Metabolism and Export of ¹⁴C-Labelled Photosynthate From Water-Stressed Leaves

Abstract

The effect of water stress on the translocation of ¹⁴C-labelled photosynthate from leaves was examined in four contrasting plant species, Sorghum sudanense (Piper) Stapf. (C₄, monocot), Triticum aestivum L. (C₃, monocot), Amaranthus caudatus L. (C₄, dicot) and Helianthus annuus L. (C₃, dicot). Water stress reduced the rate of loss of ¹⁴C from the leaves of all species following a pulse application of ¹⁴CO₂, but the effects of stress were most apparent in sorghum and sunflower, the species with the highest control rates of export.

Slower labelling of sucrose, following the application of ¹⁴CO₂, could explain the reduced rate of export of ¹⁴C from stressed wheat leaves, but this would not explain the response in sorghum. An examination of the products of ¹⁴CO₂ fixation in wheat leaves suggests that the changes in metabolism of ¹⁴C under stress may be due to reduced CO₂ levels, resulting from stomatal closure. In contrast the change in metabolism of ¹⁴C-labelled photosynthate due to water stress in sorghum was very different from that expected in response to reduced CO₂ levels.

Prevention of translocation of photosynthate out of a leaf of either wheat or sorghum, by steam killing the tissue at the base of the blade, altered the pattern of labelling of metabolites with ¹⁴C following a pulse of ¹⁴CO₂. However changes in translocation due to reduced export associated with a reduction in growth under water stress did not appear to play a part in the responses observed in these experiments.