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 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