Study On Physiological And Biochemical Mechanisms Of Early Monitoring And Warning In Water Stress Of Newhall Navel Orange Trees

Water stress is the main natural disasters in China,which causes serious reduction in yield and quality of economic crops in China.Therefore,it is of great significance to understand the physiological and biochemical response of citrus trees to different irrigation schemes during different phenological phase with the aim to timely monitor the plant water status,and these results will provide theoretical basis to improve citrus yield and fruit quality.In this investigation,sixty 15-year-old Newhall navel orange?Citrus sinensis CV.Newhall?trees on P.trifoliate are treated with irrigation density of100%,65%and 35%of local evapotranspiration under rain shelters,and the natural growth whithout rain shelters be used as a control?CK?.The response of soil and plant water status,leaf gas exchange parameters,chlorophyll fluorescence parameters,reactive oxygen metabolism and osmotic adjustment substances during different phonological phases are evaluated.The main findings are as follows:1.Effects of different irrigation schemes on fruit yield and qualityDuring flower bud differentiation,moderate water deficient irrigation?35%-65%of local evapotranspiration?promotes flower bud differentiation,increases total number of flowers and thus helps to increase fruit bearing number.However,from full bloom period to physiological fruit dropped period severe water deficient irrigation promotes the loss of flowers and young fruit,resulting in a large number of flower and fruit droping.During fruit expanding period,water deficient irrigation improves fruit internal quality?total soluble solids and Vc?and external fruit quality;however,severe water deficient irrigation?ET35?is not conducive to titratable acid degradation and fruit expanding,and the weight per fruit and juice yield significantly reduced.In all,ET65 treatment has the best influence on fruit yield and quality under the experiment condition.2.Effect of different irrigation schemes on leaf water potential and gas exchange parametersThe soil moisture content and leaf water potential decreased significantly,and the water stress index is significantly increased with the decrease of irrigation density under rain shelters.Among them,the chlorophyll SPAD value,transpiration rate,stomatal conductance and photosynthetic rate were all significantly decreased in ET35;however,although the moderate water deficit of ET65 treatment also significantly decreased in soil water content and leaf water potential and increased in water stress index,the stomatal conductance,transpiration rate and photosynthetic rate and the full irrigation treatment difference was not significant different from ET100 treatment;moreover,during fruit enlargement period the leaf gas exchange parameters even improved significantly.3.Effect of different irrigation density on the chlorophyll fluorescence parametersDuring fruit enlargement period to fruit coloring period,sever water deficit treatment of ET35 suppressed the leaf PSII receptor side in electronic from QA to QB and QA content in receptor side increased,the number of QA reduction decreased significantly under shelter cultivation.In addition,the capture energy in absorption and dissipation,transfer per reaction center?or per area?at t=0 monment are significantly increased;however,the capture energy in absorption and dissipation,transfer per reaction center?or per area?at t=M monment are significantly decreased.Which resulted in ET35 treatment significantly lowering leaf gas exchange parameters;However,there are no significant difference in the above parameters between ET 65and ET100 treatments.4.Effects of different irrigation schemes on the antioxidant enzyme activities and osmotic adjustment substances content in leafDuring summer fruit enlargement period,ET35 treatment significantly decreased the antioxidant enzyme activities of POD and SOD in leaves,and increased the malondialdehyde content in leaves;however,the moderate water deficit of ET65treatment has higher antioxidant enzyme activities,and there is no significant difference in leaf malondialdehyde content beween ET65 and ET100 treatments.In addition,CK treatment has a significantly higher in proline content than rain shelters cultivation groups during the second physiological fruit drop period,which may be due to the decrease in light and temperature by rain shelters cultivation.Moreover,the content of soluble sugar in leaf slowly increased from the flowering period to fruit enlargement perod,then decreased significantly.Among them,ET35 and CK treatment have a significantly higher soluble sugar content in leaf than the other treatments,which may be resulted in the increase of photosynthetic products with good lighting conditions for CK treatment,while ET35 treatment deals with the reaction caused by drought stress.5.Screening of key parameters for yield factors and fruit quality of Newhall navel orangePartial least squares method be used to screen the key parameters and establish the monitoring model for yield factors from full bloom stage to 2^nd physiological fruit drop period by using Simca-P 14.1 software.Among them,the monitoring model of total amount of flowers?R²Y-0.74 and Q²-0.63?,the amount of falling flowers?R²Y-0.72 and Q²-0.61?and the 1^st physiological fruit drop?R²Y-0.74 and Q²-0.60?are established by using the parameters of full blooming period.And the monitoring model of 2^nd physiological fruit drop?R²Y-0.47 and Q²-0.47?and fruit bearing number?R²Y-0.75 and Q²-0.62?can be used in the prediction of final fruit bearing monitoring.In addition,the physical and chemical indicators of fruit enlargement period can be use to establish the monitoring model for fruit appearance brightness L?R²Y-0.95 and Q₂-0.91?,the final color index a/b?R²Y-0.57 and Q²-0.37?,the edible rate?R²Y-0.64and Q²-0.57?,the fruit T/A?R²Y-0.52 and Q²-0.52?and weight per fruit?R²Y-0.77 and Q²-0.69?,and these models had good predictive effects and can be used to predict the fruit internal and external qualities.