Effects Of Rare Earth Elements On Thegrowth And Development Of Citrus

China is the country with the largest amount of rare earth elements(REEs)reserves in the world,which has the characteristics of large reserves,large varieties and wide distribution.REEs deposits are found in 18 provinces and autonomous regions across the country.Jiangxi,Guangxi,Hubei,Guizhou and Yunnan,the main citrus producing areas,have highest REEs content.In addition,since the 1970 s,China started the experiment of REEs in agriculture.The annual area of REEs used in farmland has reached 4.6 million hm2,including the fields of forestry,animal husbandry and fishery.After exogenous REEs enter the soil,more than 99.5% of the REEs are adsorbed by the solid surface of the soil,a very small amount of which are dispersed in the soil solution in an ionic or soluble organic or inorganic complex state.Long-term application of rare earth fertilizers will inevitably lead to the accumulation of rare earth in the soil.Since REEs deposits are geographically coincident with the citrus producing areas and REEs fertilizers have been universally used,the content of REEs in citrus orchard soil gradually increases.Therefore,it is important in economy to study the response of citrus seedlings to REEs and evaluate the tolerance of citrus rootstocks to REEs.In order to study the distribution of REEs in soil and tree body and the relationship between fruit quality and REEs content,soil samples and plant samples(including roots,leaves and fruits)were collected from 29 orchards in Xunwu,Xinfeng,Longnan of Jiangxi Pronvince.And,three citrus rootstocks [Ziyangxiangcheng(C.junos Sieb.ex Tanaka),Trifoliate orange(Poncirus trifoliate(L.)Raf)and Shatianyou(Citrus grandis var.shatinyu Hort)] were treated under different concentrations of lanthanum(La).The concentrations of La were set as 0 mmol/L(CK),0.125 mmol/L(0.125La),0.25mmol/L(0.25La),0.5 mmol/L(0.5La),1 mmol/L(1La),1.5 mmol/L(1.5La),2 mmol/L(2La)and 4 mmol/L(4La).The toxic index,growth index,photosynthetic pigment content,physiological and biochemical indexes under different treatments were measured respectively.The principal component analysis was performed to evaluate the tolerance of the three citrus rootstocks to La.In addition,the contents of La in the shoots and roots and the contents of Ca and CAM in leaves were measured.The main results are as follows.1.The distribution of rare earth in the soil and tree body in REEs orchards,and the correlation of REEs and fruit quality1.1 REEs were detected in all the soil samples from Xunwu,Xinfeng,Longnan 30 orchards.And the average content of total rare earth in soil of Xinfeng orchads was highest,followed by Xunwn and Longnan.The distribution of REEs in tree body was as follow: soil > root > leaf > peel >pulp.Average content of total rare earth in soil,root,leaf,peel and pulp of sampling orchard in Xinfeng was higher than Longnan and Xunwu.In the three regions,content of light REEs was higher than heavy REEs.The general distribution of REEs in soil and root was cerium(Ce)>lanthanum(La)> neodymium(Nd)> yttrium(Y)> Gadolinium(Gd)> other,but distribution of REEs in leaf,peel and pulp was La> Ce> Nd> Y> Gd> others.And La was the highest in pulps.1.2 By analyzing the correlation between the content of REEs in fruits and the fruits quality,results showed that the La content in soils and leaves was positively correlated with the fruit weight(p<0.05),and was significantly negatively correlated with the Vc content(p<0.05).Compared with the contents of REEs in soils and pulps,there was a stronger correlation between the fruit quality and the contents of REEs in leaves.2.Effects of La on citrus rootstocks2.1 Under high concentration of La treatments(? 1.0 mmol/L),citrus rootstocks showed obviously poisonous symptoms,including leaf chlorosis,water loss,wilting,and even rupture of the veins,finally branches dry until the whole plant died.At2 La and 4La treatments,the toxic symptoms of Trifoliate orange appeared first,followed by Shatianyou and then Ziyangxiangcheng.The injury index of the three citrus rootstocks was Trifoliate orange> Shatianyou> Ziyangxiangcheng.Therefore,trifoliate orange might be the most sensitive to La,followed by Shatianyou,Ziyangxiangcheng.2.2 Under the low concentration of La treatments(? 