Effects of humidity-cold combined stress at the seedling stage in direct-seeded indica rice

•Humidity-cold combined stress affected seedling growth via hampering photosynthesis and tricarboxylic acid cycle.•High humidity not low humidity destroyed photosynthetic and respiratory system of direct-seeded rice under cold stress.•Whether High or low humidity differed physiological traits induced by cold condition of cold-sensitive rice cultivar.•High humidity-cold combined stress increased antioxidant protective enzymes activity and osmotic substances.•Low humidity tended to decrease the physiological damage caused by low temperature after recovery treatment. Low temperature (LT) and extreme humidity constantly threaten direct-seeded early indica rice production during the seedling stage. However, the physiological mechanism and damage induced by LT combined with high/low humidity are still unclear. A pot experiment was conducted to study seedling growth performance and physiological traits of two indica rice cultivars (i.e., the cold-sensitive cultivar Zhongjiazao17 and the cold-tolerant cultivar Xiangzaoxian6) under LT plus low humidity treatment (LL, 8°C; relative humidity, 25 %), LT plus high humidity treatment (LH, 8°C; relative humidity, 95 %) and a control treatment (CK, 25 °C; relative humidity, 75 %). The results showed that seedling height, rubisco concentration, and APX, PDH and α-KGDH activity significantly decreased under LL and LH compared to CK, and large decreases were found under LH, especially for the cold-sensitive cultivar Zhongjiazao17. However, LH significantly increased the reactive oxygen species O2− and the soluble protein, GSH and ABA concentrations. Moreover, the APX, PDH and α-KGDH activities, rubisco and chlorophyll concentrations, and GA3 induced by LH also significantly decreased after treatment. The differences in physiological traits of Zhongjiazao17 after LL and LH were significant. During three days of recovery, the physiological activity of the two cultivars under LL tended to recover to the level of CK, and the cold-tolerant cultivar showed reduced damage caused by humidity-cold combined stress. The results suggested that the double stress of low temperature plus high humidity rather than low temperature plus low humidity could decrease the growth performance by hampering photosynthetic and respiratory systems and altering the endogenous hormonal regulation of direct-seeded early indica rice, whereas it could significantly enhance the antioxidant protective system derived from the osmotic damage of leaves.