Effect of benzimidazole and kinetin on the nicotinamide nucleotide content of senescing wheat leaves

Abstract

Assays of nicotinamide nucleotides were made on detached leaves of wheat, Triticum dicoccum var. Khapli, immediately after excision and at the end of a 16-h photoperiod and 8-h dark period at 1, 3, and 5 days on leaves floated on water, and on benzimidazole, kinetin, or both. The total nicotinamide nucleotide content of 25 nmoles per gram fresh weight was increased by treatments with these kinins. A diurnal rhythm was observed in all treatments involving an increase of NADP(H) and a decrease of NAD(H) during the photoperiod, whereas the opposite was the case in darkness. The ratio NAD(H)/NADP(H) was about equimolar after excision and decreased to a mean value of 0.4 in all treatments after the first photoperiod. Thereafter, despite fluctuations in light and darkness and increased synthesis of NADPH in the treatments, the ratio increased to a mean value of 1.5 after 5 days, with no significant difference between treatments. The transformations in light can be explained by the combined operation of NAD kinase, NADH transhydrogenase, photosynthetic electron transfer forming NADPH, and biosynthetic reductive reactions restoring NADP.Assays were also made on chloroplasts isolated in nonaqueous media from leaves treated similarly but under continuous illumination. The NAD content of chloroplasts was 11.2 nmoles per μmole chlorophyll as compared to 3.14 for NADP. The oxidized nucleotides predominated in chloroplasts. Chloroplasts from leaves floated on water had lost all their NADP after 6 days, whereas those treated with benzimidazole or kinetin increased or maintained their level of NADP. The evidence is discussed with reference to the mechanism of action of benzimidazole and kinetin in delaying the onset of visible and metabolic changes that occur during the senescence of wheat leaves.