Interactive effects of drought, elevated CO2 and warming on photosynthetic capacity and photosystem performance in temperate heath plants

Increased temperature, atmospheric CO2 and change in precipitation patterns affect plant physiological and ecosystem processes. In combination, the interactions between these effects result in complex responses that challenge our current understanding. In a multi-factorial field experiment with elevated CO2 (CO2, FACE), nighttime warming (T) and periodic drought (D), we investigated photosynthetic capacity and PSII performance in the evergreen dwarf shrub Calluna vulgaris and the grass Deschampsia flexuosa in a temperate heath ecosystem. Photosynthetic capacity was evaluated using A/C-i curves, leaf nitrogen content and chlorophyll-a fluorescence QJIP induction curves. The PSII performance was negatively influenced by high air temperature, low soil water content and high irradiance dose. The experimental treatments of elevated CO2 and prolonged drought generally down-regulated J(max) V-cmax and PItotal. Recovery from these depressions was found in the evergreen shrub after rewetting, while post-rewetting up-regulation of these parameters was observed in the grass. Warming effects acted indirectly to improve early seasonfmax, J(max) V-cmax and PItotal. The responses in the multifactorial experimental manipulations demonstrated complex interactive effects of Tx CO2, D x CO2 and Tx D x CO2 on photosynthetic capacity and PSII performance. The impact on the O-j, J-I and I-P phases which determine the response of PItotal are discussed. The single factor effects on PSII performance and their interactions could be explained by parallel adjustments of V-cmax, J(max) and leaf nitrogen in combination. Despite the highly variable natural environment, the OJIP-test was very robust in detecting the impacts of T, D, CO2 and their interactions. This study demonstrates that future climate will affect fundamental plant physiological processes in a way that is not predictable from single factor treatments. The interaction effects that were observed depended upon both the growth strategy of the species considered, and their ability to adjust during drought and rewetting periods. (C) 2011 Elsevier GmbH. All rights reserved.