Nitrogen utilisation efficiency and grain yield components of rice varieties grown under blast disease stress

The aim of this study was to analyse the relative importance of traits causing variation in grain yield and nitrogen utilisation efficiency (NUE) among rice ( Oryza sativa L.) varieties grown under blast disease ( Pyricularia oryzae Cav.). Four rice varieties with various levels of susceptibility to blast disease were grown in field experiments in 2003 and 2004. Plants were grown under high and low disease pressure, achieved by early inoculation with the blast fungus and by natural infection, respectively. Inoculation with the fungus reduced the grain weight compared with naturally infected plants. The magnitude of this reduction was proportional to disease severity. Grain nitrogen (N) concentration was higher in the inoculated plants. Increased grain N concentration resulted in a reduction of its contribution towards grain N yield variation. Inoculation reduced the NUE for biomass production (NUE b ) and the NUE for grain yield (NUE g ). NUE b was negatively correlated with leaf blast rating, suggesting that the higher disease pressure was on leaves, the lower the biomass production was per N unit. N harvest index accounted for almost all the variation in NUE g among varieties in the inoculated plants. Results indicated that the higher dilution of N by dry matter in the non-inoculated plants was responsible for the higher NUE g compared with the inoculated plants.