Camellia oleifera Abel, belonging to Camellia of Theaceae, is an economic oil crop with a wide planting area in southern China. At present, the area of Camellia oleifera planted in China has reached 6 550 hm² (Li et al., 2017), mainly distributed in southern provinces of China, such as Guangxi, Yunnan, Hainan and other places. Hainan has a long history of planting Camellia oleifera. Because of its unique geographical location, Hainan Island has its own unique Camellia oleifera resources. Camellia oleifera trees in Hainan has excellent characteristics and Camellia oleifera quality, which is deeply loved by consumers, and the price of Camellia oleifera is also higher than that in the mainland (Li et al., 2011, Bulletin of Agricultural Science and Technology, (6): 8-11). For a long time, Hainan Camellia oleifera has produced a large number of mutated tree species. Camellia oleifera trees in various production areas show obvious differences between their advantages and disadvantages, and the germplasm resources are relatively chaotic. The collection, preservation, development and utilization of germplasm resources is an essential process of Hainan Camellia oleifera breeding.

At present, because of the output of Hainan Camellia oleifera is generally low and the quality of tea oil needs to be improved, the research direction is mainly focused on the output and quality. Chen et al. (2005) investigated, collected and preserved the superior tree character resources of Hainan Camellia oleifera, providing a basis for studying the germplasm resources of Hainan Camellia oleifera (Refer to the Forestry Industry Standard "Regulations of Selection and Breeding for of Plus Tree and Superior Clone" (LY/T 1730.1-2008)); Yang et al. (2012) conducted a preliminary analysis on the introduction and fine varieties of Hainan Camellia oleifera, providing a reference plan for the selection of fine varieties of Hainan Camellia oleifera; Xi et al. (2006) made a series of explorations and analyses on excellent clones with high oleic acid content, which provided reasonable reference data for breeding superior tree varieties with high oil content and good quality; Liu et al. (2014) elaborated on fatty acids from different regions, providing basic data for the analysis of fatty acids. However, these studies are all based on Camellia oleifera superior trees in a certain area, and there are few studies on the effects of economic and biological characteristics of superior trees in different production areas on their quality.

The comprehensive evaluation and biological characteristics analysis of the fruit economic characteristics of Hainan Camellia oleifera superior trees (Refer to the Forestry Industry Standard "Regulations of Selection and Breeding for of Plus Tree and Superior Clone" (LY/T 1730.1-2008)) is an important link in Hainan Camellia oleifera breeding. In order to breed high-yield and high-quality Camellia oleifera varieties and evaluate Hainan Camellia oleifera superior trees more objectively, this study visited and investigated the main production areas of Camellia oleifera in Hainan Province. After repeated screening, the superior trees with better fruits, especially fruit characteristics, were selected, and the effects on their quality were analyzed respectively, so as to provide a reference for the breeding and comprehensive development and utilization of Camellia oleifera in Hainan.

1 Results and Analysis

1.1 Analysis and evaluation of economic characteristics of superior trees

There were significant differences in the economic characters of the fruits of Camellia oleifera superior trees (P<0.01), and there were very significant differences in some economic characters of the fruits. The excellent trees in different production areas of Hainan (including Tunchang, Ding'an, Qionghai, Chengmai and Wuzhishan) investigated in this experiment are representative, and basically cover the main production areas and main planting varieties of Camellia oleifera in Hainan. The main reason for the significant difference in economic characteristics of fruits in this study is the difference in fruit characteristics and the impact of different biological characteristics. In addition, it may be limited by local climatic environment, cultivation technology and other factors (Yang et al., 2018), which also affects the quality of Camellia oleifera. (Table 1; Figure 1).

1.2 Analysis and evaluation of growth characteristics of Camellia oleifera superior trees

The growth characteristics of the selected superior tree varieties showed different degrees of difference. TCTZ-4 has the highest plant height of 9.4 m; QHSK-1 was the smallest, 2.0 m in height; There are four types of superior trees: cluster, vertical, weeping and cross type; The leaf shape of superior trees is long oval, wide lanceolate and oval, showing certain polymorphism; The largest leaf is 8.3~12.8 cm long and 3.1~6.8 cm wide (CMFS-1), and the smallest is 5.6~9.5 cm long and 3.4~5.8 cm wide (DAST-3); There are three types of branch positions of superior trees: low, medium and high. After a series of data analysis and comparison, the growth characteristics of the superior trees are different, resulting in differences in their yield and tea oil quality (Table 2).

