The Effects Of Simulated Acid Rain On The Photosynthetic Characteristics Of Cupressus Lusitanica Mill

Mexican cypress （Cupressus lusitanica Mill.） has excellent characteristics such as cansurvive in dry and barren soil conditions or in cold region with adequate adaptability andresistance, and rare injured by pests or diseases. So it can be selected as pioneer species whenafforested in the limestone soil conditions or in rocky desertification controlling for its strongadaptability. There are lots of studies have shown the impacts of acid rain on plantphysiological characteristics, and the results are very different due to the different species andenvironmental conditions, but the effects of acid rain on the physiological characteristics ofMexican cypress has not been reported. In this study, the effects of simulated acid rain on thephotosynthetic physiological characteristics of Mexican cypress seedlings were investigated, inorder to provide a theoretical basis for the introduction of exotic plant of Mexican cypress in theacid rain stress area in southern China. The main findings are as follows:（1） Three different simulated acid rain treatments with pH3.0, pH4.0and pH5.0decreasedthe photosynthetic rates of Mexican cypress seedlings, the lower pH value of simulated acid rain,and the stronger restraint of photosynthetic rates of Mexican cypress seedlings. The netphotosynthetic rate（Pn） of Mexican cypress seedlings significantly linear correlated to airtemperature （AT）, stomatal conductance （Gs）, intercellular CO₂concentration （Ci）, transpirationrate（Tr）, leaf saturation vapor pressure deficit（Vpdl）, sun radiation intensity（PAR） and water useefficiency（WUE）（P <0.05）, showing that the diurnal Pn of Mexican cypress seedlings areclosely regulated by the changes of Gs, Ci, Tr, Vpdl, PAR, WUE and other indicators in thedaytime.（2） Three different simulated acid rain treatments all led to the elevation of the lightcompensation point （LCP） of Mexican cypress seedlings, and increased the dark respiration rates,indicating that the acid rain stress on Mexican cypress seedlings increase the consumption ofphotosynthetic products and thus reduce its ability to utilize low intensity light.（3） Three different simulated acid rain treatments did not significantly affect the lightsaturation point （LSP） and apparent quantum efficiency （AQE） of Mexican cypress seedlings, butits maximum net photosynthetic rate was significantly decreased, show that acid rain stress canreduce the ability of Mexican cypress seedlings to use high intensity light.（4） Three different simulated acid rain treatments all led to the decrease of the potential CO₂assimilation rates of WC, WJ, WPof Mexican cypress seedlings with a vary degree, which arelimited by Rubisco, RuBP, TPU respectively. And the Vcmax, Jmax and TPU of the threesimulated acid rain treatments were significantly lower than the control treatment of the Mexicancypress seedlings, showing that simulated acid rain has a restraint on the photosynthesis-CO₂response of Mexican cypress seedlings. （5） Three different simulated acid rain treatments all led to the significant decrease ofapparent carboxylation efficiency （CE） of the Mexican cypress seedlings, indicating that acid rainstress can significantly reduce the ability of Mexican cypress seedlings to use of CO₂, and there isa stronger inhibition under a lower pH value treatment.（6） Three different simulated acid rain treatments did not significantly affect the CO₂compensation point and photorespiration rate of the Mexican cypress seedlings, indicating thedefinite adaptation of Mexican cypress seedlings to the acid rain stress.（7） The transpiration rates of Mexican cypress seedlings has a consistent trends as: pH5.0>CK> pH4.0> pH3.0, which showed that mildly acid rain stress can increase the transpirationrates of Mexican cypress seedlings, while the moderate and severe acid rain stress can suppressthe transpiration rates of Mexican cypress seedlings.（8） There are significant seasonal variations of Fv/Fm values of Mexican cypress seedlings.Three different simulated acid rain treatments all decreased the maximum photochemicalefficiency （Fv/Fm） of Mexican cypress seedlings when compared with the control treatment,showing that the definite inhibition of acid rain stress on the maximum quantum yield in PS II ofMexican cypress seedlings.（9） Three different simulated acid rain treatments all increased the F_v’/F_m’ values ofMexican cypress seedlings, showing that acid rain stress can promote the capture efficiency oflight excitation energy in the open PSII reaction centers.At the same time, changes of ΦPSII, ETR, qP, NPQ had no consistent regularity under thethree different simulated acid rain treatments, indicating that Mexican cypress seedlings mayhave some physiological adaptability and ability to protect themselves under the acid rain stress.In summary, simulated acid rain produced very complex effects on the photosyntheticphysiological characteristics of the Mexican cypress seedlings, it totally inhibited thephotosynthesis of Mexican cypress seedlings, thus might decreased the potential productivity ofMexican cypress seedlings when afforested in an acid rain stress area, but there also were thephysiological adaptability of Mexican cypress seedlings to the acid rain stress. Our research canprovide theoretical basis for the introduction of Mexican cypress seedlings and afforested in theacid rain region in southern China.