Environmental drivers of bulb production in brassicas

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 series of experiments were used to clarify the growth and development of bulb turnips. Thermal time requirements for developmental phases of bulb brassicas were quantified and relationships to describe the time to bulb initiation were obtained in field and controlled environments. In the first controlled environment experiment cardinal temperatures were determined from germination of nine cultivars of forage brassicas and daily thermal time accumulation models were developed for three groups of forage brassicas and ‘Aparima Gold’ swede (Brassica napus spp. napobrassica). These models allowed the calculation of thermal time accumulation in two field experiments. The first field experiment evaluated ‘Aparima Gold’ swede and ‘Barkant’ and ‘Green Globe’ turnips sown on five dates from November 2008 to March 2009. A second field experiment evaluated ‘Green Globe’ turnip sown on four dates from December 2009 to March 2010 under two soil temperature regimes (covered with plastic sheets and uncovered). A second controlled environment experiment evaluated phyllochron and hypocotyl thickening of swede, turnips and rape cultivars sown on four dates and in three temperature regimes using two glasshouses and an outside environment from December 2011 to March 2012. ‘Aparima Gold’ swede had a thermal time requirement to 50% emergence of 83oCd (Tb = 0oC) whereas the turnip cultivars averaged 56oCd (Tb = 3.6oC). Total dry matter production of bulb brassicas was described by logistic curves where leaf production was the main component of yield until bulb development started. From this point, leaf production stabilized until leaf senescence became predominant and bulbs became the main component of yield. Dry matter yields decreased as sowing date was delayed and that was related to the total amount of radiation intercepted. Radiation use efficiency of ‘Green Globe’ turnips increased with mean temperature at a rate of 0.07 g DM/MJ total/oC, from 10 to 20oC. Bulb initiation was botanically defined as when hypocotyl was 10 mm thick and occurred at ~ 760oCd for ‘Aparima Gold’ and at ~ 390oCd for turnips. Until bulb initiation of ‘Green Globe’, increases in LAI (0.4 m2/m2/oCd) and in leaf production (0.3 kg DM/ha/oCd) were observed. From this point on, the relationship between the ratio leaf:root dry matter production and accumulated thermal time followed an exponential decline (R2 = 0.99) with a base parameter of 0.995. Phyllochron of brassicas was not constant across sowing dates. The phyllochron of ‘Aparima Gold’ swede ranged from 52 to 106oCd/leaf, ‘Goliath’ rape from 42 to 92oCd/leaf (Tb = 3.9oC) and for turnips from 20 to 67oCd/leaf across field and controlled environment experiments. Phyllochron increased with mean temperature at a rate of 3.4oCd/leaf/oC for ‘Green Globe’ and 3.7oCd/leaf/oC for ‘Barkant’ turnips from 7 to 21oC. ‘Aparima Gold’ and ‘Goliath’ also increased phyllochron with mean temperature (2.0 and 2.8oCd/leaf/oC, respectively) but further studies are recommended to uncouple the effects of temperature and photoperiod.