Leaf Abscisic Acid Accumulation in Response To Substrate Water Content: Linking Leaf Gas Exchange Regulation with Leaf Abscisic Acid Concentration

ADDITIONAL INDEX WORDS. stomatal conductance, drought tolerance, genotype variation ABSTRACT. Quantitative differences in leaf abscisic acid (ABAL) among four cultivars of red maple (Acer rubrum L.) and one freeman maple (Acer ×freemanii E. Murray) cultivar were investigated. This study tested the hypothesis that ABAL concentration can be used to compare the effects of water stress on the gas exchange response of fi ve different maple genotypes, including four red maple cultivars ('Summer Red', 'October Glory', 'Autumn Flame', and 'Frank- sred' ('Red Sunset')) and one hybridized freeman maple cultivar ('Jeffersred' ('Autumn Blaze')). Two-year-old cloned genotypes of red maple and freeman maple were subjected to two treatments: irrigated daily to container capacity or irrigation withheld for one drought and recovery cycle. Leaf abscisic acid concentration, gas exchange, and whole- tree sap fl ow measurements were conducted under well-watered and drought stress conditions. Over the course of the drought stress and recovery phase, net photosynthesis (Anet), stomatal conductance (gs), and transpiration (E) declined as ABAL and instantaneous water use effi ciency (A/gs) increased. Until severe water stress conditions were prominent, water use was higher in 'Summer Red' as compared to 'October Glory'. This study found that ABAL tracked gs and that stomatal responsiveness to substrate moisture defi cit is likely mediated by ABA accumulation in leaf tissue. This research demonstrates a leaf level physiological response to substrate volumetric water content that appears to depend on ABAL concentration. In addition, the evidence in this study indicates that ABAL may be used as a potential surrogate for the gs response to substrate water stress and could become part of a cultivar drought tolerance selection strategy for red maple and freeman maple. Red maple and freeman maple are popular ornamental shade trees. Due to the aesthetic characteristics, genotypes of these maples are among the most commonly planted street trees. The suite of abiotic stresses in metropolitan areas, particularly drought, underscores the need to improve the stress tolerance of shade trees under a wide range of environmental situations. There is considerable evidence to suggest that stomatal conductance (gs) may be regulated by chemical signals in ecotypes of red maple (Bauerle et al., 2003a, 2004a). The short-term response to chemi- cal signals could mediate gs, and the regulation of gs over the long-term could occur by a gradual change in concentration that remained reasonably constant as the soil became progressively drier (Whitehead, 1998). This observation reveals that a cultivar selection strategy, based on the concentration of chemical signals in leaf tissue, may facilitate selection of plants suited to tolerate water stress. Evaluation of genetic variability in drought tolerance requires a framework of analysis that allows for genetic differences in stomatal control (e.g., abscisic acid (ABA) concentration in comparison to gs at a given soil water status) to be distinguished from developmental differences (e.g., the effect of leaf area on the rate of soil water depletion) (Borel et al., 1997). Abscisic acid is generally considered a root-sourced hormone that moves from the roots to the shoots in the transpiration stream. Recently,