Influence of plant growth regulators on some enzymes of nitrogen assimilation in mustard seedlings

Effect of plant growth regulators (PGR) viz. gibberellic acid (GA), kinetin (KN), and abscissic acid (ABA) were investigated on growth and activities of nitrate reductase (NR) and glutamine synthetase (GS) in mustard (Brassica juncea) seedlings. All the PGRs tested promoted in vivo NR activity in cotyledons, but the magnitude differed with different treatments. Cytosolic GS in root and hypocotyl was promoted by GA treatment and inhibited by ABA and KN treatments although the latter showed slight promotion initially in hypocotyl; the trend was not clear in cotyledons. Determination of K_m value of GS extracted from 96‐h‐old cotyledons recorded lower K_m value in GA treatment (2.5 mM), while it increased in ABA treatment (4.35 mM), There was little change in K_m value in KN (3.03 mM) treatment. The kinetics of GS enzyme in cotyledons of different treatments showed marked variation in V_max . Both GA and ABA treatments inhibited GS activity while no significant effect by KN treatment was observed. It is argued that GA treatment inhibits chloroplastic GS (an enzyme which has higher K_m value), while ABA‐induced inhibition may not be specific to cytosolic or chloroplastic isoforms. Kinetin treatment was ineffective in promoting or inhibiting GS activity in cotyledons. The above conclusion is further supported by chloroplastic pigment data where inhibition is recorded in all the PGRs tested.