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Diurnal Changes in Transpiration and Daily Photosynthetic Rate of Several Crop Plants¹

Measurements of photosynthesis and transpiration of soil-grown, growth chamber-cultured cotton (Gossypium hirsutum L. cv. ‘Empire’), bell pepper (Capsicum frutescens L. cv. ‘California Wonder’), peanut (Arachis hypogaea L. cv. ‘Florigiant’), soybean (Gossypium max Merr. cv. ‘Lee’), and bermudagrass (Cynodon dactylon L.) plants were made during 14-hour photoperiods: irradiance 1.17 j. cm^-2 min^-1, day-night temperature 30 C, and CO₂ concentration 310 µliter/liter. A diurnal trend in both photosynthesis and transpiration was evidenced by the dicots but not the monocot. The sinusoidal nature of the curves suggest endogenous rhythmic changes in diffusive resistance and/or biochemical activity. As much as twofold changes in cotton and peanut photosynthesis were recorded compared to approximately 30% change in bell pepper and soybean photosynthesis. These findings jeopardize the relevancy of short-time photosynthetic measurements used in some screening techniques.

Key Words: Stomata • Growth chamber • Dicot • Monocot

² Plant Physiologist, USDA, Watkinsville, Ga. 30677.

Received for publication July 27, 1972.