DROUGHT TOLERANCE IS PARTLY RELATED TO AMINO-ACID ACCUMULATION AND AMMONIA ASSIMILATION - A COMPARATIVE-STUDY IN 2 MULBERRY GENOTYPES DIFFERING IN DROUGHT SENSITIVITY

Three-month-old mulberry (Morus alba L.) cultivars, i.e. S13 (drought: tolerant) and S54 (drought sensitive), were subjected to different re gimes of water stress conditions for a duration of 10 days. The activi ties of nitrate reductase (NR: EC 1.6.6.1), nitrite reductase (NiR: EC 1.6.6.4.), protease, glutamine synthetase (GS: EC 6.3.1.2), glutamate synthase (GOGAT: EC 1.4.1.13), glutamate dehydrogenase (NADH-GDH: EC 1.4.1.2 and NADPH-GDH: EC 1.4.1.4), aspartate aminotransferase (AAT: E C 2.6.1.1) and alanine aminotransferase (AlAT: EC 2.6.1.2.), along wit h total protein content, free amino acid level, ammonia and glutamine contents were measured in both genotypes under water stress conditions . The tolerant genotype (S13) registered a higher metabolic activity l eading to a greater accumulation of amino acids coupled with lesser am ounts of ammonia accumulation in the tissue. Further, it is realised t hat no process of endogenously generated ammonia was responsible for t he differential accumulation of ammonia between the genotypes under st ressful conditions. However, this difference in ammonia levels among t he varieties can be attributed to the difference exhibited in the reas similation of ammonia, whereby the tolerant genotype (S13) recorded a relatively better reassimilation capacity of ammonia. From these findi ngs, the drought tolerance of S13 seems to be associated at least in p art with its ability to maintain greater levels of amino acid pool cou pled with more pronounced reassimilation of ammonia.