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Lecture 4 Plant Structure, Growth and differentiation Objectives • Define plant nutrition • Outline how nitrogen is taken up by plants • Discuss the mechanism of leaf senescence and abscission. • Explain the concept of fruit and seed dispersal. • Describe process of seed germination including seed dormacy. 1. What is meant by “plant nutrition” Uptake from the soil of mineral elements “Plant nutrition” specifically does not refer to photosynthesis. In this lecture the uptake of nutrients from the soil directly by roots In the next lecture mutualistic relationships between plants and fungi and microrganisms 5. Nitrogen and the effects of soil organic matter on plant nutrition Nitrogen is the element most required by plants, in terms of weight. It is not a product of weathering of soil particles. There are two sources: fixation of atmospheric nitrogen by bacteria decomposition of organic matter, usually decaying plant material. N-fixing bacteria Most uptake from the soil is in the form of nitrate Fig. 32.13 Organic material is important in agricultural soils both as a source of nitrogen and because it can increase water holding capacity, e.g. biosolids application effects A characteristic of non-agricultural soils is accumulation of organic material and acidification of the soil. Such soils typically develop a very distinct stratification, with organic mater at the top. Spodic soil The organic layers in such soils can have a considerable total quantity of nitrogen but little may be available due to the high acidity, and sometimes lack of oxygen, in the organic layer. 6. Leaf senescence and withdrawal of nutrients to the plant Senescence is a term for the collective process leading to the death of a plant or plant part, like a leaf. Leaf senescence is a part of the process by which a plant goes into dormancy and is induced by a change in day length. Changing leaf color As daylength decreases, the plants ability to synthesize chlorophyll becomes reduced. Yellow and orange carotinoids and xanthophylls, always present within the leaf, begin to show. Water and nutrients are drawn into the stems and from the leaves. Senescing cells also produce other chemicals, particularly anthocyanins, responsible for red and purple colors. Some species, particularly oaks, contain high quantities of tannins in the leaves which are responsible for brown colors. Nutrient retention during senescence In deciduous tree species some 60 – 70% of N, 60 – 70% of P, 30% of K, 25% of Mg, and 15% of Ca are withdrawn from leaves prior to them being shed. Storage is in the bark and elements are re-mobilized in spring A decline in photosynthesis with aging can be prevented by the decapitation of the plant above the leaf in question. This implies a regulatory action by the growing point. Results from primary leaves of bean (Das, 1968). Leaf Abscission The final stage in leaf senescence is abscission ("cutting off") Abscission is controlled by a special layer of cells at the base of the petiole, the abscission layer. This layer releases ethylene gas that stimulates production of cellulase. This in turn breaks down cells walls so that eventually the leaf is held on to the plant only by xylem fibers. Wind eventually weakens these and leaf falls Another special layer of cells adjacent to the abscission layer produces cells impregnated with suberin. These form a protective layer, which is seen as the leaf scar Tyloses, as well as gums are formed inside the vessels and plug them up before abscission occurs Vascular tissue Leaf Abscission layer Developing leaf scar Axilliary bud Stem KEY CONCEPT Seeds disperse and begin to grow when conditions are favorable. Types of Fleshy Fruits Drupe (Peach) Berry (Tomato) Pome (Apple) Aggregate (Strawberry) Pepo (Cucumber) Multiple (Pineapple) Fruit Dispersal • The form of the fruit gives clues about its dispersal. • Small, dry fruits with “wings” or “parachutes” may be wind-dispersed. Fleshy fruits are often animal dispersed. Explosive fruits can fling seeds away. Floating fruits may be water dispersed. How are these Fruits Dispersed? Dandelion Coconut Maple Cocklebur Jewelweed A Thinking Question? • Why must fruits be dispersed away from the parent plant? Seeds Seed Dormancy • As seeds mature, they enter a period of dormancy. – Suspended growth and development. – Lowered metabolic rate. – Resistance to adverse environmental conditions. Seed Dormancy • Conditions that break dormancy vary among plant species. – Some germinate as soon as the environment is suitable (adequate moisture and temperature). • Conditions that break dormancy vary among plant species. – Others have additional requirements: • Initial drying (seeds within fleshy fruits) • Exposure to prolonged cold (seeds of temperate and arctic plants). • Disruption of seed coat . Seed Dormancy • Seeds can remain dormant in the soil for long periods of time. Dormancy ensures that seeds only germinate when conditions are optimal. • When we weed or cultivate a bare patch of soil, the weeds that sprout up immediately usually come from the “seed bank” already in the soil. Breaking Dormancy • Seeds require moisture and the right temperature to germinate. • In addition, some seeds germinate only after certain environmental signals: • Drying • Temperature (period of cold or heat) • Disruption of the seed coat ? Thinking Question ? • What could be the advantage of waiting for each of these signals to germinate? • Long period of cold • High heat of a forest fire • Drying out • Disruption of the seed coat. Monocot Germination • Monocot seeds have one cotyledon. – Absorbs and transfers most nutrients during seed germination. Dicot Germination • Dicot seeds have two cotyledons – Absorb nutrients from endosperm during seed development and store for future use. – Transfer stored nutrients to other parts of embryo during seed germination. Seed Germination • Emergence of the embryonic root is followed by emergence of the embryonic shoot. • Emerging roots and shoots must push through the soil without abrading the apical meristems at their tips. • Germination (resumption of growth) occurs when the seed absorbs enough water to rupture the seed coat. • Once photosynthesis begins, the plant is called a seedling. Cotyledons • Food stored in the seed (as endosperm or within cotyledons) provides the energy for sprouting. • Cotyledons of dicots with hypocotyl hooks are carried out of the soil. – – – – – Remain below ground. Digest endosperm and transfer nutrients to seedling. Become photosynthetic. Transfer stored nutrients and synthesized sugars to the seedling. Wither and fall off when reserves have been consumed. Coevolution • 43.6 What Are some Adaptations for Pollination and Seed Dispersal? – Coevolution Matches Plants and Pollinators – Fruits Help Disperse Seeds Coevolution • Some plants and pollinators have coevolved. – Each acts as an agent of natural selection on the other. • Animal-pollinated flowers must: – Attract useful pollinators able to locate flowers and extract pollen or nectar. – Frustrate undesirable visitors who might eat nectar or pollen without fertilizing the flower. Coevolution • Animal-pollinated flowers are grouped into three categories based on how they attract pollinators. – Food – Sex deceptions – A nursery Food • Many animals forage on flowers, distributing pollen from flower to flower in the process. • Examples: beetles, bees, moths, butterflies, hummingbirds. Food • Bees are attracted by sweet odors and bright colors. • Bees do not see the same range of colors that humans do. • Bee-pollinated flowers are typically white, blue, yellow, or orange, with markings that reflect UV light pointing toward the center. • Bee-pollinated flowers have structural adaptations to ensure pollen transfer. • When a bee visits a young plant, the stamens emerge from a crevice between petals, brushing pollen onto her back as her weight deflects the petals downward. • In older flowers, the sticky stigma of the carpel protrudes from the crevice, becoming pollinated by pollen-coated, nectar-foraging bees. Food • Other adaptations for drawing pollinators: – Nectar-containing tubes to accommodate the long tongues of moths and butterflies – Tubular flower shape to match the long bills and tongues of hummingbirds – Flowers that heat up to broadcast their scents • Example: skunk cabbage Sexual Deception • Some plants capitalize on the mating drive and behaviors of male wasps. • Example: some orchid flowers mimic female wasps or bees in scent and shape. – The males attempt to copulate, becoming covered with pollen in the process. – Pollen is transferred as they repeat the process with other orchids. Nurseries • Some insects fertilize a flower and then lay their eggs in the flower’s ovary. • Example: yuccas and yucca moths. – A female moth collects pollen from one flower and transfers it to another, smearing it over the stigma and laying her eggs inside the ovary. – The developing seeds provide nutrition for the developing caterpillars. Fruits Disperse Seeds • Mechanisms to disperse seeds: - Explosive fruits to eject seeds. - Light fruits with large wind-catching surfaces. • Floating fruits for water dispersal. • Clingy or tasty fruits that allow animal dispersal. In Summary. . . . • Plant Reproduction Structure and Functions • Fruit and Seed Structure and Adaptations • Coevolution THE END