H+ AND Ca2+ IN PHLOEM AND SYMPLAST: RELATION OF RELATIVE IMMOBILITY OF THE IONS TO THE CYTOPLASMIC NATURE OF THE TRANSPORT PATHS

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SUMMARY

The capacity of the symplast and the phloem to transport Ca2+ and H* (OH) is discussed in relation to the concentrations of free and bound Ca2+ and H* in these transport systems, and the general restrictions on the concentrations of free Ca2+ and H* in cytoplasm. It is concluded that the low Ca2+ and H+ transport capacity of symplast and phloem are consequences of the cytoplasmic nature of these transport systems. The significance of the low transport capacity for these two ions relative to the rate at which the plant produces or consumes H+ or can take up Ca2+ and transport it in the xylem is discussed. Other transport characteristics of the phloem can also be related to the cytoplasmic nature of the transport conduit: the nature of the transported organic C and N compounds is partly dependent on the requirement that high concentrations of these solutes can be tolerated by cytoplasmic enzymes, i.e. sucrose, mannitol and amino-acids are ‘compatible solutes'. The properties of the intracellular transport systems (symplast and phloem) are contrasted with those of extracellular transport systems such as plant xylem and animal blood.

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