Nitrogen Metabolism in Senescent Flag Leaves of Wheat (Triticum aestivum L.) in the Light

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Abstract
Nitrogen metabolism was examined in senescent flag leaves of 90- to 93-day-old wheat (Triticum aestivum L. cv. Yecora 70) plants. CO2 assimilation and the levels of protein, chlorophyll, and nitrogen in the leaves decreased with age. Glutamine synthetase activity decreased to one-eighth of the level in young flag leaves. Detached leaves were incubated (with the cut base) in 15N-labeled NH3, glutamate, or glycine in the light (1.8 millieinstein per square meter per second) at 25.degree. C in an open gas exchange system under normal atmospheric conditions for up to 135 minutes. The 15N-enrichment of various amino acids derived from these 15N-substrates were examined. The amido-N of glutamine was the first 15N-labeled product in leaves incubated with 15NH4Cl whereas erine, closely followed by the amido- and amino-N of glutamine, were the most highly 15N-labeled products during incubation with [15N]glycine. In contrast, aspartate and alanine were the first 15N-labeled products when [15N]glutamate was used. These results indicate that NH3 was assimilated via glutamine synthetase and glutamate activities and the photorespiratory nitrogen cycle remained functional in these senescent wheat flag leaves. In contrast, an involvement of glutamate dehydrogenase in the assimilation of ammonia could not be detected in these tissues.