Assimilate Movement in Lolium and Sorghum Leaves. II. Irradiance Effects on the Products of Photosynthesis

Abstract

The rate of export and longitudinal movement of ¹⁴C-labelled assimilates in the phloem was found to be greater in Sorghum sudanense than in Lolium temulentum. Sucrose was the predominant metabolite translocated from the leaf in both species in both low and high light conditions. The effect of irradiance on the rate of formation and nature of the products of photosynthesis was examined using ¹⁴CO₂ pulse-chase techniques and the differences in the primary metabolites closely followed those expected for a C₃ and a C₄ species. Reduction in irradiance reduced the rate of incorporation of ¹⁴C into sucrose, especially in Sorghum, and led to the accumulation of amino acids in both species. Although species differences in export of ¹⁴C-labelled assimilate were not apparently related to the rate of ¹⁴C incorporation into sucrose, this could account for the delay in export of ¹⁴C-labelled assimilates associated with reduced irradiance. There was a rapid initial labelling of starch and the proportion of *14C incorporated as starch was enhanced at high irradiance, particularly in Sorghum. Overall, the results support the view that there is a greater partitioning of assimilate into storage (starch) at high irradiance relative to assimilate moving into the phloem and that irradiance (in the range 20-96 W m^-2) did not directly influence vein loading.