Significant contribution from foliage-derived ABA in regulating gas exchange in Pinus radiata

The complex regulatory system controlling stomata involves physical and chemical signals that affect guard cell turgor to bring about changes in stomatal conductance (g_s). Abscisic acid (ABA) closes stomata, yet the mechanisms controlling foliar ABA status in tree species remain unclear. The importance of foliage-derived ABA in regulating gas exchange was evaluated under treatments that affected phloem export through girdling and reduced water availability in the tree species, Pinus radiata (D. Don). Branch- and whole-plant girdling increased foliar ABA levels leading to declines in g_s, despite no change in plant water status. Changes in g_s were largely independent of the more transient increases in foliar non-structural carbohydrates (NSC), suggesting that gradual accumulation of foliar ABA was the primary mechanism for reductions in g_s and assimilation. Whole-plant girdling eventually reduced root NSC, hindering root water uptake and decreasing foliar water potential, causing a dramatic increase in ABA level in leaves and concentrations in the xylem sap of shoots (4032 ng ml^-1), while root xylem sap concentrations remained low (43 ng ml^-1). Contrastingly, the drought treatment caused similar increases in xylem sap ABA in both roots and shoots, suggesting that declines in water potential result in relatively consistent changes in ABA along the hydraulic pathway. ABA levels in plant canopies can be regulated independently of changes in root water status triggered by changes by both phloem export and foliar water status.