) Orthodox seeds and
pollen have a longer mean life and succeed in germinating at greater distances from the mother plant.
5270 | Franchi et al.
Fig. 2. Changes in volume of different categories of pollen and seeds during development, presentation, dispersal, and further growth.
The graph for pollen and that for seeds are similar during development, but vary when pollen and seeds are presented for dispersal.
Dispersal always means at least partial loss of water, but the curves vary widely between orthodox and recalcitrant pollen and seeds.
(1) Cleistogamous pollen that germinates without dispersal (obligate autogamy). (2) Viviparous seeds in which dispersal (plateau) is
limited. (3) Orthodox seeds during presentation and dispersal. (3’) Orthodox pollen during presentation and dispersal; the line is wavy due
to fluctuations in environmental relative humidity. (4) Recalcitrant pollen and seeds encountering environmental water, which enables
recovery and germination. (5) Uncontrolled water loss of recalcitrant pollen and seeds. (6) Germination and fertilization of recovered
orthodox and recalcitrant pollen. (7) Germination and growth to adult plants of recovered orthodox and recalcitrant seeds. Dispersal is
absent in species with cleistogamous pollen and can be extremely reduced in viviparous seeds. Seeds do not vary in volume during
dispersal because of the large number of cells they contain and their physiology, whilst pollen varies with changes in environmental
relative humidity. Pollen and seed water loss may occur before exposure for dispersal or during exposure and the first steps of dispersal.
The timing of presentation and dispersal of orthodox and recalcitrant seeds and pollen is different, being shorter in the latter.
Fig. 3. Representative example images of orthodox pollen at
presentation. (A) Convallaria majalis L., Liliaceae. (B) Citrullus lanatus
(Thunb.) Mansf., Cucurbitaceae. (C) Cyclanthera pedata Schrad.,
Cucurbitaceae. (D) Cerinthe major L., Boraginaceae. Orthodox and
recalcitrant pollen under SEM differ because of the presence of
furrows in orthodox pollen, allowing a wider increase and decrease
in volume during dehydration and rehydration. Furrows can be one
as in Convallaria majalis, three as in Citrullus lanatus and
Cyclanthera pedata, or six as in Cerinthe major. Sizes vary widely in
orthodox and recalcitrant pollen. Cucurbitaceae has members with
both orthodox and recalcitrant pollen. Scale bar¼10lm.
property for survival of the species. For example, 115 shrub
species of the Mojave Desert, California, belonging to 29
families, produce orthodox seeds (Kay et al., 1988). In
contrast, recalcitrant species are more likely to occur in
moist habitats. Thus, many species in the genus Dipterocarpus C.F. Gaertn. growing in humid tropical rainforests have
desiccation-sensitive seeds. However, although seeds of
Dipterocarpus species growing in arid environments are not
orthodox, they have higher desiccation tolerance than those
from moist environments (Tompsett, 1987, 1992). Palms
from arid environments also have orthodox seeds, whereas
those from relatively more humid environments have
recalcitrant seeds (Dickie and Pritchard, 2002). Therefore,
it can be concluded that seeds of species typical of arid
environments such as deserts and seasonally dry tropical
savannahs are not recalcitrant, but there are some
exceptions (Dussert et al., 2000; Yu et al., 2008, and
references therein). In moist environments, some species
produce orthodox seeds, and others produce recalcitrant
seeds.
In general, analysis of the type of seed or fruit can
provide an indication of desiccation tolerance. Seeds of
species that produce achenes, multiseeded berries, or dry
dehiscent fruits such as capsules, legumes, and multiseeded
follicles are orthodox. Many species that produce siliques
and caryopses have orthodox seeds. Both recalcitrant and
orthodox seeds can be found in species that produce drupes
containing 1–4 seeds, legumes with 1–5 large seeds or many
arilate seeds, berries containing 1–10 seeds, capsules with
1–5 seeds, and nuts (for many examples, see for instance
Marzalina et al., 1998). Recalcitrance is more common in
species that produce large seeds than in those with small
seeds (King and Roberts, 1979; Chin and Pritchard, 1988;
Pollen and seed desiccation tolerance | 5271
Fig. 4. Representative example images of recalcitrant pollen at
presentation. (A) Cucurbita pepo L., Cucurbitaceae. (B) Laurus
nobilis L., Lauraceae. (C) Daphne sericea Vahl, Thymelaeaceae.
(D) Typha latifolia L., Typhaceae. Orthodox and recalcitrant pollen
under SEM differ because of the presence of furrows in orthodox
pollen (see Fig. 3). Furrows are always absent in recalcitrant pollen.
Recalcitrant pollen are devoid of furrows and may have no pores
as in L. nobilis and T. latifolia where grains are dispersed in tetrads,
or many pores as in C. pepo and D. sericea. Sizes vary widely in
orthodox and recalcitrant pollen. Cucurbitaceae has members with
both orthodox and recalcitrant pollen. Scale bar¼10 lm.
