EFFECT OF CADMIUM ON PROLINE ACCUMULATION AND RIBONUCLEASE-ACTIVITY IN RICE SEEDLINGS - ROLE OF PROLINE AS A POSSIBLE ENZYME PROTECTANT

When seedlings of two rice (Oryza sativa L.) cultivars Ratna and Jaya were raised under 100 and 500 mu M cadmium nitrate in the medium, a hi gh proline content was noted in Cd2+ stressed seedlings compared to co ntrols. Seedlings grown under 500 mu M Cd(NO3)(2) maintained increased proline level compared to non-stressed seedlings. Kinetic properties of RNase extracted from control grown and Cd2+ stressed seedlings show ed a marked alteration in Km due to Cd2+ treatment, The RNase isoforms were purified from 15-d-old rice seedlings with a total purification of 22.25 fold and 74.75 % yield using conventional biochemical techniq ues. Three RNase isoforms, namely I, II and III were eluted from DEAE- Sephacel column. The isoform RNase II had Km value of 3.2 mg(RNA) cm(- 3). The in vitro osmotic stress created by incorporation of PEG in the enzyme assay medium led to decreased affinity of enzyme towards its s ubstrate with increase in Km. This loss in affinity was partially rest ored by the addition of 1 M proline in the assay medium, suggesting th e possible protective role of proline on RNase under osmotic stress.