Modeling translocation and metabolism in lettuce

I present a whole-plant model of movement of water and nutrients that predicts growth. A plant consists of tissues; root, stem, and leaf, and each tissue has cells with compartments; apoplast, cytoplast, phloem, and xylem. Diffusion leads to rapid short-distance transport of water among compartments. All metabolism of nutrients, such as sugar, nitrate, amino acid, protein, and complex carbohydrates (structure), occurs in cytoplast, and follows the same saturation kinetics in each tissue. Volume increases according to structure. Long-distance transport between tissues is in xylem and phloem. Plants take up nitrate in root by active transport, and photosynthesis of sugars occurs in leaf. Nitrate movement from root to leaf is driven by transpiration in xylem. Sugar is moved from leaf to root in phloem by translocation. Nitrate is transformed to ammonium and amino acid. Protein and structure are synthesized in cytoplast, and cannot move between tissues. Linear relations describe water potential, short-distance movement among compartments within one tissue, long-distance transport in xylem and phloem between tissues, and metabolism with each tissue. These relations are programmed in VENSIM (Ventana Systems, Harvard MA). I examine movement of sugar, nitrate, and water in xylem and phloem, and water content and metabolite concentrations in tissues of lettuce.