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 waterstressed 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.

Implications for the hydrologic cycle under climate change due to the expansion of bioenergy crops in the Midwestern United States

“…This conclusion is drawn based on Free Air CO 2 Enrichment (FACE) experiments which have demonstrated a lack of response of photosynthesis, biomass accumulation, and yield of maize under elevated CO 2 (24,25). These experiments have also shown insensitivity of key photosynthetic enzymes of this C4 crop to elevated CO 2 (24), and have pointed to the alleviation of water stress as the primary impact of elevated CO 2 on maize productivity (26,27), in agreement with previous hypotheses on the impact of elevated CO 2 on the func-tioning of C4 plants (28). Published results for the response of miscanthus and switchgrass to elevated CO 2 are not yet available.…”

Reduced plant water status under sub-ambientpCO2limits plant productivity in the wild progenitors of C3and C4cereals

“…These observations raise the possibility of an indirect feedback on biomass accumulation at sub-ambient pCO 2 mediated via water relations because, although plants were not exposed to drought per se, they experienced a soil drying cycle between watering events on alternate days (Cunniff et al, 2008). This hypothesized mechanism is consistent with studies investigating the effects of elevated atmospheric pCO 2 (55-70 Pa) on the water relations of C 4 plants, where reduced g s and E improve plant and soil water status, and extend the active period for photosynthesis and growth (Ghannoum et al, 2000). This indirect effect of elevated pCO 2 is particularly important when C 4 plants experience some kind of water deficit (Conley et al, 2001;Wall et al, 2001;Leakey et al, 2004Leakey et al, , 2006Leakey, 2009).…”

Global food insecurity. Treatment of major food crops with elevated carbon dioxide or ozone under large-scale fully open-air conditions suggests recent models may have overestimated future yields

“…For the two major C 4 crops, there was no significant increase in yield at all in FACE compared to the chamber-predicted increase of 7% and modelled increase of 10% (table 1). Evidence from chamber experiments has been contradictory regarding direct effects of elevated [CO 2 ] on C 4 photosynthesis (reviewed in Ghannoum et al 2000). As described above, models for C 4 crops currently assume a direct With such a small sample there is the possibility that these results are unrepresentative, but the fact that they all fall below the lower 99% confidence interval established from chamber studies, suggests that this is unlikely.…”