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John Erwin, Tanveer Hussein, and David J. Baumler

The instantaneous photosynthetic rate (Pn), transpiration rate (E), and stomatal conductance (g S) were measured for 33 outdoor-grown Capsicum varieties (varying in species of origin and indigenous habitat) between 29 July and 22 Aug. 2017 using a portable gas exchange meter. Cuvette leaf temperature (Tleaf) and relative humidity (RH) were recorded at that same time. Pn differed from 3.6 to 3.7 for ‘Malawi Piquante’ and ‘Korean Long Green’ peppers to 16.3 μmol CO2/m2/s (fixed) for ‘Thai Hot’ peppers. The g S differed from 0.01 to 0.05 among 13 varieties to 0.28 mmol H2O/m2/s for ‘Thai Hot’ peppers. E differed from 0.43 to 0.59 among three varieties to 4.14 to 4.20 mmol H2O/m2/s for ‘CGN 22091’ and ‘Peruvian Purple’ peppers. Water use efficiency (WUE; Pn/E) varied from 2.92 to 3.43 among three varieties to 5.10 to 7.20 for 16 other varieties. C. annuum derived varieties had higher Pn (9.4 μmol CO2/m2/s fixed) than varieties derived from other species (4.5–8.6 μmol CO2/m2/s fixed). Varieties originating from dry climates had higher Pn (12.5 μmol CO2/m2/s fixed) than those originating from temperate or tropical climates (8.0–8.8 μmol CO2/m2/s fixed). Tleaf (27 to 33 °C) and RH (38% to 39% and 57% to 59%) differed among varieties. Pn was positively correlated with g S, E, and RH and was negatively correlated with WUE. We found that Capsicum Pn, E, and g S varied more than has been previously reported, and our data suggested that Pn, g S, and E data of outdoor-grown peppers should be used only when selecting parents for a breeding program (unless progeny is intended for greenhouse production).