Xylem flow and its driving forces in a tropical liana: concomitant flow-sensitive NMR imaging and pressure probe measurements

Flow-sensitive NMR imaging and pressure probe techniques were used for meas uring xylem water flow and its driving forces (i.e., xylem pressure as well as cell turgor and osmotic pressure gradients) in a tropical liana, Epipre mnum aureum. Selection of tall specimens allowed continuous and simultaneou s measurements of all parameters at various distances from the root under d iurnally changing environmental conditions. Well hydrated plants exhibited exactly linearly correlated dynamic changes in xylem tension and flow veloc ity. Concomitant multiple-probe insertions along the plant shoot revealed x ylem and turgor pressure gradients with changing magnitudes due to environm ental changes and plant orientation (upright, apex-down, or horizontal). Th e data suggest that in upright and - to a lesser extent - in horizontal pla nts the transpirational water loss by the cells towards the apex during the day is not fully compensated by water uptake through the night. Thus, long itudinal cellular osmotic pressure gradients exist. Due to the tight hydrau lic coupling of the xylem and the tissue cells these gradients represent (b esides the transpiration-induced tension in the xylem) an additional tensio n component for antigravitational water movement from the roots through the vessels to the apex.