Plant Responses and Adaptations

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Plant Responses and Adaptations
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Plant Responses

• Hormones chemicals that control patterns of
  growth/development and responses to environmental
  conditions
• Ex auxin, gibberellin, ethylene
• Effective in very low concentrations
• Target cell portion of organism affected by
  hormone
• Tropisms response to stimulus
• Phototropism response to light
• Gravitotropism response to gravity
• Thigmotropism response to touch

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Phototropism

• Response to light
• Positive bends toward light
• Result of auxin (from apical meristem)
  accumulating on shady side cells elongate faster
• Negative bends away from light
• Earliest studies done in 19th century by Charles
  and Francis Darwin

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Control of Apical Dominance

• Cytokinins, auxin, and other factors interact in
  the control of apical dominance
• Antagonistic mechanism b/w auxin and cytokinin
• Auxin from the terminal bud restrains axillary
  bud growth, causing
  the shoot to lengthen
• Cytokinins (from roots) stimulate
  axillary bud growth
• Auxin cant suppress axillary bud
  growth once it has begun

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• If the terminal bud is removed
• Plants become bushier
• Lower buds grow before higher ones since they are
  closer to the cytokinin source than the auxin
  source
• Auxin stimulates lateral root formation while
  cytokinins restrain it

Figure 39.9b
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Ethylene

• Plants produce ethylene
• In response to stresses such as drought,
  flooding, mechanical pressure, injury, and
  infection
• Gaseous hormone that diffuses through air spaces
  b/w plant cells
• High levels of auxin induce its release
• Acts as a growth inhibitor
• Can cause aptosis

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Effects of ethylene
Accelerates aging (senescence) breakdown of cell
walls, loss of chlorophyll. Fruits ripen, leaves
fall off.
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Gibberellins

• Stimulate stem elongation, fruit growth, and seed
  germination
• Stems elongate rapidly (bolt) to lift flowers up
  for pollination
• Seeds imbibe water gibberellins released to
  break dormancy and stimulate germination

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Photoperiodism and Responses to Seasons

• Photoperiod, the relative lengths of night and
  day
• Is the environmental stimulus plants use most
  often to detect the time of year
• Photoperiodism
• Is a physiological response to photoperiod

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Photoperiodism
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Gravitotropism

• Response to gravity
• Roots positive response
• Bend toward pull of gravity
• Stems negative response
• Bend away from pull of gravity

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Thigmotropism

• Directional response to mechanical stimulation
  (touch)
• Common in vines and other climbing plants

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Plant Adaptations

• Response to abiotic factors including access to
  water and nutrients
• Specific to environment available to plants
• Biome large geographic area with specific plant
  and animal communities
• Includes rainforest, desert, grasslands, tundra,
  taiga
• Biotic factors living (plants, animals)
• Abiotic factors non-living (soil, climate, etc.)

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• Desert adaptations
• Extensive root system
• Grow slowly need less energy
• Flowers open at night
• Waxy covering
• Spines protect from herbivores shade

• Grassland adaptations
• Extensive root system survive fire, not pulled
  out by grazing animals
• Grow from base, not tip resist grazing, fires
• Flexible, narrow leaves

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• Rainforest adaptations
• Shed water to resist fungi bacteria
• Shallow roots
• Epiphytes live on other plants collect rainwater
• Use prop roots for stability

• Aquatic adaptations
• Reduced vascular system water by osmosis
• Flexible leaves stems move with currents
• PS in top of leaf bottom of leaf waxy
• Floating seeds

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