Dynamic Changes in Organ Morphology and K+/Na+ Content of Pumpkin Seedlings Under Salt Stress

Pumpkin is widely used as a rootstock to enhance salt tolerance and improve productivity of Cucurbit crops. To date, the morphology and ion parameters of pumpkins at a certain time point under salt stress are well-known. However, the dynamic changes in organ morphology and K+/Na+ content of pumpkin under salt stress and the relationship of them remain unclear. Therefore, this study investigated biomass, root morphology, stem structure, and K+/Na+ content in salt-sensitive (JZ-1) and salt-tolerant (JYZ-1) pumpkins under 0 mM and 120 mM NaCl conditions at 2, 5, and 10 days after treatment (DAT). Our results show that at the beginning, NaCl treatment led to a sharp decrease in shoot fresh weight by 30–53% and a slight decrease in root fresh weight, plant dry weight, and total root length and affects the K+ and Na+ content both in JZ-1 and JYZ-1 at 2 DAT. Subsequently, total root volume and number of tips have changed, in which NaCl treatment resulted in a significant increase of 127% in total root volume and a significant decrease of 38.4% in number of tips in JYZ-1 at 5 DAT, but no significant difference in JZ-1 at 5 DAT was found. At the end, root fresh weight and stem structure parameters were significantly decreased by NaCl treatment at 10 DAT both in JZ-1 and JYZ-1, and stem cross-sectional area under NaCl conditions in JZ-1 and JYZ-1 at 10 DAT (2.133 and 2.316 mm3, respectively) was significantly lower than that under control conditions (2.933 and 4.441 mm3, respectively). Additionally, shoot K+ content showed a trend of first upward and then downward in JZ-1 and a slightly decreasing trend in JYZ-1, and shoot Na+ content displayed a trend of first downward and then upward in JZ-1 and a slightly increasing trend in JYZ-1. It is suggested that shoot K+ content, shoot Na+ content, and total root volume be considered as the important parameters for pumpkin salt tolerance assessment. These findings will help us better understand the mechanisms of salt tolerance and improve the efficiency of identification of salt-tolerant pumpkin.