3.1 Interactive effects of eCO2and N application rate on tillering

Compared with the ambient [CO2], eCO2 significantly increased the number of tillers (Fig. 1A). In respect to N application, the number of tillers increased with N application rate, with more enhancement at leaf-age six than at leaf-age four. Under the higher N application, eCO2 stimulated more tillers (Fig 1A). Tiller occurrence percentage and uniformity were also enhanced by eCO2 at leaf-age four (Fig. 1B), especially under eCO2 and higher N application. Apparently, the tiller enhancement effect by eCO2depended on N application rate, indicating that the N availability constrained tillering, especially under eCO2.
Similarly, eCO2 increased biomass more at the higher than lower N application rate (Fig. 1C). Though N application promoted biomass accumulation in both the ambient and eCO2conditions, the biomass accumulation reached a plateau at the N10 treatment under the ambient CO2 condition. At the N15 treatment, biomass still showed a significant increment under the eCO2 condition (Fig. 1C). It can be concluded that eCO2 promoted biomass accumulation, especially when combined with higher N application rate.