Apical abortion in calabrese is induced by periods of low temperature and results in premature differentiation of epical meristem cells

Apical abortion in calabrese (Brassica oleracea var. italica), a highly des tructive disorder which occurs in overwintered transplants, has been invest igated using a model system in which blindness (abortion of the apical meri stem) can be reproducibly and predictably induced. An initial experiment ex amined the susceptibility of 12 cultivars to apical abortion when grown thr oughout a winter period under commercial conditions. Three of those varieti es showed very high levels of blindness (100%), Subsequently, plants of the susceptible cultivar PETO 1204 were subjected to an inductive period of lo w light intensity (30 mu mol m(-2) s(-1)) and low temperature (4 degrees C) . Apical meristematic cells of all plants ceased mitotic activity within 3 d of being transferred to a regime comprising higher light intensity (100 m u mol m(-2) s(-1)) and temperature (15 degrees C). Using this system the st ructures of normal apices were compared with those which became blind. Blin dness was characterized by a cessation of leaf primordium production by the vegetative apex, the last formed primordium growing on in some cases to fo rm a mature normal leaf, or in others, a deformed structure known as a whip -tail. The inactive apical bud became embedded in the tissues of this last- formed structure. The cells of the inactivated apical bud remained alive, b ut lost their meristematic capability, becoming enlarged, highly vacuolated parenchyma cells with amyloplasts.