Influence of Age and Light on the Distribution and Development of Nitrate Reductase in Greening Barley Leaves

The relation between leaf age and the induction of nitrate reductase activity by continuous and intermittent light was studied with barley seedlings (Hordeum vulgare L. cv. Club Mariout). In general, nitrate reductase activity declined as the period of growth in darkness was extended beyond 5 days. Maximum activity was found near the leaf tip while activity was lowest in the morphologically youngest tissue near the base of the lamina. Increased activity was observed after continuous illumination of dark-grown seedlings for 24 hours. The increase in activity in response to light was greatly reduced when the dark pretreatment period was extended beyond 8 days. The amount of nitrate reductase activity present in the different sections of the leaf was closely related to the amount of polyribosomes present. The pattern of chlorophyll accumulation closely parallelled that of increases in nitrate reductase activity. The initial lag in the induction of nitrate reductase activity was removed by a 10-minute light treatment 6 hours before placing dark-grown barley seedlings in light. The enzyme was also induced under flashing light with various dark intervals. These induction curves closely resembled those of chlorophyll accumulation under the same conditions. The development of photosynthetic CO₂ fixation follows the same induction pattern in this system. Our results suggest that photosynthetic products may be required for the induction of significant levels of nitrate reductase activity in leaves of dark-grown seedlings, although other light effects may not be discounted.