EFFECT OF BLAST DISEASE ON NITROGEN ACCUMULATION AND REMOBILIZATION TO RICE GRAIN

The interaction between mineral nutrients and foliar diseases is an important factor affecting grain yield and quality of cereal crops. This study was conducted to investigate the effect of blast disease (Pyricularia oryzae Cav.) on nitrogen accumulation and remobilization to grain of rice plants (Oryza sativa L.), and to assess the possible nitrogen losses occurring between anthesis and maturity in response to disease stress in the field. Four rice cultivars with various levels of susceptibility to blast disease were grown in 2003 and 2004 under high and low disease pressure, achieved by early inoculation with the blast fungus and by natural infection, respectively. Inoculation affected the overall nitrogen metabolism of rice plants and in most cases the effect was proportional to disease pressure. High disease pressure increased the tissue nitrogen concentration and the total nitrogen content, but decreased the proportion of aboveground nitrogen that was partitioned to the grain. Post anthesis nitrogen accumulation was lower in inoculated plants, and therefore, their grain filling depended more on the existing nitrogen reserves stored in vegetative parts compared to the non-inoculated plants. Nitrogen translocation was higher in inoculated than non-inoculated plants, mainly due to the greater amount of translocation available nitrogen rather than to the higher efficiency of nitrogen translocation. The great amount of nitrogen accumulated until anthesis by the inoculated plants, in combination with the low grain demands for nitrogen, due to their lower grain yield, resulted in higher nitrogen losses between anthesis and maturity.