Effects of NaHCO_3 Stress on the Growth and Physiological Indexes of Pumpkin Seedlings

Few studies have investigated the alkali-based salt resistance aspect of plants. Here, we used 2 pumpkin cultivars ('Cucurbita ficifolia Bouch.' and 'Chuandilong') to study the growth and physiological response mechanisms of pumpkin to different levels of NaHCO_3 stress. After second true leaf development, pumpkin seedlings were treated with varying levels of NaHCO_3 stress (0, 30, 60 and 90 mmol/L) during sand culture. The chlorophyll content, electrolyte leakage, malondialdehyde (MDA) contents, proline contents, soluble sugar contents and superoxide dismutase (SOD) and peroxidase (POD) activities of both cultivars were investigated. Finally, the growth and physiological indexes of pumpkin seedlings under NaHCO_3 stress were explored. The results showed that the chlorophyll a, chlorophyll b and total chlorophyll of both pumpkin seedlings increased with exposure to low NaHCO_3 concentrations (0-30 mmol/L) and then decreased with exposure to 30-90 mmol/LNaHCO_3 and all reached their minimum values with exposure to 90 mmol/L NaHCO_3. Under the latter stress, the rate of decrease in total chlorophyll content of 'C. ficifolia Bouch.' was higher than that of 'Chuandilong'. However, electrolyte leakage, MDA content, proline content, soluble sugar content and SOD and POD activities increased with the escalation of NaHCO_3 concentration and peaked after exposure to 90 mmol/LNaHCO_3. The rates of increase in electrolyte leakage and MDA contents of 'C. ficifolia Bouch.' were higher than those of 'Chuandilong' at this NaHCO_3 concentration. These results suggested that under greater NaHCO_3 stress, these pumpkin cultivars showed significant inhibition of growth, with 'C. ficifolia Bouch.' being more sensitive than 'Chuandilong' to NaHCO_3 stress. Injury caused by NaHCO_3 stress was most likely mitigated by increasing the antioxidant activities of SOD and POD and accumulating proline and soluble sugar contents to facilitate osmotic adjustment.