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Dependency of Nitrate Reduction on Soluble Carbohydrates in Primary Leaves of Barley under Aerobic Conditions ¹

Plant Growth Laboratory and Department of Agronomy and Range Science, University of California, Davis, California 95616

Nitrate reduction was studied as a function of carbohydrate concentration in detached primary leaves of barley (Hordeum vulgare L. cv Numar) seedlings under aerobic conditions in light and darkness. Seedlings were grown either in continuous light for 8 days or under a regimen of 16-hour light and 8-hour dark for 8 to 15 days. Leaves of 8-day-old seedlings grown in continuous light accumulated 4 times more carbohydrates than leaves of plants grown under a light and dark regimen. When detached leaves from these seedlings were supplied with NO₃^– in darkness, those with the higher levels of carbohydrates reduced a greater proportion of the NO₃^– that was taken up. In darkness, added glucose increased the percentage of NO₃^– reduced up to 2.6-fold depending on the endogenous carbohydrate status of the leaves. Both NO₃^– reduction and carbohydrate content of the leaves increased with age. Fructose and sucrose also increased NO₃^– reduction in darkness to the same extent as glucose. Krebs cycle intermediates, citrate and succinate, did not increase NO₃^– reduction, whereas malate slightly stimulated it in darkness.

In light, 73 to 90% of the NO₃^– taken up was reduced by the detached leaves; therefore, an exogenous supply of glucose had little additional effect on NO₃^– reduction. The results indicate that in darkness the rate of NO₃^– reduction in primary leaves of barley depends upon the availability of carbohydrates.