Plant responses to abiotic stress

• what is abiotic stress?

  • negative impact of non-living factors on the living compounds/organisms in a specific environment
  • e.g. temperature extremes, day length changes, lack of water, wind, salinity
• why do deciduous trees lose their leaves in winter?

  • point comes when the amount of glucose required for respiration to maintain the leaves, and to produce chemicals from chlorophyll that might protect them against freezing is greater than the amount of glucose produced by photosynthesis
• what is photoperiodism?

  plants are sensitive to a lack of light in their environment e.g. timing of flowering in a plant
• what is the mechanism of sensitivity to day length?

  • light - sensitive pigment called phytochrome
  • exist in 2 forms - Pr and Pfr - each absorbs a different type of light and the ratio of Pr to Pfr changes depending on levels of light
• what is senescence?

  leaf turns yellow/brown and dying
• what is abscission?

  leaves falling off
• how do hormones prevent leaf loss in summer?

  • auxin produced in leaf tip increases level of cytokinins
  • cytokinins prevent leaf senscence by making the leaf a sink for phloem transport - leaves receive good nutrient supply e.g. sucrose keeping them alive
  • auxin inhibits abscission by reducing ethene levels and making cells in abscission zone less sensitive to ethene
• what are hormones that cause leaf loss in autumn?

  • senescence caused by decrease in auxin production in leaf tip as result of decrease in day length
  • drop in auxin causes cell in separation zone to become more sensitive to ethene
  • decrease in auxin also causes increased production of ethene - ethene causes increased production of cellulase enzyme, cellulase digests cell walls (hydrolyse cellulose) in separation zone, petiole separates from stem during abscission
• what happens in the protective layer during abscission?

  • vascular bundles sealed off, fatty material is deposited in cells on stem side of separation layer
  • protective layer formed, preventing pathogens entering, and water leaving
• what are the different ways plants cope with a decrease in temperature?

  • cytoplasm and vacuolar sap contains solutes which lower freezing point
  • produce sugars, polysaccharides, amino acids, and even proteins which act as antifreeze to prevent cytoplasm from freezing
  • different genes suppressed and activated in response to fall in temp, preparing plants to withstand frost
• how does ABA cause stomatal closure in response to abiotic stress?

  • levels of soil water fall
  • roots produce ABA (abscisic acid)
  • ABA transported to leaves
  • ABA binds to receptors on plasma membrane of stomatal guard cells
  • activates changes in ionic concentration of guard cells
  • increase water potential of guard cells
  • decrease cell turgor (K+ then water moves out)
  • guard cells close stomata
  • less water loss by transpiration
• what is stomatal resistance?

  • resistance of the stomatal to gas exchange
  • higher resistance - more closed stomata