Cadmium-induced Changes in Diamine Oxidase Activity and Polyamine Levels in Vigna radiata Wilczek Seedlings

We recently reported the diarnine oxidase (DAO: E.C. 1.4.3.6.) activity in three mungbean cultivars during seed germination and early seedling growth and that the etiolated seedlings showed significantly more enzyme activity as compared with those grown in light conditions (Choudhary and Singh, 1997). We report the same in another mungbean (Vigna radiata (L.) Wilczek), cultivar P105. The maximum enzyme activity was detected after 1 day of seed sowing, so the studies related to the effect of heavy metal salt, i.e. CdCl2, were done with 1-day-old mungbean seedlings. The supply of CdCl2 (0.1-1.5 mmol/L) decreased the total and specific activities of DAO in 1- and 2-day-old seedlings of mungbean. The total soluble proteins and endogenous free polyamines (PAs), viz. putrescine (Put) and spermidine (Spd), increased in the seedlings at 0.1 mmol/L CdCh supply. Potassium salts (10 mmol/L) supplemented with Cd+2 (0.1 mmol/L) salt could ameliorate the metal-caused decrease in the seedling growth and DAO activity, and correspondingly it restricted the enhanced accumulation of the endogenous PAs. However, phosphate salts used (10 mmollL) along with Cd+2 could only enhance the total protein content and only K2HPO4 could ameliorate the enzyme activity and PA content. Parallel experiments were done with another heavy metal salt (Pb(COO)2, 1–4 mmol/L). The two metal salts show a similar trend in the enzyme activity and recovery of DAO activity and PA accumulation when supplemented along with the inorganic salts. No metal-based differences were noticed in DAO activity and PA accumulation in the mungbean seedlings except for Cd+2, being 10-times more toxic than Pb+2; therefore, we report the data of CdCl2 only. This paper reports the occurrence of DAO activity in the mungbean seedlings during seed germination in cultivar P105, which shows a significant but reversible loss of the activity in the presence of Cd+2. A corresponding change in the endogenous PA levels under the metal toxicity is not always correlated to the activation of DAO in mungbean seedlings. The possible physiological and biochemical reasons for the metal-caused DAO activity inhibition and PA accumulation have been discussed.