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Evolution (Terrestrial Plants - Agricultural Crops) -- Summary
In a review article published a few years ago, Sage and Coleman (2001) discussed what we know about plant responses to both increases and decreases in the air's CO2 content. Very simply, they noted that plants photosynthesize at reduced rates and produce less biomass at lower-than-current atmospheric CO2 levels, but that they photosynthesize at enhanced rates and produce more biomass at higher-than-current CO2 concentrations. At optimal temperatures for C3-plant photosynthesis, for example, the two researchers wrote that "reducing atmospheric CO2 from the current level to 180 ppm [which represented an approximate 50% reduction at that time] reduces photosynthetic capacity by approximately half," while it simultaneously "causes biomass to decline by 50%." Doubling the atmosphere's CO2 concentration, on the other hand, typically increases photosynthesis and biomass production by 30 to 50%. In addition, as Sage and Coleman reported, "high CO2 concentrations reduce the impact of moderate drought, salinity and temperature stress, and can indirectly reduce low nutrient stress by promoting root growth, nitrogen fixation and mycorrhizal infection," which phenomena boost the basic CO2-induced productivity increase still more, as Idso and Idso (1994) had also noted in an earlier review of the literature.

All of these observations, of course, are common knowledge among plant biologists; and in the present instance, they but serve as introductory material for Sage and Coleman's hypothesis that modern bioengineering techniques might enable us to make plants even more responsive to increases in the air's CO2 content and thereby make a good thing even better.

Their thinking runs this way. During the peak of the last ice age -- and throughout the bulk of all prior ice ages of the past two million years -- atmospheric CO2 concentrations have tended to hover at approximately 180 ppm. This value, say Sage and Coleman, might not be much above the "critical CO2 threshold at which catastrophic interactions occur." Hence, they reasonably speculate that plants of the late Pleistocene "might have been adapted to lower CO2 concentrations than currently exist."

In light of the short period of evolutionary time that has elapsed since the last of these low-CO2 conditions prevailed, the two researchers advance the logical thought that "many if not most plants might still be adapted to CO2 levels much lower than those that exist today," even though literally thousands of laboratory and field experiments have demonstrated that earth's vegetation responds in dramatic positive fashion to atmospheric CO2 enrichment far above what is characteristic of the CO2 conditions of the present. Hence, they conclude that as good as things currently are, and as significantly better as they are expected to become as the air's CO2 content continues to rise, there may well be additional "substantial room for natural selection and bioengineering to remove the constraints [of low CO2 adaptation], thereby creating novel genotypes able to exploit high CO2 conditions to best advantage."

That such is possible -- indeed, even probable -- is borne out by the study of Ziska et al. (1996), who grew well watered and fertilized plants of 17 cultivars of rice from seed to maturity within glasshouses maintained at atmospheric CO2 concentrations of either 373 or 664 ppm in soil-filled pots kept at two different sets of day/night temperatures (29/21C and 37/29C), after which they measured both total plant biomass and grain biomass. In doing so, they found that the degree of CO2-induced enhancement of total plant biomass and the number of cultivars showing significant enhancement with elevated CO2 decreased at the higher and more stressful set of growth temperatures, dropping from 12 out of 17 cultivars with an average biomass stimulation of 70% at the lower set of temperatures to 8 out of 17 cultivars with a mean biomass stimulation of 23% at the higher set of temperatures. In addition, with respect to both total plant biomass and grain biomass production under the more standard lower set of temperatures, a few cultivars exhibited no significant changes in response to the specific degree of atmospheric CO2 enrichment employed in the study, while the most responsive cultivar exhibited a whopping 265% increase in total plant biomass and an astounding 350% increase in grain biomass. In light of this huge variability in the degree of productivity enhancement exhibited by the 17 rice cultivars in response to the experimental increase in the air's CO2 content employed in this experiment, it would appear there is a tremendous potential to select rice varieties to best take advantage of the aerial fertilization effect of the ongoing rise in the air's CO2 content, which endeavor, in the words of the researchers who conducted the study, "could maximize productivity as CO2 concentration increases."

