C 4 plants can assimilate CO 2 at greater rates than C 3 plants. Evaporative cooling reduces risk of leaf temperatures becoming too high for enzymes to function. If transpiration exceeds delivery of water by xylem, plants wilt.
Plants can adjust to reduce risk of wilting.
Regulating the size of stomatal openings also reduces transpiration. Cells that flank stomata and control stomatal diameter by changing shape. By integrating internal and external environmental cues, guard cells open and close, balancing the requirements for photosynthesis with the need to conserve water from transpirational loss. Driving photosynthesis in guard cell chloroplasts, making ATP available for the ATP-driven proton pumps. Decrease of CO 2 in leaf air spaces due to photosynthesis in the mesophyll.