Study Of Proton Release From Tea Plant Roots Induced By Aluminum And Nitrogen

Tea plant?Camellia sinensis?is a major commercial crop in our country with long cultivation history.In agricultural soils,a certain proportion of soils is utilized to cultivate tea plant in the southern regions of our country.It is well known that cultivating tea plant leads to soil acidification and aggravate acidification along with cultivation time.An acidic environment is favorable for tea plant growth.However,excessively low soil pH would have negative impacts on tea plant.Therefore,studying the mechanism of tea garden soil acidification is important for increasing yield and quality of tea and soil management.This research included the following four aspects:1.Solution culture experiments were conducted and the protons released were measured with an automatictitration system to determine the main factors affecting proton release by tea plant roots.Results indicated that the higher were the cation concentrations,the more protons were released from the roots,suggesting that tea roots took up a large amount of cations during growth,and then released protons to maintain charge balance of the plant body.The amount of protons released from tea roots at constant pH was much higher than that at a non-constant pH.Stepwise multiple linear regression analysis showed that both NH⁴⁺and Al³⁺ played distinct roles in proton release by tea plant roots.2.Proton release from tea plant roots induced by Al³⁺ in solution culture experiments was examined with an automatic titration system to evaluate the effect of Al³⁺ uptake by tea plants on soil acidification.Results indicated that the uptake of Al³⁺ by tea plants led to proton release from tea plant roots and thus an increase in soil acidification.The uptake of Al³⁺ by tea plants and the amount of protons released from tea plant roots was greater at pH 4.5 than at pH 5.0 and 4.0 and increased with increasing initial Al³⁺ concentration in the culture solutions.At the same initial pH,the amount of protons released from tea plant roots at a constant pH was much higher than that at a non-constant pH.The presence of NH⁴⁺ increased the amount of protons released from tea plant roots.3.The effect of A1 on the root-induced acidification of rhizosphere,activity of the plasma membrane H+-ATPase and cation-anion balance from tea plant were investigated.Proton release from plant roots with Al treatment cultured for a week showed a higher release rate than that from without Al roots.Plasma membrane H⁺-ATPase activity increased with Al concentration and also involved in the Al-induce proton release from tea plant roots.After cultured with different concentration Al at pH 4.5 for a week,all treatments which contained control,0.05 mmol L^-1 Al,0.25 mmol L^-1 A1 showed an excess of cations over anions and the amount of excess cation up take increased with the increasing A1 concentration.Thus,the results obtained support the view that A1 enhances the activity of plasma membrane H+-ATPase,excess cation uptake and concomitant proton release.4.In order to explore the mechanisms for soil acidification induced by tea plant roots,the effect of different nitrogen forms on proton release from tea plant roots under constant pH condition was investigated with solution culture experiments and automatic potentiometric titrations.The results showed that tea plant roots released protons in the presence of ammonium,and the amount of protons released from tea plant roots in single ammonium solution was greater than that in the solution containing ammonium and nitrate with mole ratio of NH⁴⁺/NO3-=1:1;while hydroxyl was released from tea plant roots in the presence of nitrate.The amount of proton release increased with the increasing of the initial concentration of ammonium and there was significant correlation between the uptake of ammonium and proton release by tea plant roots.In the range of pH from 4.5 to 5.5,the maximum proton release occurred in the treatment with initial pH 5.0,followed by the treatment with initial pH4.5.The uptake of both ammonium and nitrate increased with the incubation time when tea plant cultivate in ammonium nitrate solution and the similar changing trend was observed for proton release,while the amount of ammonium absorbed by tea plant was greater than nitrate during the incubation experiment.