Carbohydrates and Sucrose Metabolizing Enzymes in the Leaves of Vigna mungo Genotypes as Influenced by Elevated CO2 Concentration

Effect of different CO2 concentrations on sucrose metabolizing enzymes and on carbohydrate metabolism was studied for eight blackgram (Vigna mungo L. Hepper) genotypes grown in open top chambers under ambient (380 µmol mol-1) vs. elevated CO2 (550 and 700 µmol mol-1) levels. The higher acid invertase activity over neutral invertase indicated the major role of acid invertase in sucrose breakdown. Higher acid invertase activity over Sucrose Synthase (SuSy) suggested the major role of invertase in sucrose breakdown and sucrolysis. Sucrose Phosphate Synthase (SPS) activity did not match with sucrose pool sizes in mature leaves and rather varied among genotypes. Plants exposed to higher CO2 concentrations showed higher starch and sucrose contents as compared with those exposed to ambient CO2. Leaf starch content being found several-folds higher than sucrose throughout the study indicated its major role in regulating assimilate partitioning. Increase in glucose vs. fructose concentrations for genotypes grown under elevated CO2 conditions ranged from 20 to 90% and from 10 to 140%, respectively. The hexoses/sucrose ratio for elevated CO2 concentration was approximately 0.8-1.6, however for ambient CO2 content it approximately amounted to unity. Genotypes IC436720, IC519805, IC343952, and IC282009 with low hexose/sucrose ratio representing high CO2 assimilation along with high sucrose formation indicated better tolerance to elevated CO2 for carbon partitioning and carbohydrate metabolism. The up-regulation of leaf carbohydrate metabolizing enzymes of low hexose/sucrose as well as low sucrose/starch ratios for the genotype IC436720 (as compared with other genotypes) improved its photosynthetic capability which coupled with its better efficiency of carbon partitioning (indicative of better acclimation to elevated CO2) could prove beneficial to its growth and productivity in the future change of climatic conditions.