Xylem and Phloem Transport of Mineral Nutrients fromSolanum tuberosumRoots

The xylem and phloem transport of mineral elements from stem nodal roots to the stem and stolon of growing potato (Solanum tuberosum L. cv. ‘Russet Burbank’) plants was investigated. Adventitious roots, originating from below-ground nodes of the stem of potato seedlings, were exposed to solutions of SrCI2 or MnSO4. Relative elemental concentrations were measured in the conductive tissues using energy dispersive X-ray analysis. After a 5 h daylight uptake period, Sr (a Ca-transport analogue) levels were elevated in the stem xylem tissue, but Sr did not increase in the stem phloem, nor was it present in either of the conductive tissues of stolons located 1–2 nodes above the treated roots. In contrast, elevated levels of Cl, S, and Mn were found in stolon xylem and phloem tissue during the same period. The absence of Sr in the stolon after 5 h suggests that no xylem flow into the stolon occurred during the uptake period and, furthermore, phloem flow is responsible for the transport of the Cl, S, and Mn into the stolon. Elevated levels of these mobile nutrients in the xylem of the stolon were attributed to xylem-to-phloem transfer in the stem or leaves, transport to the stolon in the phloem, and phloem-to-xylem transfer in the stolon. During a 19 h uptake period, some Sr was observed in the phloem tissue of the stem, demonstrating slow exchange of Sr with sieve elements or proximal phloem parenchyma and companion cells.