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Articles

Effect of Water Stress on Proline Synthesis from Radioactive Precursors ¹

Don Aspinall^b and Leslie G. Paleg^b

^a Department of Botany and Plant Pathology, Iowa State University, Ames, Iowa 50010, Department of Plant Physiology, Waite Research Institute, University of Adelaide, Glen Osmond, South Australia 5064

Barley (Hordeum vulgare L. var. Prior) leaves converted more ¹⁴C-glutamic acid to free proline when water-stressed than when turgid; neither decreased protein synthesis nor isotope trapping by the enlarged free proline pools found in wilted tissue seemed to account for the result. This apparent stimulation of proline biosynthesis in wilted leaves was not observed when radioactive ornithine or P5C (¹-pyrroline-5-carboxylate, an intermediate following glutamate in proline synthesis) were used as proline precursors unless proline levels were high as a result of previous water stress. We interpret this to mean that any stimulation of proline synthesis by water stress must act on P5C formation rather than its reduction to proline. Experiments showing greater apparent conversion of ¹⁴C-glutamate to proline do not unequivocally prove that proline synthesis is stimulated by water stress, as P5C feeding studies show that proline oxidation is inhibited under comparable conditions. This inhibition could account, at least in part, for increased proline labeling, and must be considered an alternate possibility.

¹ This work was supported by grants from the Australian Research Grants Committee (to D. A. and L. G. P.) and Iowa State University Graduate College (to S. B.).

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