Effects of Water Stress on Photosynthesis and Chlorophyll Fluorescence of the Sugar Beet

To investigate the effect of water stress and rewatering on sugar beet yield and its corresponding photosynthetic parameters, and to provide the basis of water management for the sugar beet fields, pool experiments in an artificial proof canopy were set up to observe changes of beet net photosynthetic rate(Pn), transpiration rate(Tr), water use efficiency(WUE) and stomatal limitation (Ls), intercellular CO2 oncentration(Ci), and PSⅡ maximum quantum yield(Fv/Fm). The results indicated that the diurnal variation of Tr and Pn in CK treatment (whole growth period replenishment) at different times near "unimodal" type; and water shortage treatments presented "twin peaks" change. Diurnal transpiration capacity(DTC) under water stress at sugar accumulation stage reduced by 70.16%~74.81% and diurnal photosynthetic capacity(DPC) was 63.48%~69.96% lower than that of CK, while diurnal water use efficiency(WUEd) increased by 19.28%~22.39%. Rehydration helped Tr and Pn recovery, but did not reach unstressed levels. Ls changes under extremely dry environment had a midday trough "twin peaks" feature, and Ci was at "double-dip" in consistent with the timing of Ls; Water stress inhibited and inactivated photochemical reaction center of midday PSⅡ. Water stress led to irreversible decrease in the Pn and Tr, and prolonged the Pn inefficient period, which become the important factor of influencing the sugar beet yield.