Plant Responses to Environmental CuesTropisms, Photoperiodism, and PlantHormonesPlant Responses to Environmental CuesPhototropism - plant growth response to lightshoots bend toward light - positive phototropismroots sometimes bend away from light - negative phototropismallows shoots to capture more lightmediated by the plant hormone auxinGravitropism - plant growth response to gravityshoots bend away from gravity - negative gravitropismmediated by auxin - causes lower side of stem to elongateroots grow toward gravity - positive gravitropismmediated by gravity sensing cells in root capThigmotropism - plant growth response to touchcauses coiling of tendrilsmediated by auxin and ethyleneTurgor MovementTurgor is pressure within a livingcell resulting from water diffusion.After exposure to a stimulus, changes in leaf orientation aremostly associated with rapidturgor pressure changes inpulvini - multicellular swellingslocated at base of each leaf or leafletturgor movements are reversibleCircadian clocks are endogenous timekeepers that keep plantresponses synchronized with the environment.circadian rhythm characteristics• must continue to run in absence of external inputs• must be about 24 hours in duration•can be reset or entrained•can compensate for temperature differencesPlant HormonesAuxin - indole acetic acid (IAA) - causes stem elongation andgrowth - formation of adventitious and lateral roots, inhibits leafloss, promotes cell division (with cytokinins), increases ethyleneproduction, enforces dormancy of lateral budsproduced by shoot apical meristems and other immature partsCytokinins - stimulate cell division (with auxin), promotechloroplast development, delay leaf aging, promote formation ofbuds, inhibit formation of lateral rootsproduced by root apical meristems and immature fruitsGibberellins - promote stem elongation, stimulate enzymeproduction in germinating seedsproduced by roots and shoot tips, young leaves, seedsPlant HormonesEthylene - controls abscission (shedding) of leaves, flowers, fruits,promotes fruit ripeningproduced by apical meristems, leaf nodes, aging flowers, ripening fruitAbscissic acid - inhibits bud growth, controls stomate closing, enforces seed dormancy, inhibits other hormonesproduced by leaves, fruits, root caps, and seedsAuxinResponsible for phototropismCharles and Francis Darwinwondered what caused plants tobend toward lightThey demonstrated that growingtips of plants sense lightThe ability to sense light is notpresent in areas behind the shootapexWent demonstrated that a chemical produced in the shoot tip isresponsible for the shoot bending - he called it “auxin”An agar block can absorb chemicals below a growing shoot tipWhen the block is applied to an immature shoot, the shootelongates more on the side where the agar block is appliedAuxin is produced uniformly by growing shoot tips but istransported to the unlighted side of the shootIt causes cells on the unlighted side to elongate more than cells onthe lighted side - it does this by making cell walls softer and moreeasily stretched by expansion of the cell’s cytoplasmOther effects of auxinsStimulates formation of fruitspollen contains large amounts of auxin - pollen’s auxin is a chemical signal that pollination has happened and fruit formation can begin - synthetic auxins can cause fruit formation without pollinationAddition of synthetic auxins to cuttings stimulates formation of roots in plant cuttings - “rooting hormone”Auxin inhibits the growth of lateral buds in shoots - production of auxin by the shoot apex stops growth of neighboring lateral buds - “apical dominance”Synthetic auxins can be used to control weedy dicots through the inhibition of growth of shoots - it doesn’t harm monocots - most commonly used synthetic auxin is 2,4D - often used in lawn “weed and feeds”Apical Dominance - the tip of a growing shoot (apical bud)produces auxin that inhibits the growth of lateral budsbelow the apical budCytokininsProduced in root apical meristem and by fruitsInhibits lateral root growth and stimulates lateral bud growthThe combination of auxin, which inhibits lateral bud growth, andcytokinin, which stimulates lateral bud growth, produce thegrowth form of a plantGibberellinsProduced in apical portions of roots and shootsCause elongation of internodes in stems (with auxin)EthyleneProduced in mature fruit and in some apical meristemsInitial observation of ethylene gas inducing defoliationSuppresses lateral bud formation when combined with auxinSuppresses stem and root elongationPlays major role in ripening of fruitFruit forms separation layer at base of leaf petiolesHastens ripening, increases respirationComplex carbohydrates broken down into simple sugarsChlorophylls broken downCell walls become softVolatile chemicals produced, associated with flavor and scentof ripe fruitEthylene used commercially to ripen green fruits -Carbon dioxide has opposite effect, fruit is often shippedin CO2 atmosphere, ethylene applied at destinationAbscissic AcidProduced by aging leaves and fruitsApplication on leaves causes yellow spots and premature agingMay induce formation of winter budsSuppresses growth of buds and formation of bud “scales” for protectionSuppresses growth of dormant lateral buds (with ethylene)Counters effects of gibberellinsPromotes senescence (decline with age) by countering auxinCauses dormancy of seedsControls opening and closing of stomata - produced when plantsare stressed - causes loss of K+ from guard cellsPhotoperiodism - plant responses to day and/or night lengthLong-day plants flowerin the late Spring andearly Summer, whendays are long and nightsare shortShort-day plants flowerin the late Summer andearly Fall, when days areshort and nights are longA single flash of lightduring a long night willundo the normal effectof a long nightLong days: 12 - 16 hours, short nights 8-12 hoursShort days: < 14 hours, long nights > 8 hoursDay and night length are oftenmanipulated in greenhouses toproduce flowering out of seasonPoinsettias normally flower inthe Spring when day length isincreasing - they can be grownindoors under artificial lightingthat mimics the light conditionsof Spring, just in time forChristmasChemical Basis of the Photoperiodic ResponseTwo light wavelengths important in the responseRed 660 nmFar-red 703 nmChemistry: two forms of phytochrome: Pr and PfrPfr is biologically active, Pr is biologically inactivePr absorbs red light, converted quickly to