Cell structure changes

There have been numerous studies on the structural changes at the apex that take place in response to the assumed arrival of a flowering stimulus. Tissues which initiate floral morphogenesis in response to the flowering stimulus have been called the target tissues and become sites of activity in the evoked meristem. In plants where the flowers are not terminally located, the central zone of the plant remains virtually unchanged but there is an increase in activity at the axillary bud sites. In dicotyledons, the target tissues usually include the central and peripheral zones of the apical meristem but, in grasses, the primary target areas are the bud sites in the axils of the leaf primordia (Fig. 8.6). Such structural changes might suggest that the reorganisation of growth patterns represents the primary response of the apex to the day-length signal. However the meristems of some photoperiodic plants also alter their

FIG. 8.6. Target tissues in the apical meristem of dicotyledonous species (right) and the shoot tip of grasses (left). CZ, central zone; PZ, peripheral zone; PM, pith-rib meristem; LP, leaf primordia; SP, potential spikelet sites. After Bernier 1971.

FIG. 8.6. Target tissues in the apical meristem of dicotyledonous species (right) and the shoot tip of grasses (left). CZ, central zone; PZ, peripheral zone; PM, pith-rib meristem; LP, leaf primordia; SP, potential spikelet sites. After Bernier 1971.

organisation when kept for protracted periods under conditions which do not allow flowering (Bernier, 1988). Some of the changes are similar to those occurring during the floral transition, although they are usually slower and less pronounced. Evocation can be considered to be complete at the time at which the meristem is irreversibly committed to flower. In Sinapis the point at which inhibitors can no longer prevent flowering occurs at the time when most histological and morphological changes begin and more than 12 h before the first structural signs of flower initiation can be observed (Bernier, 1986; Bernier et al., 1981). The key processes in evocation are, therefore, likely to be the earliest cellular and molecular events rather than structural changes.

Was this article helpful?

0 0

Post a comment