Physiological and biochemical characteristics of cucumber seedlings under different levels of drought stress (PEG 6000 concentrations)

The effects of different drought stress levels on the plant growth, physiological and biochemical characteristics of cucumber (Cucumis sativus L.) seedlings were investigated using different mass fractions of polyethylene glycol (PEG) 6000 (0, 2.5, 5, 10%) to simulate the drought stress. The results showed that the plant height, root length, leaf dry weight (DW), stem DW, root DW, and plant DW all showed no significant difference under mild (2.5% PEG 6000) drought stress, while they exhibited a decreasing trend with an increasing level of PEG. The changing trend in the photosynthetic efficiency, chlorophyll a, and carotenoid content was consistent with the growth index under drought stress. Besides, the content of chlorophyll b in the cucumber seedlings increased under moderate (5% PEG 6000) drought stress, but decreased under severe (10% PEG 6000) drought stress. However, the antioxidase activities, soluble protein content, malondialdehyde (MDA) content, total phenols, and total flavonoid content in the cucumber seedlings increased gradually with the increase in the mass fraction of PEG. The results indicated that higher the level of drought stress, the higher inhibition on the seedling growth or more severe damage on the cell membrane, and higher contents of total phenols and flavonoids were obtained.