Dynamics of concentrations and nutrient fluxes in the xylem of Ricinus communis - diurnal course, impact of nutrient availability and nutrient uptake

Diurnal courses of nutrient transport in the xylem and their response to ex ternal availability of nutrients were studied, In soil culture, maximal con centrations in all analysed substances were observed during night-time. Ove r experimental periods of up to 20 d, concentrations of some ions increased , most by accumulation in the soil. Stringent nutrient conditions were esta blished in a novel pressure chamber. An aeroponic nutrient delivery system inside allows the sampling of xylem sap from intact plants under full contr ol of the nutrient conditions at the root. Analysis of xylem transport unde r these highly defined conditions established that (1) diurnal variations i n concentrations and fluxes in the xylem are dominated by plant-internal pr ocesses; (2) concentrations of nutrients in the xylem sap are highly but sp ecifically correlated with each other; (3) nitrate uptake and nitrate flux to the shoot are largely uncoupled; and (4) in continuous light, diurnal va riations of xylem sap concentrations vanish. Step changes in nitrate concen trations of the nutrient solution established that (5) the concomitant incr ease in nitrate concentration and flux id the xylem is delayed by 2-3 h and is only transient. Diurnal variations of xylem sap composition and use of the new technique to elucidate xylem-transport mechanisms are discussed.