Effects of different plant growth regulators on fruit quality in Jinyan kiwifruit

Abstract: 【Objective】Kiwifruit (Actinidia spp.) originated in China, is an important part of China’s fruit food in the“Big Food View”perspective. Kiwifruit has become one of the most popular fruits because of its unique flavor and rich nutrients such as vitamin C, amino acids, organic acids, soluble sugars and minerals. Plant growth regulators have many physiological effects, such as enhancing plant resistance, regulating fruit size, pigment accumulation level, saccharic acid component content, and maturation and senescence process. Improper use of plant growth regulators can easily lead to high malformed fruit rate, dark skin color, early physiological ripening, large inventory loss and low fruit quality, which leads to the decline of comprehensive quality of postharvest kiwifruit. The present experiment was undertaken to explore the effects of different plant growth regulators on the growth and postharvest quality of kiwifruit.【Methods】Jinyan kiwifruit was treated with different concentrations of GA3 (50,100 and 200 mg · L-1 ), CPPU (5, 10 and 20 mg · L-1 ), triacotanol (5, 10 and 20 mg · L-1 ), NAA (50, 100 and 200 mg · L-1 ), IAA (20, 40 and 80 mg · L-1 ), 6-BA (25, 50 and 100 mg · L-1 ), melatonin (10, 20 and 40 mg ·L-1 ) and tryptophan (100, 200 and 400 mg·L-1 ) after 25-30 d of pollination. The water treatment served as control (CK), and three plants were treated with each concentration of every plant growth regulator as three biological replicates. After 140 d of pollination, 20 fruits were selected every 2 d for the determination of soluble solid content. When the SSC content was higher than 6.5%, it was recorded as the physiological maturity stage of the treatment. At the physiological maturity stage of corresponding treatment, 120 healthy fruits were randomly selected from each plant for index determination. They were placed at room temperature until soft ripeness (80% of the fruits were less than 1.2 kg · cm- 2 in hardness), and 9 fruits were randomly selected every 2 d for hardness index determination. After soft ripening, soluble solid content and titrable acid content were determined. The remaining fruits were crushed and mixed with liquid nitrogen and stored at -80 ℃ for the subsequent determination of physiological indexes. The fruit weight, fruit shape index, fruit hardness and starch content were measured at the physiological maturity stage of Jinyan kiwifruit. The soluble solid content, titrable acid content, ascorbic acid content, sucrose, fructose, glucose, citric acid, quinic acid, malic acid, total phenol and total flavone contents were measured at the soft ripening stage of the fruit. The effects of different plant growth regulators and concentrations on the above indexes were compared and analyzed.【Results】 Different plant growth regulator treatments could significantly increase the single fruit weight, NAA treatment had the best effect, and compared with CK single fruit weight increased by 43.07%-55.78%. Tri and CPPU treatments were followed by 34.56%-43.07% and 24.07%-31.62% respectively. Compared with CK, CPPU treatment could advance the physiological maturity by 18-20 d, and NAA treatment could delay the physiological maturity by 6-8 d. Compared with CK, CPPU treatment could increase starch content by 8.53%-17.32%. Different concentrations of MT, Trp and NAA, 5 mg · L- 1 of Tri, 25 mg · L- 1 and 50 mg · L- 1 of 6-BA treatment could significantly improve fruit hardness. Different concentrations of NAA, 200 mg·L-1 of GA3 and 25 mg·L-1 of 6-BA could significantly reduce the softening rate of fruit. NAA treatment had the best effect, the softening rate was reduced by 10.96%-16.44% compared with CK, and the softening period was extended by 3-5 d. CPPU treatment could promote the softening of fruit, and the softening rate increased by 13.69%-26.03% compared with CK, and the softening period was advanced by 4-6 d. Different concentrations of NAA, 100 mg·L^-1 of 6-BA, 5 mg·L^-1 of CPPU, 50 mg ·L^-1 and 100 mg ·L^-1 of IAA, 20 mg ·L^-1 of Tri, 100 mg ·L^-1 and 400 mg ·L^-1 of Trp could significantly increase the contents of titrable acid in fruit at soft ripening stage. Different concentrations of CPPU, NAA and Trp significantly reduced the ascorbic acid content at the soft ripening stage, and the treatment effect of CPPU was the best, which decreased the ascorbic acid content by 10.53%- 14.47% compared with CK. Trp and NAA treatments decreased by 9.21%-11.84% and 6.24%-8.34%, respectively. 200 mg·L-1 of GA3, 200 mg·L-1 of NAA, 10 mg·L-1 and 20 mg·L-1 of MT treatments significantly increased the fructose content at soft ripening stage. The most significant effect was 10 mg·L-1 of MT, the fructose content increased by 17.43%, followed by 200 mg ·L-1 of GA3, 20 mg ·L-1 of MT and 200 mg·L-1 of NAA, and the fructose contents increased by 13.47%, 11.82% and 10.64%. Different concentrations of CPPU, 40 mg · L- 1 of MT, 5 mg · L^-1 of Tri and 400 mg · L^-1 of Trp could significantly increase sucrose content at soft ripening stage. Compared with CK, CPPU treatment could increase the sucrose contents by 34.23%-41.23%. Different concentrations of MT, 20 mg · L^-1 and 40 mg · L^-1 of IAA treatments could significantly increase the glucose contents at soft ripening stage, and 20 mg·L-1 of IAA treatment had the best effect, followed by 10 mg·L-1 of MT treatment, which could increase the glucosecontents by 28.39% and 26.59%, respectively. Different concentrations of CPPU and MT significantly reduced the contents of citric acid, quinic acid and malic acid at soft ripening fruits. Compared with CK, the contents of citric acid decreased by 8.75%-10.63% and 8.75%-15.01%, the contents of quinic acid decreased by 10.77%-15.38% and 9.23%-12.30%, and the contents of malic acid decreased by 14.29%-20.01% and 23.8%-24.76%, respectively. Different concentrations of NAA, MT, 20 mg·L^-1 of Tri and 80 mg · L^-1 of IAA increased the total phenol contents. Different concentrations of CPPU and MT increased the accumulation levels of total flavonoids by 22.56%-28.75% and 8.75%-15.00%, respectively, compared with CK.【Conclusion】Different concentrations of 6-BA, CPPU, GA3, IAA, MT, NAA, Tri and Trp could significantly increase fruit weight, but decrease fruit shape index. The contents of soluble solid, ascorbic acid, soluble sugar, organic acid, total phenol and total flavone were promoted or inhibited by different plant growth regulators or concentrations. CPPU treatment of 10 mg·L^-1 had the best effect on the accumulation of starch and soluble solids, NAA treatment of 100 mg ·L-1 had the best effect on the appearance and storage performance of fruits, and MT treatment had the best effect on the improvement of fruit flavor quality. The suitable concentrations of CPPU, NAA and MT combined treatments were expected to improve the comprehensive quality of kiwifruit.