Assimilate Movement in Lolium and Sorghum Leaves. Iii. Carbon Dioxide Concentration Effects on the Metabolism and Translocation of Photosynthate.

Abstract

Raising the CO2 concentration of air to 720 µl I-¹ increased the rate of net CO2 uptake by the leaf of Lolium temulentum, a C*3 species, more than in Sorghum sudanense, a C*4 species. In Lolium, but not in Sorghum, high CO2 over a 6 h period resulted in relatively more of the additional leaf photosynthate being partitioned into storage rather than to translocation. Removing CO2 from the air passing over a 5 cm length of leaf before and after a pulse application of 14CO2 resulted in a reduced labelling of sucrose and a slower rate of export of 14C-labelled photosynthate in both species. With the fall in net CO2 exchange by the leaf section deprived of CO2 there was a compensating increase within this section in retention of photosynthate derived from the distal part of the leaf. Evidence provided by 14CO2 pulse chase experiments and CO2 exchange studies confirmed the relative enhancement of photorespiration under low CO2 concentrations in Lolium, but not in Sorghum. The CO2 depletion experiments provide a useful base for comparison with the effect of low light and water stress on photosynthate metabolism and translocation in that both these conditions reduce CO2 uptake. There is support for the suggestion that the change in photosynthate metabolism under water stress in C*3 species may result from reduced CO2 entry into the leaf due to stomatal closure.