The effects of drought stress on free amino acid accumulation and protein synthesis in Brassica napus

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Abstract
Changes in the concentrations of free amino acids and specific organic acids were analysed during the induction of drought stress in Brassica napus. Most of the amino acids showed a characteristic linear increase with the induction of drought stress in Brassica leaves, increasing an average of 5.9‐fold over control levels, followed by a reduction in concentration upon rehydration of the plants. Pyruvate concentrations doubled after 4 days of drought stress whereas 2‐oxoglutarate concentrations remained relatively constant. The activities of two of the enzymes involved in amino acid biosynthesis, alanine aminotransferase (EC 2.6.1.2) and aspartate aminotransferase (EC 2.6.1.1), were also measured. Neither enzyme showed any increase in activity, except when the plants were rehydrated. This suggests that the increase in both alanine and aspartate levels results from the increase in their precursors pyruvate and glutamate and may not require increased enzyme activity. The effect of drought stress upon changes in protein synthesis was analysed by sodium dodecyl sulfate polyacrylamide gel electrophoresis. We found that there was an overall decrease in protein synthesis with the induction of drought stress, followed by a resumption of synthesis upon rehydration. In addition, the synthesis of a number of specific polypeptides was found to decrease upon water loss in the leaves.