EXPLAINING CURD AND SPEAR GEOMETRY IN BROCCOLI, CAULIFLOWER AND ROMANESCO - QUANTITATIVE VARIATION IN ACTIVITY OF PRIMARY MERISTEMS

Brassica oleracea L. is highly polymorphic and includes varieties whic h exhibit a headed phenotype (a large preinflorescence): the curd of c auliflower and 'romanesco' (var. botrytis), and the spear of broccoli (var. italica). This headed phenotype results from highly iterative pa tterns of activity at the primary meristems. Differences in the morpho logy of curds and spears are accounted for by three quantitative varia bles: the rate of production of branch primordia on the flanks of the apical meristems (RPP); the number of branch primordia produced before the first formed begin producing their own branch primordia (the iter ation interval, ITI); and the duration of the preinflorescence stage ( before production of flower primordia). Relatively stable iteration pa rameters (RPP and ITI) during curd development lead to the production of semi-spherical curds with a smooth surface in cauliflower and brocc oli, whereas in 'romanesco' RPP and ITI increase throughout curd devel opment, inducing a pyramidal curd with an angular surface. A relativel y long preinflorescence stage in cauliflower and 'romanesco' results i n the curd surface being composed largely of branch primordia, whereas in broccoli this-stage is short and the spear surface is made up of f lower buds. Simplified growth models for these three headed types are presented. The implications for the genetic control of the B. oleracea L. headed phenotype and the relationships between shoot apical merist em size, phyllotaxis and curd/spear morphology are discussed.