Bundle sheath cells of small veins in maize leaves are the location of uptake from the xylem

Rb+ as a tracer for K+ was used to test the hypothesis that uptake of K+ from xylem vessels of small veins into the symplast of maize leaves occurs at the xylem/bundle sheath cell interface. 22.5 min after immersing cut leaves into 20 mM RbCl+1 mM KCI, Rb+ appeared in the cells of the leaves. Sections of these leaves were freeze-dried. In cryo-thin sections (5 mum), Rb-85(+) and K-41(+) content was determined by laser microprobe mass analysis with a large resolution of about 1 mum. Determining the ratio of Rb-85(+) to K-41(+) in the cell walls and cytosols of bundle sheath cells, mesophyll cells, and in the cells between the xylem elements resulted in the following picture: In small veins, Rb+ entered the symplast directly at the xylem/bundle sheath cell interface.