Plant Responses to Abiotic Environment

Biological orientation of plants Tropisms: when growth toward stimulus it is positive and vice versa. Taxes: occur in algae which swim with flagella and swim toward light. Nastic responses: turgor response to diffuse stimulus. Not fully understood. Tropisms are growth responses and are generally not reversible, nasty’s are reversible.

Control of plant growth Achieved by hormones and chemicals. Auxin controls bending of plant due to light. Series of experiments done:

Found that: auxin produced in tip of shoot Is water soluble and diffuses through agar blocks but not through mica Causes elongation of cells thus promoting upward growth If light shone on side, cells on dark side elongate and grow faster so shoot bends.

Low concentrations of auxins stimulate roots and lateral buds, high conc. Inhibit. The reverse is true for stems. –Auxin migrates to dark side of stem –Main auxin is IAA (indole acetic acid), it is turned into actively growing plant tissue. –Artificial auxins can be used as weed killers.

Apical Dominance Auxin at top of plant is produced in high concentrations so shoot grows fast but lateral buds stay dormant unless they are lower down the plant where the concentration is lower. This gives many plants and trees their triangular shape. Cytokinins from roots stimulate lateral buds making them grow more at the bottom of the plant.

Geotropism in seeds Gravity causes an uneven movement of auxin to bottom of root and top of shoot to cause bend. Plants can tell up from down by using statoliths which move from side to side.

Other tropisms Hydrotropism: very strong in roots, stronger than geotropism as a root will grow to surface if that is where water is. Thigmotropisim: tendrils of climbing plant bend round any object they touch, growth is slowed on the side that touches an object. Chemotropism: roots often move towards or away from certain chemicals.

Other effects of auxin Initiates root formation (used in powders to make roots form). Stops roots growing long Suppression of lateral buds Stimulate mitosis in stems Start flowering of some plants Stops premature abscission of leaves and fruit.

Other plant hormones Gibberellins –Produced in growing tip and cause rapid elongation, can promote germination. Cytokinins –Work with auxin to promote cell division. High auxin + low cytokinin = roots, low auxin + high cytokinin = stems, equal amounts = callus tissue. Ethene gas –Ripens fruit

Abscisic acid (ABA) –Tends to inhibit all other hormones. Induces leaf fall and winter dormancy in seeds. Etiolation –Response to light, leaves grow small and yellow.

Biological timing responses Daily rhythms (circadian) –Some phytoplankton have three daily rhythms: when agitated by waves it will glow but only at night, photosynthesis only occurs during the day, all cells divide during a one hour period just before dawn. –Opening of flowers in day and closing at night. –Nectar secreted only at certain times of day –Flowers give out perfume at different times of day

Circamonthly –No obvious ones in plants –Flowers which turn face to follow sun –Leaves of many plants droop at night. Circatidal –Release of egg and sperm for reproduction. Circannual –Flowering, dormancy, growth patterns, abscission of leaves

Flowering of plants Many show photoperiodism: the response to changes in day length such as flowering or dropping leaves. The most improtant factor in when a plant flowers is length of darkness not light. This means plants can be divided in 3 groups. –Short Day plants: require a short day and a long night, these plants flower in winter, early spring and autumn.

–Long Day plants: require a long day and short night so flower in summer. –Day neutral plants: relatively unaffected by daylight hours e.g. tomato.

Phytochrome System The ability of plants to activate the photoperiod is controlled by the pigment phytochrome. This pigment detects light. It exisits in two forms coreresponding to the two wavelengths of light: P 665 and P 725. When P 665 absorbs red light it is quickly changed to P 725. When P 725 absorbs far red light it is quickly turned into P 665. In the dark P 725 is slowly converted into P 665 The significance is that P 725 is biologically active while P 665 is inactive. It seems that P 665 is the way the plant stores the potentially active form of the phytochrome.

Diag.

Other plant responses to abiotic environment Vernalisation –Seeds that require a period of cold before germinating Dormancy –Seeds that are metabolically inactive after forming, dries out to 5% water of body weight then will only germinate in the right conditions

Stratification –A moist seed exposed to low temperatures for many days breaks the dormancy. Scarrification –Some seeds need coat scratched or burnt/treated to germinate. Ephemerals –Many desert plants contain chemicals that stop germination and until there is enough rain to get rid of the chemicals there will be no growth..

Abcission –Leaf fall prevents water loss in the cold months and is triggered by auxin at certain temperatures. Autumn colour in leaf –In autumn the chlorophyll breaks down exposing the other pigments Growth forms of plants –To cope with seasonal changes there are annuals, biennials, perennials (which overwinter by having bulbs etc).