Selection of hydroxyproline resistant proline accumulating mutants of cauliflower for improved frost resistance

Differences in frost tolerance of four F1 hybrid varieties of cauliflower correlated with their heading dates. The extent of the varietal differences was small indicating a relatively low degree of variability from which to select by conventional plant breeding methods. The development of hydroxyproline resistant cauliflower lines by mutagenesis of curd pieces was demonstrated. Six thousand curd pieces (3mm) were treated with the mutagen N-nitroso-N-ethylurea (0.3mM) and selected on hydroxyproline (3mM) containing medium. Thirty one shoots were successfully regenerated and 23 of these survived sub-culturing. One resistant line showed elevated levels of proline up to 4.7 times that of controls although this fell to 1.6 times that of controls after 10 culture passages. Leaf strip assays showed that resistant shoots were invariably chimeras. Despite an accumulation of proline during hardening of both control plants and proline accumulating line, no difference in frost resistance could be found when plants were tested as in-vitro grown shoots.