Also noting there is "considerable variability among current rice cultivars in their responses to CO2 and temperature (Ziska and Teramura, 1992; Ziska et al., 1996; Moya et al., 1998) leading to the possibility of selecting rice cultivars against these two environmental variables for yield increases and/or stability in a possibly warmer, but almost certainly higher future CO2 world," Baker grew the Southern United States rice cultivars Cocodrie, Jefferson and Cypress for an entire season in outdoor, naturally-sunlit, controlled-environment chambers at a constant day/night temperature of 28C at CO2 concentrations of 350 and 700 ppm, while in the following year he grew the cultivar Lamont under the same conditions, but at day/night temperatures of 27/23C, as both a main crop and as a ratoon crop. In the first of these experiments, grain yield per plant rose by 46%, 57% and 71% in response to the doubling of the air's CO2 content in the Cocodrie, Jefferson and Cypress cultivars, respectively, while in the second experiment with the Lamont cultivar it rose by 12% when the rice was grown as a main crop but by 104% when it was grown as a ratoon crop. Based on these findings, Baker concluded that "the wide range in grain yield responsiveness to CO2 enrichment found among these four US rice cultivars points to the potential for selecting or developing high yielding US rice cultivars with the ability to take advantage of expected future global increases in CO2," noting that "CO2 enrichment could have potentially large positive effects on ratoon crop yields."

How important are these ideas? Sage and Coleman state that the low CO2 levels of the past "could have had significant consequences for much of the earth's biota." In fact, they suggest that the origin of agriculture itself "might have been impeded by reduced ecosystem productivity during low CO2 episodes of the late Pleistocene." Since that time, however, the increase in the air's CO2 content has essentially doubled the biological prowess of the planet's vegetation; and projected increases in the air's CO2 content could readily lead to a tripling of the paltry productivity of earth's ice-age past. On top of these phenomenal benefits, all three of the studies reviewed here suggest there may be other opportunities to improve plant performance even more, by using modern bioengineering techniques to overcome genetic constraints linked to adaptations to low levels of CO2 that may persist in many of earth's plants. Indeed, Sage and Coleman suggest that, for agriculture, "this could be a major opportunity to improve crop productivity and the efficiency of fertilizer and water use."

Truly, we are living in an age of unparalleled biological promise, which to a person of the distant past -- or even some of us -- would appear to be almost beyond belief. The fullness of that promise, however, has yet to be achieved; and how effectively we exploit the opportunities to do so, in the words of Sage and Coleman, "will depend on our ability to conduct the basic research [needed] to identify the genes controlling acclimation and adaptation to CO2 variation."

This effort, together with the public education effort required to stem the tide of irrational pessimism promulgated by climate alarmists intent on living in the past, must be strongly supported if we are to successfully meet the challenges that confront us. Without the benefits of the aerial-fertilization and transpiration-reducing effects of atmospheric CO2 enrichment, along with the development of plant genotypes that can take full advantage of these phenomena, we are almost assured of being unable to feed the growing human population of the planet but a few short decades from now without usurping vast tracts of tropical and temperate forests, savannas and grasslands -- as well as most of the planet's remaining freshwater resources -- and thereby destroying most of what little remains of "wild nature." Morality clearly dictates that we cannot allow that to happen.

References
Baker, J.T. 2004. Yield responses of southern US rice cultivars to CO2 and temperature. Agricultural and Forest Meteorology 122: 129-137.

Idso, K.E. and Idso, S.B. 1994. Plant responses to atmospheric CO2 enrichment in the face of environmental constraints: a review of the past 10 years' research. Agricultural and Forest Meteorology 69: 153-203.

Moya, T.B., Ziska, L.H., Namuco, O.S. and Olszky, D. 1998. Growth dynamics and genotypic variation in tropical, field-grown paddy rice (Oryza sativa L.) in response to increasing carbon dioxide and temperature. Global Change Biology 4: 645-656.

Sage, R.F. and Coleman, J.R. 2001. Effects of low atmospheric CO2 on plants: more than a thing of the past. TRENDS in Plant Science 6: 18-24.

Ziska, L.H., Manalo, P.A. and Ordonez, R.A. 1996. Intraspecific variation in the response of rice (Oryza sativa L.) to increased CO2 and temperature: growth and yield response of 17 cultivars. Journal of Experimental Botany 47: 1353-1359.

Ziska, L.H. and Teramura, A.H. 1992. Intraspecific variation in response of rice (Oryza sativa) to increased CO2 - photosynthetic, biomass and reproductive characteristics. Physiologia Plantarum 84: 269-276.

Last updated 19 December 2007