Hong and Ellis, 1996). However, seed size alone is not
a good predictor of recalcitrant versus orthodox seeds, and
any such prediction is only a probability, as shown by Daws
et al. (2008) for pioneer plants in the neotropics. For
example, although the recalcitrant seeds of Cocos nucifera L.,
Aesculus hippocastanum L., and Castanea sativa Mill. are
quite large, the recalcitrant seeds of Salix caprea L. and
Populus spp. are small (Table 1). Seed water content at
dispersal varies from 36% to 70%, occasionally higher, in
recalcitrant seeds, 23% to 55% for those in the intermediate
category, and <20% to 50% for orthodox seeds (Baskin and
Baskin, 1998; Pammenter and Berjak, 1999). However, if
water content is >60% at dispersal, the seed is probably
recalcitrant. If water content is <20%, the seed is more likely
to be orthodox than recalcitrant. If seed water content at
maturity is 25–55%, storage behaviour cannot be very
accurately predicted, but it seems likely that seeds with
a water content of <35% would be orthodox. Thus, although
a combination of information on ecology, taxonomy, type of
seed/fruit and dimensions, mass, and water content at
dispersal of seeds/fruits provides a basis for quite a good
evaluation of seed storage behaviour, standard laboratory
tests need to be conducted to confirm (or not) desiccation
sensitivity of seeds/fruits.
Adaptational advantages and disadvantages of orthodoxy
and recalcitrance are the same for pollen and seeds, and
belonging to one or the other category implies different
times between maturation and germination. There are
certain marked divergent differences between recalcitrant
seeds and pollen in angiosperms. For example, in recalcitrant seeds, the embryo is fully developed at dispersal and
therefore immediately ready for germination. On the other
hand, recalcitrant pollen can be either two-celled (e.g.
pumpkin with a tube cell and a generative cell) or threecelled (e.g. grasses with a tube cell and two sperm cells) at
dispersal. In the first case, the completion of development
(second haploid mitosis to form sperms) occurs as the
pollen tube grows through the style (Franchi et al., 2002).
Another big difference to consider between pollen and seeds
is their time of survival under natural conditions after
dispersal. Recalcitrant pollen only survives for a few hours
after anther opening, while recalcitrant seeds may rarely
survive up to some weeks. Orthodox pollen rarely remains
viable longer than a few days, whereas orthodox seeds may
live for years (Nepi et al., 2001; Franchi et al., 2002). The
best longevity record for seeds is 1288 years, and this is for
Nelumbo nucifera Gaertn. fruits collected from an old lake
bed in South Manchuria, China. Fruits were germinated,
and then their age was determined via radiocarbon
14
C using pieces of pericarp from those that germinated
(Shen-Miller et al., 1995).
Mechanism of desiccation tolerance
A suite of mechanisms or processes currently are thought to
be associated with desiccation tolerance, and these help to
ensure survival of orthodox seeds in the desiccated condition. These mechanisms include cell and intracellular
physical characteristics, intracellular de-differentiation, metabolic ‘switching off’, efficient antioxidant systems, accumulation of putatively protective molecules such as late
embryogenic abundant (LEA) proteins, sucrose and certain
oligosaccharides, amphipathic molecules, the peripheral
oleosin layer around lipid bodies, repair mechanisms during
rehydration, and others yet to be identified. The presence of
some of the mechanisms/processes, or their absence or
partial expression, has been considered in the context of the
varied responses to dehydration shown by recalcitrant seeds
(Bailly et al., 2001; Berjak and Pammenter, 2003).
Recalcitrant seeds are not equally desiccation sensitive,
and different species tolerate different degrees of dehydration. Variability in desiccation sensitivity implies that the
processes or mechanisms that confer desiccation tolerance
are variously developed or expressed in the recalcitrant
condition. Since different mechanisms may be involved in
acquisition of desiccation tolerance and maintenance of
viability of dehydrated orthodox seeds, any one of the suite
of characters may be absent, or be present but ineffective, in
recalcitrant seeds. Another important consideration is that
desiccation tolerance is probably controlled by the interplay
of mechanisms or processes, and not by any one acting in
5272 | Franchi et al.
Table 1. Four categories of angiosperms grouped according to recalcitrance and orthodoxy of pollen and seeds
Seed
Pollen
Orthodox
Recalcitrant
Orthodox
Recalcitrant
Woody–OSOP
Acer platanoides L., A. saccharum Marshall,
A. circinatum Pursh (Aceraceae)
Arbutus unedo L. (Ericaceae)
Ceratonia siliqua L. (Fabaceae)
Fabaceae p.p.
Fraxinus spp. (Oleaceae)
Myrtus communis L. (Myrtaceae)
Olea europea L. (Oleaceae)
Prunus spp. (Rosaceae)
Quercus emoryi Torr. (Fagaceae)
Rosmarinus officinalis L. (Lamiaceae)
Pandanus spp. (Pandanaceae)
Phoenix dactylifera L. (Arecaceae)
Herbaceous–OSOP
Apiaceae
Bryonia dioica Jacq. (Cucurbitaceae)
Citrullus vulgaris Schrad. ex Eckl. & Zeyh. (Cucurbitaceae)
Cyclanthera pedata Schrad. (Cucurbitaceae)
Fabaceae p.p.
Helianthus annuus L. (Asteraceae)
Lamiaceae (almost all species)
Luffa aegyptiaca Mill. (Cucurbitaceae)
Papaver rhoeas L. (Papaveraceae)
Ricinus communis L. (Euphorbiaceae)
Woody–RSOP
Acer pseudoplatanus L., A. saccharinum Wangenh.
