Effect of Atmospheric Humidity on Rate of Carbon Fixation by Plants

Abstract

Rates of C fixation by attached leaves and the entire above ground portion of squash, wax bean, cineraria, Primula, geranium, cabbage, and tomato plants were measured first in moist, then in relatively dry air while other environmental factors were maintained constant. Wide variations in atmospheric moisture caused no appreciable hanges in rates of C fixation providing the leaves remained turgid. Leaves of most plants studied remained turgid at 25[degree] C although subjected to humidities of 8-10% for periods of 2-15 hrs. Geraniums absorbed the most CO2 at approximately 30[degree] C either in moist or relatively dry air. Numerous microscopic observations showed that the stomata of geranium and tomato leaves closed when the plants were subjected to humidities below 25-30% although the leaves remained turgid. That these plants absorb CO2 when the stomata appear closed has been demonstrated by 3 methods: the amount of CO2 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; starch was found by means of the usual iodine test to accumulate in leaves exposed to relatively dry air with their stomata apparently closed; acid-hydrolizable carbohydrates in leaves of geraniums grown in relatively dry air increased approximately 41% during a period of 6 hours, although the stomata of the leaves appeared to be closed.