Plasma membrane H+-ATPase, succinate and isocitrate dehydrogenases activities of vessel-associated cells in walnut trees

In winter and spring, walnut trees exhibit variations of sugar content in t he vascular sap. According to their location, the vessel-associated cells ( VACs, also called contact cells) could control nutrient exchanges between t he storage parenchyma and the xylem vessels. According to the literature, t he recovery of sap (influx) occurs at the VAC/vessel interface via an H+/su gar symport that depends on the transmembrane pH gradient generated by the plasma membrane H+-ATPase. The aim of this study was to investigate the ATP ase activity, using a perfusion technique that allows the use of several ef fectors: carbonyl cyanide m-chlorophenylhydrazone (CCCP) and fusicoccin (FC ). During winter, the uncoupler CCCP revealed a low pH gradient between the xylem vessels and the vessel-associated cells. Under these conditions, FC, an activator of the H+-ATPase, had no effect on the pH of the perfusion so lution, suggesting that the enzyme could be lightly active. In contrast, cl ose to bud break, a high pH gradient was revealed by the use of CCCP, and a n acidification of the perfusion solution was observed in the presence of F C. Moreover, cytochemical investigation showed high activity of two respira tory enzymes located in mitochondria: NAD-dependent isocitrate dehydrogenas e and succinate dehydrogenase. The hypothesis is that in spring this high r espiratory activity of VACs provides a consequent increase in available ATP that can be utilized by the plasma membrane H+-ATPase.