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 Capitalised Species Not capitalised Variety Capitals Cultivar 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 Water Xerophyte Dry Halophyte Salty Cryophyte 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 Tall perennials with a resting bud > 25cm above ground level Crytpophyte 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) Therophyte 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 Bulbs Onion Spines Gooseberry Stem Corm Cyclamen Rhizome Ginger Succulents Prickly Pear Tuber Potato Runners / Stolon Strawberry Swollen Stem Swede Root Swollen Tap Root Carrot Root Tubers Jerusalem Artichoke Reproductive Organ Flower Nasturtium / Lavender Seed Coriander Bud Caper Fruit Apple Climbers Root Ivy Hook Blackberry Twining Runner Bean Tendrils Pea

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.

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:

Monocots Dicots Floral Arrangement 3’s 4’s and 5’s Leaf venation Parallel Net Vascular bundles Scattered Ring Habit Herbaceous Herbaceous & Woody Roots Fibrous Taproot Growth Primary only Primary & Secondary Examples Grasses, maize, Everything else? alliums

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 sieve- tube elements.

The Typical Plant Body

 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.