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With sunlight as a source of energy and atmospheric CO2 as a source of carbon, terrestrial plants have evolved with assimilatory organs that enable acquisition of both sets of resources. Planar foliage facilitates CO2 diffusion to fixation sites and maximises interception of sunlight per unit volume of photosynthetic tissue. Whether light interception or gas exchange was the more important driving variable for evolution of leaf form is a moot point, but a leaf morphology that facilitates gas exchange could imply that atmospheric supply of CO2 limits carbon gain. Indeed, present examples of photosynthetic and growth response to CO2 enrichment confirm that late twentieth century plants commonly operate well below their potential, provoking a question as to how they came to evolve with an inherent capacity for carbon fixation that generally remains underutilised.

Land plants appeared on terra firma 350–400 million years ago or thereabouts, when atmospheric CO2 concentration would have been about 2000 ppm. Such a high partial pressure of this crucial substrate shaped options for a biological assimilation system up to that time, based on Rubisco. A debate continues as to whether the Rubisco of modern-day plants is really maladapted or simply misunderstood. This huge bifunctional enzyme nevertheless remains pivotal to photosynthetic carbon metabolism at a time when atmospheric partial pressures of CO2 are almost an order of magnitude lower.

Broad variation in atmospheric CO2 partial pressure has resulted in photosynthetic adaptation, and a progressive fall to a minimum value about 100 million years ago (even below the levels of the 1990s) saw evolution of C4 photosynthesis. In those species so adapted, an internal concentrating mechanism for CO2 precedes assimilation via Rubisco, which now operates in bundle sheath cells of C4 plants under conditions that ensure near saturation of its catalytic capacity. In evolutionary terms, nature found it more expedient to enhance performance of an existing Rubisco than to engineer an alternative catalytic system.

Regardless of whether Rubisco is really maladapted or simply misunderstood, performance in vivo is enhanced by CO2 enrichment, providing an opportunity to analyse plant carbon metabolism and identify genetic and environmental limitations on carbon assimilation and growth. Such issues are analysed here within a context of global carbon budget and ecosystem gas exchange (Section 13.1), then at leaf level (Sections 13.2, 13.3), and finally in terms of environmental interactions on photosynthetic and growth responses to elevated CO2 (Section 13.3). Practical applications of CO2 enrichment in horticulture follow (Section 13.4) with a closing discussion (Section 13.5) on responses of tropical plants and savanna/woodland ecosystems to increased CO2.

A note on units

Atmospheric CO2 concentration is commonly expressed as a volume percentage which is known to have increased from a pre-industrial value of around 0.0295% to about 0.0356% by the late twentieth century. For convenience, that concentration is commonly reported as parts per million by volume (ppmv) or more simply ‘ppm’ with the volume term implicit. Late 1990s levels would thus be around 356 ppm.

Calculation of physiological variables such as stomatal conductance and analysis of leaf gas exchange via A:pi curves are facilitated if driving variables for assimilation and transpiration are expressed as mol fractions. According to that convention, 356 ppm would be represented as 356 µmoles of CO2 per mole of air, abbreviated in Chapter 13 to µmol CO2 mol–1 or simply µmol mol–1.

As an additional issue, biochemical events such as CO2 assimilation are an intrinsic function of the partial pressure of CO2 at fixation sites, rather than CO2 concentration by volume. CO2 partial pressure at fixation sites is approximated by intercellular CO2 partial pressure (represented by pi) which will be somewhat higher than the actual CO2 partial pressure at fixation sites within chloroplasts, but A:pi curves are most commonly referenced to this intercellular value. For practical purposes, and to simplify present comparisons between whole-plant physiology and leaf-level processes, an atmospheric pressure of 1 bar can be assumed, so that 356 ppm is then equivalent to either 356 µmol mol–1 or 356 µbar bar–1, and in ambient air at 1 bar, simply 356 µbar or 35.6 Pa.