Rapid flood-induced stomatal closure accompanies xylem sap transportation of root-derived acetaldehyde and ethanol in Forsythia

Plants grown in containers frequently suffer from difficulties in managing their water status due to either insufficient or too much water. In the case of the latter, little information is available regarding how container-grown woody plants respond to anaerobic media. The aims of this work were therefore to use Forsythia as a model woody plant system to provide a mechanistic understanding of the physiological events and their timing during soil flooding. Exposure of pot-grown Forsythia to root hypoxia had a dramatic effect on leaf growth and stomatal conductance. Within 24h of flooding a decline in leaf growth rate was detected along with a reduction in stomatal conductance. The effects of hypoxia appear initially with older leaves, but if flooding is prolonged (> 2 days) younger expanding leaves are affected. These responses and their timing have not been described for woody perennial plants but appear comparable to those described for herbaceous plants such as tomato and castor bean. Measurements of stem and leaf tissue and during flooding showed large time dependent increases in the concentrations of acetaldehyde and ethanol; products associated with anaerobic respiration. Both of these chemicals were shown to be root-derived and are produced in a significant amount only as flooding time increases and the decline in leaf growth and conductance become apparent. Xylem sap was collected at a range of flow rates to measure whole root system conductance and determine changes in delivery of sap flux constituents. Calculated delivery rates of acetaldehyde and ethanol changed little with sap flow, particularly in well-drained control plants, while root hydraulic conductance declined when measured, 4 days after flooding. However, neither acetaldehyde nor ethanol. when used in a detached leaf transpiration bioassay, at physiologically realist concentrations (as determined from sap collection) failed to induce a dramatic reduction in leaf transpiration rates. The reasons for this discrepancy are discussed. (c) 2008 Elsevier B.V. All rights reserved.