Dynamic studies of phloem and xylem flow in fully differentiated plants byfast nuclear-magnetic-resonance microimaging

A fast nuclear-magnetic-resonance imaging method was developed in order to measure simultaneously and quantitatively the water Row velocities in the x ylem and the phloem of intact and transpiring plants. Due to technical impr ovements a temporal resolution of 7 min could be reached and flow measureme nts could be performed over a time course of 12-30 h. The novel method was applied to the hypocotyl of 35- to 40-day-old leafy plants of Ricinus commu nis which were subjected to different light-dark regimes. The results showe d that the xylem Row velocities and the xylem Volume flow responded immedia tely to light on-off changes. Upon illumination the flow velocity and the v olume flow increased as expected in respect to literature. In contrast, the phloem Row velocity did not change in response to the light-dark regimes. Interestingly, though, the volume flow in the phloem increased during darkn ess. These findings can be explained by assuming that the conducting area o f the phloem becomes enlarged during the dark period due to opening of siev e pores.