Effects of superabsorbent polymer on the physiological characteristics of wheat under drought stress and rehydration

To explore the mechanism of superabsorbent polymer (SAP) function and its effect on the photosynthetic characteristics of winter wheat before and after rehydration during drought stress, the soil moisture and photosynthetic characteristics of winter wheat at jointing stage was analyzed in potted plants under the application of 0 (control group, CK), 27, 54, and 81 mg/kg of SAP as well as under normal irrigation (CK2, 70 to 85% of water holding capacity without SAP) respectively. The results show that under drought stress, SAP reduces the net CO(2) assimilation rate (net photosynthetic rate, Pn), transpiration rate (Tr), and stomatal conductance (Gs) of wheat leaves but increases the water use efficiency (WUE); three days after rehydration, wheat leaf Pn among different treatments from high to low were as follows: CK > 54 mg/kg > CK2 > 81 mg/kg > 27 mg/kg; Tr and Gs were ranked as follows: CK2 > 54 mg/kg > CK > 81 mg/kg > 27 mg/kg; but the rank was reversed for WUE. Four days after rewatering, the results were different corresponding to the treatments. Five days after rehydration, the level of Pn among different groups was ranked as follows: 27 mg/kg > 81 mg/kg > CK2 > CK > 54 mg/kg; Tr and Gs were ranked as follows: 27 mg/kg > CK2 > 81 mg/kg > CK > 54 mg/kg; and the rank of WUE was as follows: 81 mg/kg > 27 mg/kg > 54 mg/kg > CK> CK2. Meanwhile, there were certain correlations between leaf stomatal exchange parameters, but the extent of the responses to different amount of SAP may vary among different parameters. The wheat Gs is most closely correlated with the Pn and Tr in 54 mg/kg group, while the highest level of correlation of Pn with Tr and WUE was in 81 mg/kg SAP group. In conclusion, the treatments with 54 and 81 mg/kg of SAP exhibited significant effect on the correlations among photosynthetic parameters of wheat leaves.