Paper Reviewed
Aranda, I., Cadahía, de Simón, B.F. 2020. Leaf ecophysiological and metabolic response in Quercus pyrenaica Willd seedlings to moderate drought under enriched CO2 atmosphere. Journal of Plant Physiology 244: 153083.

Pyrenean Oak (Quercus pyrenaica) is a moderately-tolerant drought species found in the Mediterranean region of Europe. According to Aranda et al. (2020), it is of high ecological interest as it "represents the transition from the typical evergreen sclerophyllous Mediterranean woodlands to broadleaved temperate forests." However, there is concern how this species will respond to climate change. Consequently, it was the aim of the three Spanish researchers to investigate the response of Pyrenean Oak seedlings to elevated concentrations of CO2 under well-watered and water-stressed conditions.

The experiment was conducted in a controlled-environment chambers where Q. pyrenaica acorns were germinated under ambient (400 ppm) or elevated (800 ppm) levels. Following germination, the seedlings were all grown under well-watered conditions for two and a half months. Thereafter, half the seedlings in each CO2 treatment chamber continued to receive adequate water whereas the other half were subjected to water stress for a period of 53 days. At the end of this period a number of physiological measurements were conducted on the plant seedlings in order to determine the interactive effects of CO2 and water stress.

The key results are summarized in the figure below. As indicated there, elevated CO2 alone had a stimulatory effect on net photosynthesis (a 62.6% increase) whereas water stress alone suppressed it (a 20% decrease). When combined, the positive effects of elevated CO2 were strong enough to overpower the negative effects of water stress, such that net photosynthesis increased by 10% relative to the control (well-watered and ambient CO2) treatment. The authors also report that the total biomass of the seedlings was stimulated by elevated CO2 in like direction to that observed for photosynthesis, although the magnitude of that stimulation was not reported.

Aranda et al. additionally report that stomatal conductance was reduced by both water stress and elevated CO2, leading to higher water use efficiencies as shown in Figure 1 (right panel). Indeed, plant water use efficiency for the oak seedlings rose under elevated CO2 by 122% and 88% in the well-watered and water-stressed treatments, respectively. And in light of all of the above findings, the authors conclude that "CO2 improves carbon uptake capacity of Q. pyrenaica seedlings, buffering partially the negative impact of moderate water stress." And that is wonderful news for this European tree species.

Figure 1. Net photosynthesis (Anet) and water use efficiency (WUEi) of Pyrenean Oak (Quercus pyrenaica) seedlings grown under well-watered (WW) or water-stressed (WS) conditions at ambient (400 ppm) or elevated (800 ppm) CO2 conditions. Adapted from Aranda et al. (2020).