Effects of elevated ozone on maize under varying soil nitrogen levels: Biomass, nitrogen and carbon, and their allocation to kernel.

Effects of ozone (O) on maize have been increasingly studied, but only few studies have focused on the combined impacts of O and nitrogen (N) on this important crop with C carbon (C) fixation. In this study, a maize cultivar with the largest acreage in China was exposed to two O treatments (NF: ambient air O concentration; NF60: NF plus 60 ppb O) and four N levels (farmers' N practice: 240 kg N ha yr; 150%, 50% and 25% of farmers' N practice). Generally, O and N significantly influenced biomass, N and C, but did not change their allocation to kernel. There were significant interactions between O and N in stem biomass, C concentration and uptake, and leaf biomass and C uptake, with significant O effects mainly occurring at N120 and N240. Based on the coefficient of determination (R), root C:N ratio rather than the most commonly used leaf C:N ratio was the best trait to indicate maize productivity. Furthermore, O significantly increased the regression slopes between root C:N ratio and kernel N uptake, kernel C uptake and plant N uptake, strengthened the correlation of C:N ratio and kernel C uptake, and weakened the correlation of C:N ratio and hundred-kernels weight. These suggest that O pollution can change the relationship of C:N ratio and productivity in maize. The weak correlation between kernel harvest index (HI) and N harvest index (NHI) indicated that future breeding researches should consider how to improve the coupling between biomass and N-related nutrition allocations in crop edible parts. Our results not only are helpful to accurately estimate O impacts on maize with consideration of N but also provide a new insight into the relationship between plant traits and its productivity under O pollution.