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Enriching the Air with CO2 Enables Plants to Sequester Carbon at Higher Temperatures Than They Do Currently
Photosynthesis, which is the physiological process by which plants remove CO2 from the atmosphere and incorporate it into their tissues, is the first step in a sequence of events that ultimately leads to the sequestration of carbon in soils. Without it, nothing else matters. Hence, anything that either enhances photosynthesis or enables it to proceed under conditions that would normally inhibit it, will ultimately lead to the removal of more CO2 from the atmosphere and a slowing of the rate of rise of the air's CO2 content.

It is instructive to consider the effects of daily temperature changes on this process. As a plant warms from an initial state of early-morning coolness, its rate of net photosynthesis - the difference between gross photosynthesis (CO2 uptake) and respiration (CO2 release) - generally rises, until it reaches a maximum at what is called the optimum temperature for that plant, i.e., the temperature at which the plant exhibits peak performance in terms of growth or net CO2 uptake. Then, if the air temperature rises higher still, the plant's rate of net photosynthesis decreases; and if the temperature rises high enough, the plant's rate of net CO2 uptake will drop all the way to zero at what is generally referred to as the plant's upper limiting temperature, above which thermal death can occur there if is no relief from the high-temperature stress.

Because the air's CO2 content is currently on the rise, it is only natural to wonder if that global-change phenomenon will have any impact on this basic 'fact of life'? That is a question we asked ourselves several years ago, when we and some of our colleagues measured the net photosynthetic rates of leaves of sour orange trees growing out-of-doors in clear-plastic-wall open-top chambers maintained at 400 and 700 ppm CO2 under the searing summer sun of Phoenix, Arizona (Idso et al., 1995). It was so hot during that experiment that most of our measurements were made at temperatures above the trees' optimum temperature. As a result, foliage net photosynthetic rates dropped lower and lower as the air temperature climbed higher and higher into the middle of each afternoon. In fact, they dropped so low that at a leaf temperature of 47C, the net photosynthetic rates of the leaves on the trees growing in air of 400 ppm CO2 dropped all the way to zero and actually became negative thereafter, as the temperature rose higher still. In contrast, leaves on the CO2-enriched trees continued to exhibit positive rates of net photosynthesis until a leaf temperature of 54C was reached. Thus, the extra 300 ppm of CO2 to which the CO2-enriched trees were exposed allowed them to continue to remove CO2 from the air until it had warmed by an additional 7C.

An even more dramatic example of the plant "heat relief" provided by atmospheric CO2 enrichment is described in another publication of ours (Idso et al., 1989). Throughout the summers of 1985 and 1986 - again in Phoenix, Arizona - we grew floating mats of tiny water ferns on the surfaces of sunken metal stock tanks filled with water and located out-of-doors within clear-plastic-wall open-top chambers maintained at atmospheric CO2 concentrations of ambient and ambient plus 300 ppm CO2. In both years, plant growth rates (assessed as weight gains per week) in the ambient CO2 enclosures first decreased, then dropped to zero, and finally became negative when the air temperature rose above 30C. In the CO2-enriched enclosures, however, the debilitating effects of high air temperature were significantly reduced. In one experiment the plants exhibited a much less severe negative growth rate; in another they experienced only a short period of zero growth rate; and in a third instance they suffered no ill effects at all, in spite of the fact that the plants growing in ambient air all died!

Midway through one of these experiments, we measured rates of net photosynthesis every hour of the day approximately one week after we first began to detect high-temperature-induced reductions in plant growth rates in the ambient CO2 enclosures. Our data revealed that the net photosynthetic rates of the plants growing in ambient air went from negative to positive at 8 o'clock in the morning, peaked at approximately 10 o'clock, and dropped back to zero at noon, becoming negative thereafter. Consequently, with only four hours of positive net photosynthesis during a 24-hour period, the plants growing in ambient air lost a significant portion of their biomass each day. In the CO2-enriched plants, on the other hand, net photosynthesis went from negative to positive just after 7 o'clock in the morning, peaked at about 11 o'clock, and - being better able to withstand the high afternoon temperatures - did not decline to zero until just before 5 pm. Averaged over a full 24-hour period, these CO2-enriched plants took up about as much CO2 during the day as they gave off at night, enabling them to maintain their biomass during this stressful high-temperature period that saw dramatic weekly weight losses in the plants growing in ambient air.

These experiments on tiny water ferns and large sour orange trees vividly demonstrate the ability of atmospheric CO2 enrichment to enable plants to better withstand the physiological ravages of high temperatures, which for some plants occur both daily (in the afternoon) and seasonally (in the summer) in nearly all parts of the world where plants grow. In fact, they demonstrate that elevated levels of atmospheric CO2 can sometimes mean the difference between life and death itself. And if one is concerned about carbon sequestration, it doesn't take much gray matter to realize that dead plants have done all they'll ever do in the way of removing CO2 from the atmosphere. One of the important keys to greater carbon storage, therefore, is to keep plants both living and growing as long as possible; and in this regard, elevated levels of atmospheric CO2 seem to be just what the plant doctor ordered.

Dr. Sherwood B. Idso Dr. Keith E. Idso

References
Idso, S.B., Allen, S.G., Anderson, M.G. and Kimball, B.A. 1989. Atmospheric CO2 enrichment enhances survival of Azolla at high temperatures. Environmental and Experimental Botany 29: 337-341.

Idso, S.B., Idso, K.E., Garcia, R.L., Kimball, B.A. and Hoober, J.K. 1995. Effects of atmospheric CO2 enrichment and foliar methanol application on net photosynthesis of sour orange tree (Citrus aurantium; Rutaceae) leaves. American Journal of Botany 82: 26-30.