Plant Responses
PLANT HORMONES
Growth and development of plants influenced by: Genetic factors External environmental factors Chemicals Plants respond to chemicals that are naturally inside them and to synthetic chemicals
Groups of Hormones Hormone chemical messenger that affect plant s ability to respond to environment Organic compounds Effective even at low concentrations May be made in one part of plant and transported to another (similar to endocrine system)
Hormones interact with specific target tissues to cause physiological (physical) responses Growth Fruit ripening Each response may be two or more hormones acting together
b/c hormones stimulate or inhibit plant growth, they are referred to as growth regulators Many can be made in the lab increases quantity of hormones available for commercial use
Botanists recognize 5 groups of hormones 1.Auxins 2.Giberellins 3.Ethylene 4.Cytokinins 5.Abscisic acid
Auxins hormones involved in plant-cell elongation, shoot and bud growth, and rooting IAA (indolacetic acid) is well-known natural auxin Stimulates development of fleshy structures like fruit
Experiment on strawberries shows evidence about role of IAA in plants
Removing seedcontaining parts from strawberry prevents fruit from enlarging If IAA is then applied, strawberry (without seeds) enlarges normally
IAA is made in actively growing shoot tips and developing seeds Before cell can elongate, cell wall must be less rigid so it can expand IAA triggers increase in plasticity (stretchability) of cell walls
Synthetic Auxins Auxins have variety of possible effects, so used for several reasons in gardening, commercial agriculture, research Napthalene acetic acid synthetic auxin used to promote root formation on stem and leaf cuttings
When NAA sprayed on young fruits of apple and olive trees some fruits drop off so that rest of fruit grows larger Other like pears and citrus several weeks before picking NAA prevents fruits from dropping off trees before they mature
So, auxins can have opposite effects cause fruit to drop or prevent dropping proves important point: Effects of hormone on plant often depend on stage of plant s development
NAA used to prevent undesirable sprouting of stems from base of ornamental trees Stems contain lateral bud at base of each leaf Buds fail to sprout as long as shoot tip is intact apical dominance
If shoot tip removed lateral buds grow If IAA or NAA applied to cut tip of stem, lateral buds stay dormant NAA used commercially to prevent buds from sprouting on potato tubers during storage
Another important synthetic auxin is 2,4- D, a weed killer At certain concentrations it kills dicots without hurting monocots Food crops are mostly monocots
Agent Orange mixture of 2,4-D and another auxin used to defoliate jungles in Vietnam war
A nonauxin contanimant in Agent Orange throught to have caused health problems in people exposed
Giberellins In 1920s Japanese scientists discovered substance produced by fungi of genus Gibberella caused fungus-infected rice plants to grow abnormally tall Giberellin later found to be produced in small quantities by plants themselves
Have many effects on plants, but primarily stimulate elongation growth Spraying plants may cause them to grow larger than normal
Like auxins, giberellins have important commercial applications Many seedless grapes sprayed to increase size of fruit Increases alcohol content of beer by increasing amount of starch converted to sugar during brewing Also used to treat seeds, because they break seed dormancy and promote uniform germination
Ethylene Plays role in ripening of fruits Unlike other hormones it s gas as room temp Diffuses easily through air
one bad apple spoils the barrel One rotting apple will make ethylene, which stimulates others to ripen then spoil
Commercially ethylene applied in solution of ethephon Breaks down to release ethylene gas Used to ripen bananas, honeydew melons, tomatoes
Oranges, lemons, grapefruit often stay green when ripe Taste good, people won t buy Ethylene turns them desirable colors orange, yellow, etc.
