| Summary: | The aim of this study was to determine the effect of temperature and CO<sub>2</sub> on seed emergence, seedling quality, and phenological stage of <i>Capsicum chinense</i> and <i>Capsicum annum</i> cultivated in four controlled growth chambers (C1: 30 °C and 400 μmol CO<sub>2</sub> mol<sup>−1</sup>; C2: 40 °C and 1200 μmol CO<sub>2</sub> mol<sup>−1</sup>; C3: 30 °C and 1200 μmol CO<sub>2</sub> mol<sup>−1</sup>; C4: 40 °C and 400 μmol CO<sub>2</sub> mol<sup>−1</sup>). Neither temperature nor elevated CO<sub>2</sub> influenced seed emergence, although differences were observed in seedling mortality, with high temperature affecting seedling survival in both species; the mortality rate at 40 °C was 20 and 53% in <i>C. annuum</i> and 45 and 58% in <i>C. chinense</i> at 400 and 1200 μmol CO<sub>2</sub> mol<sup>−1</sup>, respectively. Differences were also observed in growth parameters, where positive effects were observed on leaf area, which reached 45.9 cm<sup>2</sup> in <i>C. annuum</i> and 23.9 cm<sup>2</sup> in <i>C. chinense</i> with elevated CO<sub>2</sub> at 30 °C, but negative effects were observed with high temperature. CO<sub>2</sub> enrichment increased flower and fruit production per plant. However, high temperature delayed flower phenology, increased flower abortion and inhibited fruit set. Elevated CO<sub>2</sub> counteracted the detrimental effects of high temperature on growth parameters and flower number, but this was not sufficient to prevent flower abortion and the detrimental morphological characteristics of fruit caused by a temperature of 40 °C.
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