Measurement of Nitrate Efflux from Roots and Its Relation to Nitrate Accumulation in Two Oilseed Rape Cultivars

Abstract

Studies have reported significant differences in nitrate accumulation among genotypes within a crop species, but the reason for these differences is not clear. This study investigated nitrate (NO3) efflux from roots of two oilseed rape cultivars (Brassica napus L. cvs. ZY821 and D89) and the relationship between nitrate efflux and plant nitrate accumulation. Nitrate efflux was estimated by measuring nitrate released from roots into nitrate-free nutrient solutions 58 days after sowing. The solutions were buffered with either 2.0 mM (n-morpholino) ethanesulfonic acid (MES) or 0.05 mM phosphate. Whole-plant and petiole nitrate accumulations were significantly greater for ZY821 compared to D89. Nitrate efflux varied diurnally, and the difference between cultivars was greater in the morning than in the afternoon. Data suggested that the relatively high pH of the phosphate-buffered solutions increased nitrate efflux rates from oilseed rape plants; therefore phosphate buffer should not be used when making direct determinations of nitrate efflux. Maximum cumulative nitrate efflux was 0.97 μmol g−1 fresh-weight (FW) root for ZY821 and 1.9 μmol g−1 FW root for D89. Maximum nitrate efflux rate was 0.084 μmol g−1 FW min−1 for ZY821 and 0.097 μmol g−1 FW min−1 for D89. These results indicated that between the two cultivars, ZY821 had the greatest petiole nitrate nitrogen concentration and the lowest total nitrate efflux. We suggest that cultivars with low nitrate efflux rates are able to translocate and store greater amounts of nitrate in aboveground plant organs, especially in petioles.