Thermal acclimation of leaf photosynthetic traits in an evergreen woodland, consistent with the co-ordination hypothesis

Abstract

Ecosystem models commonly assume that key photosynthetic traits, such as carboxylation-capacity measured at a standard temperature, are constant in time. The temperature responses of modelled photosynthetic/respiratory rates then depend entirely on enzyme kinetics. Optimality considerations suggest this assumption may be incorrect. The ‘co-ordination hypothesis’ (that Rubisco- and electron-transport limited rates of photosynthesis are co-limiting under typical daytime conditions) predicts instead that carboxylation (<i>V_cmax</i>) and light-harvesting (<i>J_max</i>) capacities, and mitochondrial respiration in the dark (<i>R_dark</i>), should acclimate so that they increase with growth temperature – but less steeply than their instantaneous response rates. To explore this hypothesis, photosynthetic measurements were carried out on woody species during the warm and the cool seasons in the semi-arid Great Western Woodlands, Australia, under broadly similar light environments. A consistent linear relationship between <i>V_cmax</i> and <i>J_max</i> was found across species. <i>V_cmax</i>, <i>J_max</i> and <i>R_dark</i> increased with temperature, but values standardized to 25 ˚C declined. The <i>ci:ca</i> ratio increased slightly with temperature. The leaf <i>N:P</i> ratio was lower in the warm season. The slopes of the relationships of log-transformed <i>V_cmax</i> and <i>J_max</i> to temperature were close to values predicted by the co-ordination hypothesis, but shallower than those predicted by enzyme kinetics. </jats:p>