Combined effects of CO₂ enrichment and elevated growth temperatures on metabolites in soybean leaflets: evidence for dynamic changes of TCA cycle intermediates

Soybean (Glycine max [Merr.] L.) was grown in indoor chambers with ambient (38 Pa) and elevated (70 Pa) CO₂ and day/night temperature treatments of 28/20, 32/24 and 36/28 °C. We hypothesized that CO₂ enrichment would mitigate the deleterious effects of elevated growth temperatures on metabolites in soybean leaflets. Net CO₂ assimilation rates increased incrementally with growth temperature and were enhanced up to 24 % on average by CO₂ enrichment. Stomatal conductance about doubled from the lowest to highest temperature but this was partially reversed by CO₂ enrichment. Metabolites were measured thrice daily and 19 and 28 of 43 total leaf metabolites were altered by the 32/24 and 36/28 °C temperature treatments, respectively, in both CO₂ treatments. Polyols, raffinose and GABA increased and 23 nonstructural carbohydrates, organic acids and amino acids decreased when the temperature was increased from 28 to 36 °C under ambient CO₂. Citrate, aconitate and 2-oxoglutarate decreased over 90 % in the 36/28 °C compared to the 28/20 °C temperature treatment. Temperature-dependent changes of sugars, organic acids and all but three amino acids were almost completely eliminated by CO₂ enrichment. The above findings suggested that specific TCA cycle intermediates were highly depleted by heat stress under ambient CO₂. Mitigating effects of CO₂ enrichment on soybean leaflet metabolites were attributed to altered rates of photosynthesis, photorespiration, dark respiration, the anaplerotic pathway and to possible changes of gene expression.