Abstract
Rice hakare symptoms indicate a unique paddy-rice disorder observed only in the northern mountainous region of Sasebo city, Nagasaki. Research on rice hakare symptoms since the 1960s has not yet clearly identified the main causal factors. Drought stress caused by a foehn-like dry wind and acidic fogs containing high concentrations of nitrate and ammonium ions have previously been implicated in its onset. In the present study we aimed to clarify the effect of excess nitrogen load on rice plants in the affected areas. We collected samples of rice plants and soils from affected and unaffected paddy fields as well as samples of fog and air rich in gaseous ammonia emitted from cattle sheds. We then examined the possibility that rice leaves incorporate nitrogen compounds from the atmosphere. To do so, we measured the nitrogen content and δ^<15>N values (natural ^<15>N abundance) in the rice leaves, air and soil samples to assess the source of nitrogen enrichment in the rice leaves. To investigate the ammonia flux between rice plants and the atmosphere, we measured atmospheric ammonia concentrations inside and outside the paddy fields at different plant development stages. Affected paddy fields generally consisted of nitrogen-rich and fertile soils, and the rice plants grown in these paddy fields tended to contain higher levels of nitrogen than those in unaffected paddy fields. At the maximum tillering stage, atmospheric ammonia concentrations inside the affected paddy fields were lower than those outside the fields (above bare ground), indicating the absorption of ammonia from the air by rice plants. In addition, damaged leaves collected at this stage showed higher nitrogen contents and lower δ^<15>N values than those of healthy leaves, which suggests that atmospheric nitrogen compounds are directly incorporated into rice leaves, because atmospheric nitrogen compounds generally have lower δ^<15> values than those derived from soils and livestock manure. In contrast, at the heading stage and subsequent ripening stage, damaged leaves showed higher δ^<15> values than those of healthy leaves, suggesting ^<15>N fractionation, wherein ^<14>N is emitted from the leaves as ammonia gas. Indeed, greater emissions of ammonia gas from rice plants were observed in the affected paddy field at these stages than at earlier stages. We conclude that excess nitrogen in rice leaves had been directly supplied from the atmosphere and this could cause drought stress during strong wind associated with typhoons or a foehn-like dry wind-the drought stress would occur because the excess nitrogen triggers extensive stomatal opening to liberate ammonia, and therefore accelerates transpiration rates. Therefore, high concentrations of nitrate and ammonium ions in acidic fogs and gaseous ammonia emitted from nearby cattle sheds, combined with desiccating winds, appear to be significant factors causing or exacerbating rice hakare symptoms.
