Limitation of photosynthesis by changes in temperature : Factors affecting the response of carbon-dioxide assimilation to temperature in barley leaves.

The aim of this work was to examine the effect of abrupt changes in temperature in the range 5 to 30°C upon the rate of photosynthetic carbon assimilation in leaves of barley (Hordeum vulgare L.). Measurement of the CO2-assimilation rate in relation to the intercellular partial pressure of CO2 at different temperatures and O2 concentrations and at saturating irradiance showed that as the temperature was decreased photosynthesis was saturated at progressively lower CO2 partial pressures and that the transition between the CO2-limited and ribulose-1,5-bisphosphate-regeneration-limited rate became more abrupt. Feeding of orthophosphate to leaves resulted in an increased rate of CO2 assimilation at lower temperatures at around ambient or higher CO2 partial pressures both in 20% O2 and in 2% O2 and it removed the abruptness in the transition between the CO2-limited and ribulose-1,5-bisphosphate-regeneration-limited rates. Phosphate feeding tended to inhibit carbon assimilation at higher temperatures. The response of carbon assimilation to temperature was altered by feeding orthophosphate, by changing the concentrations of CO2 or of O2 or by leaving plants in the dark at 4°C for several hours. Similarly, the response of carbon assimilation to phosphate feeding or to changes in 2% O2 was altered by leaving the plants in the dark at 4°C. The mechanism of limitation of photosynthesis by an abrupt lowering of temperature is discussed in the light of the results.