Simultaneous measurement of water flow velocity and solute transport in xylem and phloem of adult plants of Ricinus communis over a daily time courseby nuclear magnetic resonance spectrometry

A new method for simultaneously quantifying rates of flow in xylem and phlo em using the FLASH imaging capabilities of nuclear magnetic resonance (NMR) spectrometry was applied in this study. The method has a time resolution o f up to 4 min (for the xylem) and was used to measure the velocity of flows in phloem and xylem for periods of several hours to days. For the first ti me, diurnal time course measurements of flow velocities and apparent volume hows in phloem and xylem in the hypocotyl of 40-d-old Ricinus communis L w ere obtained. Additional data on gas exchange and the chemical composition of leaves, xylem and phloem sap were used to assess the role of leaves as s inks for xylem sap and sources for phloem. The velocity in the phloem (0.25 0 +/- 0.004 mm s(-1)) was constant over a full day and not notably affected by the light/dark cycle. Sucrose was loaded into the phloem and transporte d at night, owing to degradation of starch accumulated during the day. Conc entrations of solutes in the phloem were generally less during the night th an during the day but varied little within either the day or night. In cont rast to the phloem, flow velocities in the xylem were about 16-fold higher in the light (0.401 +/- 0.004 mm s(-1)) than in the dark (0.255 +/- 0.003 m m s(-1)) and volume flow varied commensurately. Larger delays were observed in changes to xylem how velocity with variation in light than in gas excha nge. The relative rates of solute transport during day and night were estim ated on the basis of relative flow and solute concentrations in xylem and p hloem. In general, changes in relative flow rates were compensated for by c hanges in solute concentration during the daily light/dark cycle. However, the major solutes (K+, NO3-) varied appreciably in relative concentrations. Hence the regulation of loading into transport systems seems to be more im portant to the overall process of solute transport than do changes in mass flow. Due to transport behaviour, the chemical composition of leaves varied during the day only with regard to starch and soluble carbohydrates.