XVI International Botanical Congess

Leaf expansion rate varies with temperature, evaporative demand and soil water status. (i) Time courses of relative leaf expansion rate and of epidermal cell division rate are conserved among a large range of environmental conditions provided that durations and rates are expressed in thermal time. Maximum rates are common to all zones of a leaf and to all leaves of a plant. (ii) In dicots, a water deficit imposed during the first half of leaf development affects the relative expansion rate and division rate during the deficit only, but permanently affects the absolute rates. (iii) Expansion rate is related to evaporative demand and to the concentration of ABA in the xylem sap, with relationships which apply to field and laboratory. (iv) Tissue expansion and cell division behave as independent processes which determine cell area.