Optimal Duration of Drought Stress Near Harvest for Promoting Bioactive Compounds and Antioxidant Capacity in Kale with or without UV-B Radiation in Plant Factories

Abstract

Among abiotic stresses, both drought and UV-B radiation effectively trigger the accumulation of secondary metabolites, and can be widely applied in plant factories. The objectives of this study were to investigate antioxidant accumulation under drought stress alone, or in combination with UV-B radiation near harvest, and to determine an optimal treatment time for maximum antioxidant production. Kale (Brassica oleracea L. var. acephala) plants were grown in a plant factory and harvested at 42 days after transplanting. The single and combination treatments lasted for 7 to 1 days and 4 to 2 days before harvest, respectively. The results of both F_v/F_m (maximal photochemical efficiency in photosystem II) and leaf water potential could ensure the function of photosynthesis and maintain normal leaf moisture in single drought treatments of less than 4 days. The total phenolic and flavonoid contents and antioxidant activities were significantly increased in both single and combination treatments for 3 to 4 days, compared to other treatments. The supplementary UV-B treatments showed no extra formation of antioxidants compared to the single drought treatments. As a result, drought for 3 days before harvest could achieve the highest potential value of kale as a source of natural antioxidants.

Keywords: UV radiation; antioxidant capacity; drought stress; flavonoid; kale; phenolic compound.

Figure 1

(a) Experimental design; (b) light spectra of the light emitting diode (LED) and UV-B tube. In experiments 1 and 2, the plants were harvested 42 days after transplanting (DAT). Drought treatments were imposed by releasing all of the nutrient solutions from the root system (blue bar). Combination treatment groups were subjected to drought with UV-B exposure (orange bar). The arrows indicate the type and date of each measurement.

Figure 2

Leaf water potential (Ψ_l) of kale under the control and drought stress treatments started 7, 6, 5, 4, 3 or 1 days before harvest (T7, T6, T5, T4, T3 and T1). The error bars represent one standard deviation (SE); n = 3. The different letters indicate significant differences according to the Tukey test (p < 0.05).

Figure 3

The maximal photochemistry efficiency of PSII (F_v/F_m) of kale under the control and drought stress treatments started 7, 6, 5, 4, 3 or 1 days before harvest (T7, T6, T5, T4, T3 and T1, respectively). The error bars represent 1 SE; n = 3. The different letters indicate significant difference at harvest according to the Tukey test (p < 0.05).

Figure 4

Total phenolic content (TPC), total flavonoid content (TFC), and total antioxidant capacity (TAC) per dry weight in kale leaves at 42 days after transplanting (DAT) under the control and single drought stress treatments started 4, 3, 2 or 1 days before harvest (T4, T3, T2 and T1, respectively). The error bars represent 1 SE; n = 3. The different letters indicate significant difference according to the Tukey test (p < 0.05).

Figure 5

The maximal photochemistry efficiency of PSII (F_v/F_m) of kale under control and drought stress alone and in combination with UV-B radiation started at 4, 3 and 2 days before harvest (D4, D3, D2, D+UV4, D+UV3 and D+UV2, respectively). The error bars represent 1 SE; n = 3. The different letters indicate significant difference at harvest according to the Tukey test (p < 0.05).

Figure 6

Total phenolic content (TPC), total flavonoid content (TFC), and total antioxidant capacity (TAC) per dry weight in kale leaves at 42 DAT under control and drought stress alone and in combination with UV-B radiation started at 4, 3 and 2 days before harvest (D4, D3, D2, D+UV4, D+UV3 and D+UV2, respectively). The error bars represent 1 SE; n = 3. The different letters indicate significant difference according to the Tukey test (P < 0.05).