As the atmospheric levels of CO₂ continue to rise, the concentration of carbon sources available for the plants also increases photosynthesis. The rise in atmospheric CO₂ is associated with global temperatures, resulting in an array of plant responses. Studies evaluating plant response to CO₂ treatments of 350 plant species show that plant responses are highly variable and species dependent, with the majority of studies being conducted on vegetable and grain crops, leaving a gap in understanding how tree crops respond. Mandarin (C. reticulata) trees are an economically important crop produced in California subject to climate change. This study used nursery trees of cv. Tango budded on C35 rootstock to determine the effect of elevated CO₂ exposure (400 ppm ambient vs. 800 ppm elevated) at elevated temperature (28°C ambient vs. 45°C elevated) to identify the physiological and metabolomic plant responses associated with each treatment. A longitudinal investigation was conducted over eight weeks in growth chambers (Conviron A1000, Winnipeg, CA) utilizing four treatment groups: ambCO2/Tamb (Control: CO2 400 ppm/Temperature 28°C); eCO2/Tamb (elevated CO2 at 800 ppm/Temperature 28°C); ambCO2/eTemp (CO2 at 400 ppm/Temperature 45°C); and eCO2/eTemp (CO2 at 800 ppm/ Temperature 45°C). Every two weeks, four replicates were collected from each treatment group. physiological measurements, photosynthesis response surveys, and metabolomic analysis were performed. Phenotypic measurements such as plant height, branches, leaf area, and leaf count showed that plant growth was impacted by treatment. Plant height, dry weight, and leaf count were significantly lower in the ambCO2/eTemp treatment as compared to the control (ambCO2/Tamb) as well as the other two treatments, indicating heat stress. Under elevated CO2 levels, trees were able to exhibit similar growth behaviors as the control treatment even under heat stress, signaling that the trees under eCO2 could compensate for a higher burden on carbon balance under heat stress. eCO2/Tamb trees accumulated significantly higher starch than all other treatments, whereas trees under both eTemp treatments showed significantly higher soluble sugars while significantly lower starch levels. This indicates that the trees under supplemental CO2 accumulate higher photoassimilates when they are free from heat stress, and the trees under heat stress exhibit starch conversion to soluble sugars as a stress response. Metabolomics analysis using Nuclear Magnetic Resonance (NMR) will provide valuable insight into the impact of treatment groups of eCO₂ and eTemp impact on Mandarin trees.
