Dry matter accumulation of three cultivars of turnip (Brassica campestris L.) sown in Canterbury on five sowing dates

Abstract

Dry matter production of three cultivars of Brassica campestris L. (cv Appin, York Globe, and Green Globe) sown on five dates (28 January, 11 February, 26 February, 11 March, 27 March) was studied in the field during one cropping season. Sowing date had a large effect on maximum dry matter production, with yields of 1540 g DM m⁻² to 595 g OM m⁻² being recorded for plots sown on 28 January and 27 March respectively. Maximum yields of 1105, 1185, and 1327 g OM m⁻² were achieved by cultivars of Appin, York Globe, and Green Globe respectively. Dry matter accumulation was linearly related to intercepted PAR for all sowing dates. However, there was some variation in efficiencies of dry matter accumulation, with 11 February sown crops producing 3.29 g OM MJ PAR' intercepted, compared to 2.5, 2.3, and 1.6 g OM MJ PAR' for the 26 February, 11 March, and 27 March sown crops. There was no difference in efficiency of dry matter production between cultivars, at most sewings. However, Green Globe was more efficient at producing dry matter at the last sowing. Highest yielding crops also had the best WMAGR of about 21 gDMm⁻²d⁻¹. Time to canopy closure was faster for crops sown on 11 February, and 26 February than for the later sowing date of 11 March (58 DAS v's 82 DAS). Plots sown on the 27 March never achieved canopy closure. Reduced canopy development was shown to occur due to a reduced rate of leaf appearance and leaf expansion. Appearance rate differed over a 53% range for the four sowing dates tested, with new leaves appearing every 2.9 days (plants sown on 11 February), 3.6 days per leaf (sown 26 February), 4.7 days per leaf (sown 11 March) and 5.6 days per leaf (sown 27 March). Comparisons between sowing dates in thermal time showed no difference (p>0.05), with one leaf appearing every 40°C.d. Final leaf length was greatest in higher order leaves. Leaf 2 expanded to 35% of the total length achieved by leaf 6 from plants sown on 11 February. Varying temperatures caused large differences (p<0.05) in the duration (days) of expansion between sowing date, with plants sown on the 11 February requiring an additional 8 days for complete expansion of leaf 6. When expressed in thermal time above a base temperature (0°C), individual leaves showed no difference (p<0.05) in the duration of linear growth with leaf 2 requiring 323.8 °C.d for expansion verses 346. 7 of leaf 6. No difference was observed in growth rate between sowing dates for the expansion of leaf 2. The results are discussed in relation to the determination of phenological development by temperature and radiation, and their impact on the growth of turnips.