Photosynthesis - Responses of Plants to Stress

Photosynthesis is also inhibited due to low water potential in the leaves. The leaf water potential is the sum of turgor pressure osmotic potential and matric potential. In very simple words, it represents the degree of saturation of the tissue with water. At low leaf water potentials, photosynthesis is inhibited by:

1. Stomatal closure (limiting the gas exchange), and
2. Decrease in photosynthesis.

During dehydration both quantum efficiency (i.e., moles of CO₂ fixed per mole of light quanta absorbed) and the rate of photosynthesis are inhibited in leaves exposed to either high or very low light intensity. This phenomenon is called photo inhibition of photosynthesis. Photo inhibition is probably caused by accumulation of excessive excitation energy at photosystem II reaction centre complex. In this way exposure to light adds to the water stress when the plant's response already would be to decrease photosynthesis.