Effects of drought stress on photosystem Ⅱ and photosynthesis in Beta vulgaris during the vegetative development period

In order to explore the response mechanism of the photosynthetic apparatus and function in sugar beet leaves to drought stress, cultivars ‘XJT9907’ (drought sensitive type), and ‘XJT9916’ (drought tolerant type) were used to determine the chlorophyll a fluorescence induction curve of each treatment using a plant efficiency analyzer (Handy-PEA) when the field water capacity was 45%～50% at the sugar beet leaf cluster stage over seven days. The chlorophyll a fluorescence induction curve in each treatment was measured simultaneously using the OJIP-test data analysis method. The standardized data, specific activity parameters, performance index, total performance index, and energy distribution ratio of the related optical system Ⅱ (PSⅡ) were obtained. The results showed that drought stress: 1) increased the variable fluorescence intensity and decreased the electron transfer efficiency of leaves (Ψ_EO, φ_EO, φ_RO); 2) decreased the absorption, capture, dissipation, and transfer of light energy per unit area (ABS/CS_O, DI_O/CS_O, ET_O/CS_O, RE_O/CS_O), but had little effect on drought tolerant varieties; 3) decreased the energy connectivity between PSⅡ units and electron transfer between these photosystems; 4) led to the (oxygen-evolving complex, OEC) inactivation of the PSⅡ oxygen evolution complex and imbalance of electron transfer between the PSⅡ acceptor and donor sides; 5) The net photosynthetic rate of leaves of XJT9907 and XJT9916 decreased by 32.2% and 10.3%, respectively; 6) the drought-tolerant cultivar, XJT9916, prevented over-excitation of energy by strong energy dissipation (DI_O/CS_O, DI_O/RC), resulting in a lower reduction in the net photosynthetic rate of the leaves than in the drought-sensitive cultivar, XJT9907. When the soil water content of XJT9907 and XJT9916 beet leaf clusters was 45%～50% of the field water holding capacity, the photochemical activity of the leaf photosynthetic system was reduced, the electron transfer reaction and the Calvin-Benson cycle was out of balance, and the photosynthetic mechanism stability and light energy utilization efficiency were reduced, leading to leaf light inhibition, thereby reducing the photosynthetic capacity of the leaf. The reduction in the photosynthetic capacity of sugar beet under drought stress is related to the down-regulation of PSⅡ function. Compared with F_v/F_m, the performance index, PI_total, can be used as a sensitivity index for screening drought-resistant genotypes.