Alternative oxidase pathway is likely involved in waterlogging tolerance of watermelon

Watermelon (Citrullus lanatus) is an important crop worldwide and its growth is highly susceptible to waterlogging. However, it remains unknown how tolerant and intolerant genotypes may respond to waterlogging. To address this issue, we examined a waterlogging tolerant cultivar ‘YL’ in the field and compared it with a sensitive cultivar ‘Zaojia8424’ under waterlogging. ‘YL’ exhibited higher activity of alternative oxidase (AOX), enhanced levels of AOX, and increased KCN-resistant respiration rate, as compared with the sensitive cultivar ‘Zaojia8424’. Accordingly, ‘YL’ had lower contents of O₂^[rad]−, H₂O₂ and malonaldehyde, compared with ‘Zaojia8424’. Furthermore, inhibition of AOX pathway by salicylhydroxamic acid exacerbated inhibition of plant growth, promoted the accumulation of O₂^[rad]− and H₂O₂, decreased the respiration rate in both cultivars, and diminished the positive effects by AOX observed in ‘YL’. We observed no significant differences between ‘YL’ and ‘Zaojia8424’ in terms of the rate of anaerobic respiration and cytochrome respiration, and expression levels of antioxidant genes. Thus we concluded that the high activity of AOX pathway contributed to the waterlogging tolerance in watermelon via maintaining of the respiratory and alleviation of oxidative damage.