Relationships between phenolic acid concentrations, transpiration, water utilization, leaf area expansion, and uptake...

Phenolic acid treatments of cucumber seedlings (Cucumis sativus cv "Early Green Cluster") inhibited transpiration, water utilization, leaf area, and absolute and relative rates of leaf expansion. The cinnamic acids, ferulic and p-coumaric acids, were two to five times more inhibitory than the benzoic acids, p-hydroxybenzoic acid and vanillic acid. When phenolic acid concentrations were maintained at inhibitory concentrations through multiple successive treatments, percent inhibition of water utilization remained relatively constant for a given concentration and phenolic acid, percent inhibition of leaf area initially increased and then leveled off to a constant percent, and percent inhibition of transpiration and rates of leaf area expansion declined over time. Subsequently, p-coumaric acid was chosen as the model compound for further study. When p-coumaric acid was inhibitory, percent inhibition of transpiration, water utilization, and rates of leaf area expansion of actively growing leaves rapidly declined (i.e., was lost) as p-coumaric acid concentrations surrounding roots decreased. Absolute and relative rates of leaf expansion, for example, declined approximately 12 and 14%, respectively, for every 0.1 mM decline in p-coumaric acid concentration. Uptake of p-coumaric acid by cucumber seedling roots was continuous over the 24- or 36-hr periods monitored, but was not consistently related to the initial p-coumaric acid treatment concentrations. However, declining p-coumaric acid concentrations monitored at 6- or 12-hr intervals over the 24- or 36-hr periods continued to be highly correlated to the initial p-coumaric acid treatment concentrations. A 25% depletion by 1 3-d-old cucumber seedlings took 8.5, 12, 19.5, 25, and 29.5 hr for 0.125-, 0.25-, 0.5-, 0.75-, and 1-mM treatments, respectively. Uptake during periods when phenolic acid concentrations and root uptake (depletion from solution) were related appeared to represent periods dominated by apoplastic movement into the intercellular spaces of roots. Uptake during periods without this relationship likely represented periods dominated by symplastic movement. The ability of cucumber seedlings to modify active phenolic acid concentrations surrounding their roots suggests that cucumber seedling can directly influence the magnitude of primary and secondary effects of phenolic acids through feedback regulation.