Study On The Regulatory Effects And Physiological Mechanisms Of Irrigation, Tillage And Nitrogen Fertilizer On Water And Fertilizer Utilization Of Dryland Whea

Tropospheric ozone(O3)pollution is a critical air pollution issue worldwide.Isoprene(ISO,C5H8,2-methyl-1,3-butadiene),a biogenic volatile organic compound emitted by vegetation,is an important precursor to O3 generation.On the one hand,isoprene contributes significantly to O3 generation and severe O3 pollution events in regional atmosphere.On the other hand,its emission is also significantly affected by O3 concentration.To effectively control O3 pollution and reduce carbon compound emissions such as ISO,it is necessary to clarify the characteristics of isoprene emission under elevated O3(EO3)and the underlying mechanism.To date,there are a few studies that explored the impact of O3 on isoprene emission rate(ISOrate)and the mechanism from the perspective of ISO synthesis.However,most are based on leaf-level observation and lack a comprehensive understanding about the biochemical and physiological mechanisms of how increasing O3 concentration affects ISOrate.To address this gap,five common isoprene-emitting tree species were fumigated with two concentrations of O3 using an open top chamber(OTC)platform.The effect of O3 concentration on ISOrate was investigated using a dynamic enclosure and a small clip-on leaf chamber.Furthermore,several key physiological processes,including ISO synthesis and metabolism,were investigated,and then the interspecific difference of ISOrate response to elevated O3 and its regulatory mechanism were analyzed.The main findings of this research are presented as follows:Field observations showed that the response of ISOrate to elevated O3 varied between the leaf and whole plant levels.At the leaf level,elevated O3 significantly inhibited ISOrate in the lower leaf of hybrid poplar ’546’(Populus deltoides cv.55/56 × P.deltoides cv.Imperial)and in the middle and lower leaves of Chinese white poplar(Populus tomentosa Carr.),while the ISOrate in the upper and top leaves of Chinese white poplar was not affected,and ISOrate in the upper leaves of the hybrid poplar ’546’ was even stimulated after short-term O3 exposure.Compared to CF(charcoal-filtered ambient air)treatment,EO3 significantly decreased leaf mass per area,number of leaves of hybrid poplar ’546’ seedlings,and leaf biomass,but increased leaf nitrogen content and individual leaf size.Due to such compensatory responses,increased ISOrate in upper leaves whilst decreased ISOrate in lower leaves were observed in hybrid poplar ’546’ seedlings,thus the whole-plant ISOrate showing no significant fluctuation.The results of whole-plant ISOrate also showed that the simplistic estimation approaches based on the linear regression between chlorophyll content and leaf-level ISOrate could not accurately reflect the true response of the whole plant to elevated O3.The physiological mechanism of inhibitive effects of EO3 on ISOrate was investigated by using two Chinese white poplar varieties(diploid and triploid).In both varieties,EO3 significantly decreased photosynthetic rate(Asst)and stomatal conductance(gs),as well as ISOrate of middle and lower leaves.In the diploid varieties,the reduction of ISOrate was ascribed to a down-regulation of ISO synthesis capacity,including decreased contents of the isoprene precursor dimethylallyl diphosphate,decreased isoprene synthase protein content and activity.In contrast,the negative effect of O3 on ISOrate of the triploid varieties was not due to the inhibited ISO synthesis capacity,but rather from increased ISO oxidative loss within leaves.The interspecific variation of ISOrate sensitivity to O3 and the underlying driving mechanism were investigated in four urban greening tree species.The results showed that EO3 decreased the ISOrate by an average of 42.5%across species.Among the species examined,the highest sensitivity of ISOrate to EO3 was observed in Salix matsudana,followed by Sophora japonica and hybrid poplar clone ’546’,while Quercus mongolica showed the least sensitivity.Further analysis showed that EO3 had no significant effect on palisade tissue thickness and spongy tissue thickness.However,principal component analysis demonstrated that the ISOrate sensitivity to O3 was driven by the concurrent effects of O3 on ISO synthesis ability(dimethylallyl diphosphate and isoprene synthase contents)and stomatal conductance.Our findings reveal the influence characteristics of O3 on ISOrate and uncovered the underlying reasons,and further analyzed the mechanism of intra-species difference of ISOrate in response to EO3,and preliminarily clarify the inter-specific difference of ISOrate in response to EO3 and its regulation mechanism.The results provide a scientific basis for incorporating the effect of O3 on ISOrate into the ISO emission model and accurately evaluating the contribution of ISO emissions to O3 pollution in the future.Additionally,our research provides guidance for decision-making regarding the allocation of urban green tree species in regions affected by O3 pollution.