General

Precise quantitative assessments on the relative importance of solute transport in the phloem and xylem into parts or organs of plants are difficult to make. For such assessments not only the concentrations of solutes are required, but also the velocity of transport and the cross-sectional area of the conducting vessels, according to the following relationship:

Specific mass transfer = velocity x concentration (g h-1 cm""2) (cm h-1) (mgmr1)

The velocity of transport in the xylem and phloem varies enormously. On average, velocities between 10 and 100 cm h_1 are often found, and the rates in the phloem are usually much lower than in the xylem. In fruit stalks of white lupins, maximal velocities of 22 cm h~1 in the phloem and 147 cm h~1 in the xylem have been reported (Pate et al., 1978).

Our present knowledge on the relative importance of xylem and phloem import and export of mineral elements into plant parts or organs is mainly based on detailed analyses of phloem and xylem sap in different shoot parts of individual plants and the corresponding mineral element contents in the shoot parts at sequential harvests (Jeschke etal, 1987; Jeschke and Pate, 1991a,b, 1992).

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