Soil bulk density impacts on root water potential and ABA export in drying soil

Abscisic acid (ABA) is a phytohormone known to regulate leaf gas exchange and water loss by inducing stomatal closure. ABA is synthesised in response to a variety of abiotic stressors in soil, particularly water deficit. Previous work demonstrated that across a range of soil textures, root water potential better predicted xylem ABA concentration than soil matric potential. However, the impacts of soil management practices (e.g. cultivation, compaction, organic matter addition) on the relationship between root water potential and xylem ABA concentration, when texture is held constant, has not been investigated. A loam-based growing substrate was compressed to three bulk densities (1.1, 1.3 and 1.4 g cm-3) in pots designed to fit in a Scholander-type pressure chamber, allowing the water potential of the bulk root system to be measured. After measuring root water potential, additional pressure was applied to collect root xylem sap at flow rates that matched transpirational flow. This allowed accurate determination of ABA concentrations and delivery. Low bulk density enhanced the increase in xylem ABA concentration as root water potential declined. Increasing bulk density de-sensitised the relationship between root water potential and xylem ABA concentration. Further study is required to determine whether changes in soil structure due to field management regimes will alter the relationship between root and soil water potential.