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 ion concentrations within the leaf apoplast because it opens up the possibility of differentiating between free and bound cations. The K⁺ concentration in the apoplast of intact leaves of Vicia faba reflects the nutritional status with respect of this ion and can be monitored by the infiltration-centrifugation 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. Regardless of K⁺ supply, young leaves had higher K⁺ concentrations in the apoplast than older ones. Data on diurnal variations in apoplastic K⁺ suggest that the leaf apoplast is a transient pool for K⁺ accumulation within the leaf prior to absorption into the symplast or retranslocation to other growing tissues.