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Lecture 9

BIOB31 Lecture 9

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Connie Soros

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HYDROPHYTES CONT’D Aerenchyma  Modified parenchyma tissue  Large intercellular air spaces  Found in aquatic species  Helps w/ gas exchange + maintain buoyancy Hydrophytes  Submerged leaves o Highly divided  large SA for photosynthesis + absorption o Minimizes H2O resistance… less damage to leaves o Different from emerging (less division) or floating leaves Nutrient Availability and Root Growth  Plants can’t move at all in bad conditions  local enviro adaptations to extreme fluctuations  seek good conditions w/in reach o Above ground, light, temp + humidity changes during day  CO2 + O2 levels stay about the same o Soil buffers roots from high  low temps  CO2, O2, H2O + nutrients [ ] varies temporally + spatially TROPISM Tropisms  Plant movement in the direction of an enviro stimulus (e.g. light) o Positive tropism – movement twd stimuli o Negative tropism – movement away from stimuli (e.g. vines) Phototropism  Plants move in a certain direction in response to light  Shoot tips grow twd light source o Auxin move twd shaded part of shoot  cells elongate more on this shaded area  shoot bends twds light  positive tropism  In plant stems, growth inhibitors develop on illuminated area of shoot when light is around o Phototropism not caused by auxin or movement Thigmotropism  Plants grows after touching solid object o Cells touch structure  shortens slightly + cells on other side elongate  might involve auxin + ethylene o Tendrils wrap around objects, enables climbing + clinging (rapid response)  might involve auxin + ethylene o Allows vines to climb onto other objects  receive more sunlight o Enables roots to navigate around rocks Gravitropism  Plant grows in response to gravity o USUALLY, root grows downward, stem upward o Roots – positive gravitropism (with gravity) o Stems – negative gravitropism (against gravity)  Hypothesis! 1. Seedling placed horizontally 2. Auxin + cytokinin builds up on lower side of root + stem 3. [Hormone] stimulates cell elongation along lower side of stem  Another [hormone] inhibits elongation along lower side of root 4. Stem grows upward while roots grows downward  Role of cytokinin + auxin  Both regulate gravitropism  Cytokinin - early, rapid phase near the root apex  Auxin - later, slower phase farther from root apex Hydrotropism  Plants moves partly in response to H20 o Roots always grow towards H20  Does hydrotropism guide the growth of pollen tube on surface of the stigma? o Adding (aq) phase sparked germination of pollen grain  later on, pollen tube penetrated (aq) phase o Thus, It was shown that tubes grow twd an (aq)-oil interface Chemotropism  Plant grows in response to chemicals  Example: pollen tube growth in response to chemicals produced by ovule
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