Photosynthetic response of sweet sorghum to drought and re-watering at different growth stages.

Sweet sorghum (Sorghum bicolor) is a C4 drought resistant species with a huge potential for bioenergy. Accentuated reductions in water availability for crop production and altered rainfall distribution patterns, however, will have direct impact on its physiological attributes, metabolic functions and plant growth. The objective of this study was to evaluate the effects of drought and re-watering on the photosynthetic efficiency of sweet sorghum. Durable or short transient drought stress periods were imposed at early and late growth stages and compared with well-watered plants. In spite of very similar drought levels at early and late growth stages (Ψsoil  = -1.6 and -1.7 MPa), the decrements in maximum quantum yield (ϕPo ) and performance index (PI) were about twice at late than at early growth stages. All the PI components, that is, density of active reaction centers (RCs), excitation energy trapping and conversion of excitation energy into electron flow followed a similar decreasing pattern. Upon re-watering and regardless the duration and growth stage of the drought period, all the photosynthetic functions, and particularly those of photosystem II (PSII), fully recovered. Such effective self-regulating functional activity by PSII photochemistry likely contributes to both high drought resistance and photosynthetic recovery capacity of sweet sorghum. At vegetative growth stages, the down regulation of the photochemistry seems to be the main photoprotective/regulative mechanisms, while at late growth stages, the accumulation of compatible solutes likely has a more preponderant role. The observed sugar concentration increments likely contributed to prevent permanent photo-oxidative destruction of the PSII RCs of mature droughted sweet sorghum plants.