Download THE SOIL ASSOCIATION APPRENTICESHIP SCHEME

Future Growers 2013
Module 3: Plant biology, classification and
seeds
Briefing paper
Overview
1. Plant Biology and Classification
- Classification by life cycle
- Classification by ecological growth form
- Classification by life form
- Classification by plant growth patterns
- Binomial nomenclature
- Plant part modifications
- Plant hormones
- Tropisms and nastic movements
2. Plant Structure
- xylem and phloem
- pressure flow hypothesis
- typical plant body
3. Introduction to seeds
4. Seed regulations
5. How seeds are produced
6. Varieties
7. Trialling varieties
8. Seed saving
2
Plant Biology and Classification
There are various classification methods for identifying the difference between
plants. They are:
- Life cycle
- Ecological growth form
- Life form (Raunkiaers system)
- Plant growth patterns
- Binomial nomenclature
Binomial names are usually italicised when printed and underlined when written
Genus
Species
Variety
Cultivar
Capitalised
Not capitalised
Capitals
Inverted commas
Botanical names positively identify plants and distinguish between related varieties.
Common names may differ regionally e.g. the vegetable whose botanical name is
Solanum melongena is called an eggplant in North America and an aubergine in the
UK.
Crop rotation requires knowing if crops are in the same family. Plants in the same
botanical group have common growth requirements and are susceptible to the same
diseases and pests.
Plants in the same species can cross pollinate which is important when deciding
where to plant crops.
Classification by Life Cycle
There are 4 groupings of herbaceous plants based on life cycle. They are:
 Ephemeral: can go through more than one life cycle form from seed
germination to seed production and death in one growing season.
 Annual: can go through their entire life cycle from seed germination to seed
production and death in one growing season.
 Biennial: have a two year growing cycle. The first year includes germination,
leaf, root, compact stem and stored food production. The plant lives through
winter and then the second year it forms a vertical stem, flowers, fruits and
seeds. Then the plant dies.
 Perennial: live for many years forming flowers and seeds each year. The
above ground parts die back in winter and re-grow in spring from the roots.
Classification by Ecological Growth Form
This classification system is based on the climate to which the plants have adapted:
Mesophyte
Temperate
3
Hydrophyte
Xerophyte
Halophyte
Cryophyte
Water
Dry
Salty
Cold
Classification by Life Form (Raunkiaers System)
These groups are based on the belief that the prevailing condition in a given area
determines, by natural selection, which type of plants grow there.
The key to classification is the form a plant takes in the most unfavourable system.
So, the system classifies the plants according to the position of their resting bud.
Phanerophyte
Crytpophyte
Therophyte
Tall perennials with a resting bud > 25cm
above ground level
Resting bud below ground level –
subdivided according to habitat (Geophyte
– below ground e.g. tubers, bulbs and
Hydrophyte – at the bottom of clear water
e.g. Nuphar lutea)
Pass unfavourable season as seed
Classification by Plant Growth Patterns
The vascular system (xylem and phloem) are different in certain groupings of plants:









Monocotyledons have the conducting tubes scattered throughout the stem
Dicotyledons have them arranged in a ring
The arrangement affects the stem’s ability to grow in girth
Monocotyledons leaves are narrow with parallel veins
Dicotyledons have broader leaves with net veining
Monocotyledons thrust up one seed leaf
Dicotyledons push out two seed leaves
Monocotyledons include grasses, sedges, lilies and onions
Dicotyledons include trees, shrubs, most flowers and vegetables
Binomial Nomenclature
This classification system is made according to the number and arrangement of floral
reproductive organs on a plant. However, this narrow focus often produced poor or
unnatural groupings.
Modern classification takes into account each plant’s overall morphology (that is the
form and structure of its roots, leaves, flowers, seeds, fruit and habit) as well as a
plant’s natural distribution (where it grows).
The two names used in this system are genus and specific epithet. There are also
higher taxa or classifications.
4




The genus name is always first and capitalised
The genus is a cluster of plants with common characteristics that are easily
recognised
The specific epithet is second and not capitalised
The species is a group of plants that can generally interbreed only among
themselves
They show persistent difference from members of closely related species. Together
these two names describe the particular plant species.
Binomial nomenclature can give descriptive clues about the plant e.g. colour, form,
who discovered it and where it originated.
