lecture 6 Flashcards Preview

ecology > lecture 6 > Flashcards

Flashcards in lecture 6 Deck (19)
Loading flashcards...

Plant Adaptations to Environmental Variability

→ plants have evolved a variety of adaptations to successfully grow, reproduce and survive across the entire range of environmental conditions on Earth


• Photosynthesis:

process where energy from the sun is used to transform CO2 into carbohydrates (simple sugars) and O2
– Photosynthesis takes place in specialized cells (mesophyll cells) in the leaf – Chlorophyll (light absorbing pigment) traps light energy → synthesizes ATP
→ this energy drives CO2 → O2 + sugars
– this chemical reaction is dependent on enzyme called rubisco



in the mitochondria of cells (plant & animal) - carbohydrates are broken down to generate energy (ATP), releasing CO2


• Plants both use and produce CO2 and the difference in the rates of these two processes is:

Net Photosynthesis = Photosynthesis – Respiration
(= carbon uptake – carbon loss)
• CO2 diffuses into the leaf through openings in the surface of the leaf, called stomata (diffusion = movement of a substance from areas of higher to lower concentration)
– As CO2 diffuses into the leaf, water diffuses out of the leaf (= transpiration) • CO2 enters: atmosphere >> leaf
• Water leaves: atmosphere << leaf
• water lost must be replaced with water taken by roots from the soil
• Plants must acquire essential resources: light, CO2, water, nutrients •Leaf tissue - photosynthesis (uptake of CO2)
•Stem tissue - structural support (gain access to light)
•Root tissue - water and nutrient uptake from the soil


Individual Responses of Plants
• Plants adapt to different environmental conditions to keep:

Photosynthesis > Respiration (+ carbon balance→ grow) Photosynthesis > Transpiration (+ water balance→ survive)


• Individuals must tolerate environmental conditions that influence the acquisition of all essential resources

• Therefore, balance adaptations to multiple environmental conditions all at once
→ adaptations that allow a plant to successfully grow, survive and reproduce under one set of environmental conditions may limit its ability to do equally well under different environmental conditions


Plants: Photosynthesis vs Water Loss

• A plant must open stomata to photosynthesize (↑ grow) - but it loses water when stomata are open (↓ survival)
• Balance between photosynthesis and transpiration and photosynthesis and
respiration governs the evolution of terrestrial plants


How do terrestrial plants respond to different levels of moisture in their environment?

1. short time scales
2. Moderate time scales
3. Long time scales (evolution)


Short time scales:

– Regulate opening and closing of stomata during different parts of the day
eg. close stomata during hottest part of the day when highest water loss through evaporation
– Leaf curling or wilting – reduces the surface area of the leaf exposed to solar radiation and, thus, water loss


Moderate time scales:

– Individuals can balance leaf vs root tissue
• Wet conditions (ideal) → ↑ leaf tissue & ↓ root and shoot
– Increase the photosynthetic surface (maximizes CO2 uptake and
photosynthetic rates → growth)
– No increase in other tissues (ie. shoot, root) because this increases the rate
of respiration (CO2 loss)
• Dry conditions → ↑ root tissue & ↓ leaf and shoot
– Increases the volume of tissue in the soil to extract water
– Reduces the surface area of leaf tissue to reduce water loss


Long time scales (evolution):

– modified forms of photosynthesis to increase water-use efficiency
• C4 & CAM plants
– additional step in the conversion of CO2 into sugars → higher maximum
rate of photosynthesis
– higher rate of photosynthesis requires stomata to be open less time - so
less water is lost
– leaf morphology adaptations to dry conditions:
• smaller and thicker leaves (water storage)
• smallerstomata
• cover leaves in wax, resin, little hairs (eg. cactus)


Plants – Light Conditions

Plants – Light Conditions
• Plants are either adapted to low light (shade-tolerant) or high light (shade-intolerant)


• In shade:

photosynthesis is limited by availability of light, not the amount of
photosynthetic tissue
– Shade-tolerant (low light):
• lower production of rubisco in leaf tissue (do not expend energy producing high amounts of rubisco)
→ Lower maximum photosynthetic rate


compensate low light by

– higher production of chlorophyll
– higher leaf surface area
– higher growth of leaves than roots
• increases the photosynthetic surface area to offset the decrease in photosynthetic rate (due to lower rubisco)



high growth rates under sunlight, but low rates in shade



grow similarly under sunlight and shade
– cannot increase growth dramatically in sunlight because limited by rates of photosynthesis (concentration of rubisco)


Within individuals...
• Tree Top

(direct sunlight)
– smaller, thicker leaves
→ reduces water loss in direct sunlight


• Tree bottom

– larger, thinner
→ increases photosynthetic rate in shade


Plants – Cold Temperature
• Temperatures drop below freezing – Response:

• If temperatures drop slowly...
– Ice formation in the cells of leaves
– Cells dehydrate (can be reversed when temperature rises)
• If rapid drop in temperature...
– ice crystals form within the cell without dehydration
→ can puncture cell membranes and cell contents spill out during thaw
• Frost hardening – genetically controlled characteristic
– Form protective compounds that act as antifreeze
» lower the temperature at which freezing occurs
» Require a considerable amount of energy and
eg. needle-leaf evergreen species
» Avoid these costs by shedding leaves eg. deciduous species