Exposure of loblolly pine seedlings to up to 150 ppb (300 Hg/m ) 03 (5 h/day; 5 days/week; for 6 or 12 weeks) resulted in a linear dose-response relationship with regard to mycorrhiza formation (134). Both exposure to ozone and soil water deficit decreased the amount of available carbohydrates, but only ozone caused a suppression of ectomycorrhizas. In summary, there is good evidence that ozone at concentrations which are easily reached on sunny summer days (100 to 200 pg/m3) decreases carbon translocation to the root system.

Elevated CO2 can cause an increase of the concentration of soluble sugars in roots (for loblolly pine (Pinus taeda L.) seedlings see (135). Generally, the response of plants, and especially of ECM-forming trees, toward elevated CO2 depends on nutrient availability and the degree of utilization of photoassimilates. There are at least short term responses of metabolism and carbon allocation (136-137) which apply mainly to plants with limited sink capacities. These plants do not take advantage of an increased supply of CO2 because there is a feedback regulation (decreased gene expression) of carbon fixation by accumulating sugars (12, 54). Mycorrhizal plants which are depleted of photoassimilates by their fungal partner should therefore perform better under conditions of increased CO2. Indeed, seedlings of Pinus echinata exhibited a higher degree of root colonization by mycorrhizal fungi when grown under elevated C02 (138). Similarly, pine seedlings inoculated with the ECM-forming fungus Pisolithus tinctorius exhibited a much faster fungal growth under elevated CO2 (600 (imol mol"1), while no effect on shoot biomass was found (139).

4.6. Carbon allocation and root senescence

Roots of Douglas fir (Pseudotsuga menziesii (Mirb.) Franco) seedlings grown under drought conditions initially contained a certain quantity of starch which was gradually depleted until the roots died. This led to the theory that the life time of roots could be determined by their initial starch content (140). There are, however, also opposite views which suggest that roots have an indeterminate longevity and are eventually killed as a consequence of adverse environmental conditions (141). A cost benefit analysis showed that senescence of a root located in an unfavorable soil environment is of advantage to the rest of the plant because of the reduced carbon cost (142).

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