COUPLING OF A STEM, FRUIT AND LEAF MODEL TO PREDICT STEM DIAMETER VARIATIONS IN TOMATO

Stem diameter variations have recently been identified to contain invaluable information about the plant water and carbon relations. Although continuous measurements of stem diameter variations are relatively easy to carry out, the interpretation is quite difficult, because of interacting effects of water and carbon status and possible variations in hydraulic conductance within the plant. Therefore mechanistic modelling is required to interpret the variations in stem diameter. In this research, a flow and storage stem model for tomato which links the plant water uptake, plant water status and stem diameter variations, was coupled to a tomato fruit model describing water and carbon transport towards the fruit and a leaf model predicting actual photosynthesis rates, carbohydrate metabolism and sucrose loading towards the stem phloem. Model inputs were microclimatic measurements such as air temperature, relative humidity, photosynthetically active radiation and ambient CO₂ concentration, and continuous measurements of sap flow. Governing equations use the simulation of the total, osmotic and hydrostatic water potential in each compartment to model water and carbon transport between the respective compartments. The mechanistic nature of the different sub-models allowed a smooth coupling to each other and enabled to visualise integrated effects of microclimate on plant behaviour. Effects on stem diameter variations of variable carbon status could be distinguished from variations in water status.