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© 1987 Oxford University Press

RESEARCH-ARTICLE

In-Phase Cycling of Photosynthesis and Conductance at Saturating Carbon Dioxide Pressure Induced by Increases in Water Vapour Pressure Deficit

JAMES A. BUNCE

Plant Photobiology Laboratory, USD A-Agricultural Research Service, Beltsville Agricultural Research Centre Beltsville, Maryland 20705, U.S.A.

Plant Photobiology Laboratory, USDA-ARS, Beltsville Agriculture Research Centre, Beltsville, Maryland 20705, U.S.A.

Bunce, J. A. 1987. In-phase cycling of photosynthesis and conductance at saturating carbon dioxide pressure induced by increases in water vapour pressure deficit.—J. exp. Bot. 38: 1413–1420.

The leaf to air water vapour deficit was increased suddenly from about 1·0 to 2·5 IcPa for single leaves of soybean (Glycine max L. Merr.) plants held at 30 °C, 2·0 mmol m –2 s–1 photosynthetic photon flux density (PPFD) and carbon dioxide pressures saturating to photosynthesis. After a lag of about 10 min, photosynthetic rate and stomatal conductance to water vapour began to decrease, and then cycled in phase with each other. The period of the cydes was about 20 min. During these cycles the substomatal carbon dioxide pressure was constant in the majority of leaves examined, and was always above saturation for photosynthesis. Epidermal impressions showed that most stomata changed in aperture during the cycles, and that very few were ever fully closed. Water potential measured on excised discs changed by at most 0·1 MPa from the minima to the maxima in transpiration rate. In contrast, for leaves of sunflower (Helianthus animus L.) grown at low PPFD, the increase in VPD led to leaf wilting and decreased photosynthesis, followed by recovery of turgor and photosynthesis as stomatal conductance began to decrease. In these leaves photosynthesis and conductance then cycled approximately 180° out of phase. It is suggested that in soybeans decreased leaf conductance induced by high VPD provided a signal which decreased the rate of photosynthesis at carbon dioxide saturation by a mechanism that was not related to a water deficit in the mesophyll.

Key words: Photosynthesis, stomatal conductance, cycling, vapour pressure deficit


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