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Environmental and Stress Physiology

Low Growth Temperature-Induced Increase in Light Saturated Photosystem I Electron Transport Is Cation Dependent ¹

Department of Plant Science, University of Western Ontario, London, Ontario N6A 5B7, Canada

Thylakoid membranes isolated from cold tolerant, herbaceous monocots and dicots grown at 5°C exhibit a 1.5-fold to 2.7-fold increase in light saturated rates of photosystem I (PSI) electron transport compared to thylakoids isolated from the same plant species grown at 20°C. This was observed only when either water or reduced dichlorophenolindophenol was used as an electron donor. The apparent quantum yield for PSI electron transport was not affected by growth temperature. The higher light saturated rates of PSI electron transport in 5°C thylakoids had an absolute requirement for the presence of Na⁺ and Mg⁺². The accessibility of reduced dichlorophenolindophenol to the donor site was not affected by growth temperature since 5°C and 20°C thylakoids exhibited no significant difference in the concentration of this electron donor required for half-maximal PSI activity. The cation dependent higher rates of light saturated PSI activity were also observed when rye thylakoids were developed under intermittent light conditions at 5°C. Thus, this cation effect on PSI activity appeared to be independent of light harvesting complex I and II. The extent of the in vitro reversibility of this cation effect appeared to be limited by an inherent decay process for PSI electron transport. The rate of decay for PSI activity was greatest when thylakoids were isolated in the absence of NaCl and MgCl₂. We conclude that exposure of plants to low growth temperatures induces a reorganization of thylakoid membranes which increases the light saturated rates of PSI electron transport with no change in the apparent quantum efficiency for this reaction. Cations are required to stabilize this reorganization.

¹ This research was supported by the National Science and Engineering Research Council of Canada.