Effects of foliar application of plant growth regulators at different flowering stages on fruit setting and quality in pomegranate

Abstract:【Objective】Low fruit setting is one of the major barrier restricting the production of pomegranate.High ratio of functional male flowers and physiological dropping are two essential biologicalreasons for low fruit setting in pomegranate. In previous studies, we have found the effective approachesto increasing the ratio of bisexual flowers, but the effective ways to prevent fruits from physiologicaldropping keep unsolved. Most of the studies on fruit setting rate focus on improving the fruit setting offlowers blooming at the second stage, and there are no effective approaches to improving the fruit settingaccording to the difference in flowering characteristics of the first and second blooming stages. Theobjective of this study was to figure out more effective ways to increase the fruit setting of pomegranate by spraying plant growth regulators at different flowering stages.【Methods】The experiments were performedduring 2017—2018.‘Tunisiruanzi’, one of the most popular varieties in China with seriousfruit dropping problem, was selected as the sample materials. In 2017, completely random block treatmentwas designed with GA3 (10.0, 20.0, 30.0 mg · L^-1), NAA (10.0, 30.0, 40.0 mg · L^-1), CPPU (5.0,10.0, 15.0 mg·L^-1), and 2,4-D (10.0, 30.0, 40.0 mg·L^-1) to test the effects of different concentrations ofGA3, NAA, CPPU and 2,4-D on fruit setting rate during the first blooming stage (May 5) and the secondblooming stage (May 18). Every kind of plant growth regulators was sprayed with four kinds ofways: spraying once on May 5, spraying twice on both May 5 and May 18, spraying once on May 18,spraying twice on both May 18 and June 6. In 2018, complete randomized block design was also adoptedto verify and optimize the effective methods that were used in 2017. Treatments with GA3 (10.0,20.0, 30.0 mg·L^-1) +2,4-D (10.0 mg·L^-1) were applied to bisexual flower stalks, respectively. Fruit settingrate was counted. Fruit weight, L/D index, fruit surface finish, pericarp thickness, hundred- grainweight of seeds, seed soluble solid content, seed color, and seed hardness were measured, respectively.【Results】In 2017, the effects of different concentrations of GA3, NAA, CPPU and 2,4-D on the fruitsetting were studied. The results showed that fruit setting rate of the spraying once with GA3 30.0 mg·L-1at the first blooming stage (May 5) harbored 48.3% higher than that of the control, and the fruit settingrate increased significantly by 16.0%±0.32% (p < 0.01). spraying once with GA3 (10.0 and 20.0 mg·L-1)on May 18 increased the fruit setting rate by 20.8% and 31.5% than control, respectively, and significantlyincreased the fruit setting rate (p < 0.05). However, the female sterility flowers could not dropnaturally with this treatment, which led to nutrition wasting. Twice spraying with GA3 (10.0 and 20.0mg · L- 1) on May 5 and May 18 could not significantly improve the fruit setting rate. Twice sprayingwith GA3 (10.0 and 20.0 mg· L- 1) on May 18 and June 6 could also not significantly improve the fruitsetting rate. Foliar spraying with NAA could not significantly improve the fruit setting rate. Sprayingwith 2, 4-D on the leaf surface could not significantly improve the fruit setting rate, but led to fruit malformationwith extremely thick peel. Twice spraying with CPPU on May 5 and May 18 resulted in a significantreduction in fruit setting rate. The results of spraying once with GA3 30 mg · L^{- 1} on May 5 in2018 showed a significant increase in fruit setting rate by 28.5% compared with the control (p < 0.05),which was consistent with the results in 2017. Soluble solid contents in fruits with GA3 30.0 mg · L^-1 treatment increased significantly and the three types of application of GA3 (10.0, 20.0, 30.0 mg · L^{- 1})with 2,4-D (10.0 mg · L^{- 1}) to the flower stalk could increase the fruit setting rate by 52.5%, 30% and42.5% than the control, which significantly improved the fruit setting rates without affecting the externalappearance. Seed hardness of fruits applied with GA3 (10.0, 20.0, 30.0 mg·L^-1) + 2, 4-D (10.0 mg·L^-1)increased to be 1.82±0.52, 1.85±0.52, 1.74±0.37 kg·cm^-2, which were significantly higher than the control(1.55±0.32 kg · cm^{- 2}). The seed size, seed color and soluble solid content were not significantlychanged.【Conclusion】Foliar spraying with GA3 (30.0 mg·L^-1) at the first blooming stage (May 5) caneffectively improve the fruit setting rate with significant increase of soluble solid contents. NAA, CPPUand 2, 4-D could not significantly improve the fruit setting rate. It was not suitable to increase the fruitsetting rate by spraying plant growth regulators during the second blooming stage. Compared withspraying with GA3 (30.0 mg·L^-1) at the first blooming stage (May 5), application of different concentrationsof GA3 (10.0, 20.0, 30.0 mg·L^-1) + 2, 4-D (10.0 mg·L^-1) to the flower stalk could achieve higherfruit setting rate, but would lead to the seed hardness higher than the control. Our results may providenew insights and solutions into the improvement of fruit setting in pomegranate.