3 - Long-Distance Transport in the Xylem and Phloem and Its Regulation

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The chapter discusses how the long-distance transport of water and solutes—mineral elements and low-molecular-weight organic compounds—takes place in the vascular system of xylem and phloem. Xylem transport is driven by the gradient in hydrostatic pressure (root pressure) and by the gradient in the water potential. In contrast to the xylem, long-distance transport in the phloem takes place in the living sieve tube cells and is bidirectional. Xylem transport covers the composition and concentration of mineral elements and organic solutes in the xylem sap that depends on factors such as plant species, mineral element supply to the roots, assimilation of mineral nutrients in the roots, and nutrient recycling. The chapter also explains that the major interactions are exchange adsorption of polyvalent cations in the cell walls, and resorption (uptake) and release of mineral elements and of organic solutes by surrounding living cells (xylem parenchyma and phloem). An increase in the concentration of elements in the nutrient medium can enhance the effect of transpiration rate on their uptake and translocation. Within the shoot, it depends on both the transpiration rates and the duration of transpiration. Phloem transport outlines the principles of transport and phloem anatomy, composition of phloem sap, and mobility in phloem. There is good evidence that callose can swell rapidly and fill the pores, thus blocking long-distance transport in the sieve tubes. In the regulation of long-distance transport, exchange of solutes between the two conducting systems is very important. Information is scarce about the opposite process, phloem-to-xylem transfer.

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