METABOLISM OF FREE SUGARS IN RELATION TO THE ACTIVITIES OF ENZYMES INVOLVED IN SUCROSE METABOLISM AND NITROGEN ASSIMILATION IN THE DEVELOPING NODULES OF CHICKPEA

Metabolism of free sugars in relation to the activities of sucrose cle aving- and nitrogen assimilating enzymes in the developing nodules of chickpea (Cicer arietinum) was investigated. Throughout nodule develop ment, sucrose was found to be the principal sugar of the host cytosol and this sugar accumulated in the nodules towards their maturity. On f eeding uniformly-labelled [C-14]sucrose or [C-14]glucose or [C-14]fruc tose through cut system (intact with roots and nodules), the major pro portion of C-14 appeared in sucrose of stem and root tissues showing c onversion of hexoses into sucrose in stem before their transport to ro ots and nodules. Incorporation of C-14 into sucrose on feeding [C-14]g lucose or [C-14]fructose directly to nodules also indicated the capaci ty of nodules to convert hexoses into sucrose. The nitrogen fixation c apacity in nodules was high at 45 and 60 DAS (days after sowing). Alka line invertase (EC 3.2.1.26) and sucrose synthase (cleavage direction, EC 2.4.1.13) showed maximum activities in nodule cytosolic fraction a t 45 DAS and 60 DAS, respectively. The activity of sucrose synthase (s ynthesis direction) in nodule cytosol was high at or near maturity. Ac tivities of both aspartate aminotransferase (EC 2.6.1.1) and glutamine synthetase (EC 6.3.1.2) in nodule cytosol were high at the time of ma ximum nitrogenase activity in nodules. NADP(+)-dependent isocitrate de hydrogenase (EC 1.1.1.41) activity was 10-15 times more in cytosol tha n in the bacteroids. The possible role of isocitrate, dehydrogenase an d a minimum requirement for 2-oxoglutarate synthesis to support ammoni a fixation has been discussed.