Dark and daylight activity of glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase in the leaves of nitrogen-limited spinach and soybean plants

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 enzym es glucose-6-phosphate dehydrogenase (G6PDHase) and 6-phosphogluconate dehy drogenase (6PGDHase) were monitored and compared in extracts of mature leav es from inorganic nitrogen-sufficient (N-Suff) and nitrogen-limited (N-Lim) spinach and soybean plants. Activities of G6PDHase and 6PGDHase in leaf ex tracts 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 o f those enzymes in leaf extracts of N-Suff control plants. G6PDHase and 6PG DHase 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 f rom N-Lim plants exhibited a more pronounced light-mediated inactivation. T his 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 prol onged periods of N limitation, elevated levels and activity of G6PDHase and 6PGDHase would provide an increased amount of NADPH in the dark to help su pport foliar anabolic metabolism, plant growth, and ultimate reproductive a ctivity, e.g., soybean pod fill. This would be important in N-Lim plant lea ves because photosynthetic NADPH production is lower in the light than in l eaves of plants receiving sufficient N.