Effects of simulated acid rain on diurnal changes of mulberry (Morus alba) photosynthesis

In the present study, the response characteristics of gas exchange parameters for the diurnal variation of the mulberry leaf to simulated acid rain with different pH have been studied to reveal the mechanism and the effects of simulated acid rain on plant photosynthetic productivity. The results showed that net photosynthesis rate (Pn) present obviously photosynthetic depression at midday in treatment of CK (no acid rain), while three treatments of simulated acid rain could increase Pn at midday. During photosynthetic noon-break, stomatal conductance (Gs) and stomatal limitation value (Ls) decreased, but the intercellular CO2 concentration(Ci) increased, the results of correlation analysis showed that Pn and Ci was significantly negative correlation, which indicated that CO2 assimilation have been inhibited. The corresponding parameters under acid rain treatments showed similar change trends with CK excepted with that the Ci was lower than that of CK, and Gs was higher than CK, indicating that the simulated acid rain treatments could promote stomatal opening and CO2 assimilation of mulberry leaf during midday depression. Atmospheric temperature (T) significantly positively correlated with leaf saturation water vapor pressure deficit (Vpdl) and transpiration rate (Tr), respectively. However, there was significant negative correlation between T and atmospheric relative humidity (RH). Meanwhile,the value of RH was near the minimum value during midday depression of photosynthesis, which indicated that CO2 assimilation inhibition most come from water shortage. While Tr, Gs and Ls in treatments of simulated acid rain with pH≥4.5 were higher than that of CK, and the water use efficiency (WUE) in simulated acid rain had no significant difference with CK which indicated that simulated acid rain could promote mulberry leaf intercellular water supplying. In treatments of simulated acid rain with pH 3.5, the photosynthetic assimilation total (PT), average Pn and Gs were significantly lower than that of CK, while the corresponding parameters of pH 5.6 and pH 4.5 were higher than that of CK. These results suggested that treatments of simulated acid rain with pH≥4.5 had significant promoting effects on the photosynthesis of mulberry. In the region with mild acid rain pollution, the mulberry could be employed as plants for returning farmland to forest and landscape to absorb atmospheric SO2 and nitrogen oxides and adapt or lighten the atmosphere of acid rain pollution.