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 (C ucumis sativus cv. Bellpuig). Nitrate reductase activity (NRA) was stu died in excised cotyledons of cucumber seedlings grown in distilled wa ter and in darkness for seven days at 24 +/- 0.5 degrees C. All experi ments were performed in the dark and a dim green safelight was used du ring analyses. In etiolated cucumber cotyledons NRA was induced by nit rate and a brief irradiation (ii min) with red light (R) resulted in 6 2% increase in NRA. This effect was nullified when R was followed imme diately by a brief (5 min) far-red light (FR). NRA also showed a semid ian (12 h) rhythmicity. Both P-FR, and nitrate effects were age depend ent. Calcium seemed to be involved since the phytochrome effect was on ly observed when calcium was supplied in the external solution. The ef fect 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 th an that caused by R alone. This suggested that intracellular free calc ium was involved.