Using rootstock to increase watermelon fruit yield and quality at low potassium supply: A comprehensive analysis from agronomic, physiological and transcriptional perspective

Grafting is a widely used technique in watermelon production. How grafting affects watermelon fruit yield and quality at low potassium supply and the mechanism remains unclear. This study addresses the question from agronomic, physiological and transcriptional perspective. Watermelon plants [Citrullus lanatus (Thunb.) Matsum. and Nakai, cv. Zaojia 8424], either self-grafted or grafted onto the rootstock 'Yongshi' (Citrullus lanatus sp.), 'Jingxinzhen No.1' (Lagenaria siceraria Standl.), and 'Qingyanzhen No.1' (Cucurbita maxima × C. moschata). Plants were subjected to 6.0 mM K (normal K) and 0.1 mM K (low K). Compared with plants treated with 6.0 mM K, those supplied with 0.1 mM K produced less fruit yield as indicated by the decrease in single fruit weight of all plants; however, a smaller decrease was observed in plants grafted onto 'Yongshi' (10%), 'Jingxinzhen No.1' (15%) and 'Qingyanzhen No.1' (19%) than the self- grafted watermelon (38%). The K+ concentration of stem, leaf, fruit peel and flesh were obviously higher in the rootstock- grafted plants than in the self- grafted ones under 0.1 mM K. In addition, rootstock- grafted plant had significantly higher leaf zeatin riboside and chlorophyll content. Fruit quality, including contents of total soluble solid, sucrose, vitamin C, lycopene, β-carotene, were significantly decreased in the self- grafted plants under 0.1 mM K. However, 0.1 mM K treatment did not result in an obvious decrease in most of the measured fruit quality parameters in the rootstock- grafted plants. Fruit transcriptome analysis showed that there were 670, 27, 16 and 15 differential expressed genes (DGEs) responded to 0.1 mM K in the self- grafted, 'Yongshi', 'Jingxinzhen No.1' and 'Qingyanzhen No.1'- grafted plants, respectively, indicating that rootstock grafting decrease the sensitivity of watermelon fruit to 0.1 mM K at the transcriptome level, GO and KEGG analysis showed that most of the DGEs were enriched in cellular metabolic process and metabolic pathways. Low K (0.1 mM K) significantly increased the gene expression of potassium channel (Cla020934), but decreased the gene expression of phytoene synthase (Cla009122- responsible for lycopene synthesis) in the fruit flesh of self-grafted watermelon. Taken together, the above results suggested that compared with self- grafted plants, watermelon grafted onto rootstock can enhance plant fruit yield, quality and decrease the sensitivity of fruit flesh transcriptome to low K. The mechanism of improved performance under low potassium is discussed.