Effects of different stocks on the accumulation of cadmium and fruitquality in watermelon

Abstract:【Objective】Watermelon (Citrullus lanatus) is loved by people because of its juicy and sweet in⁃terior flesh, especially in summer. Watermelon has long been and widely cultivated in Hunan, with about140 000 hm² planting area per year, ranking the 4th in China. Because of the considerable economic bene⁃fits, watermelon is becoming an important horticultural crop in the province. Along with the adjustment ofcrop structure in heavy metal contaminated areas in Hunan province, trials of watermelon planting inthese areas are underway. For sustainable development of local industry, it is important to use proper root⁃stocks. Studies of the characters of cadmium (Cd) accumulation in grafted watermelon are necessary forsustainable development of watermelon industry in heavy metal polluted areas.【Methods】We used topplug for grafting,‘8424’as the scion, and eight rootstocks including 3‘Yelang’lines (T1-T3),‘Qiang⁃zhen 1’(T4),‘Xijiaqingsheng’(T5),‘Jingxinzhenwang’(T6),‘Qingpingtiemujia’(T7) and‘Huangjin⁃dadang’(T8). The grafted plants were trained into a 2-truss trellis system with each truss bearing onefruit. When fruit ripened, cadmium contents in root, stems at different positions, leaves and fruit were mea⁃sured, and data of single fruit weight, fruit shape index, and quality traits were collected.【Results】The re⁃sults showed that root Cd content in different rootstocks had significant difference. T5 and T8 roots had acadmium content 32.6% and 36.9% lower than the non-grafted plants, respectively. Cd content in T4 rootwas 0.109 mg·kg^-1, 77.7% higher than the seedling roots. The basal stem in grafted watermelon plants hada Cd content in the range of 0.028 6-0.086 6 mg·kg^-1, i.e. 27.9%-84.4% of that of the seedlings (0.102 6mg·kg^-1), and Cd content in T2, T3 and T5 was the lowest, being 0.043, 0.038 6 and 0.028 6 mg·kg^-1, respectively. However, the Cd content in the central stem of T4 rooted plants increased significantly compared to the self-rooted seedlings, while that in those on other stocks was lower than in the seedlings atvarying degrees. The Cd content in the central leaves in T1, T2 and T5 rooted plants was 54%, 55.7 and54% lower than in the seedlings, respectively. The Cd content in the upper stems was different among rootstocks. In T4, it was 0.047 mg·kg^-1. Other rootstocks reduced the Cd content in the upper stems at variousdegrees, and reduction in T5 was the largest, followed by those in T1 and T8. There were significant differ⁃ences between the self-rooted seedlings and plants on different rootstocks in leaf Cd content. In T4, thebasal leaf Cd content reached up to 0.3 mg·kg-1 and was significantly higher than in the self-rooted seed⁃lings. In T1, T2 and T5, the basal leaf Cd content was 0.06, 0.068, and 0.057 mg·kg^-1 lower compared tothat in the seedlings, respectively. In T4, the Cd content in the central leaves was 0.233 mg·kg-1, and significantly higher than that in the seedlings, while in plants on other rootstocks, Cd content in the centralleaves was reduced at different degrees compared with that in the self-rooted seedlings. T1, T2 and T3were 70%, 70% and 68% lower, respectively. The effects on Cd content in the upper leaves differed according to rootstocks. For example, T3 and T4 significantly increased but others reduced it. Among them,the decrease in T5 was the greatest. Different grafting combinations displayed influence on Cd content inthe fruit. T1, T2 and T5 reduced it by 60.5%, 61.3% and 60.5% respectively, while T4 increased it by41.6% compared with the self-rooted control. Among different treatments, the highest Cd content in fruitwas 0.006 4 mg·kg^-1, lower than the limitation of 0.05 mg·kg^-1 defined by GB 2762—2012. Rootstocks al⁃so showed influence on fruit weight. T2 and T6 had a lower fruit weight than the control, but the differencewas not significant. Fruit weight in T5 increased significantly and reached 5.79 kg. T2 and T3 greatly inluenced fruit shape index, but others showed no influence. Rootstocks affected skin thickness, and the effect was significant in T1 and T4. T5 had the highest sugar content (10.53%) in central fruit tissue. T4 hadthe highest sugar content (8.03%) in side fruit tissues , and T2 had the lowest value of 6.07%.【Conclusion】Under Cd stress, grafted watermelon on different rootstocks had different Cd absorption capacity andCa distribution among organs. The Cd content in different plant positions followed a descending order ofbase>middle> upper part. In root, Cd content is influenced by rootstock genotypes. Our results showedthat T1, T2 and T5 had the lowest fruit Cd content and T1 and T5 significantly reduced the Cd content inall plant parts and improved fruit weight with no significant effect on fruit quality. These rootstocks couldbe suitable for grafted watermelon planted in heavy metal polluted areas.