K⁺ ions improve hydraulic conductance of grapevine shoots for sucrose loading aimed at manipulation of vine carbohydrate reserves

Introduction - Poor fruitfulness of subtropical table grapevines can reflect a low carbohydrate (CHO) status of the plant during floral bud formation and fruit development. CHO reserves can ameliorate poor fruitfulness. Such supplementation can be achieved by sucrose injection into the grapevine trunk; however, poor hydraulic conductance of stem tissues limits the success of this procedure. The objective of this study was to evaluate the co-addition of KCl to the sucrose solution to improve the hydraulic conductivity of grapevine shoots. Materials and methods - The hydraulic conductance of 15 cm long current season cane cuttings was assessed under a pressure gradient of 7.84 kPa during perfusion with water, KCl (1 to 50 mM) and sucrose solutions (2.5%, 5% and 7.5% w/v) in 0 and 10 mM KCl. Experiments were conducted over both short (about 2 hours) and long (about 78 hours) term. Additionally, transpiration of dye solutions enabled estimation of direct xylem conduits was undertaken. Results and discussion - Hydraulic conductance increased by about 20% with perfusion of ≥ 10 mM KCl compared to water, with the effect reversible and repeatable. Sucrose solutions caused a decrease in conductance relative to water. Co-addition of KCl mitigated the sucrose-mediated decrease in conductance. There was no evidence of physical vessel blockage such as tyloses or microbiota. Conclusion - The results are consistent with the hypothesis that cations cause shrinkage of pectins, increasing the conductance of pit membranes.