Molecular-based studies of thermotolerance have rarely been performed on wild plants, although this trait can be critical for summer survival. Amano et al. use a chlorophyll bioassay combined with gene-expression analyses to examine thermotolerance in two allied aquatic species of the genus Potamogeton (pondweeds) that differ in survival on land. Acquired thermotolerance is detected in heat-tolerant P. malaianus whereas plasticity in the minimum threshold temperature of heat-sensitive P. perfoliatus is observed. The latter response may be beneficial in moderate growth environments with only minor elevations in daily temperature.