(Aceraceae)
Aesculus hippocastanum L. (Hippocastanaceae)
Camellia thea Link ¼ C. sinensis Kuntze (Theaceae)
Castanea sativa Mill. (Fagaceae)
Eriobotrya japonica (Thunb.) Lindl. (Rosaceae)
Fagus sylvatica L. (Fagaceae)
Litchi chinensis Sonn. (Sapindaceae)
Quercus spp. (Fagaceaee)
Salix caprea L. (Salicaceae)
Cocos nucifera L. (Arecaceae)
Woody–OSRP
Bauhinia forficata Link (Fabaceae)
Betula alba L. ¼ B. pendula Roth. (Betulaceae)
Betula alnus L. ¼ Alnus glutinosa Medik. (Betulaceae)
Celtis laevigata Willd. (Ulmaceae)
Liquidambar styraciflua L. (Hamamelidaceae)
Opuntia spp. (Cactaceae)
Ostrya carpinifolia Scop. (Corylaceae)
Pistacia vera L. (Anacardiaceae)
Woody–RSRP
Carya oliviformis (Michx.) Nutt. (Juglandaceae)
Corylus avellana L. (Corylaceae)
Juglans regia L. (Juglandaceae)
Laurus nobilis L. (Lauraceae)
Mangifera indica L. (Anacardiaceae)
Persea gratissima C.F. Gaertn. ¼ P. americana Mill.
(Lauraceae)
Populus tremula L. (Salicaceae)
Populus spp. (Salicaceae)
Ulmus campestris L. ¼ U. minor Mill. (Ulmaceae)
Herbaceous–RSRP
Nuphar spp. (Nympheaceae)
Nymphaea spp. (Nympheaceae)
Posidonia spp. (Zosteraceae)
Spartina anglica C.E.Hubb. (Poaceae)
Zostera spp. (Zosteraceae)
Herbaceous–OSRP
Callitriche truncata Guss. (Callitrichaceae)
Celosia cristata L. ¼ Amaranthus purpureus Noronha (Amaranthaceae)
Cucurbita pepo L. (Cucurbitaceae)
Helianthus tuberosus L. (Asteraceae)
Nelumbium speciosum Willd. ¼ Nelumbo nucifera Gaertn.
(Nelumbonaceae)
Parietaria judaica L. (Urticaceae)
Spinacia oleracea L. (Chenopodiaceae)
Thalictrum minus L. (Ranunculaceae)
Orchidaceae (only massulatae orchids)
Poaceae (almost all)
Herbaceous–RSOP
Sechium edule Sw. (Cucurbitaceae) (Lira and Nee, 1999)
Crinum capense Herb. (Amaryllidaceae)
The examples are derived from original observations and from literature cited in the text. Where possible, nomenclature is according to Index
Kewensis (available online). OSOP, orthodox seed and orthodox pollen; RSOP, recalcitrant seed and orthodox pollen; OSRP, orthodox seed and
recalcitrant pollen; RSRP, recalcitrant seed and recalcitrant pollen. The categories for pollen in this table are inferred from personal observations,
from Nepi et al. (2001), and from Franchi et al. (2002). Those for seeds are from personal observations and are also from Baskin and Baskin
(1998), Dickie and Pritchard (2002), Farnsworth (2000), Hong et al. (1996, 1998), and Piotto and Di Noi (2003).
isolation. Thus, the absence or incomplete expression of any
factor proposed to confer dehydration tolerance could have
profound consequences on the ability of a seed to withstand
dehydration below a particular level (Berjak and
Pammenter, 2003). For instance, Ismail et al. (2010) report
the presence of dehydrins in the semi-viviparous seeds of the
Pollen and seed desiccation tolerance | 5273
mangrove Rhizophora mucronata Lam., suggesting that the
possible presence/role of dehydrins in recalcitrant seeds
should be reconsidered. In the case of pollen, recalcitrant
pollen can survive desiccation only if the water content
decreases slowly, as demonstrated by Barnabás and Rajki
(1981) in corn.
Therefore, biochemical mechanisms permitting and regulating orthodoxy and recalcitrance of seeds are fairly well
known and could presumably be at least partly the same as
in pollen. Differences in desiccation tolerance mechanisms
between pollen and seeds could be largely due to adaptations necessary for survival of such an extremely small
structure as pollen during dispersal. Protective sugars as
well as other protective molecules also have been found in
the cytoplasmic membrane and small vacuoles of pollen
(Buitink et al., 2002; Hoekstra, 2002; Pacini et al., 2006).
LEA proteins, also known as dehydrins, are found in seeds
and also in pollen of at least some species, for example
Typha latifolia L. (Wolkers et al., 2001) and lilies (Mogami
et al., 2002). In addition, antioxidants probably are also
active in pollen and seeds (Almaraz-Abarca et al., 2007;
Ohta et al., 2007; Frank et al., 2009; Moore et al., 2009). In
Typha pollen, LEA proteins are thought to act with other
sugars such as sucrose to form a ‘network in the dehydrating cytoplasm, thus conferring long-term stability’ (Wolkers
et al., 2001). According to Kosová et al. (2007), LEA
proteins also have a role in the general response of plants to
cold. Moreover, the presence of carbohydrates in pollen
walls, especially thick walls, can sustain changes in cell
volume due to changes in water content (Moore et al., 2006,
2008). With regard to pollen and seeds, developmental
arrest enables faster and safer dispersal without the medium
of water and hence under a variety of environmental
conditions. Developmental arrest involves the appearance
of new genes for adaptation to life in a completely different
environment. Among these genes, those enabling homeostasis of certain biological parameters, and hence survival
during dispersal, are fundamental (Footitt and Cohn, 2001).
