Photosynthetic advantage without growth superiority: Sorghum’s paradoxical response to future climate stresses compared to maize

Rising atmospheric CO2 is projected to reach 700 μmol/mol by 2100, increasing global temperatures by ∼3 °C and exacerbating drought frequency. Sorghum (Sorghum bicolor), a drought-tolerant alternative to maize (Zea mays), may gain relevance under future climates. Here we grew sorghum and maize in growth chambers under ambient (400 μmol/mol CO2, 24/18 °C day/night) and future (700 μmol/mol CO2, 27/21 °C day/night) environmental conditions, with and without drought. Drought was imposed by withholding water until soil moisture reached 20 % field capacity, which was then maintained for one week. Under high CO2 and high temperature, although both maize and sorghum achieved comparable biomass, growth was only promoted in sorghum by 67 %. Its stomatal anatomical traits reduced cumulative transpiration by 18 % while maintaining carbon assimilation, thereby improving water-use efficiency. With combined high CO2, high temperature and drought, maize and sorghum exhibited comparable biomass, but sorghum maintained gas exchange and quantum yield of photosystem II (PSII). Additionally, sorghum increased root growth, further mitigating future drought impacts on its growth compared to future control conditions. In contrast, maize photosynthesis and the quantum yield of PSII were severely impaired, thereby restricting growth. Thus the photosynthetic and water-use efficiency strategies of sorghum show its considerable potential as a future alternative crop, but further studies should verify whether these strategies persist during the reproductive stage and contribute to enhanced yield stability.