Augmentation of Antioxidant Constituents by Drought Stress On Roots In Leafy Vegetables

Plants can synthesize some antioxidants including L-ascorbic acid (AsA) and polyphenol in response to environmental stresses. Antioxidants perform not only their specific functions to detoxify reactive oxygen species in plants but also in human health against some diseases associated with oxidative damage and aging. Although there have been reported that the accumulation of antioxidants by stress as a phenomenon, its practical utilization has not yet been developed. The reason for this might be in difficulties of a trade-off that the overstressed plants reduce their growth. Especially in leafy vegetables, losses of photosynthetic products are directly linked to their yield. In this study, we tried to establish a novel hydroponic treatment to increase the contents of some valuable components in leafy vegetables without the growth retardation. Seedlings of leaf lettuce (Lactuca sativa L. var. crispa) were grown hydroponically in nutrient solution. Before harvesting (28 days after seeding), adult plants were subjected to drought stress to a rhizosphere by lowering of the water level in the solution tub. The total amount of AsA increased in proportion to a distance between a water level and styrofoam bed which supports the seedlings. Stress period was optimized during 14 days’ treatment and the most increased level of the AsA was peaked at 7th day. Drought stress treatment for 7 days at a distance of 4 cm resulted in higher AsA and polyphenol content of 1.24 and 1.50 times each in comparison with control at 0 cm. Brix is also increased 1.17 times higher by the treatment. The method of stress treatment to a rhizosphere could be applied to some other leafy vegetables. AsA was accumulated with exposure to drought stress in potherb mustard (Brassica rapa L. var. nipposinica), komatsuna (Brassica rapa L. var. perviridis) and spinach (Spinacia oleracea L.). These results suggested that the developed method has a universal potential to produce the functional leafy vegetables that increase some valuable components without yield reduction. This simple and effective method is useful for practical cultivation in controlled environments, such as a plant factory.