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

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.