Effects of forcing cultivation on the leaf morphology, fruit ripening and quality of Myrica rubra

Abstract：【Objective】Chinese bayberry (Myrica rubra) is an unique and valuable fruit crop cultured in southern China. As the most characteristic and prized fruit cultivated in Zhejiang province, it exhibits high levels of both nutritional benefits and economic value, which has significantly improved its market competitiveness. Main cultivated varieties of bayberry, like Dongkui and Biqizhong, mature from early June to early July. However, the bayberry during the maturation period usually encounters the rainy season, which results in severe decline in both yield and quality of bayberry, and further constrains the development of the bayberry industry. Thus, efforts to facilitate more reliable bayberry cultivation have the potential to improve its quality and output. Compared to traditional open-field cultivation, approaches to enhanced cultivation can increase temperatures and reduce the phenological periods for each stage of development, thus mitigating the potential harm caused by the plum rains. In addition, these approaches to protected cultivation can further improve commercial bayberry fruit outputs and promote its economic benefits.【Methods】In order to testify the benefits of protected cultivation for M. rubra, the leaf morphology, fruit ripening and quality of Biqizhong and Dongkui were studied, which were subjected to three different cultivation methods: open field (OF), single- layer- film- covered greenhouse(SLFG), and double-layer-film-covered greenhouse (DLFG). The cultivation of SLFG entailed the construction of a steel frame structure covered by a transparent plastic film based on the shape of the mountain slope, using triangular, arched, or flat roof multi-span steel frames as appropriate. The cultivation of DLFG entailed a novel and efficient cultivation method based on the use of SLFG triangular or arched roof multi-span steel frames on slopes, together with the introduction of a steel frame beam at a distance of 1.0 m from the top, followed by fixing a second transparent plastic film layer. After these protected cultivation models were established, temperature changes, phenological periods, and the relationships between temperature changes and phenological periods were assessed, and the leaf phenotypes and fruit quality were also measured.【Results】Compared to traditional OF cultivation, the average temperatures under DLFG and SLFG cultivation were 5.1 ℃ and 2.5 ℃ higher, respectively, significantly above the ambient temperatures. The forcing cultivation results in higher daily average temperatures, with a daily average temperature ≥ 10 ℃ lasting for 147 days under DLFG, as compared to 115 days under DLFG and only 90 days under OF cultivation. The cumulative temperature under DLFG cultivation (2 592.1 ℃ ) also significantly increased, compared to SLFG and OF cultivation, with the increase reaching 74.3%. The phenological and maturation periods for M. rubra were significantly earlier under these forcing cultivations, with the maturation periods of DLFG and SLFG being 40 and 22 days earlier than those of OF cultivation. The harvest time under DLFG and SLFG cultivation was up to 57 days, which was 3.4 times longer than that under OF cultivation. These forcing cultivation methods also significantly improved single fruit weight, soluble solid and flavonoid contents, taste, nutritional value, as well as storage and transportation capacity. Although these forcing cultivation methods reduced light intensity, the available light was sufficient to meet the needs for these bayberry trees. Thus, the normal plant growth and photosynthetic activity were not adversely impacted under either DLFG or SLFG cultivation. Moreover, the leaf morphology, including leaf length, width, area, perimeter, relative chlorophyll content, and chlorophyll fluorescence parameters increased with both Biqizhong and Dongkui. In addition, forcing cultivation significantly improved the fruit yields and quality. The results displayed that the average yields of Biqizhong under DLFG and SLFG cultivation were 2 312.5 kg · 666.7 m^-2 and 2 261.7 kg · 666.7 m^{- 2} , respectively, 40.0% and 37.0% higher than those under OF cultivation. Furthermore, the average commercial fruit rates were 85.1% and 79.8% under DLFG and SLFG cultivation, 2.8- and 2.6-times higher than those under OF cultivation. Similarly, the average yields of Dongkui under DLFG and SLFG cultivation were 1 655.2 kg · 666.7 m^-2 and 1 568.5 kg · 666.7 m^-2 , respectively, 2.9- and 2.8-times higher than those under OF cultivation. The commodity fruit rates and economic benefits surpassed 1.6- and 7.1-fold under DLFG and SLFG cultivation, respectively.【Conclusion】The forcing cultivation of M. rubra is an effective technique that can significantly advance the maturity period, extend the harvest period, improve fruit quality and promote economic benefits, without affecting the normal growth of M. rubra. Based on the above benefis of DLFG and SLFG cultivation, it is of a great value to further popularize and apply these bayberry cultivation techniques throughout China.