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Lateral Transport of Ions into the Xylem of Corn Roots

II. Evaluation of a Stelar Pump ¹

^a Department of Soils and Plant Nutrition and Department of Vegetable Crops, University of California, Davis, California 95616

When an excised corn (Zea mays) root pretreated with chloride was exposed for 10 minutes to pulse labeling with ³⁰Cl and then transferred to unlabeled chloride, the activity in the xylem exudate reached a maximum about 4 minutes after pulse labeling was discontinued and then declined sharply. The rate at which labeled chloride was transported across the root into the xylem and basipetally therein was on the order of 75 to 250 centimeters per hour. Consequently, symplasmic movement of chloride in corn roots is fast and may not be rate-limiting in transfer from the root surface to the xylem. Experiments on pulse labeling with ²²Na gave similar results. A large fraction of the absorbed ²²Na was not translocated into the exudate but was tightly sequestered in a cell compartment, probably the vacuole.

Electron probe analysis was used to reveal the pattern of potassium distribution in cross sections taken 10 to 11 millimeters from the tip. The cytoplasm and vacuoles of the xylem parenchyma cells accumulated potassium to a much greater extent than cortical and other stelar cells. Ultrastructural studies showed that the cytoplasm of the xylem parenchyma cells contains numerous membrane systems. It was concluded that the xylem parenchyma cells secrete ions from the symplasm into the conducting vessels, and it was suggested that this secretion is driven across the plasmalemma by a carrier-mediated transport.

¹ Supported by a travel grant from the American-Swiss Foundation for Scientific Exchange to the senior author, and a grant from the Office of Saline Water, United States Department of the Interior, to E. Epstein. A preliminary report of this investigation was presented at the Annual Meeting of the American Society of Plant Physiologists in Bloomington, Ind., August 25, 1970.

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