WATER TRANSPORT IN PLANTS - ROLE OF THE APOPLAST

The present state of modelling of water transport across plant tissue is reviewed. A mathematical model is presented which incorporates the cell-to-cell (protoplastic) and the parallel apoplastic path. It is sh own that hydraulic and osmotic properties of the apoplast may contribu te substantially to the overall hydraulic conductivity of tissues (Lp( r)) and reflection coefficients (sigma(sr)). The model shows how water and solutes interact with each other during their passage across tiss ues which are considered as a network of hydraulic resistors and capac itances ('composite transport model'). Emphasis is on the fact that hy draulic properties of tissues depend on the nature of the driving forc e. Osmotic gradients cause a much smaller tissue Lp(r) than hydrostati c. Depending on the conditions, this results in variable hydraulic res istances of tissues and plant organs. For the root, the model readily explains the well-known phenomenon of variable hydraulic resistance fo r the uptake of water and non-linear force/flow relations. Along the c ell-to-cell (protoplastic) path, water flow may be regulated by the op ening and closing of selective water channels (aquaporins) which have been shown to be affected by different environmental factors.