Effects Of Low PH On Growth And Related Physiological Characteristics In Citrus Seedlings

Approximately 30%of the world's ice-free land is acidic,and?12%of crops are planted on acidic soils.What's worse,soil acidity is rapidly increasing due to improper farm practice and environmetal problem.Citrus belong to evergreen fruit trees and are cultivated mainly on acidic or strong acidic soils affected easily by low pH(H+).So far,data available on the effects of low pH on Citrus physiology and biochemistry are very rare.In this study,seedlings of 'Xuegan'(Citrus sinensis)and'Sour pummelo'(Citrus grandis)grown in pots filled with clean river sand were supplied daily with nutrient solution at a pH of 6,5(control),4,3 and 2.5 for nine months.Thereafter,the effects of low pH on seedling growth,leaf gas exchange,ribulose-1,5-bisphosphate carboxylase/oxygenase(Rubisco),chlorophyll(Ch1)a fluorescene and Ch1 content;the levels of nutrient elements(N,P,K,Ca,Mg,S,Fe,Mn,Cu,B and Zn)in roots,stem and leaves;and total soluble protein content,relative water content(RWC),electron leakage,and the metabolsims of reactive oxygen species(ROS),including H2O2 and superoxide anion production,malondialdehyde(MDA)content,antioxidants[ascorbate(ASC),dehydroascorbate(DHA),ASC/(ASC + DHA)ratio,reduced glutathione(GSH),oxidized glutathione(GSSG)and GSH/(GSH +GSSG)ratio],the activities of enzymes(ASC peroxidase,guaiacol peroxidase,monodehydroascorbate reductase,dehydroascorbate reductase,glutathione reductase,superoxidase dismutase,catalase,ASC oxidase,phosphomannose isomerase,ATP sulfurylase,5'-adenosine phosphate sulfate(APS)reductase,sulfite reductase,glutamine synthase and y-glutamylcysteine synthetase]involved in ROS detoxification,and of methylglyoxal(MG)including MG content,glyoxalase I and glyoxalase II activities in roots and leaves were investigated.The objectives of this study were to reveal the causes for low pH-toxicity and-tolerance in Citrus at the physiological and biochemical levels,and to provide scientific basis for Citrus cultivation of high yield and good quality.The main findings are as follows:1.Compared with pH 5,only pH 2.5 greatly decreased root,stem,leaf and whole plant dry weight(DW);pH 3 slightly inhibited seedling growth;pH 4 had almost no influence on seedling growth;and seedling growth reached a maximum at pH 5.No seedling death was observed for the two Citrus species at each given pH through the whole experiment.These results clearly demonstrated that Citrus seedlings were insensitive to low pH.This is also supported by our data that many physiological parameters involved in gas exchange,Ch1 a fluorescence,ROS and MG metabolisms were altered only at pH 2.5.2.At pH 2.5,many rotted fibrous roots were observed,and the living roots had become abnormally dark brown.ROS and MG metabolsims(ASC metabolism)were affected by low pH more in roots than those in leaves.Impaired ASC metabolism might be responsible for the low pH-induced root death and root growth inhibition.Firstly,low pH directly impaired Citrus root growth and function.This in turn affected the uptake of water and nutrients,thus limiting shoot growth.3.The observed lower leaf CO2 assimilation at pH 2.5 might be primarily due to non-stomatal factors because pH 2.5 increased or did not significantly affect leaf intercellular CO2 concentration and leaf CO2 assimilation decreased with increasing intercellular CO2 concentration.The pH 2.5-induced decrease in leaf photosynthetic capacity was probably due to the combinations of impaired photosynthetic electron transport chain and decreased electron transport rate(ETR),increased ROS production,decreased uptake of water and nutrients such as N,P,Ca and Mg.4.Mottled bleached leaves were observed only in pH 2.5-treated C.grandis seedlings.Furthermore,the effects of pH 2.5 on leaf CO2 assimilation,water-use efficiency(WUE),Ch1,Ch1 a fluorescence(OJIP)transients;and total soluble proteins,electrolyte leakage,ROS and MG metabolisms in roots and leaves were more pronounced in C.grandis seedlings than those in C.sinensis seedlings.Obviously,C.sinensis was slightly more tolerant to low pH than C.grandis.To conclude,impaired ASC metabolism might account for the low pH-induced root death and root growth inhibition.Firstly,low pH directly impaired root growth and function.This in turn affected the uptake of water and nutrients,thus limiting shoot growth.