28. The rate of photosynthesis decreases at temperatures above
30 degrees Centigrade because at those temperatures water loss due to evaporation
through the stomata (transpiration) is so great that the stomata begin to close.
This decreases the amount of carbon dioxide in the leaf and photosynthesis slows
down.
In addition, higher temperatures will denature the enzymes
responsible for photosynthesis, which consequently slows the reactions down
too.
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Long & Slichter