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

Abstract

Abstract. Ecosystem models commonly assume that key photosynthetic traits, such as carboxylation-capacity measured at a 15 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 (V cmax ) and light-harvesting (J max ) capacities, and mitochondrial respiration in the dark (R dark ), should acclimate so that they increase with growth temperature -but less steeply than their instantaneous 20 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 V cmax and J max was found across species. V cmax , J max and R dark increased with temperature, but values standardized to 25˚C declined. The c i :c a ratio increased slightly with temperature. The leaf N:P ratio was lower in the warm season. The slopes of the relationships of log-transformed V cmax and J max to temperature were close to 25 values predicted by the co-ordination hypothesis, but shallower than those predicted by enzyme kinetics.

Functional trait variation related to gap dynamics in tropical moist forests: A vegetation modelling perspective

“…However, leaf N also has structural and defensive components that are roughly proportional to LMA and represent a largely independent source of variation in Narea (Dong et al, 2017). So far, these predictions have been supported for seasonal variations within individual plants (Togashi et al, 2017), and for spatial variations along environmental gradients (Prentice et al, 2014;Dong et al, 2017). Here we extend their application to biotically conditioned, microenvironmental variation within forest environments.…”

“…The observed relationships among Narea, Parea and LMA, and the weaker correlations of these traits with primary metabolic traits, reflect the fact that a substantial part of the N and P content of leaves is not directly tied to photosynthetic functions (Dong et al, 2017). Although strong linear relationships between Vcmax (at a reference temperature) and Narea seem to be widely expected, they are not always found (Prentice et al, 2014;Togashi et al, 2017), perhaps due to the overpowering effect of variation in structural and/or defensive components of leaf N. The photosynthetic component of Narea however is expected to be proportional to incident PAR. This expectation is supported by the analysis of Dong et al (2017), and by our finding of highest Narea among climax species.…”

“…Recent empirical analyses aiming to inform the development of 'nextgeneration' DGVMs have focused on the predictability of key quantitative traits as a function of environmental variation (Yang et al, 2018). The 'least-cost' and 'coordination' (Maire et al, 2012) hypotheses together suggest a degree of predictability for the air-to-leaf CO2 drawdown (χ, the ratio of leaf-internal to ambient CO2) (Prentice et al, 2014;Wang et al, 2017), Vcmax and the electrontransport capacity Jmax (Togashi et al, 2017), and Narea (Dong et al, 2017) across environments and clades. Both hypotheses have a much longer pedigree than indicated by the recent references cited here, but systematic testing of these hypotheses has only been undertaken quite recently.…”