13.5.4  Water x CO2

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CO2 enrichment generally reduces stomatal density, stomatal conductance and transpiration rate per unit leaf area. In addition, total leaf area per plant and root:shoot ratios might increase, while solute content of leaves (carbohydrates and inorganic ions) could also be affected. All of these factors can influence plant water status via effects on water uptake and/or water loss.

In a study of Maranthes corymbosa, a tropical monsoon vine forest tree, leaf water potential was consistently higher under CO2-enriched conditions than in control conditions through--out the day (Figure 13.9). Similarly in Eucalyptus tetrodonta, a savanna species of tropical Australia, pre-dawn water potential was also higher under CO2-enriched conditions. These results were attributed to decreased transpiration rate per plant. However, during daytime, leaf water potential of E. tetrodonta growing with CO2 enrichment was equal to or lower than that of plants growing in ambient CO2 conditions. These apparent discrepancies probably resulted from different responses of root:shoot ratios between these two species (Eamus et al. 1995). In M. corymbosa, the root:shoot ratio was the same in ambient as in CO2-enriched plants. Roots were able to supply enough water for transpiration from leaves with reduced gs. In contrast to M. corymbosa, the root:shoot ratio of E. tetrodonta declined in response to CO2 enrichment and by implication there may not have been enough roots to maintain water potential higher than that of control values.

Whole-plant water use

The response of whole-plant water use to CO2 enrichment is variable. In some studies the rate of transpiration per plant can be almost identical for plants growing in ambient or CO2-enriched conditions. Thus the reduction in transpiration rate per leaf area is balanced by increased leaf area per plant and the rate of depletion of soil water is the same for ambient and CO2-enriched plants (Gifford et al. 1984). In contrast Reekie and Bazzaz (1989) found that in all five tree species studied, soil water content was higher and the water requirement for growth (grams H2O transpired per gram biomass formed) was lower in CO2-enriched trees.