Phylogenetic distribution of desiccation
tolerance
Pollen of almost all extant gymnosperms is orthodox, with
the exception of a few species of Araucariaceae and Gnetum
(Faegri and van der Pijl, 1979; Caccavari, 2003; Sousa and
Hattemer, 2003; Yao et al., 2004; Bittencourt and Sebbenn,
2007, 2008). Similarly, seeds of almost all living gymnosperms are orthodox, with the exception of certain Podocarpaceae (which, however, have an orthodox saccate
pollen) and Araucariaceae (Farrant et al., 1989; Fountain
et al., 1989; Del Zoppo et al., 1998; Dickie and Pritchard,
2002).
In angiosperms, on the other hand, the occurrence of
recalcitrance seems to be more related to environmental
constrains than to taxonomy, although, at least for what
concerns seeds, some families are very uniform (Hong et al.,
1996).
Vivipary, cleistogamy, and recalcitrance
The final phase of pollen and seed development before
release into the environment is characterized initially by
attainment of final dimensions, after which pollen and
orthodox seeds undergo processes such as the secession of
cell division prior to being dispersed. Growth and development would not be feasible during dispersal. Developmental arrest in seeds of Arabidopsis exhibits two
phases. Both are controlled by specific genes involved in
hormone synthesis and other processes such as cell division
activities, but in certain Arabidopsis mutants with developmental arrest these processes may not occur (Raz et al.,
2001). One of the big problems with trying to store
recalcitrant seeds is that many species have non-dormant
seeds; that is, metabolism is continuous (Berjak and
Pammenter, 1997), and seeds may begin the initial steps for
germination prior to dispersal (Brown et al., 2001). Thus,
unless sufficient water is present after seed dispersal to allow
germination to continue, seeds will die very quickly
(Farrant et al., 1986; Fig. 2).
Under high humidity, pollen of some species germinates
inside the anther due to failure of developmental arrest
(Pacini and Franchi, 1982). This was observed in noncleistogamous species, with either orthodox (e.g. Malus
domestica Borkh. and Olea europaea L.) or recalcitrant (e.g.
Arum italicum Mill., Philodendron spp., and Cucumis melo L.)
pollen. The occurrence of germinated pollen inside anthers
was explained as a failure of anther desiccation and
dehiscence due to rain just before or during flower opening.
Another independent or concomitant cause of pre-dispersed
pollen germination seems to be the occurrence of genetic
anomalies (originating mostly as spontaneous mutations)
that are preserved in vegetatively reproduced cultivated
plants. For instance, only seven of 48 cultivars of
O. europaea had a certain percentage (1–5%) of pollen
germinated inside the anthers. The amount of germinated
pollen grains, as well as the length of pollen tubes, is very
variable from cultivar to cultivar. Also, in the other species
with pre-dispersal pollen germination there is much variability from species to species (Pacini and Franchi, 1982), and in
A. italicum it may be up to 60% (Pacini and Franchi, 1982).
In every case, and independently from their hydration status,
these germinated grains are lost for reproductive purposes. In
contrast, in the case of cleistogamous species, all the grains
germinate inside an anther, and pollen tubes are able to
reach the stigma and fertilize the egg in the ovule (Lord,
1981; Pacini and Franchi, 1982; Culley and Klooster, 2007).
Culley and Klooster (2007) state that our understanding of
the genetics of cleistogamy is still in an early stage.
Developmental arrest with consequent desiccation does
not occur in extreme conditions of recalcitrance, namely
cleistogamy in pollen (Culley and Klooster, 2007) and
vivipary in seeds (Lee and Harmer, 1980; Elmqvist and
Cox, 1996; Farnsworth, 2000; Cota-Sánchez, 2004). In
cleistogamy, the pollen is not dispersed, whereas in vivipary
the germinated seed may be dispersed, for short or long
distances, depending on the species (Fig. 1). It is noteworthy
5274 | Franchi et al.
that cleistogamy is not always the general rule for a certain
species, and it is often triggered by temporary conditions
(Culley and Klooster, 2007). However, although vivipary of
seeds is the rule for some species (e.g. Rhizophora mangle L.
that grows in tropical intertidal areas), it also may be
induced in seeds of rice and wheat by extreme environmental conditions such as high humidity, thus preventing
dehydration prior to dispersal. In other cases in which the
causes are genetic, vivipary is restricted to certain varieties
of a species (Elmqvist and Cox, 1996).
Many comparisons have been made between recalcitrant
and viviparous seeds (e.g. see Farnsworth, 2000), because
the characteristic common to both is intolerance to drying.
On the other hand, there have been no genetic or biochemical studies linking the behaviour of pollen of cleistogamous plants with recalcitrance. Pollen recalcitrance and
cleistogamy are different evolutionary outcomes that should
not be confused, even if they are induced by similar
environmental pressures such as return to a permanently or
temporarily wet environment or to a totally or partially
submerged habit.
In viviparous seeds dispersed after germination, there are
nevertheless mechanisms to keep the developing embryo
from drying out, though in a much more limited percentage
than for orthodox seeds. In mangroves, for example,
survival is ensured by the fact that the germinated seeds fall
into the sea/lagoon water. In contrast, in a normally arid
environment such as that colonized by Cactaceae, survival
of the germinating viviparous embryo is ensured by the fact
that it is inside a fleshy fruit (Cota-Sánchez, 2004; CotaSánchez et al., 2007).
