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Roots (Miscellaneous) Summary
In reviewing the scientific literature pertaining to atmospheric CO2 enrichment effects on belowground plant growth and development, Weihong et al. (2000) briefly summarize what is known about this subject.  They report that atmospheric CO2 enrichment typically enhances the growth rates of roots, especially those of fine roots, and that CO2-induced increases in root production eventually lead to increased carbon inputs to soils, due to enhanced root turnover and exudation of various organic carbon compounds, which can potentially lead to greater soil carbon sequestration.  In addition, they note that increased soil carbon inputs stimulate the growth and activities of soil microorganisms that utilize plant-derived carbon as their primary energy source; and they report that subsequently enhanced activities of fungal and bacterial plant symbionts often lead to increased plant nutrient acquisition.

In a much more narrowly-focused study, Crookshanks et al. (1998) sprouted seeds of the small and fast-growing Arabidopsis thaliana plant on agar medium in Petri dishes and grew the resulting immature plants in controlled environment chambers maintained at atmospheric CO2 concentrations of either 355 or 700 ppm.  Visual assessments of root growth were made after emergence of the roots from the seeds, while microscopic investigations of root cell properties were also conducted.  In pursuing this protocol, the scientists learned that the CO2-enriched plants directed a greater proportion of their newly-produced biomass into root, as opposed to shoot, growth.  In addition, the young plants produced longer primary roots and more and longer lateral roots.  These effects were found to be related to the CO2-induced stimulation of mitotic activity, accelerated cortical cell expansion, and increased cell wall plasticity.

In another unique study, Gouk et al. (1999) grew an orchid plantlet, Mokara Yellow, in plastic bags flushed with 350 and 10,000 ppm CO2 for three months to study the effects of elevated CO2 on this epiphytic CAM species.  They determined that the super-elevated CO2 of their experiment enhanced the total dry weight of the orchid plantlets by more than two-fold, while increasing the growth of existing roots and stimulating the induction of new roots from internodes located on the orchid stems.  Total chlorophyll content was also increased by elevated CO2 -- by 64% in young leaves and by 118% in young roots.  This phenomenon permitted greater light harvesting during photosynthesis and likely led to the tissue starch contents of the CO2-enriched plantlets rising nearly 20-fold higher than those of the control-plantlets.  In spite of this large CO2-induced accumulation of starch, however, no damage or disruption of chloroplasts was evident in the leaves and roots of the CO2-enriched plants.

A final question that has periodically intrigued researchers is whether plants take up carbon through their roots in addition to through their leaves.  Although a definitive answer eludes us, various aspects of the issue have been described by Idso (1989), who we quote as follows.

"Although several investigators have claimed that plants should receive little direct benefit from dissolved CO2 (Stolwijk et al., 1957; Skok et al., 1962; Splittstoesser, 1966), a number of experiments have produced significant increases in root growth (Erickson, 1946; Leonard and Pinckard, 1946; Geisler, 1963; Yorgalevitch and Janes, 1988), as well as yield itself (Kursanov et al., 1951; Grinfeld, 1954; Nakayama and Bucks, 1980; Baron and Gorski, 1986), with CO2-enriched irrigation water.  Early on, Misra (1951) suggested that this beneficent effect may be related to CO2-induced changes in soil nutrient availability; and this hypothesis may well be correct.  Arteca et al. (1979), for example, have observed K, Ca and Mg to be better absorbed by potato roots when the concentration of CO2 in the soil solution is increased; while Mauney and Hendrix (1988) found Zn and Mn to be better absorbed by cotton under such conditions, and Yurgalevitch and Janes (1988) found an enhancement of the absorption of Rb by tomato roots.  In all cases, large increases in either total plant growth or ultimate yield accompanied the enhanced uptake of nutrients.  Consequently, as it has been suggested that CO2 concentration plays a major role in determining the porosity, plasticity and charge of cell membranes (Jackson and Coleman, 1959; Mitz, 1979), which could thereby alter ion uptake and organic acid production (Yorgalevitch and Janes, 1988), it is possible that some such suite of mechanisms may well be responsible for the plant productivity increases often observed to result from enhanced concentrations of CO2 in the soil solution."

