Effects of plant growth regulators on the seedless rate and fruit quality of 'Shine Muscat' grape

Abstract: 【Objective】Grape is one of the most important economic fruits in the world with wide range of uses. In 2017, China ranked first in the world for production and second for planting area(FAO). The popular cultivar'Shine Muscat'('Akitsu-21'×'Hakunan') was releaased in Japan in 2003 and introduced to China in 2009.'Shine Muscat'has a strong rose fragrance, high sugar and low acidity at maturity, but bears seeded berries and sparse clusters prone to fruit rust. However, treatment with plant growth regulators significantly improves fruit quality. In this study, different plant growth regulator combinations were applied at two developmental stages to examine their impact on overall fruit quality.【Methods】'Shine Muscat'inflorescences were treated with three regulator combinations, i.e. 25 mg·L-1 GA3+200 mg · L-1 Streptomycin(SM), 2.5 mg · L-1 TDZ+25 mg · L-1 GA3+200 mg · L-1 SM, and 2.5 mg ·L-1 CPPU+25 mg · L-1 GA3+200 mg · L-1 SM, which were applied at 3 days before flowering(3 DBF) or 2 days after flowering(2 DAF). A control with no regulator treatment was set. Fruit quality traits including cluster weight, number of berries per cluster, berry weight, total soluble solid content, total acid content and berry firmness were measured. SPSS software was used to determine principal components of the indicators so as to comprehensively evaluate the different treatments.【Results】The GA3 treatments showed significant increases in the berry shape index(longer berries), soluble solids, total acids and berry firmness, but the cluster weight was lowered with sparse clusters and reduced berry retention. GA3+SM treatment at 2 DAF increased the cluster weight and berry number compared with the treatment at 3 DBF, but the berry weight had no significant difference from the control. The rate of seedless berries was 80% in the treatment with GA3+SM at 3 DBF, and was 100% when applied at 2 DAF. The treatment with addition of TDZ or CPPU increased the cluster weight significantly. The cluster weight in treatment at 2 DAF was higher than in the treatment at 3 DBF, with an average of 550 g. The number of berries per cluster was significantly increased by addition of TDZ or CPPU at the two stages, and there was no significant difference between CPPU and TDZ. The shape index reduced significantly, and the degree of reduction with TDZ was greater than with CPPU. The berries in the treatments with the addition of CPPU or TDZ were almost all seedless. The content of soluble solids was increased by adding CPPU at 3 DBF but decreased at 2 DAF. However, the treatment with TDZ had a lower content of soluble solids. Adding TDZ or CPPU reduced the total acid content, and the treatment at 2 DAF had a lower total acid content than the treatment at 3 DBF. With the addition of TDZ or CPPU, the firmness of flesh was reduced, and the two cytokinins had similar effect. The overall fruit quality in different treatments in descending order was 2.5 mg·L-1 CPPU+25 mg·L-1 GA3+200 mg·L-1 SM(2 DAF) > 2.5 mg·L-1 TDZ+25 mg · L-1 GA3+ 200 mg · L-1 SM(2 DAF) > 2.5 mg · L-1 TDZ + 25 mg · L-1 GA3+ 200 mg · L-1 SM(3 DBF) > 2.5 mg · L-1 CPPU+25 mg · L-1 GA3+200 mg · L-1 SM(3 DBF) > Control >25 mg · L-1 GA3+200 mg · L-1 SM(3 DBF) > 25 mg · L-1 GA3+200 mg · L-1 SM(2 DAF).【Conclusion】Different combinations of plant growth regulators and treatment at different times had different effects on the fruit quality of'Shine Muscat'grape. According to the comprehensive evaluation of all the treatments, the treatment with 2.5 mg · L-1 CPPU+25 mg · L-1 GA3+200 mg · L-1 SM at 2 DAF had the highest score. In this treatment, the berries were almost all seedless with relatively high TSS/acid ratio and flesh firmness, and the cluster weight met the market demand.