Presentation and dispersal
For both pollen and seeds, there are two phases between
maturation and germination: presentation to dispersing
agents by the parent plant and dispersal away from the
parent plant (Figs 1, 2). If the dispersal agent is wind,
however, pollen and seeds may be dispersed as soon as the
anther or fruit opens, bypassing the presentation phase. In the
presentation phase, completely ripe pollen or seeds are
exposed to the environment, for example seeds of dehiscent
fruits. The term presentation is normally used for pollen
(Pacini, 2000; Pacini and Hesse, 2004) and less so for seeds,
although its meaning is clear for seeds. In some species, pollen
and seeds await dispersal, protected in the anther or fruit
(often indehiscent and/or fleshy). During presentation and
dispersal, pollen may absorb or lose water within speciesspecific limits. The ability to gain and lose water is due to
special wall structures of pollen and colloidal properties of the
cytoplasm, and is known as harmomegathy. This term
was coined to indicate grain-controlled volume and shape
variations due to loss of water during anther and pollen
dehydration and absorption of water supplied by the stigma
upon pollination (Wodehouse, 1935; Pacini, 1990). To date,
harmomegathy has been demonstrated only in orthodox
pollen (Lisci et al., 1994; Guarnieri et al., 2006). During
pollen presentation and dispersal, on the other hand,
orthodox grains can maintain a constant water content. At
least in some species, carbohydrates may be converted from
one form to another in response to changes in environmental
variations, thereby avoiding uncontrolled uptake or loss of
water. For instance, a decrease of relative humidity determines the hydrolysis of sucrose, increasing the turgor pressure
of the grain and hindering water loss (Guarnieri et al., 2006).
On the other hand, recalcitrant pollen loses water rapidly and
dies, especially in dry environments (Nepi and Pacini, 1993;
Franchi et al., 2007; Fig. 2). The ability of orthodox pollen to
regulate the loss or uptake of water presumably does not have
an equivalent in seeds. Seeds with water-permeable coats
imbibe and lose water in response to fluctuations in moisture
levels of the substrate, and, if the seed is dormant and
orthodox it could be imbibed and dehydrated many times
before it germinates. On the other hand, seeds (or fruits) with
water-impermeable coats slowly lose water as they mature
and after they become impermeable cannot imbibe water until
the water gap on the seed (or fruit) coat opens. Orthodox
seeds have either water-permeable or water-impermeable
coats, but recalcitrant seeds only have water-permeable coats
(Baskin and Baskin, 1998). Recent research has shown that
transcriptional and post-transcriptional processes occur even
in dry seeds (Holdsworth et al., 2008).
Developmental arrest is a mechanism that enhances
dispersal by limiting damage to pollen and seeds. Without
the mechanisms described in the section ‘Hydration status’
that enable lasting arrest, seeds and pollen are recalcitrant
or desiccation sensitive and have to be dispersed in a fast
and efficient manner if they are to survive. Hence the time
and manner of dispersal are important in relation to
whether or not there is developmental arrest.
If death is to be avoided during dispersal in unfavourable
and changing environments, pollen must have homeostatic
mechanisms, and seeds must be desiccation tolerant. Seed
dormancy does not substitute for desiccation tolerance, and
some recalcitrant seeds are dormant and require prolonged
periods of moist conditions for dormancy break to occur.
Strategies to reduce damage during pollen dispersal differ in
relation to climate, herbaceous or tree habit, and plant
sociality; that is, whether they grow isolated or in dense
populations in which pollen can be dispersed quickly over
short distances. These conditions that affect natural dispersal of pollen can have great repercussions on the
reproductive efficiency of a species.
During dispersal of pollen and seeds, cell differentiation,
development, and division do not occur, whereas metabolic
status may vary; the water content of pollen and seeds may
vary from 2% to >60% (Baskin and Baskin, 1998; Franchi
et al., 2002). However, there are large species variations
among seeds and among pollen. Seeds are complex structures in which not all components (and not even the various
parts of the embryo) always have the same water content
(Connor and Sowa, 2004; Evered et al., 2007), whereas
pollen should have a more uniform water content due to its
limited cell number and small size.
Pollen and seed desiccation tolerance | 5275
From a physical point of view, dispersal of pollen and
seeds over long distances should occur at the end of
dehydration, when they have reached their minimum mass.
However, there are exceptions of pollen and seeds being
dispersed before they have dehydrated, and these are
dispersed: (i) by biotic vectors (e.g. pollen in massulae of
orchids and groups of seeds dispersed inside fruits); (ii) over
short distances (pollen of plants such as many annual
Poaceae growing close together); or (iii) within a particular
environment (pollen of Cactaceae, coconuts). The above
examples refer to special adaptations facilitating a fast
dispersal of recalcitrant pollen and seeds; in particular,
many features help retain water. (i) Pollen and seeds are not
exposed in such a way that they lose water but are protected
by reproductive structures. Typical of recalcitrant pollen is
the massula of orchids (i.e. a group of up to hundreds of
thousands of grains packed together) that lies protected in
the anther until a pollinating insect touches the viscidium,
a viscous appendage responsible for ejection of the massula
from the anther that sticks to the insect body until it visits
another flower (Pacini, 2009a). A similar situation occurs in
massulate pollen of the Asclepiadaceae (Dannenbaum and
Schill, 1991; Franchi et al., 2002; Pacini, 2009b). Seeds
produced in fleshy fruits are protected from dehydration by
the water content of the flesh. Avocados and mangos are
good examples, with a single large recalcitrant seed inside
the fruit with a well developed mesocarp, and the loquat is
another example with 2–5 seeds. (ii) Pollen and seeds have
water reserves in their outermost parts. Water reserves
occur in pollen grains when they have a particularly thick
intine (the inner layer of the pollen walls) containing pectins
or other polysaccharides that retain water. Examples are the
taxoid pollen (wingless and with a thick intine) of gymnosperms (Pacini et al., 1999; Chichiriccò and Pacini, 2008)
and pollen of some or all members of certain angiosperm
families, including Lauraceae, Salicaceae, and various
monocots. In these taxa, the exine (the outer flexible layer
of pollen walls) of the pollen grain is discontinuous,
reduced, or absent, while the intine is thick and contains
carbohydrates such as pectins that retain water even during
presentation and dispersal (Kress, 1986).
