The critical period for yield and quality determination in canola (Brassica naps L.)

Despite its global significance as an edible oil and biofuel, the critical period for yield determination in canola (edible oilseed rape - Brassica napus L.) has not been determined in the field. Field experiments were conducted at two contrasting sites in southern Australia where 100 degrees Cd shading periods (15% PAR transmitted) were applied from early vegetative growth until maturity to identify the developmental period when the crop was most sensitive to stress. Despite the significant difference between the two sites for yield in the unshaded control (450 gm(-2) in New South Wales, and 340 gm(-2) in South Australia), the critical period was consistent at both sites extending from 100 to 500 degrees Cd after the start of flowering (BBCH60), and centred 300 degrees Cd after BBCH60. Seed number (seed m(-2)) was reduced by an average of 48% in the critical period, generated in equal parts by reduced pod m(-2) in the early part of the period, and reduced seed pod(-1) in the latter part. Reduced seed number was partially compensated by an increase in seed size of 29%. These trends were similar on the branches and main stem. On the main stem, the timing of the critical period moved from earlier to later from lower to upper pods linked to the timing of their development. Seed oil content declined and protein content increased under shading in the critical period, while both oil and protein yield (kgm(-2)) were reduced by 40-50% and 30-40% respectively. The critical period is coincident with the greatest number of near-open buds and newly opened flowers, which are highly sensitive to assimilate supply for ovule development. Both pod abortion and restricted capacity for compensatory growth of surviving pods are consequences of assimilate restriction on developing ovules. Identification of the critical period provides a useful target for breeding and management strategies to maximize productivity.