Dark and Daylight Activity of Glucose‐6‐Phosphate Dehydrogenase and 6‐Phosphogluconate Dehydrogenase in the Leaves of Nitrogen‐Limited Spinach and Soybean Plants

Abstract

Mature leaves of nitrate‐ and ammonia‐limited soybean plants often have an elevated activity of carbohydrate metabolizing enzymes (de Veau et al. 1992). In this study, activities of the pentose phosphate oxidative cycle enzymes glucose‐6‐phosphate dehydrogenase (G6PDHase) and 6‐phosphogluconate dehydrogenase (6PGDHase) were monitored and compared in extracts of mature leaves from inorganic nitrogen‐sufficient (N‐Suff) and nitrogen‐limited (N‐Lim) spinach and soybean plants. Activities of G6PDHase and 6PGDHase in leaf extracts of N‐Lim plants monitored just subsequent to a 10‐h dark period and before illumination were higher per unit protein than were the activities of those enzymes in leaf extracts of N‐Suff control plants. G6PDHase and 6PGDHase activities in extracts prepared from leaves of control N‐Suff spinach and soybean plants exposed to light were only slightly lower than had been observed before illumination. However, G6PDHase and 6PGDHase in extracts from N‐Lim plants exhibited a more pronounced light‐mediated inactivation. This indicated that there were isoenzymes of G6PDHase and 6PGDHase that were more active in the dark period in N‐Lim plant leaves than in N‐Suff plant leaves. The results also indicated that in leaves of plants exposed to prolonged periods of N limitation, elevated levels and activity of G6PDHase and 6PGDHase would provide an increased amount of NADPH in the dark to help support foliar anabolic metabolism, plant growth, and ultimate reproductive activity, e.g., soybean pod fill. This would be important in N‐Lim plant leaves because photosynthetic NADPH production is lower in the light than in leaves of plants receiving sufficient N.