EVIDENCE THAT PLANTS FROM HIGH-ALTITUDES RETAIN THEIR GREATER PHOTOSYNTHETIC EFFICIENCY UNDER ELEVATED CO2

1. Herbaceous plant species native to low and high altitudes in the Alps evolved under CO2 partial pressures (P-a) that differ as much as pre-industrial P-a differs from present day P-a at low altitude (e.g. 21% for a 2000-m difference in altitude). 2. In a previous study we showed that the efficiency of CO2 uptake (ECU) in typical high-altitude species is generally greater than in low-altitude species. Here we investigate whether this difference prevails under longer-term exposure to altered P-a. 3. Alpine and lowland species (mainly Ranunculus glacialis/R. acris and Geum reptans/G. rivale) were grown under various CO2 regimes in full daylight growth chambers at their respective natural growth temperature and photoperiod. When they were grown at twice the present CO2 level only moderate downward adjustment of photosynthesis was observed in both groups of species. The adjustments were not enough to compensate for the effect of increased CO2 supply. These trends prevailed under reciprocally exchanged alpine/lowland partial pressure of CO2 at the same total atmospheric pressure. 4. Irrespective of altitudinal origin, greatest downward adjustment of photosynthesis was found in species with the most pronounced accumulation of non-structural carbohydrate and dilution of leaf nitrogen when grown under elevated CO2 (e.g. in G. rivale). 5. These results suggest that, at least initially, the alpine plant species studied may attain relatively greater carbon gains in a CO2-enriched atmosphere than comparable lowland plant species.