Plants are also grouped together within a species. These subgroups are called
varieties and cultivars (cultivated varieties). A variety is a group of plants (showing a
variation within a species) that developed new characteristics through naturally
occurring hybridisation. They are indicated by “var.” in the name.
A cultivar is a new or hybrid plant that is hybridised by human manipulation.
Cultivars don’t reproduce true to form naturally without human intervention i.e. if
you plant a seed from a cultivar you won’t necessarily get the same plant. The
cultivar name is always enclosed in single quotes
Plant Part Modifications
Plants are made up of leaf, reproductive organ, stem and root. All other parts are
modified from one of these.
Leaf
Stem
Root
Reproductive Organ
Climbers
Bulbs
Spines
Corm
Rhizome
Succulents
Tuber
Runners / Stolon
Swollen Stem
Swollen Tap Root
Root Tubers
Flower
Seed
Bud
Fruit
Root
Hook
Twining
Tendrils
Onion
Gooseberry
Cyclamen
Ginger
Prickly Pear
Potato
Strawberry
Swede
Carrot
Jerusalem Artichoke
Nasturtium / Lavender
Coriander
Caper
Apple
Ivy
Blackberry
Runner Bean
Pea
5
Plant Hormones
Plant hormones are also known as plant growth regulators (PGRs) and
phytohormones. They are chemicals that regulate plant growth. Plant hormones are
distinct from animal hormones in that they are often not transported to other parts
of the plant and production is not limited to specific locations.
Plants lack tissues or organs specifically for the production of hormones. Plant
hormones shape the plant, affecting:
- seed growth
- time of flowering
- sex of the flowers
- senescence of leaves and fruits
- which tissues grow upward and which grow downward
- leaf formation
- stem growth
- fruit development
- ripening
- plant longevity
- plant death
Hormones are vital to plant growth and lacking them plants would be mostly a mass
of undifferentiated cells.
Abscisic acid (ABA) effects bud growth, seed and bud dormancy. It mediates change
within the apical meristem causing bud dormancy and the alteration of the last set of
leaves into protective bud covers. Without ABA, buds and seeds would start to grow
during warm periods in winter and be killed when it froze again.
ABA dissipates slowly from the tissues and its effects take time to be offset by other
plant hormones so there is a delay in physiological pathways that provides some
protection from premature growth.
It accumulates within seeds during fruit maturation, preventing seed germination
within the fruit, or seed germination before winter. ABA’s effects are degraded
within plant tissues, during cold temperatures or by its removal by water washing in
out of the tissues, releasing seeds and buds from dormancy.
In plants that are water stressed, ABA plays a role in closing the stomata. ABA exists
in all parts of the plant. Plants start life as a seed with high ABA levels, just before the
seed germinates ABA levels decrease; during germination and early growth of the
seedling, ABA levels decrease even more.
As plants begin to produce early shoots with fully functional leaves ABA levels begin
to increase, slowing down cellular growth in more mature areas of the plant.
6
Auxins
Auxins are compounds that positively influence cell enlargement, bud formation and
root initiation. They promote the production of other hormones and in conjunction
with cytokinins, they:
- control the growth of stems, roots, flowers and fruits
- affect cell elongation by altering cell wall plasticity
- decrease in light and increase where its dark
- Stimulate cambium cells to divide
- Cause secondary xylem to differentiate in stems
- Act to inhibit growth of buds lower down the stems, affecting a
process called apical dominance
- Promote lateral and adventitious root development and growth
- Promote flower initiation, converting stems into flowers
Seeds produce auxins that regulate specific protein synthesis as they develop within
the flower after pollination, causing the flower to develop a fruit to contain the
developing seeds.
Ethylene
Ethylene is a gas that forms from the breakdown of methionine which is in all cells.
Its effectiveness as a plant hormone is dependent on rate of production versus rate
of escape into the atmosphere. Ethylene is produced at a faster rate in rapidly
growing and dividing cells especially in darkness.
New growth and newly germinated seedlings produce more ethylene than can
escape the plant, which leads to elevated amounts of ethylene which inhibit leaf
expansion.
As the new shoot is exposed to light, reactions by photochrome in the plant’s cells
produce a signal for ethylene production to decrease allowing leaf expansion.
Ethylene affects cell growth and cell shape; when a growing shoot hits an obstacle
while under ground, ethylene production greatly increases, preventing cell
elongation and causing the stem to swell.
