Summary Effects of moderate heat on growth and photosyn- thesis were investigated in two clonal genotypes of Acer rub-

thesis were investigated in two clonal genotypes of Acer rubrum L., originally collected from the thermally contrasting habitats of Florida and Minnesota, USA, and known in the horticultural trade for sensitivity and insensitivity to heat, respectively. Under both common garden and warm greenhouse conditions (day/night temperature of 33/25 °C), the Florida genotype exhibited more growth than the Minnesota genotype. To determine the physiological parameters associated with this response, plants were acclimated to ambient (27/25 °C) or moderately elevated (33/25 °C) temperatures for 21 days before measurement of net photosynthesis at temperatures ranging from 25 to 48 °C. In vivo measurements of gas exchange and chlorophyll a fluorescence of ambient-acclimated plants revealed that, compared with the Minnesota genotype, the Florida genotype maintained a higher photosynthetic rate, higher stomatal conductance, more open PSII reaction centers, a greater PSII quantum yield and a lower quantum requirement for photosystem II (φPSII) per mole of CO2 fixed (φCO 2 ) throughout the measurement temperature range. When both genotypes were acclimated at 33/25 °C and measured at 33 °C, analysis of the response of net photosynthesis to calculated intercellular CO2 concentration indicated that the maximal rate of Rubisco carboxylation (Vcmax) decreased more in the Minnesota genotype than in the Florida genotype in response to elevated temperature. Additionally, φPSII/φCO 2 at 33 °C was markedly higher for Minnesota plants under photorespiratory conditions, but similar to Florida plants under non-photorespiratory conditions. The results indicate that the higher net photosynthetic rate at 33/25 °C of the Florida genotype compared with the Minnesota genotype could be a result of several mechanisms, including the maintenance of a higher Vcmax and a more efficient quantum requirement of PSII per mole of CO2 fixed, which is likely the result of lower photorespiration.