Lateral Transport of Ions into the Xylem of Corn Roots

Abstract

When an excised corn (Zea mays) root pretreated with chloride was exposed for 10 minutes to pulse labeling with 'Cl 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 'Na gave similar results. A large fraction of the absorbed 'Na 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.According to the classical Crafts-Broyer hypothesis on lateral transport of ions in roots (11), the stele does not participate actively in transfer from the symplasm to the xylem vessels. The membranes of the stelar cells are deemed permeable to ions, and hence ions simply leak into the vessels. The Casparian strip of the endodermis functions to prevent ions from diffusing backwards into the outer space of the cortex.Laties (25), Laties and Budd (26), and Luittge and Laties

Species differences in calcium and potassium distributions within plant roots

“…This pattern of K distribution supports the concept that K moves freely in the symplasm and penetrates the endodermis (28). K was found to be concentrated at the parenchyma cells around the xylem, suggesting that the xylem parenchyma cells may play an important role as an ion-pump in the radial transport of ions (18,20,21,28).…”

“…The distributions of more important nutrients such as K and Ca have also been studied in relation to their radial transport (4, 5,12,14,20,37). However, various unresolved problems still remain.…”

Trans‐root potential, xylem pressure, and root cortical membrane potential of ‘low‐salt’ maize plants as influenced by nitrate and ammonium

“…Using intact maize seedlings, Läuchli, Spurr & Epstein (1971) found that the K + concentration is significantly higher in the stelar cells than in the cortical cells. This would prevent symplastic K + transport into the stele by diffusion (unless the gradient found by these authors represents vacuolar, rather than cytoplasmatic concentrations).…”

Simultaneous recording of xylem pressure and trans‐root potential in roots of intact glycophytes using a novel xylem pressure probe technique

“…the so-called trans-root potential, provide a way to elucidate the contribution of electrical forces to solute and water transport through the root (Etherton & Higinbotham 1960;Bowling & Spanswick 1964;Shone 1969;Davis & Higinbotham 1969;Läuchli, Spurr & Epstein 1971;Cortes 1992;Rygol et al 1993;Clarkson 1993;Marschner 1995). Reliable data on these forces and their coupling to transpiration-induced changes in xylem pressure, and to turgor pressure of the hydraulically coupled root cells (Andrews 1976;Stahlberg & Cosgrove 1995;Schneider, Zhu & Zimmermann 1997a), are also urgently needed to explain the generation of radial turgor and osmotic pressure gradients through the root cortex of transpiring plants (Rygol & Zimmermann 1990).…”