The external layer of some seeds contains mucilage,
which expands after coming into contact with water and
potentially might act as a water reserve that can be used to
complete or accelerate rehydration and/or initiate and
complete germination. It has been suggested that mucilage
prevents seeds from complete desiccation and protects
against drought stress during germination and early seedling growth (Gutterman, 2001). When seeds are released
from fruits on to moist soil, mucilage allows seeds to adhere
to the soil crust as drying occurs, thereby preventing the
seeds from being taken by seed predators such as ants
(Gutterman, 2001; Thapliyal et al., 2008).
Elaiosomes (seed appendages containing food rewards for
dispersal by ants) of seeds such as Ricinus also have a role
during dehydration, rehydration, and germination of seeds
because they temporarily store water in thick pectocellulose
walls (Bianchini and Pacini, 1996a; Pacini et al., 2008).
Other examples of dispersing strategies in recalcitrant pollen
and seeds are discussed below. During presentation to
dispersing agents and/or dispersal, weather conditions may
not remain constant. If the environment is very dry,
recalcitrant pollen and seeds lose water, and a subsequent
increase in humidity does not always lead to replenishment
of water that was lost. Beyond a certain level of dehydration that varies from species to species, damage to the
cellular structure of both recalcitrant pollen and seeds is
irreversible and so extensive that death occurs (Fig. 2).
Four combinations of orthodoxy and
recalcitrance in seeds and pollen of plants
The combinations of orthodoxy and recalcitrance of pollen
and of seeds make up four categories: (i) orthodox seeds and
pollen (OSOP); (ii) recalcitrant seeds and pollen (RSRP); (iii)
recalcitrant seeds and orthodox pollen (RSOP); and (iv)
orthodox seeds and recalcitrant pollen (OSRP). Table 1
shows some representative examples of species, genera, and
families belonging to these four categories. Herbaceous
species and trees are separated because habit and size can
influence dispersal mode and distances travelled by dispersed
structures, especially those that are orthodox.
Of these four categories of plants, the most frequent one
is species with orthodox pollen and orthodox seeds,
a condition in which seeds and pollen are programmed
physiologically to last a long time and to travel far. In
desiccation-sensitive pollen or seeds, pollination and/or
dispersal is faster and relies on particular devices and
strategies. When both pollen and seeds are recalcitrant,
adaptations vary in relation to habit. For example, trees
and shrubs have faster dispersal of pollen and seeds, with
mechanisms enabling exploitation of a fast vector and/or
choice of the best environmental conditions for dispersal. In
herbaceous plants, there are different examples of evolution
involving a return to aquatic life (e.g. Nymphaea and
submerged monocots) (Cook, 1990). Species with desiccation-intolerant pollen or seeds, or both, are more exposed to
failure of sexual reproduction because these structures
survive for a shorter period of time; herbaceous plants with
this characteristic are often perennial. Vegetative reproduction is a consistent feature in perennial herbs (and many
trees) having recalcitrant pollen or seeds. Normally, recalcitrant seeds are not dormant, but there are some
exceptions such as Aesculus hippocastanum (Pritchard et al.,
1996). However, contrary to the expectation that seeds of
aquatic plants (both marine and freshwater) are likely to
have recalcitrant seeds, Hay et al. (2000) reported that seeds
of only 6.9% of 87 species were recalcitrant, while 74.7%
were orthodox; the remaining 18.4% had intermediate
behaviour.
Species in the same family or genus often belong to
different categories of desiccation sensitivity or tolerance of
seeds and pollen (Table 1, Figs 3, 4). From a taxonomic
point of view, recalcitrance has a scattered pattern of
distribution, and it rarely follows phylogeny. Indeed,
5276 | Franchi et al.
irrespective of systematic relationships, ecological pressure
induced by a given environmental niche seems to favour
recalcitrance (Pammenter and Berjak, 2000). In some cases,
a phylogenetic approach leads to correct predictions about
desiccation sensitivity, but there are generally too many
exceptions to establish the behaviour of a seed on the basis
of systematic criteria alone.
An example of a systematic/phylogenetic correlation
prediction is found in the Poaceae, which are thought to be
the ancestors of marine monocots. The Poaceae have
spherical recalcitrant pollen that is considered to be the
starting point for the trend leading to species having
underwater pollination such as the Cymodoceaceae with
advanced filiform exineless (recalcitrant) pollen (McConchie
and Knox, 1989). For example, rice pollen (Poaceae) is
‘short-lived, with most pollen grains losing viability after
;5min under typical environmental conditions’ (Oka,
1988). Grass pollen morphology changes dramatically after
shedding, initially being spherical but collapsing within
minutes due to water loss (Lansac et al., 1994, and
references therein). The collapse of the pollen coincides with
a measurable loss of viability (Pacini et al., 1997).
A distinction should be made between aquatic species
pollinating in air and those with submarine or surface
(floating) pollination. The former have partially or only
slightly dehydrated pollen, and the latter have fully hydrated
pollen and reduced, discontinuous, or no exine (Ackerman,
1995; Pacini and Franchi, 2000; Franchi et al., 2002). From
this point of view, the genus Callitriche is interesting because
it contains species at different stages of this evolutionary
pathway: terrestrial, amphibious, and obligate submerged.
