Radioactive Decay The amount (in grams) of a sample of lead 214 present after t years is given byA(t) = 500e?0.31t.Find the rate of change of the quantity present after each of the following years.a. 4________________b. 6________________c. 10________________d. What is happening to the rate of change of the amount present as the number of years increases?________________e. Will the substance ever be gone completely?
Plant Responses to Internal & External Stimuli Plant Hormones Function similarly to animal hormones; transported in vascular system Most aspects of plant growth & development under hormonal control Often several hormones interact to affect growth & development Auxin Any chemical that promotes elongation of shoots Produced mostly in shoot tips; transported only from tip to base of shoot Found in roots, but usually obtained from shoots Auxin & Cell Elongation Causing cell elongation in developing shoots is one of auxin’s primary functions Most likely binds to receptors in cell membranes Only stimulates growth at concentration 10 to 10 M; above this, it can inhibit growth by inducing ethylene production Also... • rapidly alters gene expression, causing elongating cells to produce new proteins • causes elongating cells to make more cytoplasm & wall material Auxin & Development Affects pattern formation of developing plant; spatial organization of plant organs • carries info. about development, size, & environment of individual branches • reduced flow from a branch tells that the branch is being insufficiently productive, so lateral buds below are stimulated to grow Interacts w/ other hormones to control apical dominance; application of auxin to decapitated shoot resuppresses lateral bud growth Cytokinins Chemical substances that stimulate cytokinesis Produced in actively growing tissue; roots, embryos, fruits Interacts w/ auxin at certain concentrations to cause cell growth, division, & differentiation • shoots form if cytokinin level higher than normal • roots form if auxin level higher than normal Slow aging of some plant organs • inhibits protein breakdown • stimulates RNA & protein synthesis • mobilizes nutrients from surrounding tissues Gibberellins Affects cell elongation, fruit growth, seed germination Young roots & leaves major sites of production Stimulates stem & leaf growth by enhancing cell division and elongation Work w/ auxin to cause more stem elongation Must be present along w/ auxin to cause fruit to develop Important application is spraying of seedless grapes • individual grapes grow larger (consumers like this) • internodes of grape bunch elongate, allowing more space b/w each grape • more space b/w each grape means less likelihood of infection from yeasts, etc. Causes seed to break dormancy & germinate once water is taken up In cereal seedlings, cause synthesis of digestive enzymes to utilize endosperm Brassinosteroids Similar to sex hormones in animals • cause cell division & elongation in stems at very low concentrations -12 (10 M) • slow leaf abscission • promote xylem differentiation Function so similar to auxins that they were first classified as such Slows growth, often antagonizing actions of growth hormones Ratio of ABA to growth hormones determines whether growth occurs ABA & Seed Dormancy Presence of ABA inhibits premature seed germination • while still inside fruit • immediately upon falling to the ground in autumn • before temperature, moisture, & sunlight levels are optimal Also causes production of proteins that help seeds withstand stresses of maturation Dormant seeds commonly germinate when ABA is removed or inactivated. • heavy rains wash it out of dormant desert seeds How could this be adaptive >>Seeds only germinate when rains occur, so death by dehydration less likely Dormant seeds commonly germinate when ABA is removed or inactivated. • light level, cold period, or acidity inactivates it in some seeds How could this be adaptive Seeds don’t germinate when there’s not enough light, ...or before their hard seed coats have been softened by frost and warmer weather is near, ...or before they’ve passed through an animal’s intestines and have feces for fertilizer Dormant seeds commonly germinate when ABA is removed or inactivated. • low levels are adaptive for some species; red mangrove seeds germinate early b/c of low ABA How could this be adaptive >>>Seeds have pointed roots ready to anchor them in mud below parent tree as soon as they fall; less likely to get washed away w/ tide ABA & Drought Tolerance When some plants begin to wilt, ABA accumulates in leaves, causing stomata to close rapidly This reduces further water loss & plant is more likely to survive drought conditions Ethylene Produced in response to stresses • drought • flood • mechanical pressure • injury • infection And as part of normal life cycle • fruit ripening • programmed cell death Ethylene & Mechanical Stress The “Triple Response”- example scenario, pea seedling growing through soil encounters a rock; the stress on its shoot tip causes ethylene production 1. stem elongation slows (stops the stress) 2. stem thickens (increases strength) 3. stem grows horizontally (increases chance of getting past obstacle) The “Triple Response”- example scenario, pea seedling growing through soil encounters a rock; the stress on its shoot tip causes ethylene production • ethylene produced in a burst, the effects of which fade with time • when effects wear off, shoot attempts vertical growth again • if barrier still present, triple response repeated • this continues until shoot is free of barrier, or plant dies of nutrient/ water deficiency Ethylene & Senescence Senescence: programmed death of certain cells, organs, or entire plant • leaves dropped (at once in deciduous trees, continuously in evergreens) • death of annual plants after flowering • shriveling of flower petals Apoptosis: process of cell death (remember development lecture), burst of ethylene usually required Ethylene & Leaf Abscission Leaf Abscission: detachment of leaf from stem following