Effects of drought stress on diurnal changes and related characteristics of sugarcane photosynthesis

【Objective】To investigate the effects of drought stress on the daily trends of CO₂ exchange and H₂O exchange, photosynthetic capacity and physiological characteristics of sugarcane. This study would provide a scientific reference for the selection and breeding of drought-resistant varieties of sugarcane and the investigation of their drought tolerance mechanisms.【Method】Yunzhe 05-51 and Yuetang 93-159 were used as the test materials. A pot experiment was conducted with four drought treatments of 1,5,9 and 10 d with re-watering after 9 d of drought, pot plants watered normally served as the control. The CO₂ and water exchange patterns of sugarcane leaves were monitored using PTM-48A plant photosynthetic physiological and environmental monitoring system. Photosynthetic(photosynthesis,chlorophyll fluorescence,SPAD value) and physiological(osmotic permeability,relative water content of leaves,etc.) indicators associated with drought were measured and correlations between the indicators were analyzed.【Result】Environmental changes affected the CO₂ and water exchange capacity of sugarcane leaves. Under normal watering growth conditions with high temperature and low humidity as well as high vapor pressure difference,the CO₂ exchange rate and H₂O exchange rate of Yunzhe 05-51 and Yuetang 93-159 performed better,and sugarcane leaves had higher stomatal conductance and net rate of photosynthesis. Drought stress led to the inhibition of CO₂ exchange and H₂O exchange in Yunzhe 05-51 and Yuetang 93- 159. At 5 d of drought,the CO₂ exchange rate and transpiration of both varieties were significantly reduced,and 9 d of drought resulted in near stagnation of CO₂ exchange and H₂O exchange. Compared with the control,the net photosynthetic rate(P_n),stomatal conductance(G_s) and transpiration rate(T_r) of Yunzhe 05-51 and Yuetang 93-159 were significantly reduced during drought stress(P<0.05, the same below),but the reduction observed in Yunzhe 05-51 was less severe than that of Yuetang 93-159. The treatments of 5 and 9 d of drought affected the chlorophyll fluorescence,SPAD value,soil water content and the relative leaf transpiration of sugarcane. There were significant correlations between photosynthetic systems,chlorophyll fluorescence systems,soil water content and leaf relative water content,SPAD value and plasma membrane permeability under drought stress. After rehydration,the photosynthetic and physiological effects of both varieties recovered significantly from drought stress,while CO₂ exchange and transpiration recovered to a lesser extent,with the recovery of all indicators being higher in Yunzhe 05-51 than in Yuetang 93-159.【Conclusion】High temperature and low humidity as well as high vapour pressure differentials are conducive to promoting photosynthesis and water-air exchange in Yunzhe 05-51 and Yuetang 93-159. Drought stress leads to the inhibition of CO₂ exchange rate and H₂O exchange,photosynthetic capacity,chlorophyll fluorescence system and leaf water related physiological activities of Yunzhe 05-51 and Yuetang 93-159. Yunzhe 05-51 has relatively higher stomatal conductance and transpiration under drought stress,and its drought resistance and recovery ability after rehydration are better than those of Yuetang 93-159.