Steps toward an improvement in process-based models of water use by fruit trees: A case study in olive

We applied two process-based models in a hedgerow olive orchard with the aim of understanding the limitations and mechanisms behind the control of transpiration in olive trees under drip irrigation. One model is based on the biophysics of water flow through the porous media of soil and xylem. The other is a hydromechanical model based on the observed dependence of stomatal aperture on whole-plant and epidermis water relations. The experiments were made in a hedgerow olive orchard (1667 trees ha−1) planted with 5-year-old 'Arbequina' trees. Measurements were made in control trees irrigated to replace 100% of the crop water needs, and in trees under regulated deficit irrigation (RDI) strategy, in which irrigation replaced ca. 30% of the control. Soil physical properties, root distribution, leaf area, sap flow, leaf osmotic pressure and key variables of leaf gas exchange and water status were measured and models were applied. Results show how in our orchard, with a shallow root distribution and very coarse soil, most of the limitation to transpiration was imposed by the hydraulics of the rhizosphere. The model shows how this limitation was related to the ratio of root to leaf area, and how this ratio can be managed by canopy pruning or by changing the number of drippers. Likewise, osmotic adjustment occurred similarly in both irrigation treatments, despite differences found on leaf water potential. Water stress largely affected plant hydraulic conductivity of RDI trees. A potential involvement of regulating signals, other than purely hydraulics, was evident in both treatments, although our data suggests that these signals were not regulated by the soil water status only.