The resulting thicker stem can exert more pressure against the object impeding its
path to the surface. If the shoot does not reach the surface and the ethylene
stimulus becomes prolonged it affects the stems natural geotropic response, which is
to grow upright, allowing it to grow around an object.
Studies seem to indicate that ethylene affects stem diameter and height, when
stems of trees are subjected to wind causing lateral stress, greater ethylene
production occurs resulting in thicker, more sturdy tree trunks and branches.
Ethylene affects fruit ripening, normally when the seeds are mature, ethylene
production increases and builds up within the fruit resulting in a climacteric event
just before seed dispersal.
7
Gibberellins
Gibberellins or Gas include a large range of chemicals that are produced naturally
within plants and by fungi. They play a major role in seed germination affecting
enzyme production that mobilises food production that new cells need for growth.
Gas produce bolting of rosette forming plants, increasing inter-nodal length. They
promote flowering, cellular division and in seeds, growth after germination.
Cytokinins
Cytokinins or CKs are a group of chemicals that influence cell division and shoot
formation. They also affect inter-nodal length and leaf growth. CKs counter the
apical dominance induced by auxins, they in conjunction with ethylene promote
abscission of leaves, flower parts and fruits.
Tropisms and Nastic Movements
Plants may respond to directional stimuli such as gravity or sunlight this is known as
a tropism. They may also respond to non-directional stimuli such as temperature or
humidity this is known as a nastic movement.
Tropisms in plants are the result of differential cell growth, in which the cells on one
side of the plant elongate more than those on the other side, causing the part to
bend toward the side with less growth.
Phototropism is the bending of the plant toward a source of light. It allows the plant
to maximise light exposure in plants which require additional light for photosynthesis
or to minimise it in plants subjected to intense light and heat.
Geotropism allows the roots of a plant to determine the direction of gravity and
grow downwards.
In contrast to tropisms, nastic movements result from changes in turgor pressure
within plant tissues and may occur rapidly. Thigmonasty (response to touch) in the
Venus fly trap is a good example of this. The traps consist of modified leaf blades
which bear sensitive trigger hairs. When the hairs are touched by an insect or other
animal, the leaf folds shut.
Although the trap is rapidly shut by changes in internal cell pressures, the leaf must
grow slowly in order to reset for a second opportunity to trap insects.
2. Plant Structure
Angiosperms are flowering plants and there are over 275,000 named species. Within
the angiosperms there are two plant groups, monocots and dicots. There are some
general trends which allow for distinction between the two groups outlined below:
8
Floral Arrangement
Leaf venation
Vascular bundles
Habit
Roots
Growth
Examples
Monocots
3’s
Parallel
Scattered
Herbaceous
Fibrous
Primary only
Grasses, maize,
alliums
Dicots
4’s and 5’s
Net
Ring
Herbaceous & Woody
Taproot
Primary & Secondary
Everything else?
Xylem and Phloem
Xylem comes from the Greek word for wood. It transports water from the root up
the plant and is composed mainly of dead cells. It is mainly responsible for the
transportation of water and mineral nutrients throughout the plant.
Xylem sap consists mainly of water and inorganic ions, although it can contain a
number of organic chemicals as well. Two phenomena cause xylem sap to flow. They
are:
- Transpirational Pull
- Root pressure
Transpirational Pull is the evaporation of water from the surface cells to the
atmosphere. It causes a negative pressure in the xylem that pulls the water from the
roots and soil.
If the water potential of the root cells is more negative than the soil, usually due to
high concentrations of solute, water can move by osmosis into the root. This may
cause a positive pressure that will force sap up the xylem towards the leaves. In
extreme circumstances the sap will be forced from the leaf in a phenomenon known
as guttation. Root pressure is most common in the morning before the stomata open
and cause transpiration to begin.
Phloem comes from the Greek word for bark. In vascular plants, phloem is the living
tissue that carries organic nutrients to all parts of the plant where needed and is
composed of still living cells that transport sap. The sap is a water based solution
which is rich in sugars made by the photosynthetic areas of the plant. These sugars
are transported to non-photosynthetic parts of the plant e.g. roots or into storage
structures e.g. tubers or bulbs.
Because phloem tubes sit on the outside of the xylem in most plants, a tree or other
plant can be effectively killed by stripping away the bark in a ring on the trunk or
stem.
With the phloem destroyed, nutrients cannot reach the roots and the tree/plant will
die.