1.3 Analysis and evaluation of flower buds of Camellia oleifera superior trees

The flower buds of Camellia oleifera superior trees are all apical; The number of flower buds per plant of DAST-1 is 14 at most, and the number of flower buds per plant of superior trees QHSK-1 and TCTZ-4 is 5 at least; The number of flower buds per plant of each superior tree variety showed different degrees of difference, which affected its total yield. The flower bud differentiation and growth of each excellent tree need to consume more nutrients, so it is necessary to pay attention to supplementing the fertilizer in the critical period of its fertilizer requirement, so as to increase the number of flower buds, improve the fruit yield and increase economic benefits (Tian et al., 2019). The investigated superior tree varieties are basically free of diseases and pests, which also creates good conditions for them to obtain high yield and high quality (Table 3).

1.4 Analysis and evaluation of diseases and insect pests of Camellia oleifera superior trees

The pests and diseases of superior trees DAST-3, DAJC-1, DAJC-2, CMFS-2, CMFS-4, QHSK-1, QHSK-2, QHSK-3, QHSK-5, WZSHS-1, WZSFS-1, WZSFS-3, WZSND-1, TCTZ-1, TCTZ-3, TCTZ-4, TCTZ-5 and TCTZ-6 perform well. Basically, there is no damage to the leaves and a small number of leaves are disturbed by pests and diseases. The leaves of other superior tree varieties were damaged to varying degrees. QHSK-4 leaves were severely damaged, about 20% of which were damaged. The leaves of other superior tree varieties are damaged to a lesser extent or not. The leaves of superior trees are damaged, which affects the vegetative growth and reproductive growth of plants, thus affecting the yield and economic characteristics of superior trees and further affecting the quality of Camellia oleifera (Table 4).

1.5 Fatty acid composition and content of Camellia oleifera seed oil

The coefficient of variation of fatty acids of Hainan Camellia oleifera seed oil ranges from 6.39% to 46.12%. Linoleic acid (46.12%) has the largest coefficient of variation and oleic acid (6.39) has the smallest coefficient of variation. The degree of variation of palmitic acid, stearic acid and linolenic acid also shows obvious differences, which indirectly reflects the rich genetic variation characteristics of Hainan Camellia oleifera superior trees and has great selection potential (Liu et al., 2014). Other contents showed a moderate degree of variation (Figure 2).

1.6 Correlation comparison of main fatty acids

This study focuses on the evaluation of the quality of tea oil from the biological characteristics. Although there is a large difference in its biological characteristics and a close correlation between the content of fatty acids in Camellia oleifera seed oil, the correlation varies slightly. Oleic acid has a very significant negative correlation with linoleic acid and palmitic acid. During the formation of fatty acids, linoleic acid has a large degree of variation and is constantly transformed into oleic acid, which is consistent with the research results of Xu et al. (2018), while oleic acid has a significant positive correlation with stearic acid (Figure 3). Therefore, these correlations can be well used as auxiliary indicators when selecting Hainan Camellia oleifera varieties (Yuan et al., 2012; Zhong et al., 2015).

2 Discussion

At present, there have been studies on fruit traits and biological characteristics in a certain area, but the economic benefits are not significant and there are certain limitations (Jia et al., 2018). This study aims to improve the market competitiveness of Hainan Camellia oleifera seed oil with the breeding requirements of high yield and quality. The excellent germplasm resources and quality of Hainan Camellia oleifera were analyzed, which can improve the quality of Hainan Camellia oleifera and increase economic benefits to a certain extent. In order to select and breed high-quality varieties suitable for promotion in Hainan, this experiment focuses on the study of excellent Camellia oleifera trees with good growth, which is more conducive to improving the economic benefits and application and promotion of Camellia oleifera in Hainan. For the deficiencies of the investigated Camellia oleifera superior tree varieties, continuous optimization research is needed, which is also the direction of further research in the future.

