Photochemical efficiency and variation in photosynthetic pigments in Nothofagus pumilio seedlings growing under light intensity and soil moisture gradients

Abstract

Regeneration dynamics in forest ecosystems depend on seedlings’ ability to persist in shaded understories and, following canopy opening, to rapidly adjust physiologically to new, contrasting light and soil moisture conditions. In this study, we examined the real-time efficiency of the photosynthetic apparatus and the short- to intermediate-term acclimatory response of photosynthetic pigments on Nothofagus pumilio seedlings exposed to different light and soil moisture levels. We evaluated phenology, chlorophyll fluorescence and pigment content in leaves, on 2-3 years-old seedlings in a controlled greenhouse experiment with three levels of light intensity (low = 4%, medium = 26%, high = 64% of the natural incident irradiance) and two levels of soil moisture (optimal = 40-60%, excess = 80-100% of soil field capacity). We measured six samples per treatment (light intensity × soil moisture levels) monthly during one growing season (n = 216). Data were analyzed by multiple ANOVAs. Although maximum quantum yield remained stable (> 0.80) across treatments, seedlings exhibited differential eco-physiological responses primarily driven by light availability. Structural investment was synergistic and negatively affected by the light × soil moisture interaction, resulting in reduced leaf production, chlorophyll, and carotenoids during peak season under the high-light and excess-soil-moisture treatment. Seedlings under medium light showed optimal performance, with maximum pigment accumulation and high PSII efficiency without signs of chronic photoinhibition. N. pumilio seedlings decouple short-term functional photochemical efficiency from their structural investment strategy (pigment content), a mechanism that ensures survival under variable canopy cover but results in reduced growth under sub-optimal conditions.