The wetter the environment, the thinner the exine, which
disappears completely in totally submerged species such as
C. truncata Guss. (Cooper et al., 2000). On the other hand,
seeds of all species of Callitriche should be orthodox since the
family Plantaginaceae to which they belong is orthodox and
very uniform (Hong et al., 1996). Other examples of species
in the same genus with different pollen behaviour include
sunflower (Helianthus annuus L.) and Jerusalem artichoke
(H. tuberosus L.). Sunflower pollen is dispersed during the
high temperature period of the day in summer and is
unaffected by exposure to air during presentation (Pacini,
1996) and transport by insects. On the other hand, Jerusalem
artichoke pollen is recalcitrant, and this may be due to
flowering time, which is between September and November,
in a period when days are often rainy and relative humidity is
very high (Franchi et al., 2002). Moreover, unlike the annual
sunflower, the perennial Jerusalem artichoke relies heavily on
vegetative reproduction.
There are also genera with species showing a different
seed behaviour with respect to recalcitrance/orthodoxy. An
example is that of certain palms that grow in rainforests
[Phoenix roebelinii O’Brien and Syagrus schizophylla (Mart.)
Glassman] and have recalcitrant seeds, while congeners in
dry environments (Ph. rupicola T. Anderson, Ph. sylvestris
Roxb., Ph. theophrasti Greuter, S. botryophora Mart.,
S. flexuosa Becc., and S. yungasensis M. Moraes) have
orthodox seeds (Pritchard et al., 2004). Another case is the
genus Acer, in which species may have very different seed
storage behaviours, although their distributions overlap. In
continental Europe, Acer pseudoplatanus L. produces recalcitrant seeds, whereas A. platanoides L. has orthodox
seeds. In the eastern USA, A. saccharinum L. has recalcitrant
seeds and A. saccharum Marshall has orthodox seeds, and in
the western USA A. circinatum Pursh has orthodox seeds
and A. macrophyllum Pursh has intermediate seeds (Hong
et al.. 1998).
Interestingly, when orthodox pollen of many species is
launched, as in Ricinus, it is already dehydrated, and the
expulsion mechanism is basically linked to dead anther cells
(mechanical layer). In contrast, recalcitrant Parietaria
pollen is actively launched by the anther filament that
consists of live cells (Bianchini and Pacini, 1996b; Franchi
et al., 2007).
Grouped pollen and seeds dispersed en
masse
Pollen from the same anther and seeds from the same fruit
may be dispersed singly or clustered in various ways that
facilitate transport by vectors. Clumped pollen may give rise
to a great number of types of pollen dispersal units (Pacini
and Franchi, 1998, 2000), which facilitate not only transport but also reproduction, with specific adaptations of the
female counterpart (Pacini and Franchi, 1998). Pollen
grains may be joined or clumped together by viscous fluids
of different origins, filaments of different natures, or
common walls. Up to 13 different pollen dispersal units
were recognized (Pacini and Franchi, 1998, 1999), and
recalcitrant pollen grains are found in almost all of them
(Franchi et al., 2002), for example Typha has recalcitrant
pollen arranged in tetrads (Fig. 4C). The family Orchidaceae is emblematic of and useful to demonstrate the
passage from orthodoxy to recalcitrance in pollen, as well
as the change in how pollen is dispersed. In the more
primitive groups of this family, such as Pterostylis plumosa
Cady, pollen is orthodox and dispersed as monads; that is,
isolated grains (Pandolfi et al., 1993). In the more highly
evolved orchids, pollen is dispersed in compact groups with
very reduced spaces between the grains (the so-called
pollinia), which may contain hundreds of thousands of
grains as in Loroglossum hircinum (L.) Rich. (Pandolfi and
Pacini, 1995) and Calypso bulbosa (L.) Oakes (Proctor and
Harder, 1995). Because the pollen is tightly packaged, water
loss before dispersal is unlikely. Thus, pollen grains that
developed synchronously avoid damage due to water loss
prior to dispersal from the anther (Pacini and Hesse, 2002;
Pacini, 2009b).
For both pollen and seeds, dispersal represents a way to
go far from the mother plant and from offspring remaining
at the site occupied by the mother plant. With some
exceptions, there are always forms of competition among
pollen and among seeds during development and during
and after germination. Competition during development is
mainly for nourishment and is stressed by drought, leading
Pollen and seed desiccation tolerance | 5277
to death of some pollen and seeds/fruits, in particular those
in less advanced stages (for instance fruit thinning). In
pollen, the cytoplasm of dead grains is destroyed and
probably recycled, while only large starch grains remain
inside (personal observation). Two different orders of
competition, on the other hand, occur in pollen and seeds
after germination. In the case of seeds, it is mainly a matter
of space (for further development of plantlets), while for
pollen it depends mainly on the number of ovules per ovary
(Pacini and Franchi, 1998). Competition ceases only during
developmental arrest because exchange with ‘the outside
world’ is suspended.
