Fruit growth and development pattern of Ziziphus jujuba‘Dongzao’and the regulatory effects of exogenous hormones application

【Objective】In recent years, during the development of Dongzao (Ziziphus jujuba Mill.) industry, the facility-based cultivation model has gradually replaced the open-field cultivation, effectively solving the problem of cracking due to rainfall during the ripening period, and significantly improving the quality of the fruits. However, facility cultivation requires higher management skills. If the management is improper, a series of problems such as small fruits or deformed fruits, decreased sugar content and reduced flavor, and poor coloring will occur. The study aimed to systematically investigate the fruit growth and development patterns of Dongzao jujube under protected cultivation and the effects of exogenous plant growth regulators on fruit development, providing a theoretical basis for high-quality and efficient cultivation of Dongzao.【Methods】The study selected the Dongzao jujube trees under the shelter shed mode as the research material. During the sampling process, 10 healthy Dongzao jujube trees with consistent flowering periods and growth conditions were selected from all directions of the experimental area (north, south, east, west, center). The plants were sampled from the east, south, west, and north directions, as well as the upper, middle, lower, inner, and outer directions. The disease- free and pest- free jujube fruits were collected as the experimental materials and stored in a -80 ℃ refrigerator for future use. By monitoring the changes in the longitudinal diameter, transverse diameter, fresh fruit weight and fruit shape index of Dongzao jujube at different growth stages, a numerical simulation model was established to analyze the growth and development patterns of Dongzao jujube fruits at different periods. The Logistic, Cubic polynomial, and Quadratic polynomial models were employed to fit the dynamic changes of the fruit longitudinal diameter, transverse diameter, fruit weight, and fruit shape index. The patterns of sugar-acid accumulation were analyzed. At the fruit coloring stage (60 days after flowering, 60 DAF), four treatments were applied: ethephon (ETH, 300 mg·L-1 ), methyl jasmonate (MeJA, 50 mg·L-1 ), abscisic acid (ABA, 50 mg·L-1 ), and tap water as the references (CK) to analyze the effects of different exogenous phytohormones on the appearance and internal quality of fruits.【Results】 The growth and development of Dongzao jujube fruit exhibited a typical‘S’-shaped curve. The Logistic model provided the best fit for fruit growth indices (R2 = 0.978-0.999). Based on the growth rate changes, fruit development could be divided into three stages: slow growth period (10-30 DAF), rapid growth period (30-80 DAF), and late growth period (80-110 DAF). The sucrose content in the Dongzao jujube fruit began to accumulate at 70 DAF of fruit growth. After 70 DAF, the sucrose content showed a gradually increasing trend. By the time the fruit reaches 110 DAF, that is, at the maturity stage, the sucrose content was the highest, accounting for approximately half of the total sugar content. Dongzao jujube was a sucrose-accumulating fruit (sucrose content at maturity: 64.82 mg · g-1 ) with malic acid as the predominant organic acid (peak content: 12.96 mg · g-1 ). The vitamin C content peaked at 50 DAF (2.54 mg · g- 1 ). During the hardening period of the fruit core, a large amount of nutrients were consumed, and the content of vitamin C and total soluble sugar decreases. The exogenous ETH and MeJA treatments significantly promoted the fruit coloring (resulting in partially red surfaces), increased the soluble solids content, solid-acid ratio (a 24.5% increase with ETH treatment), and sweet-acid ratio (an 11.05% increase), while reducing the fruit firmness (4.62 N with ETH treatment). The ETH treatment also significantly increased the endogenous levels of ETH, ABA, GA3, and cytokinin (CTK) at maturity. When the fruit of Dongzao jujube was treated with exogenous MeJA (50 mg·L-1 ) during the fruit ripening period, the contents of IAA, ABA, GA3 and ETH increased, while the content of CTK decreased. When the fruits were treated with exogenous ABA (50 mg · L- 1 ), the contents of endogenous ABA and GA3 in the fruits increased, while the contents of IAA and CTK decreased.【Conclusion】The development of Dongzao jujube under the protected cultivation conformed to the Logistic growth model, during the growth and development of Dongzao jujube fruits, there was a S-shaped growth curve consisting of a gradual growth period (10-30 DAF), a rapid growth period (30-80 DAF), and a slow growth period (80-110 DAF). At 70 DAF of Dongzao jujube, the total soluble sugar content of the fruit rapidly increased. This would be the main period for sugar to be transported and accumulated in the fruit. Therefore, during this critical period and the rapid growth period, the environmental temperature and humidity in the facility should be controlled within an appropriate range to reduce non-stomatal limitations. At the same time, attention should be paid to strengthening water and fertilizer supply, so as to ensure that the fruit would have sufficient nutrients for growth and improve the fruit quality. Spraying ETH (300 mg · L- 1 ) or MeJA (50 mg · L- 1 ) on the fruits at the coloring stage could effectively improve fruit quality by regulating the balance of endogenous phytohormones. Among these, ETH treatment demonstrated a significant effect in promoting coloring and enhancing sugar accumulation.