In some plant species, ethylene promotes abscission detachment of leaves, flowers, fruits
Growers can use mechanical tree shakers if fruit sprayed with ethylene before harvest
Leaf abscission advantage Dead, damaged, infected leaves drop to prevent spreading disease or blocking light of healthy leaves Minimize water loss in winter
Cytokinins Promote cell division Made in developing shoots, roots, fruit, and seeds Important in culturing plant tissues in lab
High ratio of auxins to cytokinins in tissue culture stimulates root formation Low ratio promotes shoot formation Other cytokinins used to promote later bud growth of flower crops
Abscisic Acid Generally inhibits other hormones Originally thought to promote abscission Brings about dormancy in buds Maintains dormancy in seeds Causes stomata to close in response to drought Too expensive to make commercially
Other Growth Regulators Many used on ornamental plants Do not fit into five categories Ex. Utility (electric, water, etc.) apply growth retardants prevent plant growth Sometimes less expensive than prune (cut) trees
PLANT MOVEMENTS
Plants look immobile b/c they are rooted Time-lapse photography shows parts of plants move a lot Too slow for us to notice Move in response to several environmental stimuli
Tropisms are slow movements Nastic movements are quicker
Tropisms Response toward or away from stimulus Toward positive tropism Away negative tropism Each named for the stimulus
Phototropism Light causes auxin to move to shaded side of shoot Causes cells on shaded side to elongate more Result - shoot grows toward the light Positive phototropism
Sometimes light causes production of growth inhibitor on lighted side
Negative phototropism seen in vines that climb and there s nothing to grab onto Grow away from light toward wall
Solar Tracking Heliotropism Motion of leaves/flowers as they follow sun s movement Maximizes light for photosynthesis Not actually tropism something similar
Thigmotropism Response to touching solid object Tendrils and stems of vines coil when they touch an object Allows some to climb objects increases chance of getting to light Thought that ethylene and type of auxin involved
Gravitropism Response to gravity Root grows downward, stem upward Roots - positive gravitropism Stems negative
Appears to be at least partly regulated by auxins One hypothesis when seedling placed horizontally, auxins accumulate on lower sides
Concentration stimulates cell elongation on lower side of stem grows up Inhibits growth on lower root grows down
Chemotropism Response to chemical Ex. Pollen tube after flower pollinated Grows out from pollen grain down through stigma and style to ovule Follows direction of increasing concentrations of chemicals made by ovule
Nastic Movements Independent of direction of stimuli Regulated by changes in water pressure against cell wall (turgor pressure) of certain plant cells
Thigmonastic Movements Occur in response to touch Many are quick
Caused by rapid loss of turgor pressure in certain cells Similar to guard cells (around stomata)
Physical stimulation causes K to be pumped out of cells at base of leaflets and petioles Water moves out by osmosis As cells shrink, leaves move
Folding of leaves thought to discourage insect feeding Help prevent water loss (reducing transpiration through wind)
Nyctinastic Movements Response to daily cycle of light and dark Involve same osmotic mechanism as thigmonastic movements Changes in turgor pressure more gradual
Prayer plant leaf blades vertical at night,,resembling praying hands During the day, leaves are horizontal
SEASONAL RESPONSES
In nontropical areas, responses strongly influenced by seasonal chages Trees shed leaves in fall Most plants flower only certain times of year
Photoperiodism Plant s response to changes in length of days and nights Affects many plant processes, formation of storage organs, bud dormancy Most-studied flowering
Day Length and Night Length Important factor in flowering is amount of darkness (night length) plant receives Many have specific requirement for darkness Critical night length
Short-day plant flowers when day is short and nights are longer than critical night length
Long-day plant flowers when days are long and nights are shorter than critical night length
Day-neutral plants Largest group Not affected by day or night length
SDPs flower in spring or fall when days are shorter LDPs flower when days are long, usually in summer
Adjusting the Flowering Cycles of Plants Figure 31-10 demonstrates how length of uninterrupted darkness is important factor Even though daily total of 15 hours of darkness, SDP does not flower b/c of that one hour of light
Regulation by Phytochrome Plants monitor changes in day length with pigment called phytochrome Exists in two forms, based on wavelength it absorbs
Daylight converts Pr to Pfr In dark, Pfr converted to Pr Based on percentages of both, plant determines how much light and dark
Vernalization Low-temperature stimulation of flowering Important for fallsown grain crops (wheat, barley, rye) Farmers can take advantage and harvest before summer drought
Wheat seeds planted in fall and survive winter as seedlings Exposure to cold temp in winter causes flowering in early spring (early crop) If seeds planted in spring, it will take two months longer Cold not required for growth, but quickens flowering
Biennial Plant that usually lives for only two years Produces flowers and seeds in second year Survive first winter as large roots with small above ground plants
In spring, flowering stem elongates quickly (bolting) Most biennials undergo vernalization before they flower the second year After flowering die Treating biennial with giberellin sometimes used to substitute cold
Fall Colors Caused mainly by photoperiodic response Nights become longer in fall leaves stop making chlorophyll Carotenoids become visible
Carotenes
Xanthophylls
Anthocyanins