Chinese kale is a native Chinese vegetable, widely distributed in South China and Southeast Asia. However, little research has focused on the leaves of Chinese kale. In this study, we selected leafy Chinese kale as the research material and systematically analyzed the nutritional substances of different varieties under various cultivation seasons. The results showed that the leaves of Chinese kale contain high levels of vitamin C, chlorophyll, carotenoids, total phenols, and glucosinolates. The detected nutritional substance levels were generally consistent with those reported by previous studies in other Brassica vegetables, and some, such as the vitamin C content, were even significantly higher than those of other Brassica species [25,28,29,30,31,32,33].

Previous studies have shown that cultivation season, meaning the variable growing environment, affects the yield and nutritional quality of Brassica vegetables [34,35,36,37,38]. Chinese kale cultivated in spring has been reported to have a higher glucosinolate content than that cultivated in fall [34], and similar observations have been made in Brassica oleracea var. acephala [36]. Charron et al. [35] found that the glucosinolate content of cabbage varied between spring and fall, with the highest total glucosinolate content being observed when the crop was harvested under high temperatures and long sunlight exposure. Consistently, in the present study, total glucosinolates, total aliphatic glucosinolates, total indole glucosinolates, gluconapin, and glucobrassicin were found to be higher in the warmer cultivation seasons (spring and fall) than in the cooler season (winter). This may be due to the higher temperature inducing the accumulation of glucosinolates [39]. Moreover, the higher glucosinolate content of leafy Chinese kale cultivated in fall and spring may be related to the longer photoperiod as reported by the finding of Liu et al. [40] that glucosinolate content in cabbage increases under long-day conditions. In addition to glucosinolates, other nutritional indexes were also affected by the cultivation season. Free sugars such as sucrose, glucose, and fructose were higher in spring- and summer-cultivated broccoli compared to fall- and winter-cultivated broccoli, whereas the opposite trend was observed in kale and Portuguese cabbage [41]. Our study found that spring-cultivated leafy Chinese kale had higher levels of reducing sugars. Additionally, a higher content of carotenoids was found in the leafy Chinese kale cultivated in spring, which is consistent with the finding that longer light duration increases carotenoids content in broccoli [42]. Fall-cultivated leafy Chinese kale had higher levels of vitamin C and antioxidant capacity. This enhancement in antioxidants and antioxidant capacity may be related to the temperature changes associated with seasonal adaptations [17]. In summary, to maximize glucosinolate content in leafy Chinese kale, it is advisable to cultivate in spring or fall due to higher levels observed in these warmer seasons compared to winter. If the focus is on other nutrients like reducing sugars, soluble proteins, and carotenoids, spring cultivation is recommended, while for higher soluble solids, vitamin C, and antioxidant capacity, fall cultivation is preferable, as these nutrients tend to be more abundant in their respective seasons. Our present study implicated the significant effect of cultivation season on the contents of nutrient substances. However, only meteorological data were collected in this pilot study. Environmental data collection in the greenhouse is necessary to clarify further mechanisms behind why cultivation season affects nutritional substance content. This needs to be paid attention in a future full-scale study.

Numerous studies have shown that the nutritional substances of Brassica vegetables vary significantly depending on both species and variety. The levels of vitamin C, total phenols, and glucosinolates in vegetables such as broccoli, cauliflower, and Chinese kale have been found to differ considerably [28,29,30]. According to Verkerk et al. [43], the composition and content of glucosinolates vary greatly among different crop varieties. Vallejo et al. [32] found significant differences in the vitamin C and glucosinolate content among different varieties of Brassica oleracea. Similarly, significant variations in vitamin C, protein, and mineral content have been found across different varieties of Chinese kale shoots. Consistent with previous studies, the results of the present experiment also revealed significant differences in the nutritional substances of various leafy Chinese kale varieties. JLYC-04 exhibited high levels of vitamin C, total phenols, antioxidant capacity, total glucosinolates, total aliphatic glucosinolates, and 3-butenyl glucosinolates, while JLYC-05 was rich in soluble solids, chlorophyll, and carotenoids. In contrast, JLYC-01 showed significantly lower levels of chlorophyll, carotenoids, vitamin C, total phenols, antioxidant capacity, glucobrassicin, and total indole glucosinolates compared to other varieties. Based on this pilot study, JLYC-04 and JLYC-05 exhibited superior performance in terms of these detected nutrients, making them more suitable candidates for breeding aimed at developing nutrient-functional vegetables.