Photosynthesis‐CO 2 Relationship of Highland Barley Leaves and Its Developmental Response to Experimental Warming on the Third Pole

Abstract The Tibetan Plateau, known as the Third Pole, is experiencing rapid warming and increasing atmospheric CO 2 concentration, which profoundly influences the growth of crops such as highland barley. However, it remains unclear how highland barley photosynthesis quantitatively responds to changes in CO 2 concentration and how the photosynthesis‐CO 2 relationship responds to warming on the Tibetan Plateau. Here, we examined the photosynthesis‐CO 2 relationship of highland barley leaves at the heading stage and its developmental response to experimental warming (+1 and +2°C). The photosynthetic CO 2 saturation concentration ( C a,sat ) of highland barley leaves was 1,208.8 ± 533.4 ppm, exceeding the current atmospheric CO 2 concentration (419.3 ± 0.2 ppm in 2022, Waliguan). Experimental warming did not significantly affect the maximum photosynthetic rate ( A max , 39.0 ± 9.5 μmol m −2 s −1 ), C a,sat (1,208.8 ± 533.4 ppm), the CO 2 compensation concentration ( C a,com , 58.9 ± 13.8 ppm), the maximum carboxylation rate of Rubisco ( V cmax , 142.8 ± 12.5 μmol m −2 s −1 ), and the maximum electron transport rate ( J max , 193.9 ± 31.0 μmol m −2 s −1 ) at a measurement temperature of 25°C. At +2°C warming, the carotenoid content of highland barley leaves increased by 5%–21%, whereas chlorophyll and anthocyanin contents were not significantly affected. Our findings highlight that the CO 2 fertilization effect on highland barley leaf photosynthesis is expected to continue and experimental warming may developmentally influence highland barley leaf photosynthesis by altering the leaf carotenoid content rather than the photosynthesis‐CO 2 relationship.