Above 380c and below ooc the rate of photosynthesis decreases. propose a hypothesis to explain why this occurs.
enzyme activity decreases at these temperatures
at these temperatures plants wilt
chlorophyll cannot absorb energy from the sun at these temperatures
the cytoplasm of the plant cell cannot function

Enzyme activity decrease at these temperatures.

Explanation:

Photosynthesis is facilitated by the action of various enzymes and involves light and dark reactions.  In light reactions chlorophyll absorbs sunlight and creates ATP AND NADPH. In dark reactions carbon is fixed using the ATP and NADPH.

The efficiency of all these activities depends on the activity of enzymes which depends on the temperature. The optimum temperature range where the enzyme activity is optimal is about 10°c - 20°c. At low temperatures and high temperatures enzyme activity is less efficient.

At a temperature above 38°c and below  0°c enzyme activity decreases.