Plant Physiology 96:1018-1025 (1991)
© 1991 American Society of Plant Biologists
Environmental and Stress Physiology
Photophosphorylation in Attached Leaves of Helianthus annuus at Low Water Potentials 1
Adriana Ortiz-Lopez,
Donald R. Ort and
John S. Boyer
Photosynthesis Research Unit, U.S. Department of Agriculture, Agricultural Research Service, Urbana, Illinois 61801,
Department of Plant Biology University of Illinois, Urbana, Illinois 61801,
Department of Agronomy, University of Illinois, Urbana, Illinois 61801,
College of Marine Studies, University of Delaware, Lewes, Delaware 19958
The in situ response of photophosphorylation and coupling factor activity to low leaf water potential ( L) was investigated using kinetic spectroscopy to measure the flash-induced electrochromic absorption change in attached sunflower (Helianthus annuus L. cv IS894) leaves. The electrochromic change is caused by the formation of an electric potential across the thylakoid membrane associated with proton uptake. Since depolarization of the thylakoid membrane following flash excitation is normally dominated by proton efflux through the coupling factor during ATP formation, this measurement can provide direct information about the catalytic activity of the coupling factor. Under low L conditions in which a clear nonstomatal limitation of net photosynthesis could be demonstrated, we found a strong inhibition of coupling factor activity in dark-adapted leaves which was probably caused by an increase in the energetic threshold for the activation of the enzyme at low L. While this result supported earlier in vitro findings, we further discovered that the light-dependent reduction of coupling factor reversed any observable effect of low L on the energetics of activation or on photophosphorylation competence. Furthermore, coupling factor was reduced, even in severely droughted sunflower, almost immediately upon illumination. Based on these measurements, we conclude that the nonstomatal limitation of photosynthesis observed by us and others in droughted plants cannot be explained by impaired coupling factor activity.
1 Supported in part by U.S. Department of Agriculture Competitive Research Grant 87-CRCR-1-2381 (D.R.O.).
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