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

Net nitrate (NO3⁻) uptake represents the balance between active influx mediated by nH⁺:mNO3⁻ symporters (with n > m) and passive efflux with nitrate leaking from the cytoplasm to the apoplasm (i.e., an electrically driven uniport). Nitrate efflux is only one of the factors that determine net nitrate uptake; however, it can be very significant. Nitrate efflux can be considered a “leakage” that reduces nitrate absorption efficiency, and it may influence plant nitrate concentrations by affecting nitrate uptake.

Large genotypic differences in nitrate accumulation have been reported in many crop species, but the reason for these differences is not clear. Furthermore, little is known about whether there is a relationship between nitrate efflux and genotypic differences in plant nitrate accumulation. Therefore, nitrate 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 investigated. 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. This indicates that low nitrate efflux rates result in increased transport of nitrate from roots to shoots and increased nitrate accumulation in petioles.

The main finding has been published on Communications in Soil Science and Plant Analysis, 2012, 43: 507-518. The paper is also archived at http://www.tandfonline.com/doi/abs/10.1080/00103624.2012.639426.