The standard two-part answer to this question is that enriching the air with CO2 causes fewer stomatal openings to be created per unit area of leaf surface at the same time that the elevated CO2 reduces the apertures of the leaf stomatal openings, both of which phenomena tend to reduce evaporative water loss from the substomatal cavities of the leaf, where atmospheric CO2 is captured by the plant and water escapes to the free air.

At a somewhat mundane level of complexity, this answer is sufficient.  If one is more curious, however, it leaves much to be desired; for it can still be asked why atmospheric CO2 enrichment does these two things.

One approach to answering this question is to consider the concept of water use efficiency, which is perhaps most simply defined as the amount of organic matter produced by a leaf per amount of water it loses in the process.  It can readily be appreciated, for example, that the fewer and smaller leaf stomatal openings caused by atmospheric CO2 enrichment make it disproportionately more difficult for water to escape from a plant's leaves than for CO2 to be captured by them; for the increase in the air's CO2 concentration increases the air-to-leaf CO2 concentration gradient, and this enhanced driving force generally tends to more than compensate for the reduced ease of CO2 entry into the leaf that results from the fewer and smaller leaf stomatal openings.  The net result of these CO2-mediated alterations of plant and atmospheric properties is that plant water use efficiency is typically increased by atmospheric CO2 enrichment, which provides the plant with a very real benefit.

But why should higher concentrations of atmospheric CO2 help plants become more efficient at what they do?  To explore this question, we consider the concept of adaptation.  All organisms, be they plant or animal, generally perform best, i.e., operate most efficiently, within a certain range of environmental conditions.  If it gets too hot or too cold, too wet or too dry, too this or too that, they just don't function as efficiently as they do in the environment for which they were made, or, depending on one's view of how things came to be, the environment to which they have become accustomed over the eons.

Well, plants have been around a long time; and over much of that time the CO2 content of the air has been much greater than it is currently.  At the start of the Phanerozoic Era some 550 million years ago, for example, and lasting for approximately 100 million years, the air's CO2 content may have been as much as twenty times greater than it is today; and around 220 million years ago, it is estimated to have been approximately five times greater (1).  Consequently, even though the atmosphere's CO2 concentration periodically dropped to only half its current value during the coldest portions of the recurring ice ages of the last two million years (2), it is logical to presume that the most basic of plant physiological processes are genetically designed to operate most efficiently at higher-than-current atmospheric CO2 concentrations.

But what if we carry this reasoning to the extreme, especially with the concept of water use efficiency?  As the air's CO2 concentration rises higher and higher, might not fewer and fewer ever-less-opened stomates eventually lead to a situation where even the enhanced air-to-leaf CO2 concentration gradient could not overcome the increased difficulty of CO2 entry into the leaf, causing the plant to die of carbon starvation?  And might not this situation also reduce transpiration to such a degree that leaf evaporative cooling could not prevent the occurrence of plant death due to increased thermal stress?

Yes, the continued operation of established "business-as-usual" plant stomatal responses to atmospheric CO2 enrichment must surely pose a significant challenge somewhere along the line of ever-increasing atmospheric CO2 concentrations; but having experienced such high concentrations in the past, would not plants have also developed a genetic "safety valve" for terminating these stomatal responses at some critical value of atmospheric CO2 concentration?  Based on the evident robustness of life over the eons, it is logical to assume that plants would indeed have developed just such a strategy, and that they would have tucked away its blueprint somewhere in the hidden recesses of their genes, where it would lay latent and unobserved until it was needed.

Well, it's a great theory.  But is there any evidence that can lift it above the level of pure conjecture?

Until earlier this month, the answer - by default, due to lack of confirmatory evidence - was no; but then came the report of Gray et al. (2000), wherein a group of scientists from the United Kingdom and The Netherlands identified a gene of the small mustard plant Arabidopsis thaliana that performs just this function: it actually prevents changes in the number of leaf stomata in response to atmospheric CO2 enrichment above a specific critical value of atmospheric CO2 concentration.

In commenting on this discovery, Serna and Fenoll (2000) say "the main message of Gray et al.'s work is that plants seem to be well armed to cope with a further enrichment in atmospheric CO2," and that genes such as this one - denoted HIC for high carbon dioxide - "should ensure that, at high CO2 concentrations, changes in stomatal indices are kept to a minimum."

So, yes, to answer the question posed by the title of our editorial, plants do indeed appear to be genetically prepared - programmed, as it were - to deal with whatever challenges mankind may throw at them in the way of anthropogenic CO2 emissions.  The providence that developed them built into the progenitors of today's plants the precise genetic machinery to accommodate just such a contingency.

References and Footnotes
Gray, J.E., Holroyd, G.H., van der Lee, F.M., Bahraml, A.R., Sijmons, P.C., Woodward, F.I., Schuch, W. and Hetherington, A.M.  2000.  The HIC signaling pathway links CO2 perception to stomatal development.  Nature 408: 713-716.

Serna, L. and Fenoll, C.  2000.  Coping with human CO2 emissions.  Nature 408: 656-657.

2. See our Subject Index entry Carbon Dioxide (History - The Last 250,000 Years).