| Acid rain,one of the most severe global environmental issues,not only results in the acidification of soil and water,but also is detrimental to plant growth and cause forest decline.However,effective methods for monitoring plant responses to acid rain stress are still lacking.The hyperspectral and chlorophyll fluorescence techniques,which are non-destructive,rapid and relatively inexpensive compared with biochemical analyses,provide the effective approaches to diagnose acid rain stresses on plant.Taking Quercus glauca,a typical subtropical tree species in southern China,as the subject of the controlled experiment,this study aims to detect the influence of long-time simulated sulfuric acid rain(SAR)and nitric acid rain(NAR)stresses on Q.glauca.The controlled experiment in greenhouse includes four grades of simulated acid rain:pH 5.6(Control Group),pH 4.5,pH 3.0 and pH 2.0.In total,the experiment lasted 417 days,in which 15 periods of hyperspectral data and 14 periods of chlorophyll fluorescence data under four pH levels of SAR and NAR have been obtained.Additionally,the plant height and base diameter data are also measured.The main results are as follows:(1)The spectral reflectance of Q.glauca is significantly sensitive to acid rain stress in the range of 500-660 nm at leaf scale,a hyperspectral index(green peak area index,GPAI),which can effectively reflect the influence of acid rain on Q.glauca,is proposed based on the sensitive bands.Comparing the responses of GPAI of Q.glauca under the four pH levels of SAR and NAR,light acid rain(pH 4.5 SAR and NAR)promoted Q.glauca growth relative to the control groups(pH 5.6 SAR and NAR),and pH 4.5 NAR has more obvious promoting effect;moderate acid rain(pH 3.0 SAR)firstly promoted and then inhibited plant growth at about 300 days of acid rain treatment,while pH 3.0 NAR showed mild inhibitory effects during the experiment;heavy acid rain(pH 2.0)significantly inhibited plant growth with the increasing experimental days,and pH 2.0 SAR induced more serious damages to Q.glauca than pH 2.0 NAR.(2)The Fv/Fm and NPQ of Q.glauca significantly changed under acid rain stress,demonstrating the chlorophyll fluorescence can reflect the acid influences.The chlorophyll fluorescence parameters Fv/Fm and NPQ,which can indicate the photosynthetic capacity and light protection ability of plant,were used to analyze the acid rain stresses on Q.glauca.Compared with the control group(pH 5.6),Fv/Fm and NPQ difference of Q.glauca under light acid(pH 4.5 SAR and NAR)are positive and increase with time.This suggest light acid have an increasing promoting effect on photosynthesis of Q.glauca with time.From the values of Fv/Fm and NPQ difference at the end of experiment,the promoting effect of pH 4.5 NAR was more obvious than pH 4.5 SAR;at about 220 days of experiment,Fv/Fm and NPQ difference of Q.glauca under pH 3.0 SAR change from positive to negative value,and the influence of pH 3.0 SAR on Q.glauca from promotion to inhibition,while moderate NAR(pH 3.0)has an alternate promoting and inhibitory effect on Q.glauca with the increasing acid rain spraying time.Fv/Fm and NPQ difference suggest overall influence of pH 3.0 NAR Q.glauca is relative small;Fv/Fm and NPQ difference under heavy acid rain(pH 2.0)are negative at the initial stage of acid rain spraying and significantly decrease with time,suggesting it cause damage to PS Ⅱ in the early stage of this experiment,which resulted in the deactivation of it and then affected the photosynthesis process.With the increase of acid rain spraying time,the inhibitory effect was significantly strengthened.Compared with NAR,pH 2.0 SAR had more obvious inhibition.(3)Different acid types and intensity showed difference influence on Q.glauca.From the change of plant height and base diameter,the effect of short-term acid rain stress on Q.glauca growth was not obvious.With the increase of experiment days,the growth difference of seedlings under different pH levels acid rain become significant in the medium and late stage of this experiment.At the end of the experiment,the plant height and base diameter of Q.glauca under pH 4.5 SAR and NAR were higher than those of control group(pH 5.6);compared with control group,Q.glauca under moderate acid rain had not obvious difference,but pH 3.0 SAR treated Q.glauca had slight lower plant height and base diameter,while that of pH 3.0 NAR were slight higher;heavy acid rain(pH 2.0)made the plant height and basal diameter of seedlings lower than that of control group at the end of the experiment.Compared with Q.glauca under SAR,NAR treated Q.glauca had higher plant height and basal diameter in the late stage of this experiment,suggesting that the inhibitory effect of SAR on Q.glauca growth was more obvious than NAR.(4)GPAI difference and Fv/Fm and NPQ difference had good consistency in indicating acid rain stress on Q.glauca seedlings,after comprehensive analysis of hyperspectral and fluorescence responses of Q.glauca under four pH levels of SAR and NAR,Moreover,the hyperspectral and fluorescence responses were consistent with the height and basal diameter of Q.glauca under different pH levels acid rain stress at the end of the experiment.The results show that hyperspectral and fluorescence information can effectively and accurately indicate the influence of acid rain stress on Q.glauca.In total,the influence of different pH levels of SAR and NAR on Q.glauca could be diagnosed through hyperspectral and chlorophyll fluorescence responses.This study provides the theoretical basis and scientific reference for monitoring plant acid rain stress on a larger regional scale,and is of great significance to the restoration of vegetation and ecosystem under acid rain stress. |
