Regulation of leaf nitrate reductase activity in spinach (Spinacea oleracea L.) during light and dark transition

experiments were conducted on spinach variety jyoti to understand the role of light in regulation of nitrate reductase (NR) activity. The plants were grown under three different light intensities (a) >1200 ~ (under natural conditions), (b) about 300 ~ (in a growth chamber) and (c) about 100 ~ (in an open growth chamber). Highest in vivo NR activity was rt!corded in leaf samples harvested from (a) followed by (b) and (c) . Photosynthetic' activity, chlorophyll content and tissue sugar content also showed a similar trend. Leaf tissues of plants raised under natural conditions showed a decline in NR activity when incubated in distilled water in the dark. The activity disappeared almost totally after 36hr of incubation in dark. However, incubation of the leaves in the dark with nitrate or sucrose solution or nitrate + sucrose solution helped to maintain the activity even after 3hr. Potted plants also showed a decrease in the enzyme activity when kept in the dark. Intact plants which had lost all activity by 36hr in the dark showed recovery of NR activity only after 48hr in light. Apparently light does not regulate NR activity by any switch-on, switch-off mechanism.