Screening of new reagents for preventing fruit-drop on late-maturing Citrus

Abstract: 【Objective】Fruit drop is severe in late-maturing citrus varieties during winter and early springseasons. Therefore, how to address the fruit drop problem is very important for the late-maturing citrus in-dustry. 2,4-dichlorophenoxyacetic acid(2,4-D) is the best effective reagent preventing citrus preharvestdrop, but the toxicity of 2,4-D residues has been a concern and also controversial for a long time, and 2,4-D has been limited or forbidden to be used on citrus fruits in many countries or regions. There are no sub-stitutes for controlling citrus preharvest drop. Thus, it is necessary to investigate the efficacy of some newreagents, which have a no or low toxicity to humans and animals, on preventing fruit-drop of the late-ma-turing citrus.【Methods】This experiment was executed at the Citrus Research Institute, Chinese Academyof Agricultural Sciences during the 2014—2015 seasons. Seven year old‘Tarocco'blood orange(Citrus sinensis‘Tarocco') and‘Kiyomi'tangor(Citrus sinensis × Citrus unshiu Marc.) growing under the sameconditions were used as experimental materials, and fifty trees were divided into five test groups. Using 2,4-D as a positive control(TCK), the resistance of three preliminary screened chemicals [2-methyl-4-chloro-sodium(T1), 2-methyl-4-chloro-dimethgl amine salt(T2) and 2-methyl-4-chloro-phenoxy ace-tic acid(T3)] against citrus preharvest drop were investigated. Prochloraz and Triton X-100 were used as ablank control(CK). At four different times, sprays of reagents were applied to‘Tarocco'blood orange and‘Kiyomi'tangor every 20-30 days on sunny days from Nov. 2014 to Jan. 2015. The fruit drops of eachtree were investigated every two weeks, and the dropped fruit were removed from the orchard. The fruitdrops were divided into two types including physiological fruit drop and pathological fruit drop, and onlythe data of the physiological fruit drops were analyzed in this study. To evaluate mature fruit quality, thesoluble solid content(TSS), titration acid(TA), vitamin C and fruit chromatic aberration etc were analyzedusing a PAL-1 hand-held refractometer, naoh neutral titration, 2,6-dichloroindophenol titration, andchroma meter CR-10, respectively. The reagent residues in the peel and pulp were also determined usingQu ECh ER combined with UPLC-MS/MS. All the treatments were repeated 3 times.【Results】The resultsshowed that 2,4-D and three tested reagents significantly reduced the fruit-drop. And the percentage fruitdrop of both varieties was very low before the low temperatures arrived. In test groups, the fruit drop ratiosof‘Tarocco'blood orange and‘Kiyomi'tangor were 15.42%-19.79% and 10.80%-18.89%, respective-ly. The accumulative fruit drop ratio of CK was very high at the end of the experiment and that of 34.65%and 82.52% were observed in‘Tarocco'blood orange and‘Kiyomi'tangor,respectively. The datashowed that the fruit-drop rates of all treatments were significantly lower than that of CK, while there wereno significant differences between 2,4-D and the three tested reagents. The reagent residues of the fruitpulp were lower in both varieties than that of the fruit peel. In the pulp of the‘Tarocco'blood orange, theresidues of 2,4-D residue were 0.013 0 mg·kg^-1 and that of T1 was only 0.002 3 mg·kg^-1. The residues ofT2 and T3 were not detected. But in the peel, the reagent residues of 2,4-D were 2.2-4.7 times as muchas that of the other three treatments. Similarly, in the pulp of‘Kiyomi'tangor, the reagent residues of 2,4-D were the highest among all the treatments, and the reagent residues of TCK were 0.081 0 mg·kg^-1. Inthe peel, the reagent residues of T2 were the highest(0.082 4 mg·kg^-1), and that of 2,4-D was the lowest(0.060 8 mg·kg^-1). The GB2763—2014 results showed that the largest residues in the citrus of 2,4-Dwere 1 mg·kg-1, and that of 2-methyl-4-chloro-sodium were 0.1 mg·kg^-1. Thus, the presented reagentresidues in the pulp and peel after treatment with T1, T2, T3 and 2,4-D were within the standards set byour country. For the fruit chromatic aberration, when compared with 2,4-D, the L value, a value and b val-ue of the two varieties had no significant differences after treatment with T1, T2 and T3, and there were nosignificant differences among most indexes for the fruit quality.【Conclusion】The results showed that 2,4-D and the three tested reagents significantly reduced the fruit-drop in both of‘Tarocco'blood orange and‘Kiyomi'tangor, compared to the control. Moreover, the reagent residues in the pulp and peel were underthe national standards of the PRC, and the qualities of the fruit had no significant differences. Thus, T1,T2 and T3 reagents should be effective substitutes for 2,4-D in preventing fruit-drop of late-maturing citrus.