Proline metabolism during dehydration in two mulberry genotypes with contrasting drought tolerance

The free proline levels and activities of pyrroline-5-carboxylate reductase, proline oxidase and proline dehydrogenase and the levels of quaternary ammonium compounds and chlorophyll stability were studied during water stress in drought sensitive (S54) and drought tolerant (S13) cultivars of mulberry (Morus alba L.). The cultivar S13 accumulated relatively greater amounts of free proline compared with S54. Water stress increased about 8- and 3.5-fold in free proline content in the third leaf from the top, of S13 and S54 respectively, after 15 days of severe water stress. The pyrroline-5-carboxylate reductase was increased, while the activities of proline oxidase and proline dehydrogenase were significantly decreased in both cultivars under water stress conditions. However, the magnitude of increase in pyrroline-5-carboxylate reductase was relatively greater and the degree of decrease in proline oxidase and proline dehydrogenase activities was relatively small in S13 compared with S54. The results indicate that: during water stress, proline metabolism was altered in mulberry and the extent of alteration varied between the cultivars S13 and S54, leading to the greater levels of proline in S 13, supporting its drought tolerance. The greater proline levels in S13 were due both to the higher rates of proline synthesis and lesser magnitude of proline oxidation compared with S54. Further, the drought tolerance of S13 was evident from the greater magnitude of accumulation of quaternary ammonium compounds compared with S54 during water stress. Furthermore, the drought tolerance of S13 was supported by chlorophyll stability having comparatively lower amounts of chlorophyll breakdown.