Effects of elevated ozone on winter wheat and potential genetic and physiological mechanisms

Abstract

Wheat is an ozone-sensitive crop with substantial global yield loss. It is necessary to make assessments of ozone impacts and to investigate the tolerance mechanisms and genetic factors conferring ozone tolerance in wheat. Contrasting wheat cultivars had been pre-selected from a larger wheat population with known ozone tolerance and exposed to season-long ozone fumigation in open-top chambers. Season-long ozone fumigation was conducted at an average ozone concentration of 70 ppb with three additional acute ozone episodes of around 150 ppb. This thesis is structured into three major parts:<br /> (1) Based on the large variations of ozone responses in wheat, physiological traits contributing to yield losses or yield stability were identified under ozone stress in eighteen contrasting genotypes for modelling parameterization, that is, foliar chlorophyll content represented by normalized difference vegetation index and net CO₂ assimilation rate of young leaves during grain filling. LINTULCC2 crop model was further parametrized for two selected tolerant or sensitive varieties, respectively, with an ozone response routine. Parameters representing the distinct physiological responses of contrasting genotypes were specified to improve the accuracy of modelling simulation.<br /> (2) Carbon (C) and nitrogen (N) allocation were analyzed in straw and grains under ozone stress. Elevated ozone exposure reduced the N absorption from soil and allocation from vegetative to reproductive organs, manifested as significantly reduced indicators of N use efficiency (NUE) with the exception of N utilization efficiency (NUtE). In addition, the relationship between harvest index (HI) and nitrogen harvest index (NHI) was changed by ozone stress, and the reduced regression slope between them indicated that ozone exposure significantly affected the relationship of N and biomass allocation into wheat grain.<br /> (3) According to genetic backgrounds at ozone tolerant loci, tolerant and sensitive haplotypes represented by two genotypes, respectively, were used for gene expression, physiological and biochemical analyses. Tolerant and sensitive haplotypes showed consistently contrasting responses to ozone in terms of net photosynthetic rate, lipid peroxidation, apoplastic ascorbate, ascorbate peroxidase and peroxidase activity. Among candidate genes located within an ozone tolerant locus on chromosome 5A 592.04 - 593.33 Mb, the gene TraesCS5A01G400500 putatively involved in peroxidase activity was differently regulated in two haplotypes, with consistent sequence polymorphisms in the promoter region.<br /> Taken together, this study improved the accuracy of modelling simulation for contrasting genotypes, assessed the C and N allocation and NUE among different genotypes and investigated the potential genetic and physiological mechanisms. Further explorations were warranted to reveal the molecular mechanisms underlying ozone tolerance in wheat.