Leaf age controls photosynthetic efficiency and energy partitioning in Amazonian trees

Seasonal variability in photosynthesis across old-growth Amazonian forests challenges classical paradigms linking productivity primarily to climatic constraints. Satellite observations of solar-induced chlorophyll fluorescence (SIF) and eddy-covariance measurements indicate that ecosystem-scale photosynthesis is maintained or even enhanced during the dry season, yet the physiological mechanisms underlying this pattern remain poorly constrained. Increasing evidence suggests that canopy phenology and leaf demography, rather than changes in leaf area index (LAI), play a central role in regulating seasonal productivity. However, the functional consequences of leaf aging for photosynthetic energy allocation and its implications for SIF–GPP relationships are still largely unknown.

 

Here, we investigate how leaf age controls photosynthetic efficiency and the partitioning of absorbed light energy in Amazonian trees. We measured chlorophyll content, leaf temperature, linear electron flow (LEF), and the yields of photochemistry (ΦPSII), regulated heat dissipation (ΦNPQ), and nonregulated energy losses (ΦNO) in 2,323 leaves from 61 trees (27 species) across multiple canopy strata at two sites in Central Amazonia: the Amazon Tall Tower Observatory (ATTO) and the AmazonFACE experiment. Measurements were conducted under ambient and high photosynthetically active radiation (PAR = 2000 µmol m⁻² s⁻¹) using a MultispeQ device, allowing us to quantify leaf “light potential”, defined as the capacity to respond to rapid increases in irradiance. Leaf age was assessed both as categorical classes (young, mature, old) and, for a subset of leaves, as continuous age in days based on long-term demography monitoring.

 

Our results show that leaf age strongly regulates photosynthetic energy partitioning. ΦPSII increased during early leaf development, peaked at intermediate ages, and declined steadily in older leaves, while ΦNO increased with age. ΦNPQ decreased during early development but increased again in older leaves, indicating a shift toward enhanced photoprotective dissipation. Older leaves also exhibited greater light potential, with higher NPQ and sustained LEF under high light, suggesting increased capacity to cope with sunflecks and transient irradiance.

 

We demonstrate that leaf age is a key state variable linking canopy demography, photosynthetic energy allocation, and ecosystem-scale SIF–GPP relationships. Incorporating leaf-age dynamics into ecosystem and Earth system models is essential to improve predictions of tropical forest productivity under current and future climate conditions.