Association genetics, geography and ecophysiology link stomatal patterning in Populus trichocarpa with carbon gain and disease resistance trade-offs ATHENA D. MCKOWN,* ROBERT D. GUY,* LINDA QUAMME,* JAROSLAV KL  AP  ST  E,* † JONATHAN LA MANTIA,* C. P. CONSTABEL, ‡ YOUSRY A. EL-KASSABY,* RICHARD C. HAMELIN,* MICHAEL ZIFKIN ‡ and M. S. AZAM* *Department of Forest and Conservation Sciences, Faculty of Forestry, Forest Sciences Centre, University of British Columbia, 2424 Main Mall, Vancouver, BC V6T 1Z4, Canada, †Department of Genetics and Physiology of Forest Trees, Faculty of Forestry and Wood Sciences, Czech University of Life Sciences, Prague 165 21, Czech Republic, ‡Department of Biology, Centre for Forest Biology, University of Victoria, Victoria, BC V8W 3N5, Canada Abstract Stomata are essential for diffusive entry of gases to support photosynthesis, but may also expose internal leaf tissues to pathogens. To uncover trade-offs in range-wide adaptation relating to stomata, we investigated the underlying genetics of stomatal traits and linked variability in these traits with geoclimate, ecophysiology, condensed foliar tannins and pathogen susceptibility in black cottonwood (Populus trichocarpa). Upper (adaxial) and lower (abaxial) leaf stomatal traits were measured from 454 accessions collected throughout much of the species range. We calculated broad-sense heritability (H 2 ) of stomatal traits and, using SNP data from a 34K Populus SNP array, performed a genome-wide association studies (GWAS) to uncover genes underlying stomatal trait variation. H 2 values for stomatal traits were moderate (average H 2 = 0.33). GWAS identified genes associated primarily with adaxial stomata, including polarity genes (PHABULOSA), stomatal development genes (BRASSINOSTEROID-INSENSITIVE 2) and disease/wound-response genes (GLUTAMATE-CYSTEINE LIGASE). Stomatal traits correlated with latitude, gas exchange, condensed tannins and leaf rust (Melampsora) infection. Latitudinal trends of greater adaxial stomata numbers and guard cell pore size corresponded with higher stomatal conductance (g s ) and photosynthesis (A max ), faster shoot elongation, lower foliar tannins and greater Melampsora susceptibility. This suggests an evolutionary trade-off related to differ- ing selection pressures across the species range. In northern environments, more adaxial stomata and larger pore sizes reflect selection for rapid carbon gain and growth. By contrast, southern genotypes have fewer adaxial stomata, smaller pore sizes and higher levels of condensed tannins, possibly linked to greater pressure from natural leaf pathogens, which are less significant in northern ecosystems. Keywords: adaxial–abaxial patterning, amphistomaty, evolutionary trade-offs, genome-wide association studies, Melampsora, stomatal conductance, stomatal ratio Received 4 June 2014; revision received 29 September 2014; accepted 12 October 2014 Introduction Variation in traits arising within a species across its distribution may indicate differing selection pressures along environmental clines and functional dichotomies, or trade-offs, relating to range-wide adaptation. Correspondence: Athena D. McKown, Fax: 604-822-8645; E-mail: admckown@gmail.com © 2014 John Wiley & Sons Ltd Molecular Ecology (2014) doi: 10.1111/mec.12969