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Varietal Selection for drought: The Evidence
Background
Arabica evolved as an understory shrub in Ethiopian forests at elevations between 1700 and 2000 m and exhibits
features typical of shade-adapted plants. As might be expected for an understory tree, most of Arabica coffee’s
roots are found in the first 30 cm of soil depth – since it would be exploiting leaf-litter from overstory shade trees
in its natural habitat, rather than competing with very extensive roots systems of major tree species. Coffee has
higher stomatal resistance than rubber, cashew, rubber, but these trees do better in dry conditions probably
because of deeper roots (Alves de Sena et al., 2007).
Van der Vossen (2005) suggests that since much of the world’s Arabica stock originates from Yemen, where coffee
had perhaps 1000 years to adapt to a very dry climate there, informal selection for drought resistant stock has
already taken place. Bizarrely support for this comes from Burkhardt et al. (2006) who tested drought susceptibility
of coffee landraces in wet (west Rift) and dry (east Rift) regions of Ethiopia; they found the opposite of what they
expected, i.e. greater drought susceptibility in the dryer region.
Coffee’s maximum photosynthetic rates are low and photosynthesis saturates at low photon flux densities (Kumar
and Tieszen 1980). In its native habitats annual rainfall is 1500-2000 mm and exhibits a pronounced seasonal
distribution with several consecutive months of near rainlessness. This has contributed to the substantial drought
resistance shown by coffee (Meinzer et al. 1990b) and to its requirement of a period of reduced water availability
to trigger phenological events, such as floral bud release (Alvim 1960; Crisosto et al. 1992).
State-of-the-art
The range of response of coffee varieties to drought is a complex subject, many factors are involved and no
unequivocal advice can be given about which variety might be best chosen for a specific locality that might then
reliably exhibit drought tolerance. As D’Souza et al. (2010) note, ‘Breeding varieties for drought tolerance and to
achieve higher carbon exchange rate types in a perennial crop like coffee is a long term process and not much
progress has been achieved either in India or elsewhere.’
Response of coffee varieties to drought under field conditions will depend on:
the soil type;
the presence of shade and species of shade;
the extent to which it is tree survival under occasional severe drought that is required; or yield
maintenance during less severe episodes that is sought.
It is not even clear whether Robusta or Arabica is most drought tolerant – the older literature mostly considers
Arabica more drought tolerant than Robusta, but DaMatta (2006) states that there is sufficient variability within
each genome for this assertion to be contested.
Methodological problems exist, since some experiments have involved testing of seedlings and potted plants
which may not have validity for true field conditions.
The situation is further complicated by the likelihood that a coffee variety that is drought resistant may exhibit
greater water use efficiency through reduced sap flow, but this same trait may also render it less productive under
normal conditions.
The following partial information about previous work is provided.
Architecture
Tausend et al (2000a,b) found that the tall variety Typica was less drought resistant than Yellow Caturra and San
Ramon. Meintzer et al. (1996) found that Mokka was less drought resistant than yellow Caturra, Catuai and San
Ramon. They ascribed the differences to the more open architecture of the Typica plant compared to the dwarf
varieties.
On the other hand, Batista et al. (2010) evaluated 15 Coffea arabica cultivars on their water status and leaf
anatomy to indicate the most tolerant cultivars under water stress. The Bourbon Amarelo and Catimor were
considered the most efficient under water stress because these varieties have thicker cuticle, more palisade
parenchyma, bigger central bundle sheath, and higher stomatal density. The other varieties were Obatã, Catucaí
Amarelo 2 SL Paraíso MG 1, Pau- Brasil MG 1, Palma 2, Catuaí IAC 99, Rubi MG 1192, Topázio MG 1190, Mundo
Novo 379-19, Icatú Amarelo IAC 3282, Catiguá MG 2, Sabiá and Siriema Vermelho,.
But Siriema is an inter-specific hybrid between Arabica and Coffea racemosa and is known to be drought-tolerant
(Dias et al 2007). Hence the evidence is conflicting: Typica is supposed to be less tolerant than Catuai because of its
open architecture, but Bourbon Amarelo, another open tall plant is more resistant than Siriema, which itself is
more drought tolerant than Catuai.
Response to irrigation
Carvalho et al (2008) studied response of different Arabica varieties to irrigation in the marginal coffee area of
Coromandel, MG, Brazil. They found that irrigation caused an average 50% rise in yield across all genotypes. The
best response to irrigation was with late maturity types such as Red Obatã, Yellow Catucaí (late) line 30 cv. 2 and
Yellow Bem-te-vi, with 223.3%, 120.9% and 110.9% of yield increase, respectively. On the other hand, irrigation did
not significantly increase yield of early maturity genotypes, e.g. Catucaí 785-15, Yellow Catucaí 24/137 (early) cv.
900 and Siriema. Since the early maturity cultivars differentiate flowers in the beginning of the year, when water
availability is still not a limiting factor, and the late maturity cultivars flower differentiation occurs later on, during
the dry season, the authors suggest that the early maturity cultivars are more adapted to those conditions.
Thedata indicate that early maturity genotypes may be an interesting genetic source for the development of coffee
cultivars more adapted to regions with low rainfall during fall/winter time.
Robusta
There appears to be considerable variation in drought tolerance within robusta coffee (DaMatta and Rena, 2001;
Lima et al., 2002; DaMatta et al., 2003; Pinheiro et al., 2004). Drought-tolerance can be due to deep root systems
(Pinheiro et al., 2004), tissue water status (DaMatta et al., 2003; Pinheiro et al., 2004) or with change of leaf area
(DaMatta et al., 2003) as soil water becomes limiting.
Within C. canephora, extensive observations have indicated that Kouillou endures drought stress much better than
Robusta (Boyer, 1965; Coste, 1992). This is likely to be associated with a presumably higher stomatal sensitivity to
both soil and atmospheric droughts in the former (DaMatta and Rena, 2001). Four clones of robusta kouillou
coffee represent both drought-tolerant (14 and 120) and drought-sensitive (46 and 109A) genotypes (Praxedes et
al, 2006). These results differ from previous observations in the field (DaMatta et al., 2003) and in the greenhouse
(Pinheiro et al., 2004) in which clone 120 was better able to postpone tissue dehydration than clone 46. Clone 120
has been characterized as showing a deeper root system and a more efficient antioxidant system to protection
against both drought- and paraquat induced oxidative stress than does clone 46 (Pinheiro et al., 2004).