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. 1991 Aug;96(4):1018–1025. doi: 10.1104/pp.96.4.1018

Photophosphorylation in Attached Leaves of Helianthus annuus at Low Water Potentials 1

Adriana Ortiz-Lopez 1,2,3,4, Donald R Ort 1,2,3,4, John S Boyer 1,2,3,4
PMCID: PMC1080887  PMID: 16668292

Abstract

The in situ response of photophosphorylation and coupling factor activity to low leaf water potential (ψL) 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 ψL 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 ψL. 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 ψL 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.

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Selected References

These references are in PubMed. This may not be the complete list of references from this article.

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