Significance of root growth and activity for genotypic differences in nitrogen efficiency of rape (Brassica napus L.): II. Root production and nitrate-N uptake by two cultivars

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Pervious work demonstrated a suitable method (compartment method) for estimating root-growth and nitrate-N uptake activity in the field. In this work, the compartments were installed in the plots of two contrasting oilseed rape cultivars (N-effiicient ‘Apex’ and N-ineffiicient “Capitol”) fertilized with 0 (N0) and 227 (N2) kg N ha -1. The new developed roots and nitrate-N depletion were estimated in the compartments after the installing period (21 days at shooting and 16 days both at flowering and grain-filling stages).

Suboptimum N application to the crop (particularly N0) reduced total biomass and seed yield formation substantially (24% and 38% for ‘Apex’ and ‘Capitol’, respectively). At the shooting stage, there were no differences in root production and N uptake of both cultivars between N0 and N2. But at flowering and seed filling stages higher root production and accordingly higher N depletion was observed at N0 compared to N2. Towards later growth stages the new developed roots were characterised by reduction of root diameter and a shift towards the deeper soil layer (30–60 cm). At low N but not at high N supply, the N-efficient cultivar ‘Apex’ exhibited higher root-growth activity and accordingly depleted nitrate-N more effectively than the N-inefficient cultivar ‘Capitol’ especially during the reproductive growth phase. The calculated nitrate-N uptake rate per unit root length was maximum at flowering but showed no differences between the two cultivars. This indicated that the higher N uptake efficiency of ‘Apex’ was due to higher root-growth activity rather than higher uptake per unit of root length.