Spatial distributions of expansion rate, cell division rate and cell size in maize leaves: a synthesis of the effects of soil water status, evaporative demand and temperature

The spatial distributions of leaf expansion rate, cell division rate and ce ll size was examined under contrasting soil water conditions, evaporative d emands and temperatures in a series of experiments carried out in either co nstant or naturally fluctuating conditions. They were examined in the epide rmis and all leaf tissues. (1) Meristem temperature affected relative elong ation rate by a constant ratio at all positions in the leaf, If expressed p er unit thermal time, the distribution of relative expansion rate was indep endent of temperature and was similar in all experiments with low evaporati ve demand and no water deficit. This provides a reference distribution, cha racteristic of the studied genotype, to which any distribution in stressed plants can be compared, (2) Evaporative demand and soil water deficit affec ted independently the distribution of relative elongation rate and had near -additive effects. For a given stress, a nearly constant difference was obs erved, at all positions of the leaf, between the relative elongation rates of stressed plants and those of control plants. This caused a reduction in the length of the zone with tissue elongation. (3) Methods for calculating cell division rate in the epidermis and in all leaf tissues are proposed an d discussed, In control plants, the zone with cell division was 30 mm and 6 0 mm long in the epidermis and in whole tissues, respectively. Both this le ngth and relative division rate were reduced by soil water deficit. The siz e of epidermal and of mesophyll cells was nearly unaffected in the leaf zon e with both cell division and tissue expansion, suggesting that water defic it affects tissue expansion rate and cell division rate to the same extent. conversely, cell size of epidermis and mesophyll were reduced by water def icit in mature parts of the leaf.