The relationship between vernalization saturation and the maintenance of freezing tolerance in winter rapeseed

Vernalization requirement maintains winter annuals in a cold hardy vegetative state during the winter. Vernalization saturation has been shown to be followed by a loss of freezing tolerance under controlled experimental conditions. The objective of this study was to determine a possible relationship between vernalization saturation and the maintenance of cultivar specific freezing tolerance levels in winter rapeseed (Brassica napus L.) under field conditions. Two cultivars, 'Banjo' and 'Californium', with differing vernalization requirements were chosen after a screening of 18 cultivars under controlled conditions. Vernalization response and freezing tolerance of the two cultivars grown under field conditions were compared on five different dates throughout the winter of 2010/11 at Apelsvoll Research Station, Norway (60° 42′ N, 10° 52′ E, 250 m a.s.l.). 'Californium' reached vernalization saturation on 11 Oct. whereas 'Banjo' did 13 days later. Maximum freezing tolerance of both cultivars was attained in early December and it was maintained for 31 days in 'Californium' and 67 days in 'Banjo'. Our results indicate that there is no simple relationship between freezing tolerance and saturation of vernalization. Increasing contents of water, d-glucose and d-fructose and decreasing sucrose and starch levels in the shoot apex from mid to late winter were significantly correlated with a decrease in freezing tolerance (LT50) and shoot regrowth following a −12 °C low temperature treatment and so these parameters may be good predictors of plant survival and vitality in the spring.