EFFECTS OF SOWING TIME AND PLANT-AGE ON CRITICAL NITROGEN CONCENTRATIONS IN CANOLA (BRASSICA-NAPUS L)

Critical concentrations of NO3-N in fresh petiole tissue and total N i n the dried lamina were determined for the youngest mature leaf (YML) of field-grown canola. For dry matter yield of canola sown on 4 May, c ritical NO3-N concentration in the YML petiole at the rosette stage (R S) was 1.46 mg/g fresh wt. At the flower-buds-visible stage (BV) it wa s 0.45 mg/g fresh wt. For seed yield the values were 1.72 and 0.53 mg/ g fresh wt. Critical total N concentration in the YML lamina for dry m atter yield were 69 mg/g dry wt. at RS and 57 at BV. For seed yield th ey were 71 and 59 mg/g dry wt. Critical NO3-N concentrations in the YM L petiole of canola sown on 30 May were reduced by 50%; critical total -N concentrations in the YML lamina were not reduced to the same exten t. Despite the reductions in critical N concentrations in the YML, cri tical N fertilizer rates for vegetative growth and seed yield were una ffected by sowing date or plant growth stage.