Preliminary Study On The Mechanism Of Boron Alleviating Growth Inhibition Of Beta Vulgaris Under Salinity

Boron, as a trace element for plant growth, has several important functions to theplant. Boron significantly alleviated growth inhibition of beta vulgaris under salinity.In this thesis, we conducted a preliminary study on the mechanism of boronalleviating growth inhibition of Beta vulgaris under salinity. The main results areshowed as follows:1. The screening of salt sensitive beet varieties and the determination of the saltconcentrationThree varieties of B. vulgaris seedlings were treated with0mmol·L-1,100mmol·L-1,200mmol·L-1,300mmol·L-1NaCl for15days to screen for salt-sensitivebeet varity. The results of the salt toxity coefficient, leaf area, dry weight and freshweight showed that the most salt sensitive variety of B. vulgaris is KWS3418and300mmol·L-1is the most sensitive NaCl concentration.2. The determination of boron sensitive periodB. vulgaris seedlings were divided into two treatments respectively for normalgroup and boron deficiency from the germination. When the second true leaves of B.vulgaris seedlings were mature, we named this period as two-leaf stage; and likewise,we marked three-leaf stage and four-leaf stage. By observing the morphology ofdifferent periods and comparing the difference on fresh and dry weight, we found thatwith the increase of treating time, the influence of boron on B. vulgaris increasedgradually. Compared with normal group growth in four-leaf stage, inhibition of borondeficiency group is the most distinct. So we concluded that with the increase oftreating time, boron requirements of B. vulgaris seedlings continuously increased. Inthe following experiments, we extended the treating time to30days during which the seedlings was four-leaf stage.3. Determination of the optimum boron concentration of the B. vulgaris growthSeedlings were treated with0μmol·L-1,25μmol·L-1,50μmol·L-1,250μmol·L-1,750μmol·L-1, and1250μmol·L-1boric acid for30days. The measurement ofphysiological indicators got consistent results: compared with0μmol·L-1boric acidtreatment, as the boron concentration increased to250μmol·L-1, sugar beet growthwas promoted and the biomass was the best under the treatment of50μmol·L-1boricacid. With the further increase of the boron concentration, seedling growth wasinhibited with leaves curled. Therefore, we can determine the50μmol·L-1is the mostsuitable boron concentration for B. vulgaris KWS3418.4. The effect of different boron concentration on B. vulgaris seedlings under salttreatmentCompared with the control, the dry weight, fresh weight and leaf area weresignificantly decreased under300mmol·L-1NaCl (NaCl+25μmol·L-1boric acid),and the dry weight of shoots was as32.21%as that of the control, accordingly theroots28.97%of the control. There was no significant difference between NaCl+50μmol·L-1boric acid and NaCl+25μmol·L-1boric acid. But the growth inhibition ofNaCl+250and NaCl+750treatment was alleviated comparing with the salt groupand the proportion of relief was48.85%and80.48%of NaCl+25respectively. NaCl+1250had no significant alleviation. The same conclusion was drawn by measuringthe leaf area,content of chlorophyll and photosynthetic parameters.5. Mechanism investigation of boron alleviating growth inhibition of B. vulgarisunder salt stressThrough analyzing membrane lipid peroxidation, ion content and soluble sugarcontent of seedlings growing under the treatment of boron combined with salt, wefound that the Cl-content significantly changed. Compared with the control, the Cl-content of300mmol·L-1NaCl+25was maximum and was9.66times as much as thatin control. The Cl-content under the treatments of NaCl+50, NaCl+250, NaCl+750and NaCl+1250were6.37,3.32,1.63and1.57times respectively than control.We can conclude from the above results that compared with300mmol·L-1NaCl treatment, the Cl-content declined with the increasing of boron concentration in thetreatment of boron combined with salt treatment, with the drop rate34.1%,65.6%,83.14%and83.74%. Therefore, we inferred that the regulation of Cl-content underexogenous boron may be the main reason of boron alleviating growth inhibition of B.vulgaris under salinity.Based on the above results, we applied different concentrations of chloride ionchannel inhibitors on the seedlings of B. vulgari under salt stress. Compared with thecontrol, the fresh and dry weights under300mmol·L-1NaCl+25treatment weresignificantly inhibited, while that in NaCl+750and NaCl+50μmol·L-1ZnCl2weremarkedly relieved and the fresh weight increased as63%and52%as NaCl+25treatment. Other physiological data showed the same trends. The measurement of Cl-content illustrated that the Cl-content under NaCl+750and NaCl+50μmol·L-1ZnCl2was lower than that in NaCl+25treatment, with36.28%and41.71%reducedrate. It is predicted that the alleviation of boron alleviating growth inhibition underNaCl treatment may be the results of down-regulating Cl-content. NaCl+200μmol·L-1ZnCl2treatment have no alleviation because of the damage of high ZnCl2concentration. We speculated that the reduction of Cl-content may be the reason forboron alleviating growth inhibition under salt stress.