Although numerous experiments have been dedicated to studying plant response to elevated CO₂, almost none crossed the level of 1000 ppm. Plant responses to high CO₂ levels importantly inform our understanding of plant physiology in ultra-high CO₂ environments, e.g., in Earth history, in the case of unmitigated anthropogenic emissions, and for future colonization of Mars.</p> <p>Here, we challenged two-year old seedlings of fruit trees grown in soil in a mesocosm, with CO₂ levels of 400, 1600 and 6000 ppm, the highest of which is approximately equivalent to that of Mars&rsquo; atmosphere. Plant growth, and leaf gas exchange (transpiration, stomatal conductance, and CO₂ assimilation) were measured on a weekly basis for 3 consecutive weeks. We hypothesized that elevated CO₂ levels will induce a decrease in transpiration, primarily attributed to reduced stomatal conductance. Indeed, leaf transpiration was decreased at 1600 ppm CO₂ and remained low at 6000 ppm, concurrent with a 50 % decrease in stomatal conductance. The CO₂-induced stomatal closure appears to have saturated between 850 and 1600 ppm CO₂. Due to this effect, net assimilation was only mildly changed at 1600 ppm CO₂, but significantly increased at 6000 ppm. As a result, water-use efficiency quadrupled at 6000 ppm CO₂. Stem height increment did not change significantly across the CO₂ treatments.</p> <p>Taken together, our measurements demonstrated both the potential and limit of CO₂-induced stomatal closure, with positive implications for fruit tree growth in ultra-high CO₂ environments, as on Earth in the case of unmitigated anthropogenic CO₂ emissions and on Mars.