Water stress, in the face of climate change, is expected to reduce photosynthesis and leaf water potential in order to regulate plant water status and maintain growth by adjusting water use efficiency. We hypothesize that higher summer temperatures increase photosynthesis, however summer higher atmospheric demand decreases the efficiency of water use, being this decrease more pronounced in more stressed genotypes. Our study investigated the photosynthesis (A_net), intrinsic water use efficiency (iWUE) and water stress integral (WSI) changes in the early development of E. globulus, E. nitens and E. nitens x E. globulus hybrids (E gloni) genotypes across contrasting seasons: winter (6 months years old) to summer (13 months years old). A_net, iWUE, and WSI showed a significant interaction (p>0.001) between genotype and season. Regardless of the season, some E. globulus genotypes presented no significant changes in A_net, however, in summer higher increment in A_net was observed for E. gloni (almost 50%). In winter, the highest iWUE was 129 µmol mol^-1, while in summer was 50 µmol mol^-1. The highest reduction in iWUE values between winter and summer was observed for E. nitens genotypes (300%). The lowest iWUE values in summer were related to the lowest values of WSI, in which E. nitens were different from E. globulus and E. gloni (p=0.01). Interestingly, we observed a positive relationship between WSI and iWUE in summer; however, this relationship was the opposite in winter. Our results suggest that accumulated stress (WSI) in winter helps to promote stomatal closure which increases iWUE, since A_net presented small changes in winter. Regardless of genotype, warm periods increased A_net; however, iWUE decreased, especially in genotypes with more accumulated stress, which implies different strategies of eucalyptus genotypes/species plantations in regions with water deficit.