Photosynthesis And Antioxidative Defense During Senescence In Flag Leaves Of Field Grown Wheat

Wheat is one of the most important crops in China. The senescence of wheat flag leaves is accompanied by the grain filling process. It is of great significance to study the photosynthesis during the senescence of wheat flag leaves because it contributes greatly to grain yield. Meanwhile, some active oxygen species are also produced in chloroplast, which is harmful to the photosynthesis. So a question arises as to how the senescent leaves scavenge the active species. In this article, the characteristics of photosynthetic gas exchange, chlorophyll a fluorescence, xanthophyll cycle pigments, and the antioxidant of antioxidative system during flag leaf senescence of field-grown wheat plants were investigated.1. With senescence progressing, a significant decrease was observed in the light-saturated net CO2 assimilation rate expressed either on a basis of leaf area or chlorophyll. The apparent quantum yield of net photosynthesis decreased when expressed on a leaf area basis but increased when expressed on a chlorophyll basis. The maximal efficiency of PSII photochemistry (Fv/Fm) decreased very little while actual PSII efficiency, photochemical quenching, and the efficiency of excitation captured by open PSII centers decreased considerably. At the same time, non-photochemical quenching increased significantly. The results suggest that PSII apparatus remained functional, but a down-regulation of PSII occurred at the steady state of photosynthesis in senescent flag leaves. Such a down-regulation is associated with the closure of PSII centers and an enhanced thermal dissipation in the PSII antennae.2. A substantial decrease in the contents of violaxanthin and zeaxanthin but a slight decrease in the content of antheraxanthin was observed. However, the de-epoxidation status of the xanthophyll cycle increased with senescence due mainly to a smaller decrease in zeaxanthin than in violaxanthin. So these results imply that thexanthophyll cycle related thermal dissipation improves in the PSII antennae in flag leaves during the senescence of field grown wheat.3. During the senescence in flag leaves of field grown wheat, the MDA content and the contents of ascorbate and ASC/DASC increased, but the content of glutathione and the ratio of GSH/GSSG decreased. The results suggest that the ascorbate is the main antioxidant in the senescent flag leaves of field grown wheat.4. The activities of dehydroascorbate reductase (DHAR), monodehydroascorbate reductase (MDHAR), ascorbate peroxidase (ASPX), glutathione reductase (GR), catalase (CAT) and superoxide dismutase (SOD) during senescence in flag leaves of field grown wheat were investigated. Their activities expressed on leaf fresh weight decreased during flag leaf senescence, but the activities expressed on the protein content increased. And the severely senescent leaves had the largest amplification. These data suggest that the antioxidative enzyme's degradation is slower than the rate of total soluble protein degradation, which may relatively improve the enzyme's function of scavenging the active oxygen.5. The diurnal changes of the photosynthetic CO2 assimilation rate, Chl a fluorescence, xanthophyll cycle pigments, and antioxidant in the control, moderately senescent and severely senescent flag leaves were investigated. At three stages, the Chl content underwent little change during a day. The photosynthesis CO2 assimilation rate increased with the PPFD increase in the morning, but it was inhibited by the high light at noon. The maximal efficiency of PS II (Fv/Fm), the actual efficiency of PS II (PSII), the efficiency of the opened PS II center (exc), and the photochemical quenching (qP) suffered obvious inhibition at noon too. The non-photochemical quenching (NPQ) displayed a substantial rise at noon. The largest amplification of the Fv/Fm, psII, exc., qP, and NPQ was observed in the severely senescent flag leaves than others. The de-epoxidation status also showed the greatest increase at noon in the severely senescent flag leaves of field grown wheat. Our data support...