Acclimation of winter rye to cold-hardening temperatures results in an increased capacity for photosynthetic electron transport

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Abstract
Photosynthetic electron transport and its partial reactions were measured as a function of light intensity and temperature in isolated chloroplasts from cold-hardened and unhardened rye (Secale cereale L. cv. Puma). Chloroplasts from cold-hardened rye plants exhibited light-saturated rates for whole chain electron transport that were about 1.4-fold higher at 25 °C than those observed in chloroplasts from unhardened rye plants. This was correlated with light-saturated rates of electron transport through photosystem I that were 1.6-fold higher in cold-hardened chloroplasts than in unhardened chloroplasts. These results could not be attributed to a differential uncoupling of electron transport, nor to a differential production of a superoxide free radical which would stimulate net O2 consumption. Electron transport through photosystem II of cold-hardened and unhardened chloroplasts exhibited a differential sensitivity to temperature. The ratio of light-saturated rates in cold-hardened chloroplasts to light-saturated rates in unhardened chloroplasts increased from 0.87 to 1.40 when the temperature was decreased from 26 to 4.5 °C. The locus of this differential temperature sensitivity appeared to reside on the O2 evolving side of photosystem II prior to the site of electron donation by diphenylcarbazide. On the basis of the estimates of the number of photons required for half the maximal rates of electron transport, it was concluded that the efficiency of light utilization by photosystem I and II was unaffected by growth temperature. However, the estimated efficiency for light utilization did increase with a decrease in reaction temperature for chloroplasts from both cold-hardened and unhardened ‘Puma’ rye. Thus, it is concluded that growth at cold-hardening temperatures resulted in an increase in the capacity for photosynthetic electron transport, but did not affect the quantum efficiency for this process.