Effects of O[sub(3)] and CO[sub(2)] Alone

Due to their global importance and their contrasting effects on vegetation, plant growth responses to either 03 or C02 alone are briefly described. Primary 03 effects include subtle biochemical and ultra- structural changes in the plant cell, which may result in impaired photosynthesis, alterations of carbon allocation patterns, symptoms of visible injury, enhanced senescence, reduced growth and economic yield, altered resistance to other abiotic and biotic stresses, or reduced flowering and seed production at the whole-plant level17,8! At the ecosystem level, this may result in a loss of competitive abilities of plant species in communities along with shifts in biodiversity and impaired ecosystem functions and services like reduced carbon sequestration and altered hydrology.19! por example, current ambient (03) in many industrialized areas has been shown to suppress crop yields of sensitive species and to retard growth and development of trees and other plant species of the non-woody (semi)natural vegetation. Overall quantification of 03 effects on vegetation is complicated by large inter- and intra-specific variability in the 03 susceptibility of plants.1101

By contrast, plants of the C3 type most frequently respond to elevated C02 with a stimulation of photosynthesis accompanied by a reduced stomatal conductance and transpiration rate, an enhanced concentrations of soluble carbohydrates, and a stimulation of biomass production and economic yield.1'11 Similarly, in C4 plants, higher C02 concentrations reduce stomatal conductance and transpiration, i.e., both C3 and C4 plants may benefit from elevated CO, by improved water-use efficiency and a reduced demand for water. Under well-watered conditions, no significant growth stimulation has been found so far in C4 plants, because C4 photosynthesis is saturated under ambient C02.112,131 Growth and yield enhancements of up to 25-35% as compared to ambient C02 have been observed when crop plants were exposed to 550-750 ppmv C02. Experiments with tree species ranging from short-term studies with seedlings to long-term whole-stand manipulations have also shown that elevated C02 stimulated net photosynthesis and resulted in enhanced tree growth in almost all cases.114,151 As with О,, plant species differ widely in their response to high C02, which makes an overall assessment of its potential effects on vegetation difficult.

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