Effects of elevated CO2 concentration on transpiration and root-water-uptake of wheat

Since global atmospheric CO2 concentration is increasing and the gap between water supply and demand is becoming more and more prominent, it is very important to explore the effects of elevated CO2 concentration on water absorption and utilization of crops for meeting climate change. In this study, the effects of elevated CO2 concentration on transpiration and root-water-uptake of wheat were investigated by setting an indoor hydroponic experiment, where wheat seedlings were cultured in half-strength Hoagland nutrient solution and under three CO2 concentrations ((400±50), (625±50), (850±50) µmol/mol). During the experimental period, the dynamics of leaf stomata conductance, transpiration rate, photosynthesis rate, leaf area, biomass, root length was monitored every 8 days, and plant transpiration was measured daily by weighing the culture pots. Based on the measured data, plant growth, water uptake and consumption, water use efficiency and the capacity of root water uptake were evaluated under each CO2 concentration condition. The experiment results indicate that, with increasing CO2 concentration (from 400 to 625, 850 µmol/mol), leaf stomata are partially closed, transpiration rates at both leaf and canopy scales are significantly limited during a short period (about 3 days), and photosynthesis rate is significantly enhanced, resulting in a significant increase in water use efficiency (P<0.05). Compared to the CO2 concentration of 400 μmol/mol, the plant transpiration averaged during this short period reduced by 14% and 24% under the CO2 concentration treatment of 625 and 850 μmol/mol, respectively. With the prolonging of time under elevated CO2 condition, the effects on leaf stomata conductance, transpiration and photosynthesis are weakened, namely CO2 acclimation, but wheat growth is still significantly promoted. During this period, the positive effect of increased leaf area is almost offset by the reverse effect of stomatal closure, and thus plant transpiration was not impacted significantly, resulting in higher water use efficiency. Compared to the CO2 concentration of 400 μmol/mol, plant transpiration averaged during this long period decreased by only 3% and 4%, while plant water use efficiency increased by 59% and 89% under 625 and 850 μmol/mol, respectively. Under the situation of elevated CO2 concentration, the limitation of transpiration and the promotion of root growth lead to a significant decrease in root-water-uptake function evaluated on root length (the potential water uptake coefficient per unit root length). Compared to the CO2 concentration of 400 μmol/mol, the potential water uptake coefficient per unit root length decreased by average 13% and 31% under 625 and 850 μmol/mol, respectively. Under all the CO2 conditions, the change trends of the potential water absorption coefficient per unit root length and the nitrogen content per unit root length are very similar, and a linear function between them is found. The results supply evidences to improve root-water-uptake and crop growth models and further understand soil-plant-atmosphere continuum (SPAC), then, to help us to deal with climate change and improve water use efficiency. © 2020, Editorial Department of the Transactions of the Chinese Society of Agricultural Engineering. All right reserved.