Contrasting effects of submergence in light and dark on pyruvate decarboxylase activity in roots of rice lines differing in submergence tolerance.

Submergence tolerance of 13 doubled haploid lines of rice and their parents (submergence tolerant FR13A and submergence intolerant CT6241) was assessed using 2-week-old seedlings. Plants were scored for leaf senescence and percentage of seedlings that survived up to 15 d submergence, followed by a 12 d recovery period. Seven lines proved to be submergence tolerant, and six relatively intolerant. In all lines, activity of pyruvate decarboxylase (PDC), extracted from the apical 3-5 cm of root axes, decreased by 46-96 % and 38-76 %, respectively, during 5 or 10 d submergence under natural day/night conditions, compared with pre-submergence values (100 %). However, when the enzyme was extracted at night, submergence increased PDC activity of all rice lines (approx. 112 % on average), compared with pre-submergence values (100 %). The stimulating effect of the dark period on PDC activity was reproduced and amplified by submerging rice seedlings for up to 5 d in continuous darkness in water containing sub-ambient concentrations of oxygen (2.3 mg l(-1)). Such increased PDC activity was also observed in seedlings exposed to anoxia for 6 h (approx. 6-175 % higher than pre-submergence values). Irrespective of tolerance class, submergence decreased soluble protein concentrations under all conditions and sampling times. No positive correlation was found between PDC activity and tolerance of the various rice lines to submergence. However, PDC activity was slightly higher in submergence intolerant lines, compared with tolerant lines, under both dark submergence and anoxia. Such differences in PDC activity between the two groups of rice lines were not observed when they were submerged under the natural diurnal cycle. Increased PDC activity in roots at night demonstrated a probable incidence of tissue hypoxia or anoxia during submergence during each dark period.