Carbon assimilation is the fundamental basis of crop productivity, but this important and complex trait has not been genetically characterized and directly exploited at the commercial level to improve yield. Therefore, there is a critical need to determine natural genetic variation in carbon assimilation, to advance our knowledge of the genetic mechanisms controlling this trait and, on that basis, to develop germplasm with superior photosynthetic capacity. Sorghum is the most productive annual species for biofuel production in which leaf photosynthetic capacity has been associated with biomass yield. In this study, a set of 82 sorghum accessions was genetically characterized and phenotypically evaluated for carbon assimilation (A), stomatal conductance (g s ), transpiration rate (T), efficiency of energy capture by open PSII reaction centers (Fv 0 /Fm 0 ), effective quantum yield (U PSII ) and photochemical quenching (qP). Phenotypic variation was observed for all traits, with A ranging from 11.6 to 42.5 lmol CO 2 m -2 s -1 . The highest positive correlations were between A and U PSII (r = 0.71), g s and T (r = 0.89) and U PSII and qP (r = 0.91). The highest negative correlations were between g s and A/g s (r = -0.82), T and A/g s (r = -0.79), T and A/T (r = -0.79) and A/g s and A/T (r = -0.78). Population structure and cluster analysis clearly differentiated three subpopulations among this set of accessions with significantly different values for A, g s and Fv 0 /Fm 0 . This study demonstrates that this diverse set of sorghum accessions could be utilized to identify genes/markers associated with variation in leaf photosynthetic rate and could be exploited in breeding programs to develop germplasm with superior carbon assimilation capacity. List of symbolsA Leaf photosynthetic rate (lmol CO 2 m -2 s -1 ) g s Stomatal conductance (mol H 2 O m -2 s -1 ) T Transpiration rate (mmol H 2 O m -2 s -1 ) Fv 0 /Fm 0 Efficiency of energy capture by open PSII reaction centers (unitless) U PSII Effective quantum yield (unitless) qP Photochemical quenching (unitless)

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