EXUDATION RATE AND HYDRAULIC CONDUCTIVITY OF MAIZE ROOTS ARE ENHANCED BY SOIL DRYING AND ABSCISIC-ACID TREATMENT

When young maize seedlings were fed with abscisic acid (ABA) into their root systems, the exudation rate from roots through cut stems was enhanced more than leaf diffusive resistance. Enhanced root exudation was also observed when mildly droughted plants were re-watered. To compare the mechanisms involved in this response under these two treatments, we examined solute transport in exudates and in situ whole root hydraulic conductivity following ABA feeding to the roots or soil drying/re-watering treatment. Increases of both solute concentration and delivery rate in the exudates of pre-droughted plants appeared to account for about half of the enhanced exudation rate. ABA-fed plants showed only a marginal increase of solute delivery rate and in fact a somewhat reduced concentration of solutes in their exudates. In situ root hydraulic conductance, assessed by the pressure-flow method on undisturbed roots, was greatly increased by ABA or soil drying treatments. The results indicate that drought-induced ABA in roots improved root hydraulic conductivity and raised root pressure when droughted plants were re-watered.