THE ROLE OF LEAVES IN THE PERCEPTION OF VERNALIZING TEMPERATURES IN SUGAR-BEET

Annual and biennial sugar beet varieties require long days to induce f lowering but the biennial genotypes additionally require vernalization . Previous research has suggested that the inability of non-vernalized biennial plants to flower can be explained by a lack of competence of the leaves to respond to long days. In this study defoliation experim ents were used to investigate which leaves could perceive long dayleng ths and, in particular, whether leaves initiated from a non-vernalized shoot apical meristem could perceive vernalizing temperatures and pro duce a floral stimulus in long days. Annual and vernalized biennial pl ants flowered if young leaves (i.e. those formed during or after verna lization) were kept on the plants, but they did not flower if only old er expanded leaves (including those expanded prior to vernalization) w ere present. No evidence was obtained to indicate that the older leave s contained inhibitors of flowering and it seems most likely that ther e is a decline in responsiveness to daylength with increasing leaf age . Exposure to vernalizing temperatures accelerated flowering of the an nual and was essential for flowering of the biennial. The presence of a single leaf initiated, but not expanded, prior to the transfer of bi ennial plants to vernalizing temperatures was sufficient to induce flo wering. This indicates that expanding leaves do not need to be initiat ed from a vernalized apical meristem to become competent to produce a floral stimulus in long days.