Ontogenetic changes in potassium transport in xylem of tomato

The influence of plant ontogeny on xylem exudate K + concentrations and K + transport to the shoot was studied in both nutrient‐solution and field‐grown tomato plants ( Lycopersicon esculentum ). K + concentrations in xylem exudate from decapitated plants decreased during tomato plant development from a high of 12 m M to a low of 5 m M . In the nutrient‐solution plants, the most rapid decline occurred during the vegetative growth phase, while in field‐grown plants, the xylem K + concentrations remained high during an‐thesis and then subsequently declined. The rapid decline in nutrient‐solution plants might be related to a decrease in the absorptive efficiency of the root system. In field‐grown plants, a reduction in the availability of assimilates to the root might account in part for the decrease in xylem exudate K + concentrations. The volume (ml h −1 plant −1 ) and the net rates of K + exudation (mmol h −1 plant −1 ) decreased dramatically as the fruits approached maturity. Since only a small reduction in xylem exudate K + concentrations occurred during fruiting, the hydraulic conductivity of the root system decreased as the tomato plants aged. It is proposed that the ontogenetic changes in xylem transport of K + contribute to a reduction in leaf free space K + concentration which would explain the decline in tomato leaf K + concentrations.