9
Trees located in areas with animals such as beavers are vulnerable since beavers
chew off the bark at a fairly precise height. This process is known as girdling and it is
used in fruit production for vigour control.
The Pressure Flow Hypothesis
The Pressure Flow Hypothesis was proposed by Ernst Munch in 1930 to explain the
mechanism of phloem translocation. A high concentration of organic substance
inside cells of the phloem at a source, such as a leaf, creates a diffusion gradient that
draws water into the cells. Movement occurs by bulk flow; phloem sap moves from
sugar sources to sugar sinks by means of turgor pressure.
A sugar source is any part of the plant that is producing or releasing sugar. During
the plant’s growth period, usually during the spring, storage organs such as the roots
are sugar sources and the plant’s many growing areas are sugar sinks.
The movement in phloem is bi-directional, whereas, in xylem cells it is unidirectional
(upward). After the growth period, when the meristems are dormant, the leaves are
sources and storage organs are sinks.
Developing seed bearing organs (such as fruit) are always sinks. Because of this
multi-directional flow, coupled with the fact that sap cannot move with ease
between adjacent sieve-tubes, it is not unusual for sap in adjacent sieve-tubes to be
flowing in opposite directions. Organic molecules such as sugars, amino acids, certain
hormones and even messenger RNAs are transported in the phloem through sievetube elements.
The Typical Plant Body
10

The Root System
- usually underground
- anchor the plant in the soil
- absorb water and nutrients
- conduct water and nutrients
- food storage

The Shoot System
- usually above ground
- elevates the plant above the soil
- many functions including photosynthesis, reproduction and dispersal
and food and water conduction
- Note: the shoot system includes the leaves and the reproductive
organs, although these will be covered in more detail separately
3. Introduction to Seeds
There are currently a wide range of crop varieties available for farmers to choose
from, many of which are suitable for use in organic systems. Choice of crop and
variety is therefore extremely important. Farmers’ choice is dependent on many
factors some within their control and some outside their control so it is important to
understand some of the issues surrounding organic seed production and variety
selection.
4. Regulations
The regulations for organic farming came into force through European Commission
Regulation 2092/91.
This law feeds down into all European member states where an ‘authoritative body’
takes responsibility for ensuring the legal standards are met. In the case of the UK
this responsibility falls to DEFRA’s Advisory Committee on Organic Standards (ACOS).
The regulations regarding organic seed are encompassed within the EU regulation
2092/91 but there is also a further seed regulation that came into effect on 1
January 2004. This is EU Regulation (EC) No. 1452/2003 which regulates the use of
seeds, vegetative propagating material and seed potatoes in organic farming.
Much of the basic research into crop science is conducted by public sector research
organisations but the majority of commercial plant breeding is done within the
private sector.
11
The UK’s official seed certification system (different from organic certification) is an
independent assurance of quality to growers.Minimum standards apply for varietal
identity, purity and germination capacity. They must be distinct, uniform and stable
(DUS)
BSPB (British Society of Plant Breeders)
BSPB is the representative body for the UK plant breeding industry
The organisation was formed in 1966 after the UK Plant Varieties & Seeds Act 1964
established a legal framework for collecting seed royalties on protected varieties.
This legislation introduced a system of royalty payments on individual plant varieties
known as Plant Breeder’s Rights. The Society has two core functions – royalty
collection and industry representation. Levies paid to BSPB by seed suppliers help to
fund seed research
Garden Organic Heritage Seed Library
The Garden Organic Heritage Seed Library aims to conserve and make available
vegetable varieties that are not widely available. The HSL produces comprehensive
guidelines on seed saving for all vegetable species
www.organicxseeds.co.uk
www.organicxseeds.co.uk is an online database which is designed to help organic
producers search for organic seeds, vegetative propagation material and seed
potatoes.
Organic certification bodies also use the website to authorise derogations, and seed
companies can advertise their organic stock. Licensees registered with organic
certification bodies are registered on the website automatically and can login to
apply for prior permission to use non-organic untreated seed using their licence
number.
5. How seeds are produced
Plant breeding can directly improve the performance of crops in different ways.
Developing crop varieties which convert more of their biomass into productive yield
is the single biggest contributor to improved crop output. Creation of new varieties
of seed is a complex, costly and long process.
Techniques vary between crop species but the main principle is to use selected
parent plants that can be cross-pollinated to produce desirable characteristics e.g.
high yield or resistance to disease.
