Effects of severe drought stress on the physiology and biochemistry characteristics of Welsh onion (Allium fistulosum L. var. giganteum Makino) seedlings

The physiological regulation of Welsh onion seedlings was explored under short- and long-term drought stress. The Welsh onion cultivar 'Neicong 3' was grown in pots under drought stress (22.5-25% soil water content) or controlled watering conditions (65-67.5% soil water content). Different lengths of drought stress (7, 14, 21, and 28 days) were studied to determine the effects of the drought on the levels of the osmoregulatory substances, including proline (Pro), free amino acids (FAAs), soluble sugar (SS), and soluble protein (SP); the membrane-damage indicator malondialdehyde (MDA); the endogenous hormones indoleacetic acid (IAA), gibberellic acid (GA), abscisic acid (ABA), and jasmonic acid (JA); and the activities of the antioxidant enzymes superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD). The soil water content was restored to the control level after the drought stress to analyse the physiological responses of the Welsh onion with regards to rehydration. The results showed that the Welsh onion responded to drought stress primarily by increasing the levels of the osmoregulatory substances and the activities of the protective enzymes. The drought stress resulted in the accumulation of Pro, FAA, SS, MDA, and ABA in the pseudostems, while the IAA and GA levels decreased. Short-term drought stress led to a decrease in the SOD activity and an increase in the CAT activity, whereas the long-term drought stress led to a decrease in the POD activity. The effects of the drought stress on the SP and JA contents were negligible. The rehydration demonstrated that the short-term drought stress (7 or 14 days) did not exceed the tolerance of the Welsh onion; the substances regulating the drought tolerance were restored to normal levels after the stress conditions were relieved. In contrast, after the severe drought stress (28 days), the substances regulating the drought tolerance in the seedlings could not return to normal levels. This study provides a theoretical basis for the cultivation of the Welsh onion under early drought conditions and for breeding of drought-resistant varieties.