Different Response And Mechanism Of Different Citrus Rootstock Under Boron Stress

Boron(B)is an essential trace element for higher plants,the sensitivity and the demand to B of different crops or different varieties of one crop are different.And the phenomenon of B deficiency is common in the world,in China,the largest area of navel orange is located in the red soil region of China where the soil available boron deficiency or serious lack,which has seriously restricting the quality and yield of citrus.The threshold between deficiency and toxicity is narrow,B deficiency and B toxicity are more likely to occur in plants.In many areas of the world,there has been a decline in crop yield and quality caused by B toxicity.Rootstock plays an important role in the growth of Citrus,therefore,it is very important to select the superior rootstock to boron stress.Citrange [Citrus sinensis(L.)Osb.× Poncirus trifoliate(L.)Raf.,B-insensitivity] and trifoliate orange [Poncirus trifoliate(L.)Raf.,B-sensitivity] are considered a kind of important rootstock resource.In this study,the two rootstocks was selected as materials,experiments were conducted with different B levels.We compared the capacity of B utilization and photosynthetic pigment between different rootstocks,analyzed the differences on the microstructure in leaves and roots from the two rootstocks by the method of TEM under B stress,investigated the differences in the composition and structure of cell wall materials under B deficiency and B toxicity by Fourier-transform infrared spectroscopy(FTIR)and Solid-state 13 C Nuclear Magnetic Resonance(13C-NMR).The main results were concluded as following:(1)Citrange contained lower B concentration and B accumulation than trifoliate orange and the B effective utilization rate was higher than trifoliate orange under the same treatment,especially in the treatment of B0,the citrange used 38.6% more B compared with trifoliate orange.Under the different B treatments,R value(R=semi-bound B/free B)was much lower in trifoliate orange than citrange samples,which indicated B utilization of citrange at the cellular level was significantly higher than trifoliate orange.B deficiency reduced the photosynthetic pigment content(Chl a,Chl b,Car)in two rootstocks,there were much more significant effects of B deficiency on trifoliate orange.The net photosynthetic rate was significantly higher in citrange than trifoliate orange under the condition of B-deficiency(B0,B2,B5)and rich B(B50).Under B-deficiency treatment,moreover,a higher level of soluble sugars,sucrose,and fructose was observed in trifoliate orange than that in citrange.More carbohydrate was accumulated in trifoliate orange,and its feedback inhibition was more serious than that of citrange.(2)The growth of citrus rootstocks was inhibited by the low B and high B stress situations.Symptoms of B-toxicity were first appeared in old leaves as tip yellowing in trifoliate orange,but almost no visible symptoms occurred in citrange.B deficiency and B toxicity decreased the activity of CAT in leaves of two rootstocks,and increased the activity of POD and SOD.B deficiency significantly decreased the content of soluble protein of trifoliate orange,but not in citrange plants.High B significantly reduced the soluble protein content,which trifoliate orange decreased by 31.83%,higher than the 23.15% from citrange.The structure of cellulose glycosides,soluble sugar and ribosome in leaves changed and other components increased under low B stress in trifoliate orange,only soluble sugar changed in citrange.The structure of protein and soluble sugar in leaves changed and other compositions reduced under high B stress in trifoliate orange,and ribosome changed in citrange.Therefore,the effect of low B and high B stress on trifoliate orange is higher than that of citrange in material structure.Results show that citrange was more tolerant to low B and high B stress than trifoliate orange in terms of material composition and structure.(3)The content of cell wall materials(CWM)on a leaf fresh weight basis was generally higher in trifoliate orange than citrange under the same B treatment;B-deficiency significantly increased the ratio of B concentration in cell wall to the total B of leaves in both two rootstocks,204.94% from citrange are bigger than 178.57% from trifoliate orange.These results indicated that citrange have greater B accumulation in cell walls than trifoliate orange.TEM micrographs of the leaves showed thickened cell wall in cells,fuzzy cell morphology and disordered cell arrangement under B-deficiency and B-toxic treatments compared with the control.B deficiency led to an accumulation of starch grains in leaf cells in two rootstocks.The internal structure of the cells was damaged by high B stress,which was the most serious damage to chloroplast,and trifoliate orange affected more severely.The TEM of the roots showed that thickened cell wall in cells,and cell contents and organelles disappeared of two rootstocks under B deficiency,trifoliate orange inhibited significantly.Under high boron stress,many organelles were disappeared,the cell wall thickness was not affected,an accumulation of starch grains in trifoliate orange but not in citrange.(4)FTIR showed that B-deficient influenced the hydrogen bond among protein and carbohydrates(cellulose,hemicellulose),affected the composition of protein and polysaccharide,decreased the content of protein and increased the accumulation of carbohydrate in cell wall of trifoliate orange leaves;B-toxic changed the structural of carbohydrates such as cellulose of trifoliate orange and decreased the composition,not serious influence in citrange under both B stress.In addition,13C-NMR spectra shown that B-deficient decreased the content of protein and increased the content of carbohydrate,cellulose and phenols,especially in the cell wall of trifoliate orange.The variation in those integrations suggested B stress especially increased the accumulation of phenols of trifoliate orange,whereas non-significant changes in citrange.Furthermore,it illustrated the internal cause of citrange was more tolerant to both B deficiency and toxicity than trifoliate orange on the aspects of chemical structure of the cell wall.(5)Under different treatment conditions,the B content was characterized by low center and high tip in the leaves of trifoliate orange.B-deficiency and B-toxic significantly influenced the content of B,56.99% and 642.36% from leaf tip are bigger than 47.91% and 532.54% from leaf center.B deficiency caused the arrangement of the whole cells in the leaves to be disordered,destroyed the integrity of cell structure,which the accumulation of starch and phenolic substances in the leaf center was affected by boron deficiency.High B stress damaged the internal structure of the cells,which was the most serious damage to chloroplast,so the leaf tip are unable to carry out photosynthesis.The starch grains in the leaf center were blocked by B-toxic,which seriously affected photosynthesis.All those results demonstrated that the response under different B stress to different parts of trifoliate orange leaves is different,the middle part of the leaves was affected by boron deficiency,and the tip was affected by high boron stress.