The response of nodulated alfalfa to water supply, temperature and elevated CO₂: photosynthetic downregulation

Abstract

Plants grown in an environment of elevated CO 2 and temperature often show reduced CO 2 assimilation capacity, providing evidence of photosynthetic downregulation. The aim of this study was to analyse the downregulation of photosynthesis in elevated CO 2 (700 µmol mol −1 ) in nodulated alfalfa plants grown at different temperatures (ambient and ambient + 4°C) and water availability regimes in temperature gradient tunnels. When the measurements were taken in growth conditions, a combination of elevated CO 2 and temperature enhanced the photosynthetic rate; however, when they were carried out at the same CO 2 concentration (350 and 700 µmol mol −1 ), elevated CO 2 induced photosynthetic downregulation, regardless of temperature and drought. Intercellular CO 2 concentration measurements revealed that photosynthetic acclimation could not be accounted for by stomatal limitations. Downregulation of plants grown in elevated CO 2 was a consequence of decreased carboxylation efficiency as a result of reduced rubisco activity and protein content; in plants grown at ambient temperature, downregulation was also induced by decreased quantum efficiency. The decrease in rubisco activity was associated with carbohydrate accumulation and depleted nitrogen availability. The root nodules were not sufficiently effective to balance the source–sink relation in elevated CO 2 treatments and to provide the required nitrogen to counteract photosynthetic acclimation.