ROOT-GROWTH, DEVELOPMENTAL-CHANGES IN THE APEX, AND HYDRAULIC CONDUCTIVITY FOR OPUNTIA-FICUS-INDICA DURING DROUGHT

Developmental changes in the root apex and accompanying changes in lat eral root growth and root hydraulic conductivity were examined for Opu ntia ficus-indica (L.) Miller during rapid drying, as occurs for roots near the soil surface, and more gradual drying, as occurs in deeper s oil layers. During 7 d of rapid drying (in containers with a 3-cm dept h of vermiculite), the rate of root growth decreased sharply and most root apices died; such a determinate pattern of root growth was not du e 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 ra pid drying. During 14d of gradual drying (in containers with a 6-cm de pth 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%. R oot hydraulic of meristematic cells decreased 30%, conductivity (L-P) decreased to one half during both drying treatments; L-P 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. fi cus-indica near the surface die, whereas deeper in the substrate cell division and elongation in root apices continue. Water uptake in respo nse to rainfall in the field can be enhanced by lateral root prolifera tion near the soil surface and additionally by resumption of apical gr owth for deeper roots.