EFFECTS OF SOWING TIME ON NITRATE AND TOTAL NITROGEN CONCENTRATIONS IN FIELD-GROWN CANOLA (BRASSICA NAPUS L.) AND IMPLICATIONS FOR PLANT ANALYSIS

Abstract

Canola was grown in a field experiment in the cropping belt of central New South Wales to determine the effect of sowing time on nitrate-nitrogen (N03-N) and total nitrogen (N) concentrations in relation to assessing the N status of the crop by plant analysis. Plants were sampled at the 5–6 leaf rosette stage (5–6RS), the start of stem elongation (SE), and the start of flowering (SF). The petiole of the youngest mature leaf (YML) was used for determination of NO3-N, and total N concentrations were determined in the YML petiole and lamina and the whole shoot. Critical N concentrations were derived from the relationships between the NO3-N or total N concentrations in these organs and dry-matter production at the time of sampling or seed yield at maturity. Delaying sowing from early to late May reduced nitrate-N concentrations at 5–6RS and SE, and total N concentrations at 5–6RS by about 50% in the YML petiole, but did not reduce total N concentrations to the same extent in the YML lamina or whole shoot (mean reduction for 5–6RS, SE, and SF: 6%). Nitrate-N and total N concentrations in all organs tested declined with plant age, but were similar at each sampling for dry-matter production at the time of sampling and for predicting potential seed yield. Concentrations of both NO3-N and total N in all organs tested were suitable for assessing the N status of young canola plants. After SE, however, the concentration of total N in the whole shoot was a more reliable indicator of plant N status as NO3-N was barely detectable in the YML petiole at SF, and the YML was difficult to identify. Seed yield decreased by 45–50% between the early and late May sowings, but the just-adequate N fertilizer rates (defined as producing 90% of maximum seed yield) were similar (90–93 kg N ha−1) for both sowing times. It was concluded that sowing time and accurate determination of the stage of crop development are important factors in interpreting the results of plant testing based on NO3-N or total N concentrations in canola.