Model for Steady State Coupled Transport in Xylem and Phloem

Abstract

AbstractKnowledge of water potentials throughout the xylem and phloem elements is required for understanding effects of water stress on transport of water, solutes, and carbohydrates in plants. The interaction between water potentials in xylem and phloem is of particular concern. This knowledge is difficult to obtain through experimentation but insights can be gained by means of computer programs based on mathematical statements of the processes involved. A steady state transport model was developed to gain better understanding of this interaction. The model consists of a set of equations developed from compartmental analysis of coupled xylem and phloem transport in plant roots, stems, and leaves. The set of equations is solved using a Newton‐Raphson iterative procedure. Simulations with user specified air temperature, relative humidity, water potential at the root surface, and carbohydrate concentrations in leaf and root as input variables show the effect of water stress on rate of xylem and phloem transport at six points in time during a 1 d cycle of changing air temperature and relative humidity. Maintenance of phloem pressure gradients is shown to be very sensitive to changing environmental conditions. Using user specified input, xylem and phloem transport rates were highest at midday. At 1800 h the xylem and phloem transport rates were 83 and 30% of maximum rates that occurred for both rates at 1200 h. The model provides a basis for conceptual analysis of effects of water stress on the food distribution in plants. Further development requires a systematic sensitivity analysis and comparison with experimental results.