A NONINVASIVE MEASUREMENT OF PHLOEM AND XYLEM WATER-FLOW IN CASTOR BEAN SEEDLINGS BY NUCLEAR-MAGNETIC-RESONANCE MICROIMAGING

A flow-sensitive nuclear magnetic resonance (NMR) microimaging techniq ue was applied to measure directly the in-vivo water flow in 6-d-old c astor bean seedlings. The achieved in-plane resolution of the techniqu e allowed discrimination between xylem and phloem water flow. Both the xylem- and the phloem-average flow velocities in the intact seedling could be quantified. Furthermore, the total conductive cross-sectional area of the xylem vessels and the phloem sieve elements could be dete rmined using the non-invasive and non-destructive NMR microimaging tec hnique. Hence, it was possible to calculate the in-vivo volume flow ra tes for both xylem and phloem water How. Our non-destructive technique showed that previously used methods to measure phloem water flow affe cted the flow rate itself. In the intact seedlings we found values of 16.6 mu l . h(-1), two fold lower than those previously estimated from phloem exudation rates. Finally, our results demonstrate for the firs t time that water is internally circulated between phloem and xylem, a nd that water flow within the xylem is maintained by this internally c irculated water, even in the absence of any significant transpiration or evaporation.