In situ photosynthetic responses to light, temperature and carbon dioxide in herbaceous plants from low and high altitude

Net CO2 assimilation (A) was analysed in situ in 12 pairs of altitudinally separated, herbaceous plant species in the Austrian Alps at 600 and 2600m. Both groups of species show a similar average response to light, saturating at quantum flux densities (400-700mm) (QFD) of more than 1200 mu mol m(-2) sol(-1). Temperature optimum of QFD-saturated A differs little (3K) and corresponds to the median of air temperature at leaf level for hours with rate-saturating light conditions and not to mean air temperature which differs by 10K. Species with an exclusive high altitude distribution show steeper initial slopes and higher levels of saturation of the response of A to internal partial pressure of CO2 (CPI) than low elevation species. Mean A at local ambient partial pressure (CPA) does not differ between sites (c. 18 mu mol m(-2) sol(-1)), despite the 21% decrease in atmospheric pressure. Plants at high altitude operate at mean CPJ of 177 mu bar as compared to 250 mu bar at low altitude. The higher ECU (efficiency of carbon dioxide uptake [linear slope of A/CPJ curve]) as well as the steeper CO2 gradient between mesophyll and ambient air of alpine plants are explained by (1) greater leaf and palisade layer thickness and (2) greater nitrogen (protein) content per unit leaf area. We hypothesize that alpine plants profit more from enhanced CO2 levels than lowland plants (Fig 7).