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C3 and C4 Plant Responses to Elevated CO2 and Water Stress
Ward, J.K., Tissue, D.T., Thomas, R.B. and Strain, B.R.  1999.  Comparative responses of model C3 and C4 plants to drought in low and elevated CO2Global Change Biology 5: 857-867.

What was done
Abutilon theophrasti (C3 dicot) and Amaranthus retroflexus (C4 dicot) were grown from seed in growth chambers receiving atmospheric CO2 concentrations of 180, 270, 350, and 700 ppm.  In addition, about one month after seedling emergence, water was withheld from half the plants of each species at each CO2 concentration to study the interactive effects of elevated CO2 and water stress on their photosynthesis and growth.

What was learned
Under well-watered conditions, atmospheric CO2 enrichment increased rates of net photosynthesis in both species.  In going from 180 to 700 ppm CO2, for example, rates of net photosynthesis were enhanced by 30 and 142% for the C4 and C3 species, respectively.  However, the C4 plants exhibited absolute rates of net photosynthesis that were about 60 to 160% greater than those of the C3 plants, depending upon the atmospheric CO2 growth concentration.

Such increases in photosynthesis should logically lead to increases in total plant dry mass, which occurred for the C3, but not the C4, species.  Indeed, total C3 plant dry mass increased by 142% as the CO2 content of the air was raised from 180 to 700 ppm.  Although elevated CO2 did not significantly impact C4 plant dry mass, the absolute values of C4 plant dry mass were still 56 to 285% greater than those obtained by C3 plants, depending upon CO2 growth concentration.

Under water-stressed conditions, C3 plants reduced their leaf areas to a greater extent than C4 plants, regardless of atmospheric CO2 concentration.  Similarly, C3 plants exhibited a greater average reduction in photosynthesis than C4 plants (93 vs. 85%).  Thus, total plant dry mass was least effected by drought in C4, rather than in C3, plants.

What it means
As the air's CO2 content rises, it is likely that seedlings of Abutilon theophrasti and Amaranthus retroflexus will exhibit increases in photosynthesis and growth, regardless of soil moisture conditions.  However, in the C4 species there may not be a direct observable relationship between CO2-induced photosynthetic increases and biomass production, as C4 species are typically involved in more robust relationships with symbiotic mycorrhizal fungi and may exhibit greater exudation rates of photosynthetically-derived carbohydrates into the soil to support them, rather than use these resources directly to increase their own biomass.  It is important to note, nonetheless, that Amaranthus retroflexus exhibited a greater relative recovery to drought than the C3 species, suggesting, in the words of the authors, that "the C4 species would continue to be more competitive than the C3 species in regions receiving more frequent and severe droughts," which basically characterizes regions where C4 plants currently exist.  It is therefore unlikely that the rising CO2 content of the air will cause significant reductions in the C4 components of earth's ecosystems.  Hence, C4 plants should continue to maintain their presence, importance, and diversity as the air's CO2 content continues to rise in the years ahead.

Reviewed 2 August 2000