Developmental changes in the root apex and
accompanying changes in lateral root growth and root hydraulic
conductivity were examined for Opuntia ficus-indica
(L.) Miller during rapid drying, as occurs for roots near the
soil surface, and more gradual drying, as occurs in deeper
soil layers. During 7 d of rapid drying (in containers
with a 3-cm depth of vermiculite), the rate of root
growth decreased sharply and most root apices died; such a
determinate pattern of root growth was not due to
meristem exhaustion but rather to meristem mortality after 3 d
of drying. The length of the meristem, the duration of
the cell division cycle, and the length of the elongation zone
were unchanged during rapid drying. During 14 d of gradual
drying (in containers with a 6-cm depth of
vermiculite), root mortality was relatively low; the length
of the elongation zone decreased by 70%, the number
of meristematic cells decreased 30%, and the duration of the
cell cycle increased by 36%. Root hydraulic
conductivity (LP) decreased to one half
during both drying treatments; LP was
restored by 2 d of rewetting owing
to the emergence of lateral roots following rapid drying
and to renewed apical elongation following gradual drying.
Thus, in response to drought, the apical meristems of roots
of O. ficus-indica near the surface die, whereas deeper
in the substrate cell division and elongation in root
apices continue. Water uptake in response to rainfall in the field
can be enhanced by lateral root proliferation near the
soil surface and additionally by resumption of apical growth
for deeper roots.