Co-ordination of cell division and tissue expansion in sunflower, tobacco,and pea leaves: Dependence or independence of both processes?

Temporal analyses of cell division and tissue expansion in pea, tobacco, an d sunflower leaves reveal that both processes follow similar patterns durin g leaf development. Relative cell division and relative tissue expansion ra tes are maximal and constant during early leaf development, but they declin e later. In contrast, relative cell expansion rate follows a bell-shaped cu rve during leaf growth. Cell division and tissue expansion have common resp onses to temperature, intercepted radiation, and water deficit. As a conseq uence, final leaf area and cell number remain highly correlated throughout a large range of environmental conditions for these different plant species , indicating that cell division and tissue expansion are co-ordinated durin g leaf development. This co-ordination between processes has long been expl ained by dependence between both processes. Most studies on dicotyledonous leaf development indicate that leaf expansion rate depends on the number of cells in the leaf. We tested this hypothesis with a large range of environ mental conditions and different plant species. Accordingly, we found a stro ng correlation between both absolute leaf expansion rate and leaf tell numb er. However, we showed that this relationship is not necessarily causal bec ause it can be simulated by the hypothesis of independence between cell div ision and tissue expansion according to Green's theory of growth (1976).