Conventional plant breeding involves crossing carefully chosen parent plants and
then selecting the best plants from the resulting offspring to be grown on for further
selection. Hundreds of individual crosses are carried out to create seed for the first
filial or F1 generation.
12
The resulting F1 plants are uniform but they can produce hundreds of thousands of
plants in the following generation. The new combinations produced from each cross
are revealed at the second generation (F2). Seed from the best of the F2 plants is
grown on in small rows or plots when the best plants are selected again.
The process is repeated year after year until only the very best plants remain. Once
the best lines are purified to ensure that every plant has the same characteristics the
process of multiplying seed begins. These “inbred” lines are then ready for entry into
official trials.
Genetically modified
Genetically modified crops are prohibited in organic systems. Cell fusion techniques
are a technique of genetic modification according to the IFOAM principles.
Maintaining biodiversity is central to the process of crop improvement. In
conventional systems genetic modification is used to allow individual traits to be
added, modified or deleted from a plant variety. For example, genetic modification
has been used to combat grey mould infestation in strawberries which is not possible
through conventional breeding methods because no resistance genes exist in the
world strawberry germplasm.
Varieties produced using genetic modification must pass through a process of
regulatory scrutiny.
6. Varieties
Organic farmers are largely reliant on varieties supplied by conventional plant
breeders. Some varieties perform well in both organic and non-organic farming
systems but the majority of varieties produced by conventional plant breeders are
designed for use within systems that would routinely be using artificial fertilisers and
pesticides.
Organic growers often require characteristics in their chosen varieties that are not as
important in non-organic systems e.g. varietal vigour where nitrogen availability is
limited. Before any plant varieties can be placed on the market they must undergo
statutory testing under a process known as National Listing. Successful varieties are
placed on a National List or register of varieties approved for marketing.
Factors to consider when choosing varieties
Resilient varieties are the best option as they will provide a stable yield over a range
of conditions.
Choose a variety that is suitable for organic production systems e.g. low input. This
usually means varieties which will have a more developed root system enabling
them to interact with soil micro-organisms.
13
Choose a variety that is suitable for your soil conditions and climate.
Varieties that suppress weeds through rapid early growth and have a high
percentage of soil coverage are particularly important in organic systems as
herbicides and other chemical means of weed control are prohibited. Pest and
disease resistance should also be considered with resistance to air borne diseases
being more important than soil borne diseases which can be managed by rotation.
Consider the quality of the product for the market you are selling into. Factors such
as size, shape and taste will be important to varying degrees e.g. supermarkets and
packing houses have specific demands but box scheme customers are less fussy.
7. Trialling varieties
Variety trialling is a way that growers can select varieties suitable for their
production needs and chosen market. Before choosing which varieties to trial,
growers should have decided on which character traits they want and what pest,
disease or growing problems they expect to encounter.
These requirements should then be matched against the varieties available for
selection. The main source of varietal information is found in seed catalogues and
through the NIAB Recommended Lists. A list of current seed suppliers can be found
on www.organicxseeds.co.uk Information sharing with other organic growers is
vital.
Ways to trial new varieties
Organic growers can test varieties in their own systems by planting small areas of
new varieties, recording their performance and taking note of important
characteristics.
Promising varieties should be tested in two or more seasons to ensure reliable
results and compare performance over a number of years.
Where to access information on professional trials
Trials are normally carried out through organisations such as NIAB (National Institute
of Agricultural Botany) or agricultural colleges. They are carried out either on
research stations or larger commercial farms using replicated field trials to provide
analysable data.
They can be carried out at different sites or in different seasons to help give reliable
data and to assess the adaptability of the variety. General dissemination of trial
results is through trade press, booklets, factsheets, open days and by communication
between growers. NIAB produces the Organic Vegetable Handbook which details the
results of commercial variety trials. This is available from the Soil Association.
14
8. Seed saving
For certain crop species, particularly small grain cereals, growers can opt to save
their own seed for sowing the following year provided care is taken to ensure that
the crop remains healthy.
The concept of Plant Breeder’s Rights was extended in 1994 to cover farm saved
seed as well as certified seed. Since 1996 an industry-wide system for collecting
payments on farm saved seed has operated for certain crop species in the UK.
Payment levels are lower than royalty rates on certified seed and apply only to the
most recent varieties.
The Garden Organic Heritage Seed Library produces guidelines on seed saving for all
vegetable crops.
Seed saving that produces seed of significant quality can be difficult under organic
systems.
15