Effects of Water Stress on Carotenoid and Proline Contents in Kale (Brassica oleracea var. acephala) leaves

BACKGROUND: Environmental stress has amajor effect on the growth and yields of vegetables, and can significantly affect nutritionally important phytochemicals, causing large economic losses. METHODS AND RESULTS : The present study was aimed at exploring the effects of water stress on the carotenoid and proline contents in kale leaves to understand drought tolerance of kale plants. Kale was randomly divided into two groups at 57 days after sowing (DAS).One of the groups was well-watered (WW) and the other was water stressed (WS). Harvesting of kale leaves was started one day after treatment (58DAS) and continued for 10 days (~67DAS).We investigated the status of plant growth (leaf number, length,width, freshweight) of kale throughout the study.Carotenoid (lutein, alpha-carotene, zeaxanthin, beta-carotene) and proline contents were analyzed by high-performance liquid chromatography (HPLC). Our results showed that the total carotenoid contents ranged from 926.0 to 1,212.0 mg/kg dry wt. (at 3 and 2 days, respectively) in WW treatment and 887.8 to 1,157.4 mg/kg dry wt. (at 10 and 4 days, respectively) inWS treatment. The ratio of individual carotenoid to the total carotenoid contents of kale leaves was 51.4 for lutein, 4.44 for zeaxanthin, 2.76 for alpha-carotene, and 41.4% for beta-carotene. Total carotenoid contents showed a significant reduction from7 days (1,037.2mg/kg dry wt.) to 10 days (887.8mg/kg dry wt.) inWS treatment. The lutein content did not show a significant difference in WWbetween 7 and 10 days after treatment but showed a significant difference in WS treatment. The alpha-carotene content showed no significant difference between the treatments.However, zeaxanthin contentwas higher during 4 - 10 days and beta-carotene content was lower during 6 - 10 days inWS than inWWon each harvest day. InWW, the proline content showed no significant difference, but in WS, the proline content started to increase at 7 days and almost doubled in 10 days. CONCLUSION : The marked increase in zeaxanthin and proline contents in kale leaves indicated that the two phytochemicals are associatedwith drought tolerance in the plant.