Effect of drought stress at different development stages on fruit quality formation in Satsuma Mandarin

Abstract: 【Objective】Moderate drought is an important means to regulate fruit quality. Citrus grows in southern China, where abundant rainfall happens during cell division but less rain during fruit enlargement, which is sensitive to drought. The impact of drought on fruit quality during different periods remains unclear. In this regard, the effect of drought on fruit quality at different fruit development stages was examined in order to provide strategy for improving citrus fruit quality with water-saving irrigation.【Methods】The experiments were carried out in National Centre for Citrus Improvement, Changsha, China. Five-year-old Satsuma Mandarin (Citrus unshiu Marc.‘Yamasitaka') trees grafted on trifoliate orange [Poncirus trifoliate (L.) Raf.] were transplanted into plastic pots with a volume of 27 L.The cultivation medium was Vsawdust: Vriver sand: Vpeat = 2:1:0.5. A TDR300 portable soil moisture analyzer (Spectrum Technologies, USA) was used to measure water content in the substrate. Water content below40% (relative to maximum water holding capacity of the substrate) was considered as drought stress.Drought treatment was applied at fruit cell division stage (June-July) , fruit enlargement stage (AugustSeptember) and throughout the whole fruit development stage (June-September) in this study. Meanwhile, normal irrigation (approximate 70% of the maximum water holding capacity in the substrate) was treated as the control. The control group and each of the treatment groups had 10 trees. Nine fruit were collected at each time of sample collection, and the appearance indexes were determined immediately after fruit collection. The fruit were divided into two parts, half of which were stored in the refrigerator at-20 ℃, while the other half was quickly peeled and frozen in liquid nitrogen and stored at-80 ℃. High performance liquid chromatography (HPLC) and direct measurement were performed to analyze the changes in external and internal quality indexes during fruit development.【Results】The results showed that fruit volume decreased after drought stress during different fruit development periods.Fruits under persistent drought showed a smallest fruit size, which was 65.7% lower than that of control. Besides, fruit size under drought at fruit enlargement stage and cell division stage was 49.6% and22.5% smaller than that of the control, respectively. The results indicated that drought during fruit enlargement had a greater impact on fruit development, and was the key period affecting fruit size and yield. Analysis of the mature fruit showed that total acid and sugar increased by 86% and 6.5% under continuous drought stress, respectively. Total acid increased by 37.3% and total sugar decreased by 6%in the group with drought applied during cell division compared with the control. Meanwhile, total acid and total sugar increased by 31.8% and 26.1% in drought treatment applied during fruit enlargement, respectively. Furthermore, restoring irrigation after drought stress during cell division exhibited no obvious effect on sugar decrease and acid increase, suggesting that drought during cell division had irreversible negative effects on fruit quality. The sugar-acid ratio in mature fruit in drought stress at different fruit development periods was the highest in the control and in the drought treatment during fruit enlargement, followed by the group with drought during cell division, and the continuous drought group was the lowest, indicating that drought during fruit enlargement had a great effect on sugar and acid metabolism and thus on fruit quality formation. In addition, all drought treatments resulted in increased in organic acids in fruit. Citric acid is the major acid, accounting for over 75% of total acids. The degradation of citric acid in fruit under drought treatments was seriously hindered, leading to increased organic acid content in fruit.【Conclusion】Drought stress at different periods of fruit development may intensify citric acid synthesis and suppress its degradation. Drought during cell division causes an irreversible deterioration of fruit quality with high acid and low sugar contents. Drought during fruit enlargement has a great impact on fruit volume as well as sugar and acid contents.