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

Annual and biennial sugar beet varieties require long days to induce flowering 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 experiments were used to investigate which leaves could perceive long daylengths and, in particular, whether leaves initiated from a non-vernalized shoot apical meristem could perceive vernalizing temperatures and produce a floral stimulus in long days. Annual and vernalized biennial plants flowered if young leaves (i.e. those formed during or after vernalization) were kept on the plants, but they did not flower if only older expanded leaves (including those expanded prior to vernalization) were present. No evidence was obtained to indicate that the older leaves contained inhibitors of flowering and it seems most likely that there is a decline in responsiveness to daylength with increasing leaf age. Exposure to vernalizing temperatures accelerated flowering of the annual and was essential for flowering of the biennial. The presence of a single leaf initiated, but not expanded, prior to the transfer of biennial plants to vernalizing temperatures was sufficient to induce flowering. This indicates that expanding leaves do not need to be initiated from a vernalized apical meristem to become competent to produce a floral stimulus in long days.