Effects of drought stress and rewatering on photosynthetic physiological characteristics of sorghum

Water deficiency is a main limiting factor for plant growth and development in arid and semi-arid areas, it can severely affect a wide range of physiological processes, such as respiration and photosynthesis of plants. Chlorophyll fluorescence assays provide a rapid and noninvasive means to study the behavior of plant photosynthesis systems under drought stress. In the past, more attentions were paid on the growth morphological indexes of sorghum under water stress conditions, while the reports of using fluorescent technology to study photosynthetic characteristics were few in the process of continuous drought stress and rewatering, which is important to understand the adaptive mechanism of sorghum to variable water conditions. In this study, a sorghum variety (Sorghum, jinza4) was selected as the experimental material, and potted experiment was designed to study the dynamic changes of the leaf gas exchange parameters and chlorophyll fluorescence parameters in the process of different levels of drought stress: no stress (70%–80% of field capacity), medium stress (50%–60% of field capacity) and severe stress (30%–40% of field capacity) and rewatering. The results showed that: (1) With the drying of soil and prolongate of water stress, The leaf net photosynthetic rate (Pn), Stomatal conductance (Gs), intercellular CO2concentration (Ci), maximal efficiency of PS II (Fv/Fm), actual photochemical efficiency of PS II (ФPS II), and photochemical quenching coefficient (qP) were gradually reduced. After the forth day of rewatering, these parameters were gradually recovered to control level; (2) The non-photochemical quenching (NPQ) of sorghum leaf under water stress conditions were higher than control in the process of drought stress and rewatering, which suggested that a part light energy absorbed by light system II was dissipated and this can protected the photosynthetic tissue, alleviate the influence of water stress on the photosynthesis; (3) Under the moderate drought condition, the reduction of Pn was mainly resulted from stomatal limitation; while for severe stress, the decreased of Pnwas mainly attributed to non-stomatal factors; (4) Moderate water stress is help to enhance the single leaf water use efficiency of sorghum, and produced ultra-compensation effect in yield and water use efficiency after rewatering, severe water stress produced ultra-compensation effect in water use efficiency after rewatering.