NITRATE REDUCTASE IN COTYLEDONS OF CUCUMBER SEEDLINGS AS AFFECTED BY NITRATE, PHYTOCHROME AND CALCIUM

Regulation by the active form of phytochrome (P-FR) and the effect of Ca2+ was examined with nitrate reductase (NR) in etiolated cucumber (Cucumis sativus cv. Bellpuig). Nitrate reductase activity (NRA) was studied in excised cotyledons of cucumber seedlings grown in distilled water and in darkness for seven days at 24 +/- 0.5 degrees C. All experiments were performed in the dark and a dim green safelight was used during analyses. In etiolated cucumber cotyledons NRA was induced by nitrate and a brief irradiation (ii min) with red light (R) resulted in 62% increase in NRA. This effect was nullified when R was followed immediately by a brief (5 min) far-red light (FR). NRA also showed a semidian (12 h) rhythmicity. Both P-FR, and nitrate effects were age dependent. Calcium seemed to be involved since the phytochrome effect was only observed when calcium was supplied in the external solution. The effect of R on NRA depended on the period of calcium nitrate incubation. An external supply of calcium ionophore mimicked the effect of R and, if supplied to R-irradiated cotyledons, produced a higher NR level than that caused by R alone. This suggested that intracellular free calcium was involved.