apoptosis during senescence • helps prevent desiccation during winter in deciduous trees • nutrients taken from leaves & stored in stems • nutrients recycled into new leaves in spring Where Do Fall Colors Come From • Deciduous trees are programmed to lose their leaves all at once in autumn (senescence) • Cells in leaves die (apoptosis), and nutrients are removed to stems. Chlorophylls are disassembled first, allowing other pigments (red, yellow, orange) to be seen. • Nutrients taken from leaves and stored in stems, then leaves are dropped (abscission) Ethylene & Fruit Ripening Immature fleshy fruits are tart, hard, & green, which help to protect developing seeds from herbivores For many species, burst of ethylene triggers fruit maturation Once ripe, mature fruits attract animals which eat them & disperse the seeds in their droppings made soft by enzymatic breakdown of cell walls made sweet by conversion of starches into sugars enticing smells & bright colors advertise ripeness Photomorphogenesis Key events in plant growth & development that are triggered by light: Etiolation: morphological adaptations for growing in darkness De-etiolation (greening): when stem reaches light, shoot growth slows, leaves expand, roots elongate, chlorophyll produced Blue-Light Photoreceptors Light in the blue wavelengths initiates: • Growth toward light source • Light-induced stomatal opening • Light-induced slowing of hypocotyl elongation Phytochromes Detect red light wavelengths Red light (~600nm) instigates germination Far-red light (~730nm, dark) inhibits germination Experiments alternating flashes of red & far-red light on seeds showed that they respond to the last flash (reversible response) Phytochromes & Shade Avoidance Phytochromes also provide info about light intensity they can detect the difference in ratio of red to far-red light resulting from shading of other plants Phytochromes also provide info. about light intensity • when plant that needs bright sunlight is shaded, phytochromes trigger greater energy expenditure on growing tall Phytochromes also provide info. about light intensity • when tree is exposed to right intensity of sunlight, phytochromes inhibit vertical growth & stimulate branching Circadian Rhythms Even under experimentally-controlled conditions, many physiological processes in plants maintain an approximate 24-hour schedule opening & closing stomata production of photosynthetic enzymes raising & lowering leaves opening & closing flowers Photoperiodism Physiological response to photoperiod (relative lengths of night & day); on annual time scale Flowering schedule is important example • short-day plants bloom when day length is below a certain threshold; usually in late summer, fall, winter; chrysanthemums, poinsettias, some soybean varieties Physiological response to photoperiod (relative lengths of night & day); on annual time scale Flowering schedule is important example • long-day plants bloom when day length is above a certain threshold; usually in late spring, early summer; spinach, radishes, irises, cereals Physiological response to photoperiod (relative lengths of night & day); on annual time scale Flowering schedule is important example • day-neutral plants are unaffected by photoperiod & flower when they reach a certain stage of maturity, regardless of day length; tomatoes, rice, dandelions Flowering Hormone Leaves detect changes in photoperiod & produce signaling molecules that trigger flower bloom • only one leaf is required for photoperiod to be recognized & flowering triggered • signaling molecule is a protein, coded for by gene called Flowering Locus T (FT), which is activated when conditions are right for flowering • FT protein travels from leaf to shoot apical meristem & causes transition of bud from vegetative to flowering state Tropisms Any growth response that results in plants curving towards or away from stimuli • phototropism: light • gravitropism (geotropism): gravity • thigmotropism: touch or other mechanical perturbation (like wind) Phototropism Positive: plant organs grow toward light • most often seen in shoots • cells on “dark” side of plant elongate faster than on “light” side Negative: plant organs grow away from light • most often seen in roots • cells on “light” side elongate faster than those on “dark” side Gravitropism Useful for roots & shoots that start to grow underground, where light can’t reach • roots display positive gravitropism • shoots display negative gravitropism Response occurs as soon as seed germinates & organs grow in appropriate direction no matter how seed is oriented when it lands Statoliths: dense cytoplasmic components that settle to lower portions of cell in response to gravity One hypothesis for how this works: 1. aggregating statoliths trigger redistribution of calcium 2. redistribution of calcium causes lateral auxin transport in root 3. auxin accumulates on lower side of root (the ‘underside’) 4. at high concentration, auxin slows elongation of cells here, meaning that cells on opposite side elongate faster, causing root to curve downward Thigmotropism Comes in several forms • trees in windy habitats grow shorter & thicker than their sheltered conspecifics, making them better able to resist strong winds • plants that use objects for climbing have positively thigmotropic organs (like tendrils) • plants that close when touched, possibly to protect themselves from damage or herbivory, are negatively thigmotropic Environmental Stress Important factor determining geographic ranges of plant species • Abiotic stress: caused by non-living things like heat, light, water, etc • Biotic stresses: caused by living things like herbivores, pathogens • Drought: close stomata, close leaves, drop leaves, • Flood: produce ethylene that causes death of some cells, which then serve as ‘snorkels’ that provide air to submerged roots • Heat: heat-shock proteins produced during high temperatures, which protect other proteins from being denatured • Cold: increase unsaturated fatty acids in cell membranes