Rubisco, the primary enzyme governing carbon assimilation, is dependent upon Rubisco activase. The heat sensitivity of activase, including its expression and thermal stability, varies among species and is considered a key component governing photosynthetic performance in response to moderate heat stress (32-35 °C). However, the Rubsico-Rubsico activase association has yet to be examined among woody plants or varieties within a species, the understanding of which will assist cultivar improvement strategies. Using molecular and physiological techniques to study the role of activase in thermal regulation of photosynthesis, we found that net photosynthesis decreased in Acer rubrum L. `Northwood' at 31°C, whereas the southern variety, A. rubrum `Florida Flame', maintained optimal assimilation rates up to 36 °C. Additionally, the maximal carboxylation rate of Rubisco (Vcmax) at 35 °C was 31.7% lower for Northwood in comparison to Florida Flame. The cloned activase sequences from both cultivars show 97% nucleotide homology and 98% amino acid identity, indicating the potential for similar protein product formation and function. Interestingly, sequence analysis indicates that both cultivars produce at least two isoforms of activase derived from alternative transcript splicing. We will discuss activase mRNA processing and protein isoform abundance in relation to Rubisco kinetic properties as a function of heat tolerance in these two thermally contrasting woody plant genotypes.