Abstract:

Using Satsuma mandarin fruits at enlargement stage as the material,three stress treatments with the substrate relative water content 40%,30% and 20% were set. Afterward,fruit quality changes and expression patterns of transcription factors regulating citrate accumulation were studied under drought conditions. Using deuterium water（D₂O）tracer method,the law of water transportation and distribution in citrus tree after re-watering could be cleared. Drought stress at fruit enlargement stage inhibited fruit growth and fruit size severely. After ten days drought stress,fruit transverse diameter,longitudinal diameter and single fruit weight decreased significantly. After constant drought for 40 days,fruit diameter in treatment groups decreased by 18.33% to 23.82% comparing to the control nd single fruit weight decreased by 45.17% to 48.67%. No significant difference was found in fruit size between different drought-treatment groups. After 40 days of drought stress in each treatment,fruit total solid soluble increased significantly by 47.7% to 59.3% and fructose increased drastically by 63.25% to 78.77%,but sucrose and glucose in fruit did not change. No significant difference was found in fruit sugar components between different drought-treatment groups. Citrate content in control fruits decreased gradually with fruit development,but that increased significantly after 20 days drought stress with 1 fold higher to control at the peak. Fruit malate content was higher than that in control by 14.70% to 33.82% after drought stress. No significant difference was found in fruit acid components between different drought-treatment groups. Transcription factors CitPH3（WRKY）,CitPH4（MYB）and CitAN1（bHLH）which regulate citrate accumulaiton in citrus fruit expressed with up-regulation under drought stress. After rehydration,citrus roots absorbed water quickly in four to eight hours. Deuterium water content in main stems and perennial stems reached the highest in 24 hours. The water absorption of the fruit tended to be stable during 24 to 48 hours. In this experiment,drought duration affects fruit quality significantly while drought degree does slightly. The up-regulated expression of CitPH3,CitPH4,and CitAN1 under drought stress promotes the accumulation of citric acid,which may be an important reason for the acidification of citrus fruits. Orchard soil being moist for 24 to 48 hours after continuous drought is necessary for water reaching fruit massly.

Key words: citrus, fruit enlargement stage, drought, fruit quality, gene expression, water transportation, re-watering, D₂O