Plant Adaptations Notes

PLANT STRUCTURE

• Leaves
  • Upper and lower epidermis covered with a waxy cuticle that prevents gas exchange.
  • Stomata are openings in the epidermis that allow gas exchange.
    • Can be opened and closed by changes in turgor pressure in the guard cells.
  • Mesophyll is the tissue between the epidermal layers.
  • Aquatic plants
    • No waxy cuticle.
    • Gases can diffuse directly between the mesophyll and the surrounding water.
  • Transpiration is the loss of water vapour through the stomata.

C3 and C4 Plants

• C3 Plant
  • Mesophyll cells.
• C4 Plant
  • Bundle sheath cell.
  • Vein
  • Pore

TEMPERATURE AND PHOTOSYNTHESIS

• Photosynthesis and respiration increase as temperature increases.
• Rates of photosynthesis and respiration are measured in the same units: amount of $CO_2$ taken up or released.
• Respiration increases more slowly than photosynthesis at first.
• At high temperatures, respiration rate exceeds photosynthesis.
• Net photosynthetic rate
  • Subtracting respiration rate from photosynthetic rate.
  • Results in three important temperatures for a plant:
    • T-minimum: Lowest temperature at which $CO_2$ uptake is above zero (photosynthesis stops below this temperature).
    • T-optimal: Temperature at which $CO_2$ uptake is maximum.
    • T-maximum: Highest temperature at which $CO_2$ uptake is above zero (respiration exceeds photosynthesis above this temperature).

ADAPTATIONS TO LIGHT INTENSITY

• Plants shade one another, adapting differently to direct sunlight versus shade.
• In shade, the amount of light limits photosynthesis.
• Compared to photosynthesis in direct sunlight, photosynthesis in shade has:
  • A lower light compensation point (rate of photosynthesis matches the rate of respiration).
  • A lower light saturation point (the intensity at which additional increases in light do not increase photosynthesis).
  • A lower $T_{max}$.
  • Higher chlorophyll levels to trap as much of the limited light as possible.

Cooler Temperatures in Shade

• Leads to reduced rate of respiration, lowering of the light compensation point.

Leaf Adaptations

• Leaves are broader and thinner to expose as much area of photosynthetic tissue to light as possible (even on a single plant).
• Some plants are shade tolerant, and some are shade intolerant.
• Shade-tolerant plants show less reduction in photosynthetic rate when moved from sunlight to shade than do shade-intolerant plants.

Sun vs. Shade Plants

• Sun Plant
  • Cells: Large cells
  • Chloroplasts: Small chloroplasts
  • Grana stacks: Few/thin grana stacks
  • Chlorophyll/Rubisco ratio: Low
  • Chl a/b ratio: High
  • Leaves: Small thick leaves
  • Stomatal conductance: High
  • Photosynthetic capacity: High
  • Plants: Low leaf area ratio
  • Root:shoot ratio: High
  • Leaf orientation: Vertical
  • Photosynthetic capacity: High
  • Compensation irradiance: High
• Shade Plant
  • Cells: Small cells
  • Chloroplasts: Large chloroplasts
  • Grana stacks: Several/thick grana stacks
  • Chlorophyll/Rubisco ratio: High
  • Chl a/b ratio: Low
  • Leaves: Large thin leaves
  • Stomatal conductance: Low
  • Photosynthetic capacity: Low
  • Plants: High leaf area ratio
  • Root:shoot ratio: Low
  • Leaf orientation: Horizontal
  • Photosynthetic capacity: Low
  • Compensation irradiance: Low

TEMPERATURE, MOISTURE AND PLANTS

• Higher temperatures increase water stress because they lower the relative humidity of the air.
• The relative humidity of air spaces inside the leaf is 100%.
• The rate of water loss from the plant depends on the difference between relative humidity inside the leaf and the relative humidity of the outside air.

Short Term Changes in a Plant in Response to Water Stress:

• Closing stomata during hottest (= driest) time of day.
• Changing turgor pressure in parts of the leaf to curl leaves to shade the underside of the leaf (more stomata on the bottom of a leaf).
• Curling occurs so that the underside of the leaf is on the inside of the cylinder formed.
  • This space inside the cylinder acts as a boundary layer and slows the rate of water loss.
• Leaves wilt and collapse onto the stems (same effect as curling).

Changes in a Plant in Response to Prolonged Water Stress:

• Reduces chlorophyll production, which yellows leaves and reduces the rate of photorespiration when stomata are closed for most of the day for many days.
• Shedding of leaves to reduce water loss and photorespiration.

Adaptations in Plants That Regularly Undergo Water Shortage:

• Drought deciduous plants are adapted to drop their leaves at the onset of the dry season (e.g. Ana tree Faedherbia albida).
• Changes in morphology: sinking stomata into pits, covering photosynthetic areas with hairs.
• Plants adapted to xeric (dry) habitats often have a greater portion of their biomass allocated to below-ground structures than do plants adapted to mesic (moist) environments.