Multivariate analysis reveals physiological trade-offs and synergies under light and nutrient gradients in the herbaceous species Agastache rugosa

Agastache rugosa is an herbaceous species that shows a high degree of phenotypic plasticity in response to light and nutrient gradients, but the coordination among its leaf structural, photosynthetic, and resource use traits remains unexplored in tropical environments. We investigated the functional traits and resource use efficiencies of A. rugosa under four nutrient levels nested within two light levels. Photosynthetic rates increased under high-light, while leaf temperatures remained stable (34–37°C) across treatments, suggesting effective thermoregulation. Unexpectedly, Rubisco content was 22.4% higher under low-light, intermediate nutrient levels, indicating a compensatory mechanism. Water use efficiency increased under high-light, whereas photosynthetic phosphorus and potassium use efficiencies were higher under low-light levels. Principal component analysis showed that light and nutrients explained 71.6% of trait variation, with distinctive clustering of resource use efficiencies. Hierarchical clustering identified three functional trait groups at 90% similarity levels, comprising photosynthetic, nutrient use, and water conservation mechanisms. The species showed tight coordination between CO2 supply and demand, with strong correlations between photosynthetic traits and resource use efficiencies. Our findings demonstrate that A. rugosa employs a suite of adaptive mechanisms to optimise resource acquisition and utilisation across heterogeneous environments, advancing our understanding of plant responses to multiple resource gradients.