| China's citrus economic cultivation area is mainly concentrated between 20° and 33° north latitude,belonging to the subtropical monsoon climate zone.The seasonal drought is severe from July to October,and the spatial and temporal distribution is extremely uneven.As well as citrus cultivation in hills and mountains,lack of irrigation conditions and imperfect irrigation equipment,seasonal drought is very prominent.The drought stress time usually occurs in the citrus fruit expansion period,which not only affects the citrus yield but also has a great influence on the fruit quality,especially the accumulation of citric acid.In this study,different degrees of drought stress were used in the fruit enlargement of Satsuma Mandarin.The isotope tracer technique was used to explore the absorption,movement and distribution of water in citrus trees after drought stress,and to study in physiological,biochemical and molecular biology.The effect of both on citric acid metabolism in citrus fruits and its mechanism of action.This study aims to reveal the relationship between drought stress and citrus tree water absorption and citrus fruit citrate metabolism,and to provide a theoretical basis for regulating the quality of citrus fruit.The main research results are as follows:1.After irrigation,the citrus roots quickly absorb water and transport upwards.However,under different soil moisture conditions,the transport and transfer of water in citrus is different.After 2 h of irrigation,the turmeric abundance(?D)of the control citrus reached the highest after 2 h,and then gradually decreased.The citrus root reached the highest at 4 h after treatment with 10% soil moisture(RWC10%);RWC30 % processing is highest at 8 h.In the control group,the ?D of the main stem,perennial,annual and leaf organs increased gradually and reached the highest at 48 h.Under drought stress,the main stem,perennial branches and annual branches reached the highest at 24 h,while the leaves reached the highest at 48 h.At 8 h after irrigation,the ?D of the treated citrus fruits did not change significantly.After 24 h of irrigation,the ?D increased rapidly,and the ?D reached the highest at 48 h.2.Drought stress significantly enhanced the water absorption capacity of citrus roots.The stronger the drought stress of citrus plants,the stronger the absorption capacity of roots,the more water absorption,the higher the ?D value in each organ.The highest values of ?D in roots,main stems,perennial branches,annual branches,leaves and fruit organs of the control group were 1400‰,1230‰,1255‰,1307‰,684‰,8‰,respectively;the highest value of ?D of each organ under RWC30% stress They were 2360?‰,1855‰,1820‰,1667‰,641‰,20.88‰;the highest values of ?D of each organ under RWC10% stress were 2801‰,2527‰,1867‰,1311‰,748‰,47.89‰,citrus The ?D in each organ was positively correlated with the degree of stress.3.Under different soil moisture conditions,the distribution of water in different organs of citrus is different.RWC10% drought stress will delay the upward transport of water.Under normal management,while the water absorbed by citrus roots meets its own growth needs,it can be quickly transported to various organs to meet the normal growth of other organs.The RWC30% drought treatment was basically consistent with the control trend.After irrigation for 2 h,the upper part of the upper part contained higher salinity mass fraction.Over time,the distribution ratio of various organs in the ground gradually increased.RWC 10% treatment under water transport is delayed.After irrigation for 2 h,the proportion of citrus roots was 38%,which was significantly higher than that of the control and RWC 30%,while the proportions of the main stem,perennial branches,annual branches and leaf organs were 2.79%,0.33%,and 0.04%,respectively.0.22%,significantly lower than other treatments in the same period,8 h after irrigation,the proportion of water in the annual branches and leaves was still lower than that of the control,and the proportion of distribution in the upper part of the 48 h was significantly higher than the other two treatments..The distribution of water in plants is related to the degree of drought.Under severe drought stress,plant organs will preferentially supply their own growth and survival.4.During the fruit enlargement of Satsuma Mandarin,drought stress seriously inhibited the normal growth and development of the fruit.After the onset of drought stress,the fruit's transverse diameter,longitudinal diameter,and single fruit weight decreased significantly,reaching the minimum at September 5,which was about 20%,18%,and 45% lower than the control,and RWC40%,RWC30%,and RWC20.There was no statistically significant difference in the horizontal and vertical diameter,fruit weight and peel weight between the treatment groups,that is,the drought stress of soil with water content below 40% had the same effect on the external quality of Satsuma Mandarin.5.The drought stress in the fruit enlargement of Satsuma Mandarin has different changes in the main sugar content in the fruit.Under drought stress,the soluble solids in the fruit were significantly increased.With the deepening of drought stress,the more soluble solids increased.The sucrose content in citrus fruits was significantly lower than that in the control under different degrees of drought stress.The sucrose accumulation in the fruits under drought stress was inhibited by RWC 40%.The fructose and glucose in the drought treatment group were significantly higher than the control,and the sugar content in the treatment was not different.Significant.That is,drought stress with soil water content below 40% will inhibit the accumulation of sucrose and promote the content of fructose and glucose.6.Drought stress during fruit enlargement significantly increased the content of citric acid and malic acid in fruits.The citric acid in the control group had the highest content on July 25,and then showed a gradual decreasing trend.The citric acid in the treated group had a high content,which was significantly higher than that of the control.The citric acid degradation was slow.On September 5,there was no significant difference in citric acid content between the drought-treated groups.Malic acid was also significantly higher than the control under drought treatment.7.Under different drought stress,the content of citric acid in citrus fruits increased,and the expression of CitPH3,CitPH4,and CitAN1 genes in citrus fruits was significantly up-regulated,while the CitPH1 and CitPH5 genes did not show significant changes.The CitPH3,CitPH4,and CitAN1 genes are the WRKY,MYB,and bHLH transcription factors that regulate citric acid accumulation,respectively.The CitPH1 and CitPH5 genes encode H+-ATPase.Therefore,the dramatic increase in citric acid content under drought stress may be related to the regulation of downstream genes by transcription factors.The result of the expression. |
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