Translocation of Photosynthetic Assimilate From Grass Leaves, as Influenced by Environment and Species

Abstract

The export of ¹⁴C-labelled photosynthate from leaves of two grasses with Calvin cycle (C*3) photosynthesis, and five grasses with the C₄ dicarboxylic acid pathway, was followed by continuous monitoring through the hours in light immediately after assimilation of ¹⁴CO₂ and also through the subsequent dark period. The proportion of activity exported in the first few hours was much greater in all C₄ grasses than in the C₃ species, being greatest in the C₄ grasses best adapted to growth at cool temperatures. Large differences between species were also apparent in their rates of assimilate export per unit leaf area, but there was no consistent relation between these rates and the cross-sectional area of phloem serving a given catchment leaf area. The rate of assimilate transfer per unit phloem area in Lolium temulentum was comparable to rates previously recorded for other C₃ plants, but the rates for all C₄ grasses except Digitaria sanguinalis were considerably faster. The rate of export in Paspalum dilatatum increased progressively up to the highest light intensity used, giving no evidence of saturation of translocation capacity. It was greater in plants grown under higher light intensities, and increased with increasing sink demand, but was not increased by darkening or removing the leaf blade distal to the area exposed to ¹⁴CO₂ in Lolium temulentum. Most of the photosynthate not exported within 6 h was stored as polysaccharide in Panicum maximum, and was subsequently mobilized over a brief period during the following night. ¹⁴C assimilated at the end of the light period was mobilized first, while that assimilated at the beginning of the day was mobilized only at the end of the following night. However, the rate of mobilization could be increased by partial defoliation or by other treatments which increased the demand for assimilates from the leaf. The time to mobilization was unaffected by night temperature in Lolium temulentum, but increased markedly at temperatures below 15°C in the C₄ grasses Panicum maximum and Paspalum dilatatum. The adaptive value and some possible explanations of the differences between C₃ and C₄ grasses in their translocation of assimilates are considered.