Orthodoxy, recalcitrance, and environment
If acquisition of desiccation tolerance by seeds of plants that
conquered dry land is understandable from an evolutionary
point of view (Pammenter and Berjak, 2000), it is more
difficult to explain why there are organs having different
desiccation sensitivity in the same species (recalcitrant pollen
and orthodox seed, or vice versa). The reason for the
existence of the four categories (OSOP, RSOP, OSRP, and
RSRP) in angiosperms (Table 1) may be an evolutionary
response to one or more of the following: (i) distribution of
the species in environments with a permanently hot humid
climate without periods of stress; (ii) natural distribution of
the species in particularly severe environments, at least in
certain periods of the year, making developmental arrest of
seeds or pollen functional for survival; (iii) the presence of
species in environments particularly favourable during seed
dispersal but hostile during pollen dispersal (or vice versa);
(iv) return to water in certain systematic groups, both marine
and freshwater, such as marine monocots; (v) nocturnal
blooming with special adaptations for arid conditions (e.g.
many Cactaceae); (vi) brief pollination, often due to plant
sociality (i.e. the presence of plants of the same species in
a restricted area), short anemophilous pollen flight (e.g.
grasses, Parietaria), or movement for short distances by
pollinators; and (vii) special strategies as in C. nucifera L.,
the recalcitrant seed of which has exceptional structure and
biomass that enable it to remain undamaged during long
periods of floating in the sea.
The four categories evidently exist because pollen and
seeds have independent behaviour. The phenomenon is
probably under genetic control, and genes are activated
independently in seeds and pollen according to the biological and adaptational strategies of each species for
reproductive success. Genetic action may also be conditioned by environmental constraints (see ‘Vivipary, cleistogamy, and recalcitrance’). It may seem surprising that plants
such as mangroves have viviparous seeds and orthodox
pollen. The strategy ensures dispersal of genes of paternal
origin over long distances.
Most plants of agricultural interest in temperate climates
have orthodox seeds. The situation for pollen is more
varied. For example, all Poaceae have recalcitrant pollen in
all environments, but dispersal is over a short distance,
a few centimetres in a few seconds for annuals (e.g. wheat
or corn) or a little further for perennials such as sugarcane,
which also has vegetative reproduction. However, the
quantity of water in pollen and seeds and their resistance to
desiccation vary within the family (Hoekstra et al., 1988).
A hot humid environment favours presentation and
dispersal of recalcitrant pollen and seeds (Franchi et al.,
2002; Tweddle et al., 2003). Tweddle et al. (2003) report
that woody pioneer rainforest species have orthodox seeds,
whereas only 45.2% of non-pioneer species in the same
environment have orthodox seeds. This suggests that the
spread of pioneer species is more reliable with seeds that
tolerate stress (orthodoxy) than it would be if they had
recalcitrant seeds. Many families with primarily or exclusively humid tropical distributions such as Musaceae,
Cannaceae, and Heliconiaceae have recalcitrant pollen
(Kress, 1986; Franchi et al., 2002).
Conclusions
From the evolutionary viewpoint, the transition from desiccation sensitivity to desiccation resistance of organs and/or
tissues marks the first phases of the conquest of dry land by
plants. Gametophytes of Bryophyta are the first example of
desiccation tolerance developed in land plants (while moss
spores are always recalcitrant because of the presence of
vacuoles and chloroplasts), followed by spores of certain
ferns (Alpert and Oliver, 2002). When orthodoxy becomes
the rule, the back-transition of pollen and seeds from
orthodoxy to recalcitrance indicates an adaptation to a particular ecological factor, which plants may depend on for
surviving in extreme environments (amphibious or underwater environments) or because of certain ecological conditions
(nocturnal pollination and seed dispersal in wet periods as in
acorns of oaks released in autumn). A pause in the
development of reproductive structures, as occurs in desiccation-tolerant structures, makes it possible to wait for more
favourable conditions, avoid unfavourable moments, travel
further, and favour crossing between different partners.
When possible, orthodoxy is the more favoured condition.
Recalcitrance is taxonomically scattered in such a way
that members of a family do not always have both
desiccant-sensitive pollen and seeds. Recalcitrance means
the lack of genetic mechanisms that build desiccation
tolerance (e.g. protective substances such as LEA, efficient
antioxidant systems, or the capacity to switch off metabolism), but it is nonetheless genetically regulated and may
appear in response to particular environmental pressures.
Sometimes the environment rewards speed in reproductive
processes such as, for example, plants with short intervals
between the start of presentation and germination, thus
decreasing the risk associated with environmental adversities. On other occasions, recalcitrance becomes necessary,
as in the pollen of submerged monocots, because it cannot
desiccate. Further cases for the necessity for recalcitrance
are when there is an adaptation to environmental conditions
optimal for pollen or seed dispersal, such as at different
5278 | Franchi et al.
times of the day (Cactaceae pollen) or of the year, as in
H. tuberosus pollen and in large recalcitrant seeds (e.g. oaks,
chestnuts, and horse chestnuts) of temperate environments,
released at the onset of autumn. A similar case is that in
which there are particular structural modifications of male
and female reproductive structures for attracting vectors
that may even be species specific, as for orchids (Pacini
et al., 2008; Pacini, 2009b).
In recent years, many environmental changes have been
documented (Woodward, 1992; Cleland et al., 2007; Hedhly
et al., 2008) such as the rise of minimal temperatures,
decrease or sudden increase of annual rainfall concentrated
in short periods, and increase in air relative humidity. All
these changes greatly influence vegetative and reproductive
development of plants, as well as dispersal of pollen and
seeds. In particular, dispersal per se is influenced, without
even considering further influences of these changes on biotic
pollinators and seed dispersers. For instance, the increase in
humidity and frequency of heavy rains greatly affect flower,
anther, and dry fruit opening, anther and fruit ripening, and
seed dispersal in dry fruits. These environmental changes
differentially affect orthodox and recalcitrant pollen and
seeds and may lead to modifications in reproductive
ecophysiology of the floras in various parts of the world.
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