Elevated CO2 effects on plant carbon, nitrogen, and water relations: six important lessons from FACE

Abstract

Plant responses to the projected future levels of CO(2) were first characterized in short-term experiments lasting days to weeks. However, longer term acclimation responses to elevated CO(2) were subsequently discovered to be very important in determining plant and ecosystem function. Free-Air CO(2) Enrichment (FACE) experiments are the culmination of efforts to assess the impact of elevated CO(2) on plants over multiple seasons and, in the case of crops, over their entire lifetime. FACE has been used to expose vegetation to elevated concentrations of atmospheric CO(2) under completely open-air conditions for nearly two decades. This review describes some of the lessons learned from the long-term investment in these experiments. First, elevated CO(2) stimulates photosynthetic carbon gain and net primary production over the long term despite down-regulation of Rubisco activity. Second, elevated CO(2) improves nitrogen use efficiency and, third, decreases water use at both the leaf and canopy scale. Fourth, elevated CO(2) stimulates dark respiration via a transcriptional reprogramming of metabolism. Fifth, elevated CO(2) does not directly stimulate C(4) photosynthesis, but can indirectly stimulate carbon gain in times and places of drought. Finally, the stimulation of yield by elevated CO(2) in crop species is much smaller than expected. While many of these lessons have been most clearly demonstrated in crop systems, all of the lessons have important implications for natural systems.

Will Elevated Carbon Dioxide Concentration Amplify the Benefits of Nitrogen Fixation in Legumes?

“…3). FACE experiments with soybean (Glycine max) and clover (Trifolium repens) showed a stimulation in photosynthetic C uptake and leaf carbohydrate content at elevated [CO 2 ] (Ainsworth et al, 2003b;Rogers et al, 2004;Leakey et al, 2009;Fig. 3).…”

“…As a result the plant acclimates, adjusting its photosynthetic, respiratory, and N metabolism to improve performance under elevated [CO 2 ] (Leakey et al, 2009). The acclimation of photosynthesis is best understood in the context of the relationship between changes in intercellular [CO 2 ] (c i ) and the light-saturated rate of photosynthesis (A), otherwise known as the A-c i curve (Fig.…”

“…Legumes avoid acclimation by allocating some of the additional photosynthate produced at elevated [CO 2 ] to their symbiosis with rhizobia to stimulate N 2 fixation. In nonleguminous species where N acquisition cannot be stimulated in this manner, C and N metabolism are balanced by reducing synthesis of Rubisco, which leads to lower V c,max and A, but also decreases the photosynthetic demand for the limiting nutrient, N, making it available to other sinks (Leakey et al, 2009). These mechanisms maximize the efficiency of resource use for both functional groups, but explain how legumes can gain greater benefit from elevated [CO 2 ] than nonlegumes.…”

“…The capacity for legumes to coordinate enhanced assimilation of C and N at elevated [CO 2 ] to avoid down-regulation of photosynthetic capacity and ultimately maximize gains in productivity has been demonstrated across a range of species, environmental conditions, and ecological settings (Ainsworth and Rogers, 2007;Leakey et al, 2009). Nevertheless, there are situations where N 2 fixation cannot be stimulated in response to greater photosynthate availability at elevated [CO 2 ] because some other factor is limiting.…”

“…There is overwhelming evidence that growth at elevated [CO 2 ] reduces stomatal conductance (g s ; Fig. 3), and that this instantaneous response of stomates to CO 2 is maintained with time (Leakey et al, 2009). Recent evidence from FACE experiments, where the natural coupling between plants and the atmosphere is largely unaltered, also suggests that the decrease in g s at elevated [CO 2 ] often translates to a proportional reduction in canopy transpiration and an increased soil moisture content in a range of plants, including soybean (Leakey et al, 2009).…”