Quantifying growth and development of bulb turnips

Abstract

Bulb brassicas are used as supplementary feed in intensive pastures systems. However, there is a lack of quantitative data to define their growth and development. This has limited the creation and use of prediction models and decision support systems. Thus a field experiment measured growth and development of ‘Barkant’ and ‘Green Globe’ turnips sown on five dates from November 2008 to March 2009. In a second field experiment ‘Green Globe’ turnips were sown on four dates from December 2009 to March 2010, under two ground cover treatments that changed mean soil temperature by ∼2°C. Bulb initiation was defined botanically as when the hypocotyl was 10mm thick, at 360°Cd (±13.0) for ‘Barkant’ and 420°Cd (±13.7) (Tb=3.6°C) for ‘Green Globe’. However, the bulb participation in dry matter production occurs after an 18mm hypocotyl thickness, which occurred at ∼500°Cd for both turnip cultivars. A single base parameter of 0.995 described the exponential decline of the leaf:bulb ratio. Relationships also described how leaf production and total leaf area expansion changed up until bulb initiation. Radiation use efficiency (RUE) ranged from 1.13 to 1.33g DM/MJ total. A constant rate of total leaf area expansion (0.015m2/m2/°Cd) was obtained up to LAIc for ‘Green Globe’ turnips. A third pot experiment confirmed the thermal time requirement to bulb initiation based on direct assessment of the hypocotyl thickening of ‘Barkant’ and ‘Green Globe’ turnips. Temperature was shown as the main driver of bulb development and growth. The relationships provided could be used to improve the performance of prediction models.