Although much is thus known about many aspects of root responses to atmospheric CO2 enrichment, much remains to be learned.  Nevertheless, it is abundantly evident that plant roots, like most other plant organs, typically do better in CO2-enriched air than in current ambient air.

References
Arteca, R.N., Pooviah, B.W. and Smith, O.E.  1979.  Changes in carbon fixation, tuberization, and growth induced by CO2 applications to the root zones of potato plants.  Science 205: 1279-1280.

Baron, J.J. and Gorski, S.F.  1986.  Response of eggplant to a root environment enriched with CO2HortScience 21: 495-498.

Crookshanks, M., Taylor, G. and Dolan, L.  1998.  A model system to study the effects of elevated CO2 on the developmental physiology of roots: the use of Arabidopsis thalianaJournal of Experimental Botany 49: 593-597.

Erickson, L.C.  1946.  Growth of tomato roots as influenced by oxygen in the nutrient solution.  American Journal of Botany 33: 551-556.

Geisler, G.  1963.  Morphogenetic influence of (CO2 + HCO3-) on roots.  Plant Physiology 38: 77-80.

Gouk, S.S., He, J. and Hew, C.S.  1999.  Changes in photosynthetic capability and carbohydrate production in an epiphytic CAM orchid plantlet exposed to super-elevated CO2Environmental and Experimental Botany 41: 219-230.

Grinfeld, E.G.  1954.  On the nutrition of plants with carbon dioxide through the roots.  Dokl. Akad. Nauk SSSR 94: 919-922.

Idso, S.B.  1989.  Carbon Dioxide and Global Change: Earth in Transition.  IBR Press, Tempe, AZ.

Jackson, W.A. and Coleman, N.T.  1959.  Fixation of carbon dioxide by plant roots through phosphoenolpyruvate carboxylase.  Plant and Soil 11: 1-16.

Kursanov, A.L., Kuzin, A.M. and Mamul, Y.V.  1951.  On the possibility for assimilation by plants of carbonates taken in with the soil solution.  Dokl. Akad. Nauk SSSR 79: 685-687.

Leonard, O.A. and Pinckard, J.A.  1946.  Effect of various oxygen and carbon dioxide concentrations on cotton root development.  Plant Physiology 21: 18-36.

Mauney, J.R. and Hendrix, D.L.  1988.  Responses of glasshouse grown cotton to irrigation with carbon dioxide-saturated water.  Crop Science 28: 835-838.

Misra, R.K.  1951.  Further studies on the carbon dioxide factor in the air and soil layers near the ground.  Indian Journal of Meteorology and Geophysics 2: 284-292.

Mitz, M.A.  1979.  CO2 biodynamics: A new concept of cellular control.  Journal of Theoretical Biology 80: 537-551.

Nakayama, F.S. and Bucks, D.A.  1980.  Using subsurface trickle system for carbon dioxide enrichment.  In Jensen, M.H. and Oebker, N.F. (Eds.), Proceedings of the 15th Agricultural Plastics Congress, National Agricultural Plastics Association, Manchester, MO, pp. 13-18.

Skok, J., Chorney, W. and Broecker, W.S.  1962.  Uptake of CO2 by roots of Xanthium plants.  Botanical Gazette 124: 118-120.

Yorgalevitch, C.M. and Janes, W.H.  1988.  Carbon dioxide enrichment of the root zone of tomato seedlings.  Journal of Horticultural Science 63: 265-270.

Splittstoesser, W.E.  1966.  Dark CO2 fixation and its role in the growth of plant tissue.  Plant Physiology 41: 755-759.

Stolwijk, J.A.J. and Thimann, K.V.  1957.  On the uptake of carbon dioxide and bicarbonate by roots and its influence on growth.  Plant Physiology 32: 513-520.

Weihong, L., Fusuo, Z. and Kezhi, B.  2000.  Responses of plant rhizosphere to atmospheric CO2 enrichment.  Chinese Science Bulletin 45: 97-101.