WILL INCREASES IN OUR UNDERSTANDING OF SOIL-ROOT RELATIONS AND ROOT SIGNALING SUBSTANTIALLY ALTER WATER FLUX MODELS

Three models combining water transport and stomatal control are compar ed with experimental data to test whether, and how, water flux models that take into account root signalling or more accurate calculations o f root water status would differ from current models. Models under stu dy involve stomatal control by either the leaf water status alone, or a root message alone, or an interaction between both mechanisms. They are combined with a model allowing accurate calculation of the root wa ter status in the case when roots are not regularly disposed. The mode l involving leaf water status alone provides relatively realistic pred ictions of water relations, but only on a day-to-day timescale and for relatively constant environmental conditions. The model based on root messages alone also needs adjustments, as it does not allow control o f leaf water status during a drying period. The model involving intera ction provides simulations which are reasonably consistent with experi mental data, and applies to a range of environmental conditions withou t the necessity of adjusting its parameters for each condition. Effect s of rooting characteristics, such as those caused by soil compaction, on stomatal conductance and root messages can only be predicted if a direct calculation of root water potential is carried out, therefore a voiding the hypotheses which are implicit in Gardner's classical calcu lation.