Physiological responses to ultra-high CO ₂ levels in an evergreen tree species

Abstract. 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's history, in the case of unmitigated anthropogenic emissions, and for future colonisation of Mars. Here, we challenged 2-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' atmosphere. Plant growth and leaf–gas exchange (transpiration, stomatal conductance, and CO2 assimilation) were measured on a weekly basis for 23–25 consecutive days. We hypothesised 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 (WUE) quadrupled at 6000 ppm CO2. Stem height increment did not change significantly across the CO2 treatments. Taken together, our measurements demonstrated both the potential and the 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.