0.5 mmol/L),the plant height increment rate,dry biomass,root shoot ratio and water content of the three citrus rootstocks had no significant difference with CK.With the increase of La concentration,the fresh weight,dry weight and root shoot ratio of the three kinds of rootstock increased first and then decreased.Water content of Ziyangxiangcheng and trifoliate orange also increased first and then decreased,but that of Shatianyou decreased with the increase of La concentration.2.3 For the three kinds of citrus rootstocks,the contents of chlorophyll a,chlorophyll b,chlorophyll a+b and carotenoid under low concentration(? 2 mmol/L)of La treatment were not significantly different from those in CK group.Under 4La treatment,photosynthetic pigment contents were significantly reduced.2.4 Treatments of La induced antioxidant enzyme activity,malondialdehyde(MDA)content and soluble protein content changed irregularly.Superoxide dismutase(SOD)activities of Ziyangxiangcheng under La treatments was no significant different with CK group,while the SOD activities of trifoliate orange and Shatianyou were lower than those of the CK group.Changes of peroxidase(POD)activities of the three rootstocks showed no obvious regularity.POD activity of Ziyangxiangcheng was the highest in CK group.For Trifoliate orange,the highest POD activity was obtained under 0.125 La treatment,while POD activity of Shatianyou was highest under 1La treatment.CAT activities of the three rootstocks increased first and then decreased with the increase of La concentration.The MDA content of the three rootstocks at low concentration(< 2mmol/L)did not differ significantly from CK group,but that under 4La treatment was significantly higher than that of CK group.Soluble protein contents of three kinds of rootstocks increased first and then decreased with the increase of La concentration.2.5 To evaluate the tolerance of the three citrus rootstocks to La,principal component analysis and subordinate function were performed based on the data of La tolerance coefficients and other leaf indices.The results showed that the La tolerance of the three citrus rootstocks was Ziyangxiangcheng > Shatianyou > Trifoliate orange.3.Distribution of La in citrus rootstocks and effects of La on calcium(Ca)content and calmodulin(CAM)content in leaves3.1 With the increase of the La concentration,the La content of shoot and root of thethree citrus rootstocks went up,and the La content of root was significantly higher than that of shoot for all three rootstocks.Under the low concentration of La treatments(? 0.5 mmol/L),the La content of root was Trifoliate orange >Shatianyou > Ziyangxiangcheng,and for the shoot,La content was Shatianyou >Ziyangxiangcheng > Trifoliate orange.Under the high concentration of La treatments(?1.0 mmol/L),the content of La in root of Ziyangxiangcheng climbed rapidly with the increase of concentration,but that of Shatianyou rose slowly.Finally,La content in root of 4La treatment was Ziyangxiangcheng > Trifoliate orange > Shatianyou,and for the shoot,La content was also Ziyangxiangcheng >Trifoliate orange> Shatianyou.3.2 Under different concentrations of La treatment,Ca content of the three kinds of citrus rootstock leaves decreased with the increase of La concentration.There was a significant logarithmic negative correlation(p <0.01)between Ca content and La content in leaves.3.3 CAM content of Shatianyou was the lowest during the whole experiment,and decreased with the prolongation of treatment time.CAM content of Ziyangxiangcheng increased first and then decreased with treatment time in CK group,while in La treatments,CAM content of Ziyangxiangcheng increased with the prolongation of treatment time.For Trifoliate orange,CAM content in CK group increased with the prolongation of treatment time,while CAM content went up first and then fell down in La treatment groups.4.Conclusion4.1 The content of REEs in the soil and tree body of the REEs mining area and the surrounding orchards was in the order of soil> root> leaf> peel> pulp,with the highest content of La in pulp.The general distribution of REEs in soil and root was Ce> La> Nd> Y> Gd> other,but distribution of REEs in leaf,peel and pulp was La> Ce> Nd> Y> Gd> others.4.2 The La tolerance of the three citrus rootstocks was Ziyangxiangcheng >Shatianyou > Trifoliate orange.4.3 Ca content in leaves and calcium signaling system were affected by La treatments.