ANTIOXIDANT EFFICIENCY DURING CALLUS INITIATION FROM MATURE RICE EMBRYO

The changes in the activities of active oxygen scavenging enzymes and lipid peroxidation during callus formation from germinating mature ric e embryo was investigated. Superoxide dismutase (SOD) and catalase (CA T) activities were much lower during callus formation than during seed ling growth indicating the declining capacity of the callus tissue to scavenge O-2(.-) and H2O2 respectively. Other H2O2-utilizing enzymes s uch as guaiacol peroxidase (GPOX) and ascorbate peroxidase (APOX) had also much lower activities during the initial period of callus formati on than during normal germination but soon these enzyme activities wer e rapidly enhanced during callus growth but declined during seedling g rowth. Since H2O2 level was quite high in the callus tissue, it is pro bable that GPOX and APOX are not efficient in decomposing H2O2 in this tissue. Water soluble non-protein -SH compounds of which GSH is the m ajor component increased more rapidly during seedling growth than duri ng callus formation. This was reflected by the higher activity of glut athione reductase (GR) in the seedling tissue than in the callus tissu e. Although peroxide and malondialdehyde did not accumulate during the callus initiation period, the fast decrease in the SOD and CAT activi ties indicates that during this transition period the tissue has incre asing tendency towards an oxidative state because of the weakening of the scavenging mechanism. The cellular environment, thereafter, become s more oxidizing during callus growth when compared with the normal se edling development in the absence of 2,4-D.