Assimilate Movement in Lolium and Sorghum Leaves. I. Irradiance Effects on Photosynthesis, Export and the Distribution of Assimilates

Abstract

At low light levels (20 W m^-2 PAR), little difference was detectable in net photosynthetic rates between leaves of the C₃ grass Lolium temulentum and the C₄ grass Sorghum sudanense. At low light, both species also had similar rates of export of photosynthate per unit leaf area, although the rate of export of ¹⁴C, following the assimilation of ¹⁴CO₂ by the uppermost fully expanded leaf, was much greater in Sorghum. As light levels were increased to 96 W m^-2 PAR or greater, net photosynthesis rose more in Sorghum than in Lolium and a greater proportion of the assimilate was exported from the Sorghum leaf, while Lolium with the thicker leaf showed a much greater capacity for storage of photosynthate. High and low light pretreatments, which altered the amount of both soluble and insoluble dry matter in the leaf, had little effect on the percentage rate of export of ¹⁴C following the uptake of ¹⁴CO₂, which suggests that the bulk of the stored material may be effectively separated from the pathway of transfer from the chloroplast to the vascular system within the leaf. The pattern of distribution of ¹⁴C-labelled assimilates to other parts of the plant was only slightly affected by reduced light during the 4-h translocation period' however, a 3-day pretreatment at low light significantly reduced the proportion of the assimilates moving to the roots.