Drought tolerance is partly related to amino acid accumulation and ammonia assimilation: a comparative study in two mulberry genotypes differing in drought sensitivity.
Three-month-old mulberry (Morus alba) cultivars S13 (drought-tolerant) and S54 (drought-sensitive), were subjected to water stress for 10 days. Water stress was imposed by watering plants daily to 75, 50 or 25% of field capacity (mild, moderate and severe water stress, respectively). The activities of nitrate reductase (NR: EC 18.104.22.168), nitrite reductase (NiR: EC 22.214.171.124.), protease, glutamine synthetase (GS: EC 126.96.36.199) [glutamate-ammonia ligase], glutamate synthase (GOGAT: EC 188.8.131.52), glutamate dehydrogenase (NADH-GDH: EC 184.108.40.206 and NADPH-GDH: EC 220.127.116.11), aspartate aminotransferase (AAT: EC 18.104.22.168) and alanine aminotransferase (AIAT: EC 22.214.171.124.), along with total protein content, free amino acid level, ammonia and glutamine contents were measured in both cultivars under water stress conditions. During water stress NR and NiR activities were inhibited in both cultivars, but the percentage inhibition was greater in cv. S54 than in S13. GS, GOGAT, GDH (both forms), AAT and AIAT activities were significantly increased in response to water stress in both cultivars with the increases being greater in S13 than in S54. The tolerant cultivar, S13, had a higher metabolic activity leading to a greater accumulation of amino acids coupled with lesser amounts of ammonia accumulation in the tissue. Further, it was shown that no process of endogenously generated ammonia was responsible for the differential accumulation of ammonia between the cultivars under stressful conditions. However, this difference in ammonia levels among the cultivars can be attributed to the difference exhibited in the reassimilation of ammonia, whereby the tolerant cultivar recorded a relatively better reassimilation capacity of ammonia. From these findings, the drought tolerance of S13 seems to be associated at least in part with its ability to maintain greater levels of amino acid pool coupled with more pronounced reassimilation of ammonia.