Physiological and molecular mechanisms of elevated CO2 in promoting the growth of pak choi (Brassica rapa ssp. chinensis)

•Elevated CO2 increased the growth of pak choi with better leaf nutritional quality compared to ambient condition.•Photosynthetic traits such as net photosynthetic rate and light saturation point were increased, whereas stomatal conductance, transpiration rate and light compensation point decreased under elevated CO2 for pak choi.•Analysis of differential expressed genes revealed CO2 enrichment promoted CO2 fixing, synthesis of carbohydrate, chlorophyll and auxin, resulting in better plant growth and quality. Effects of elevated carbon dioxide (CO2) on plant growth and productivity has received considerable attention with atmospheric CO2 concentration increasing. This study examined the effects of elevated CO2 (800±50 μmol•mol−1) on plant growth, nutritional qualities, photosynthetic characters and leaf transcriptomes in pak choi (Brassica rapa ssp. chinensis) variety ‘Wuyueman’ treated from four real-leaf stage up to 40 days compared to ambient CO2 (approx. 400 μmol•mol−1). The results showed that elevated CO2 generally promoted plant growth with larger plant height, leaf size and number. Elevated CO2 also improved leaf quality with increased leaf vitamin C, chlorophyll, soluble protein, soluble sugar, and water contents by 7.70, 33.5, 3.44, 21.9 and 1.62% compared to the control, respectively, and decreased leaf organic acid content by 26.0%. Pak choi plants treated with elevated CO2 had higher net photosynthetic rate and light saturation point, and lower transpiration rate and light compensation point. Transcriptomes analysis identified 187 differentially expressed genes with 31 genes directly involved in the response to elevated CO2. Two genes involved in chlorophyll synthesis were up-regulated, indicating that elevated CO2 promoted chlorophyll synthesis. Elevated CO2 also increased chemical energy and accelerate carbon assimilation as evidenced by up-regulated genes involved in ATP and NADPH synthesis (7 genes) and Rubisco activity (6 genes). In addition, three genes involved in starch synthesis and the conversion of sugar; eight genes related to auxin synthesis and three genes related to cell growth and differentiation were all up-regulated under elevated CO2, which suggested that elevated CO2 speeded up synthesis of carbohydrates and auxin, and eventually promoted plant growth and leaf quality. This study enhances our understanding of physiological and molecular mechanism of elevated atmospheric CO2 on plant growth and quality.