A biophysical analysis of stem and root diameter variations in woody plants

A comprehensive model of stem and root diameter variation was developed. Th e stem (or root) was represented using two coaxial cylinders corresponding with the mature xylem and the extensible tissues. The extensible tissues we re assumed to behave as a single cell separated from the mature xylem by a virtual membrane. The mature xylem and the extensible tissues are able to d ilate with temperature and grow. Moreover, the extensible tissues are able to shrink and swell according to water flow intensity. The model is mainly based on the calculation of water volume flows in the "single cell" that ar e described using the principles of irreversible thermodynamics. The elasti c response to storage volume and plastic extension accompanying growth are described. The model simulates diameter variation due to temperature, solut e accumulation, and xylem, water potential. The model was applied to the pe ach (Prunus persica) stem and to the plum (Prunus domestica X Prunus spinos a) root. The simulation outputs corresponded well with the diameter variati on observed. The model predicts that variations,of turgor pressure and osmo tic potential are smaller than the variations of xylem water potential. It also demonstrates correlations between the xylem water potential, the turgo r pressure, the elastic modulus, and the osmotic potential. The relationshi p between the diameter and the xylem water potential exhibits a subtential hysteresis, as observed in field data. A sensitivity analysis using the mod el parameters showed that growth and shrinkage were highly sensitive to the initial values of the turgor pressure and to the reflection coefficient of solutes. Shrinkage and growth were sensitive to elastic modulus and wall-y ielding threshold pressure, respectively. The model was not sensitive to ch anges in temperature.