Plant Responses to Environment

Plant Responses to Environment

1. Why plants respond to the environment

   • Plants respond to stimuli
     • responding to external stimuli is how plants avoid being stressed
       • or being eaten and to help them survive long enough to reproduce
     • Tropism- directional growth response (hormones) in which direction of the response is determined by the direction of the external stimuli
       • Phototropism- shoot grows towards light (positive phototrophic). enables them to photosynthesise. (more shoots, less roots)
       • Geotropism- roots grow towards pull of gravity- anchors them in soil and helps them take up water.
         • needed for support (Turgid), photosynthesise and cool plant, water has minerals (nitrates) needed for amino synthesis
       • chemotropism- on flower, pollen tubes grow down the style, attracted by chemicals towards ovary (fertilisation occurs)
       • Thigmotropism- shoots of climbing plant (ivy) wind around other plants to gain support
       • Hydrotropism
     • Animals respond to biotic (living) and abiotic (non livivng) components of environment.
   • What controls plant hormones
     • Hormones co ordinate plant response
     • plant hormones reffered to as plant growth regulators because
       • unlike animals, hormones not produced in endocrine gland but by cells in various tissues in plant
     • Hormones move around by
       • Active transport
       • diffusion
       • Mass flow- in phloem sap/ xylem vessels
   • Plant hormones and their effects
     • Auxins (IAA- indole-3-acetic acid)
       • promotes cell elongation, inhibits growth of side shoots, inhibints leaf abscission (falling off)
     • Cytokinin
       • promotes cell division
     • Gibberellins
       • promotes seed germination and stem growth
     • Abscisic acid
       • inhibits seed germination and growth, causes stomatal closures when plant is stressed
     • ethene
       • protoes fruit ripening

2. How plants respond to the environment

   • Plant Growth
     • Cell wall of plant limit ability to expand and divide.
       • therefore, growth occurs in particular places of plants where there are immature cells that are capable of diving. these are MERISTEMS
     • Meristems...
       • Apical
         • located at tip/apices of roots, shoots.
           • responsible for roots and shoots getting longer
       • Lateral bud
         • Found in bids, give rise to side shoots
       • Lateral
         • cylinder near outside of roots and shoots. responsible for roots and shoots widening
       • Intercalary
         • between nodes where leaves and buds branch of stem.
           • growth between nodes responsible for shoot getting longer
     • cell division occurs close to apex--> cell elongation behind apex --> auxins produced in apex-->
       • auxins travel (diffusion, AT) --> cells in zone of elongation --> elongate/shoots grow
     • extent to which cell elongates is proportional to conc of auxlins.. more Auxins = increase in wall stretchiness by
       • promoting AT of H+ by ATPase on plasma membrane--> cell wall--> lowers pH therefore, optimum condition for wall loosening enzymes to work Expanisins- break cellulose bonds, increase H+ conc, disrupt hydrogen bonds with wall therefore, less rigid and expands to take in water.
   • What causes phototropism
     • 1. shoots bend towards light- phototropism--> shaded side elongates faster than illuminated--> pushes shoot end towards light.
     • 2. Auxins transported to shaded side, increases rate of elongation.
     • 3. enzymes identified- Phototropin 1, phototropin 2 (promoted by blue light).
       • there is alot of activity of P1 on light side, thought that change in gradient causes redistribution of auxins
   • Shedding leaves
     • Cytokinins stop leaves of deciduous trees senescing (ageing).
       • Leaf acts as sink for phloem transport, therefore, leaf garunteed good supply of nutrients.
     • cytokinins conc drops, supply of nutrients drop, senescing begins, leaves shed (abscission)
     • Auxins inhibit Abscission by acting on cells in abscission zone BUT..
       • leaf sensecing causes auxin production to drop at tip
       • makes cells more sensative in zone to other growing substances (ethene)
       • low auxin conc, high ethene conc
       • increase in cellulase digest cell walls in abcission zone, separating pepitole from stem

3. Controlling plant growth

   • Apical dominane
     • growing apical bud at tip inhibits growth of lateral buds futher down shoot.
       • When tip removed, auxin conc in shoot low and bud grows.
     • Hypothesis test: applied auxin containing paste to cut end of shoot, lateral end didnt grow
     • other factors may cause lateral bud to not grow- oxygen exposure to cut end could produce hormone promoting lateral bud growth
     • Thimann and Skoog applied ring of auxin transport inhibitors below shoot apex.
       • lateral bud grew. thereofre, norm auxin conc in lateral inhibits growth and decreased conc promotes growth. BUT both variables may be affected by a third.
     • Gocal- auxin conc in lateral of kidney bean plant was high when shoot cut.
       • 2 hormones invovled:
       • Absicicis acid- inhibit bud growth,
         • increase auxin conc, keep acid levels high, when tip removed, hormone decrease, bud grows
       • Cytokinins- bud growth.
         • direct application = override apical dominane effect. High auxin conc = shoot apex a sink for cytokinins in root. Meant that most cytokinin goes to shoot apex. When removed, cytokinin spread evenly around plant promoting growth
   • Gibberellins and stem elongation
     • giberellic acid responsible for plant stem growth

4. Commercial use of plant hormones

   • Auxins
     • 1. Taking cuttings- dip cut end into rooting powder before planting.
       • Encourages root growth as it contains auxins, fungicides and talcum powder
     • 2. Seedless fruits- Treating unpollinated with auxins promotes seedless fruits,
       • promotes ovule growth, triggers automatic auxin production
     • 3. Herbicides- Transported in phloem to all plants for a longer effect as there are no close fit enzymes
   • Gibberellins
     • 1. Fruit production- delays senescene in citrus, extending time for fruit picking.
       • improves apple shape, elongates grape stalks
     • 2. Brewing- Speed up Brewing process of alcohol
     • 3. sugar production- spraying sugar cane stimulates growth between nodes
       • and males stems longer therefore more sugar yield (4.5 tonnes by hectar)
     • 4. Plant Breeding- speed up growth processes by inducing seed fermentation in young trees
   • Cytokinins- prevents yellowing in pickled lettuce leaves, promotes bud and shoot growth.
     • short shoots with many branches produced
   • Ethene- speeds up fruit ripening (apple, tomatoes, citrus), promotes fruit drop (cotton, cherry, walnuts),
     • promotes female sex expression (cucumbers), decrease chances of self pollination and increase yield. lateral growth