The growth response of C-4 plants to rising atmospheric CO2 partial pressure: a reassessment

Despite mounting evidence showing that C-4 plants can accumulate more bioma ss at elevated CO2 partial pressure (p(CO2)), the underlying mechanisms of this response are still largely unclear, Iri this paper, we review the curr ent state of knowledge regarding the response of C-4 plants to elevated p(C O2) and discuss the likely mechanisms. We identify two main routes through which elevated p(CO2) can stimulate the growth of both well-watered and wat er-stressed C-4 plants. First, through enhanced leaf CO2 assimilation rates due to increased intercellular p(CO2), Second, through reduced stomatal co nductance and subsequently leaf transpiration rates, Reduced transpiration rates can stimulate leaf CO2 assimilation and growth rates by conserving so il water, improving shoot water relations and increasing leaf temperature.; We argue that bundle sheath leakiness, direct CO2 fixation in the bundle sh eath or the presence of C-3-like photosynthesis in young C-4 leaves are unl ikely explanations for the high CO2-responsiveness of C-4 photosynthesis. T he interactions between elevated p(CO2), leaf temperature and shoot water r elations on the growth and photosynthesis of C-4 plants are identified as k ey areas needing urgent research.