Influence of Temperature and O2 Concentration on Photosynthesis and Light Activation of Ribulosebisphosphate Carboxylase Oxygenase in Intact Leaves of White Clover (Trifolium repens L.)
Detached leaves of white clover (Trifolium repens L.) were kept for 1 h under various conditions of temperature, oxygen concentration and light intensity. Rates of photosynthesis were measured where appropriate and then ribulosebisphosphate carboxylase oxygenase (RuBPCO) was extracted rapidly and its initial activity measured immediately. The extracted activity increased with increased intensity of illumination of the leaves. Where leaves were pretreated at low light intensity, the lower the temperature of the leaves the higher the extracted activity of RuBPCO. At high light intensity temperature did not affect the activity of subsequently extracted RuBPCO but the light intensity which was necessary for maximum activity increased with temperature. Activity of RuBPCO from leaves pretreated in the dark was least when CO2 was low and temperature high. Leaves, pretreated at low temperatures and high light intensity in 20% O2, yielded higher activity in extracts than leaves pretreated under similar conditions but in 2% O2. A relatively weak temperature response of photosynthesis at low irradiances was associated with a decrease in extractable RuBPCO activity with increasing temperature. A strong temperature dependence of the oxygen inhibition of photosynthesis was associated with lower extractable RuBPCO activity in leaves pretreated at low oxygen concentration at low temperatures. With leaves from plants grown at low temperatures prior to treatment of leaves, oxygen inhibition of photosynthesis was less temperature dependent and activity of RuBPCO in extracts was not decreased by low O2 at low temperatures. Differences in the activation of RuBPCO appear to influence photosynthesis and account for an absence of oxygen inhibition of photosynthesis at low temperatures in plants grown in warm condition
