Seasonal variation in the relationship between leaf chlorophyll content and photosynthetic capacity.

Accurate estimation of photosynthesis is crucial for ecosystem carbon cycle modelling. Previous studies have established an empirical relationship between photosynthetic capacity (maximum carboxylation rate, V cmax ; maximum electron transport rate, J max ) and leaf chlorophyll (Chl) content to infer global photosynthetic capacity. However, the basis for the Chl-V cmax relationship remains unclear, which is further evidenced by the temporal variations in the Chl-V cmax relationship. Using multiple years of observations of four deciduous tree species, we found that V cmax and J max acclimate to photosynthetically active radiation faster (4-8 weeks) than Chl (10-12 weeks). This mismatch in temporal scales causes seasonality in the V cmax -Chl relationship. To account for the mismatch, we used a Chl fluorescence parameter (quantum yield of Photosystem II, Φ(II)) to tighten the relationship and found Φ(II) × Chl correlated with V cmax and J max (r 2  = 0.74 and 0.72 respectively) better than only Chl (r 2  = 0.7 and 0.6 respectively). It indicates that Φ(II) accounts for the short-term adjustment of leaf photosynthetic capacity to light, which was not captured by Chl. Our study advances our understanding of the ecophysiological basis for the empirical V cmax -Chl relationship and how to better infer V cmax from Chl and fluorescence, which guides large-scale photosynthesis simulations using remote sensing.