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1

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

2

• 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

3

Respiration:

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

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• 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

5

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

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

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• 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

7

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

8

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)

9

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

10

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

11

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)

12

Plants – Light Conditions

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

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• 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

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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)

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Shade-intolerant

high growth rates under sunlight, but low rates in shade

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Shade-tolerant

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

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Within individuals...
• Tree Top

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

18

• Tree bottom

(shade)
– larger, thinner
→ increases photosynthetic rate in shade

19

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
nutrients
eg. needle-leaf evergreen species
» Avoid these costs by shedding leaves eg. deciduous species