Red and Blue Lights Significantly Affect Photosynthetic Properties and Ultrastructure of Mesophyll Cells in Senescing Grape Leaves

Light quality significantly affects photosynthetic efficiency in plants. The mechanisms for how light quality affects photosynthesis in grape is poorly understood. Therefore, to investigate the effects of different light qualities on chloroplast ultrastructure and photosynthesis efficiency, two grape cultivars ‘Italia’ (slower speed of leaf senescence) and ‘Centennial Seedless’ (faster speed of leaf senescence) grown under protected and delayed conditions were used. The three treatments, replicated three times, were control (no supplemental lighting), red light and blue light. Chlorophyll content, net photosynthetic rate, and the ratio of Fv/Fm significantly increased in red light relative to the control. The opposite trend was observed in blue light in the early phase of leaf senescence. At later stages, physiological indexes were gradually higher than that of control, resulting in a delay in leaf senescence. Compared to the control, red and blue light both significantly increased the chlorophyll a/b ratio. Electron microscopy showed that blue light caused severe damage to the fine structure of chloroplasts at early stages of leaf senescence, but effects at later stages of leaf senescence became less severe compared to the control. The degradation of chloroplast ultrastructure was apparently delayed in red light throughout the experimental timeframe compared to other treatments. In this experiment, ‘Italia’ showed higher chlorophyll content, net photosynthetic rate, ratios of Fv/Fm, chlorophyll a/b and better preserved chloroplast ultrastructure relative to ‘Centennial Seedless’, resulting in a slower rate of leaf senescence.