EFFECTS OF SPERMIDINE ON THE PHYSIOLOGICAL ACTIVITIES OF MARSILEA MINUTA LINN. UNDER CADMIUM STRESS

摘要

Marsilea minuta Linn., an aquatic fern species was grown in nutrient solution with varying concentrations (0, 50, 100 and 200 uM) of cadmium (Cd) along with 2 mM spermidine (Spd) as supplementation. The accumulation of Cd in the root tissues showed concentration-dependent relationship. Moreover, significant changes of tissue organization in petiole were also observed by scanning electron micrographs (SEM). Cd was identified using electron dispersive x-ray spectroscopy (EDXS) and found mostly in the cortical tissues as well as in the peripheral zones. Dissolution of the inner wall of cortical cells and distortion of xylem lumens were maximum at the highest Cd concentration (200 uM) as compared to control (0 uM). Treatment with Spd significantly mitigated those damages accompanied by lowering of Cd absorption. At the cellular level, a significant rise of proline accumulation was accompanied by an increase of y-glutamyl kinase (y-GK) and y-glutamyl phosphate reductase (y-GPR) activity. Spd also retrieved the proline accumulation by modulation of the enzyme activities involved in its biosynthesis. Nitrate reductase (NR) activity both in shoot and root showed a similar declining trend at all concentrations of Cd tested. The activity of NADH-dependent glutamate dehydrogenase (GDH) in shoot and root was quite discriminatory at the different Cd concentrations. For NADPH-dependent GDH, the activity remained subdued irrespective of shoots and roots. In that case, the application of Spd was significantly effective in up regulation of activities in root only, but not in shoot. The activity of glutamate synthase (GS) initially showed an increase and thereafter declined in both shoots and roots. Cellular responses of the Marsilea plants for metal hyper-accumulation are discussed with their evaluation as biomarkers under Cd toxicity in aquatic environment.