Reference
Sebastiani, L., Minnocci, A. and Tognetti, R. 2002. Genotypic differences in the response to elevated CO2 concentration of one-year-old olive cuttings (Olea europaea L. cv. Frantoio and Moraiolo). Plant Biosystems 136: 199-208.
What was done
One-year-old actively-growing cuttings of two olive (Olea europaea L.) cultivars (Frantoio and Moraiolo) were grown for a second season in 2-dm3 pots in a set of FACE arrays maintained at either ambient CO2 (360 ppm) or enriched CO2 (560 ppm) under conditions of ample water and nutrient supply, throughout which time a number of measurements of various plant parameters were made.
What was learned
Net photosynthetic rates of leaves in the CO2-enriched treatment were 96% greater in Moraiolo plants and 52% greater in Frantoio plants than in their ambient-air counterparts. Also, elevated CO2 decreased stomatal conductances by 27% and 31% in Moraolo and Frantoio plants, respectively. As a result, the authors report that instantaneous water use efficiency "was significantly increased by 62% in the cultivar Moraiolo and by 39% in the cultivar Frantoio." Nevertheless, they observed that total above-ground dry mass was unaffected by elevated CO2 in the Moraiolo cultivar, while in the Frantoio cultivar it was actually reduced in the CO2-enriched treatment by approximately 33%.
What it means
It is difficult to understand how a plant that experienced a CO2-induced increase of 96% in its mean rate of net photosynthesis could not experience a CO2-induced increase in biomass production. It is also difficult to understand how a plant that experienced a CO2-induced increase of 52% in its mean rate of net photosynthesis could actually experience a 33% decrease in biomass production. In acknowledging this conceptual difficulty, the authors say "it must be pointed out that our cuttings grew in pots which may have influenced the growth response of plants to elevated CO2." Indeed, the problem of pots has been demonstrated to be substantial, severely restricting -- and in some cases even totally eliminating -- what would otherwise be a significant positive response to elevated CO2, especially in the case of woody plants (Idso, 1999). In this experiment, however, the pot effect went even further, actually leading to a CO2-induced decrease in biomass production in one of the two olive cultivars.
Reference
Idso, S.B. 1999. The long-term response of trees to atmospheric CO2 enrichment. Global Change Biology 5: 493-495.
Reviewed 30 July 2003