Role of acropetal water transport in regulation of cytokinin levels in stems of pea seedlings

Removal of the main shoot of a plant (decapitation) considerably elevates the content in the stem of cytokinins (CKs) whose synthesis is supposedly induced by the decrease in IAA level. However, the decrease in auxin concentration in the stem does not always result in the expected elevation in CK content. The decapitation impairs also the acropetal water transport (xylem transport, XT) whose role in the control over CK level was not investigated. To clarify this issue, 11-day-old seedlings of pea (Pisum sativum L., cv. Adagumsky) were decapitated at the first internode, and in some decapitated plants XT was sustained by the suction method under vacuum of −0.055 MPa assuring a xylem sap flow rate of about 0.16 µL/s. In the analyzed tissues of the upper hypocotyl and the basal epicotyl parts, the predominant forms of zeatin-type cytokinins (Z-CKs) and isopentenyladenine-type cytokinins (iP-CKs) were ribotides, ribosides, and glucosides of ribosides. The main CK in the xylem sap was zeatin riboside (ZR) whose level was stable throughout 6-h-long maintenance of XT by vacuum suction. Following seedling decapitation, the content of iP-CKs increased after 3 h (by a factor of 2 in hypocotyls and 7 in epicotyls), whereas a four-to sixfold increase in Z-CKs level was observed only after 6 h. In 18 h after decapitation, it caused a 35-fold rise in the content of ZR; iRP rose 7 times in hypocotyls and 40 times in epicotyls. Maintenance of XT for 6 h after decapitation resulted in two-fold drop in accumulation of iP-CKs in hypocotyl and epicotyl tissues, making their levels equal, as it was in 3 h after decapitation without XT, and completely suppressed changes in the content of Z-CKs. The decrease in CK accumulation by means of washing out CKs from the tissues with the xylem sap flow was unlikely, because the ribotide/riboside ratio in tissues with XT did not show a trend toward an increase in the share of nontranslocated ribotides. Thus, the results suggest that XT can be involved, along with IAA, in control over CK content in the stem; the mechanism of XT effect requires further investigation.