Effect of K+ nutrition, leaf age and light intensity on apoplastic K+ in leaves of Vicia faba

The infiltration-centrifugation method is valuable for the estimation of io n concentrations within the leaf apoplast because it opens up the possibili ty of differentiating between free and bound cations. The K+ concentration in the apoplast of intact leaves of Vicia Saba reflects the nutritional sta tus with respect of this ion and can be monitored by the infiltration-centr ifugation technique. Transfer into a nutrient solution without K+ led to a significant decrease in apoplastic K+ within two days. Under K+ deficiency, the K+ concentration in the leaf apoplast was maintained at 2 mM. Regardle ss of K+ supply, young leaves had higher K+ concentrations in the apoplast than older ones. Data on diurnal variations in apoplastic K+ suggest that t he leaf apoplast is a transient pool for K+ accumulation within the leaf pr ior to absorption into the symplast or retranslocation to other growing tis sues.