Optimal Treatment Strategies of Drought and UV-B Stresses before Harvest for Promoting Antioxidants Accumulation in Kale Grown in Plant Factories

Abstract

Kale is one of the healthiest Brassica vegetables and its cultivation is increasing for either fresh consumption or as a source for functional foods and nutraceuticals. Among abiotic stresses, drought and UV-B radiation are effective in triggering extra secondary metabolites and can be easily applied to plant factories. The objective of this study was to determine the optimal treatment times of drought and UV-B stresses before harvest to achieve its maximum amount of antioxidants. Kales (Brassica oleracea L. cv Manchoo Collad) were grown at a temperature of 20°C and a photosynthetic photon flux density of 350 μmol∙m-2∙s-1 (LEDs with red:blue:white = 8:1:1 and 16-8 h light/dark photoperiod) in a plant factory and harvested at 42 days after transplanting (DAT). Chlorophyll fluorescence (Fv / Fm), leaf water potential, dry weight, total flavonoid content, total phenolic content, and antioxidant capacity of the samples were determined. In the first experiment, for determining the optimal treatment time of drought stress to achieve its maximum amount of antioxidants, drought stress treatments lasting for 7, 6, 5, 4, 3, 2, and 1 days before harvest (T7, T6, T5, T4, T3, T2, and T1, respectively) were applied to kales. At T7, the FV / Fm gradually decreased from 0.811 on DAT 36 to 0.563 on DAT 39, followed by a drastic decline to 0.286 on DAT 40. Leaf water potential decreased from -3.863 MPa at T2 to -6.27 at T3. However, it increased slightly to -6.04 MPa at T4 and then decreased again with longer drought stress. Both results indicated that the drought stress less than 4 days could ensure the function of leaf chlorophyll fluorescence and maintain normal leaf water potentials. As to the antioxidants such as total flavonoid content and total phenolic content, T2, T3, and T4 stresses significantly induced higher amount of antioxidants compared to the control and other treatments. In the second experiment, a combined effect of drought and UV-B stresses on the accumulation of antioxidants was investigated at T4, T3, and T2 stresses from DAT 38 to 40. Compared to the control and single drought stress, leaf chlorophyll fluorescence and dry weight significantly decreased at combined stresses. The amounts of total antioxidants were higher at T3 and T4 drought stresses than any other treatments including the control, while the combined stresses showed no extra formation of antioxidants compared to those obtained at the single drought stress. Considering energy inputs, T3 drought stress could achieve the highest potential value of kale as a source of natural antioxidants.