Effect of Atmospheric Humidity on Rate of Carbon Fixation by Plants

Rates of carbon fixation by leaves of squash, wax bean, cabbage, Pelargonium, Primula, and tomato plants were not affected either by a rapid decrease in the humidity of the surrounding atmosphere or by a prolonged exposure of 15-20 hours to low humidity. 2. Attached leaves of squash, wax bean, cabbage, Pelargonium, Primula, and tomato plants remained turgid at moderate temperatures although subjected to an extremely low humidity. The leaves of Cineraria plants in some few cases wilted when subjected to dry air, and the loss of turgidity was always accompanied by a decrease in the rate of carbon fixation. 3. Air temperatures above 30⚬C. retarded the rate of carbon fixation by Pelargonium leaves in either moist or dry air. 4. Atmospheric moisture affected the opening of the stomata of tomato and Pelargonium leaves. Under the conditions used, the maximum size of stomatal opening generally occurred when the leaves were exposed to relative humidities above 60 per cent. The size of the openings decreased with a decrease in atmospheric humidity and the stomata generally appeared closed in leaves exposed to humidities below 30 per cent. 5. The process of carbon fixation was not greatly affected by wide variations in the size of stomatal openings. The leaves of tomato and Pelargonium plants absorbed carbon dioxide and accumulated carbohydrates in appreciable quantities although the stomata appeared to be closed. That these plants absorb carbon dioxide when the stomata appear closed has been demonstrated by means of three methods: (1) the amount of carbon dioxide absorbed by leaves in which the stomata appeared closed was approximately equal to the amount absorbed by the same leaves when the stomata were open; (2) starch was found by means of the usual iodine test to accumulate in leaves exposed to relatively dry air with their stomata apparently closed; (3) acid-hydrolyzable carbohydrates in leaves of Pelargonium plants grown in relatively dry air increased approximately 41 per cent during a period of six hours, although the stomata of the leaves appeared to be closed.