Comprehensive analysis of the economic and biological characteristics of the fruits of superior trees in different places of Hainan can effectively improve their yield and quality (Yang et al., 2018). Wang et al. (2016) showed that the differences in economic characteristics were mainly due to varieties, while this study showed that they were mainly affected by fruit characteristics and biological characteristics. In this study, it was found that Tunchang, Ding'an and other areas in Hainan province had the best comprehensive characteristics of fruit and the best quality of tea oil, which had obvious advantages over other areas and could be used as excellent materials for breeding. The fruit economic characters of Hainan Camellia oleifera superior trees have obvious advantages, and there are significant differences in all indicators (P<0.01), and its oleic acid content is rich, which can promote the improvement of its quality; Through the comprehensive analysis of the biological characteristics of superior trees in different places, it is shown that their growth characteristics, flower bud growth, diseases and insect pests all affect the quality of tea oil. If the biological characteristics of superior trees are good, the quality is better; otherwise, the quality is worse; It not only reflects the polymorphism of Camellia oleifera superior trees in Hainan, but also shows the rich genetic variability of Camellia oleifera in Hainan.

The comprehensive analysis of the economic and biological characteristics of the fruit proved that it had a great impact on the quality of Camellia oleifera, and also reflected that the superior trees of Camellia oleifera in Hainan had great selection potential.

3 Materials and Methods

3.1 Test materials

From August 2018 to September 2020, this experiment conducted a detailed visit and investigation on the excellent Camellia oleifera trees in Hainan Island, from Ding'an, Chengmai, Qionghai, Wuzhishan, Tunchang and other places. According to Zhu et al. (2016), 30 excellent trees were selected for high yield and large fruit. All the selected excellent trees can bloom and bear fruit normally. The tree age is generally 10~50 years, and the average tree height is 4~8 m (Table 5).

3.2 Determination of economic characteristics of fruits

Randomly select several Camellia oleifera fruits from each sample, and weigh the single fruit weight, 100 seed weight, fresh seed weight, dry seed weight and kernel weight of each Camellia oleifera fruit with a balance; Determine the dry seed rate, fresh seed rate and other main parameters, and count the number of seeds. Fresh seed rate=fresh seed weight/fruit weight×100%; Dry seed yield=dry seed/fresh seed weight×100%; Kernel yield=dried kernel weight / dried seed×100% (Song et al., 2013).

3.3 Observation and evaluation of biological characteristics of superior trees

Through on-the-spot investigation and sampling, the selected excellent trees were measured with tape measure, counter and other tools. The biological characteristics were mainly analyzed from their growth characteristics, flower bud conditions, pests and diseases. Refer to the Forestry Industry Standard "Regulations of Selection and Breeding for of Plus Tree and Superior Clone" LY/T 1730.1-2008.

3.4 Fatty acid composition and content analysis

Take 400 μL oil into 10 mL tube, and 0.5 mL benzene is added to dissolve it. Then add 0.4 mol/L KOH-CH₃OH solution 200 μL. Shake to convert all oil and fat into fatty acid methyl ester, stand for 15 minutes, add a little distilled water, take the upper liquid after layering and analyze the fatty acid composition with gas chromatograph.

3.5 Data analysis

In this experiment, Excel 2010 was used to process the mean and standard deviation of data, and R 4.0 language software was used to analyze the difference and fatty acid correlation.

Authors’ contributions

WH is the executor of the experimental design and research of this study; WH, DYJ and SJM completed the data analysis and the writing of the first draft of the paper; ZY participated in the experimental design and the analysis of experimental results; LHG is the designer and principal of the project, guiding the experimental design, data analysis, paper writing and revision. All authors read and approved the final manuscript.

Acknowledgments

This study was jointly supported by the Special Fund for World-class Discipline Construction of Hainan University (NO.RZZX201902), the Research and Development of Key Technologies and Processing Products for High Quality and High Yield of Hainan Camellia oleifera (ZDKJ2017004), and the Branch of Camellia oleifera Germplasm Resources of the National Tropical Plant Germplasm Resources Bank (HD-KYH-2020009).

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