The growth response of C4 plants to rising atmospheric CO2 partial pressure: a reassessment

Abstract

Despite mounting evidence showing that C 4 plants can accumulate more biomass at elevated CO 2 partial pressure (p(CO 2)), the underlying mechanisms of this response are still largely unclear. In this paper, we review the current state of knowledge regarding the response of C 4 plants to elevated p(CO 2) and discuss the likely mechanisms. We identify two main routes through which elevated p(CO 2) can stimulate the growth of both well‐watered and water‐stressed C 4 plants. First, through enhanced leaf CO 2 assimilation rates due to increased intercellular p(CO 2). Second, through reduced stomatal conductance and subsequently leaf transpiration rates. Reduced transpiration rates can stimulate leaf CO 2 assimilation and growth rates by conserving soil water, improving shoot water relations and increasing leaf temperature. We argue that bundle sheath leakiness, direct CO 2 fixation in the bundle sheath or the presence of C 3‐like photosynthesis in young C 4 leaves are unlikely explanations for the high CO 2‐responsiveness of C 4 photosynthesis. The interactions between elevated p(CO 2), leaf temperature and shoot water relations on the growth and photosynthesis of C 4 plants are identified as key areas needing urgent research.