Effect of nitrogen supply form on the invasion of rice roots by the root-knot nematode, Meloidogyne graminicola

Rice is capable of taking up both nitrate (NO3-) and ammonical (NH4+) forms of nitrogen (N). It is largely grown under flooded cultivation when NH4+ is the main form of available soil N. The root knot nematode, Meloidogyne graminicola, can cause serious damage to rice crops and disturb N uptake and translocation. We have investigated if the N supply form can influence the susceptibility of rice to this important pest. Roots supplied with a 100-fold lower supply of calcium nitrate (0.1 mM Ca(NO3)(2)) showed a higher level of nematode infection as measured by the gall index. Plants supplied with 2.85 mM of Ca(NO3)(2) were more infected compared with the same dose of ammonium nitrate (NH4NO3) or ammonium chloride (NH4Cl). (NO3-)-N-15 influx studies showed significantly smaller uptake of nitrate in nematode-infected roots when compared with non-infected control plants. The electrophysiological studies showed that resting cell membrane potentials of nematode-infected plants were significantly smaller (less negative) than those of non-infected control rice. Furthermore, when the cellular responses to NO3- were compared, these also showed significantly smaller nitrate transport activity in nematode-infected roots (4.7 +/- 1.2 mV) when compared with non-infected control plants (11.9 +/- 3.4 mV). Taken together, the (NO3-)-N-15 influx and electrophysiological measurements clearly showed that the root NO3- transport activity was severely decreased in nematode-infected roots. The results also show that regulation of NO3- concentration at critical periods of nematode infection of rice root can provide a non-nematicidal method of nematode management.