Assimilate Movement in Lolium and Sorghum Leaves. IV. Photosynthetic Responses to Reduced Translocation and Leaf Storage

Abstract

The net CO₂ exchange (NCE) rates of the last fully expanded leaves of Lolium temulentum L. and Sorghum bicolor Moench were examined in relation to the accumulation of photosynthate that occurred with time in continuous light (1100 μmol m^-2 s^-1 PAR). On a leaf area basis Lolium, although having a lower NCE, accumulated current photosynthate at a faster rate than Sorghum and had a greater capacity for storage. In Lolium there was an initial rapid rise in leaf solute, followed by a steady increase in the accumulation of starch, while in contrast in Sorghum there was an initial rapid rise in starch followed by a change to the accumulation of soluble photosynthate.

After 4-6 h in continuous light there was a fall in NCE in both species, with the response depending on the balance between source and sink tissue. The decrease' in NCE coincided with an increase in specific leaf weight (SLW) of about 40% and SLW showed a closer negative correlation with NCE than leaf solute potential (Ψs). The fall in NCE, associated with an accumulation of photosynthate, was primarily related to factors within the leaf and not to stomatal conductance. The fall in NCE could not be accounted for by a rise in photorespiration and did not appear to be associated directly with an increase in the partitioning of photosynthate into starch. In both Lolium and Sorghum a fall in apparent quantum yield coincided with the fall in NCE.