Evidence of altitudinal increase in photosynthetic capacity: gas exchange measurements at ambient and constant CO2 partial pressures

Issue		Ann. For. Sci. Volume 66, Number 5, July-August 2009


Article Number		505
Number of page(s)		8
DOI		10.1051/forest/2009027
Published online		09 July 2009

Ann. For. Sci. 66 (2009) 505
Number of pages: 8
DOI: 10.1051/forest/2009027

Evidence of altitudinal increase in photosynthetic capacity: gas exchange measurements at ambient and constant CO₂ partial pressures

Caroline C. Bresson¹, Andrew S. Kowalski^{2, 3}, Antoine Kremer¹ and Sylvain Delzon¹

¹ UMR BIOGECO, Université Bordeaux 1 – INRA, 33405 Talence, France
² Departamento de Física Aplicada, Universidad de Granada, 18071 Granada, Spain
³ Centro Andaluz del Medio Ambiente (CEAMA), 18006 Granada, Spain

Received 10 September 2008; Accepted 9 February 2009; Published online 9 July 2009

Abstract
• Because all microclimatic variables change with elevation, it is difficult to compare plant performance and especially photosynthetic capacity at different elevations. Indeed, most previous studies investigated photosynthetic capacity of low- and high-elevation plants using constant temperature, humidity and light but varying CO₂ partial pressures $(P_{\rm CO_{2}})$ .
• Using gas exchange measurements, we compared here maximum assimilation rates $(A_{\max})$ at ambient and constant-low-elevation $P_{\rm CO_{2}}$ for two temperate tree species along an altitudinal gradient (100 to 1600 m) in the Pyrénées mountains.
• Significant differences in $A_{\max}$ were observed between the CO₂ partial pressure treatments for elevations above 600 m, the between-treatment differences increasing with elevation up to 4 $\mu$ mol m^-2 s^-1. We found an increase in $A_{\rm max}$ with increasing elevation at constant-low-elevation $P_{\rm CO_{2}}$ but not at ambient $P_{\rm CO_{2}}$ for both species. Given a 10% change in $P_{\rm CO_{2}}$ , a proportionally higher shift in maximum assimilation rate was found for both species.
• Our results showed that high elevation populations had higher photosynthetic capacity and therefore demonstrated that trees coped with extreme environmental conditions by a combination of adaptation (genetic evolution) and of acclimation. Our study also highlighted the importance of using constant CO₂ partial pressure to assess plant adaptation at different elevations.

Résumé - Augmentation de la capacité photosynthétique avec l'altitude : mesures d'échanges gazeux à pressions partielles de CO₂ ambiante et constante.
• Les conditions microclimatiques étant très variables avec l'altitude, il est difficile de comparer les performances d'une espèce végétale à différentes altitudes, particulièrement la capacité photosynthétique. En effet, la plupart des études antérieures ont estimé le taux maximal d'assimilation à basses et hautes altitudes en maintenant la température, l'humidité de l'air et la lumière constantes mais en laissant varier la pression partielle de CO₂ $(P_{\rm CO_{2}})$ .
• Afin de comparer le taux maximum d'assimilation $(A_{\rm max})$ à pressions partielles de CO₂ constantes de basse altitude et variables, nous avons effectué des mesures d'échanges gazeux sur deux espèces d'arbres tempérés le long d'un gradient altitudinal de 1600 m de dénivelé dans les Pyrénées françaises.
• La différence entre les deux traitements de $P_{\rm CO_{2}}$ est significative au-dessus de 600 m d'altitude et atteint un maximum de 4 $\mu$ mol m^-2 s^-1. Pour les deux espèces, nous avons mis en évidence une augmentation de $A_{\rm max}$ avec l'altitude à $P_{\rm CO_{2}}$ constantes mais pas à $P_{\rm CO_{2}}$ ambiantes. Pour une modification de $P_{\rm CO_{2}}$ de 10 %, le changement du taux maximum d'assimilation est proportionnellement supérieur chez les deux espèces.
• Nos résultats montrent que les populations de hautes altitudes possèdent une capacité photosynthétique supérieure, démontrant que les arbres font face aux conditions environnementales extrêmes grâce à des adaptations génétiques ou des acclimatations. Notre étude souligne ainsi l'importance de fixer la P $_{\rm CO_{2}}$ pour comparer l'adaptation des plantes à différentes altitudes.

Key words: adaptation / altitudinal gradient / acclimation / partial pressure / photosynthetic capacity

Mots clés : adaptation / acclimatation / gradient altitudinal / pression partielle / capacité photosynthétique

Corresponding author: sylvain.delzon@u-bordeaux1.fr

© INRA, EDP Sciences 2009

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