LAUSR

Abstract This study was conducted to investigate the effects of relative air humidity (RH) on morphological and physiological traits of Pakchoi (Brassica chinensis L.) under high temperature. Pakchoi was planted in climate chambers at 35/28 °C, day / night temperature, and the RH levels were 20%, 40%, 60%, 80%, and 95%. Most growth parameters peaked at 40–60% RH under high temperature. Biomass, leaf area, net photosynthetic rate (Pn), soluble sugar, and vitamin C were the highest at 60% RH, whereas total chlorophyll content, transpiration rate, and leaf air temperature were the highest at 40% RH. Superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD) activities were the highest at 80% RH. All the above properties showed the lowest value at 95% RH. Malondialdehyde (MDA) peaked at 95% RH, and severe chlorosis was observed simultaneously. Based on our fuzzy comprehensive evaluation, the fuzzy value of production and intrinsic quality was the highest at 60% RH and lowest at 95% RH. RH does affect the temperature difference between leaf and air from the change of transpiration. Excessively high or low RH caused a significant reduction in biomass production and intrinsic quality. We found that 60% RH was the optimum level for growth of Pakchoi at high temperature. With 95% RH, the Pakchoi was damaged under high temperature; Pn, SOD, CAT, and POD activities were the lowest; and MDA content was the highest. Results suggested that proper RH level could reduce leaf temperature and maintain stomatal conductance, thereby maintaining the high yield of Pakchoi via higher photosynthetic efficiency at high temperature.