Acclimation of Charophytes to Spectral Composition of Light at Limiting Irradiances

ABSTRACT

Irradiance is regarded as a key factor determining the maximum colonization depth ( Z ₀ ) of submerged macrophytes. However, the relevance of acclimation capabilities to spectral composition under such low‐light conditions remains poorly understood. To address this gap, the present study tested whether spectral composition, in addition to light intensity, affects physiological performance in Chara globularis Thuill. and Chara hispida L., two charophyte species differing in depth distribution. Individuals were cultivated for 5 weeks under three low irradiances below the photosynthesis light saturation level (6, 12, and 24 µmol photons m ⁻² s ⁻¹ ); each irradiance given at four different spectral compositions (full spectrum and blue‐, green‐, or yellow‐wavelength dominated). Daily growth rate, biomass‐related parameters, pigment composition (chlorophylls and carotenoids), and photosynthetic light‐response parameters were quantified. Across treatments, growth declined with decreasing irradiance in both species, confirming light quantity as the primary constraint, whereas spectral composition had no significant effect on growth or biomass‐related parameters. Pigment composition differed consistently between species, with C. globularis exhibiting higher chlorophyll a and carotenoid concentrations across all treatments. Spectral effects on pigment composition were weak, restricted to the highest irradiance, and observed exclusively in C. globularis . Despite these differences, photosynthetic parameters remained largely stable across treatments. The initial slope of the photosynthesis‐irradiance curve (α) showed no interspecific differences, while the light saturation point (I _k ) was consistently higher in C. globularis than in C. hispida . Overall, physiological responses were dominated by light intensity and species identity, while spectral effects were minor and conditional. These findings indicate that charophytes, owing to their comparatively low organizational complexity and maintenance demands, do not appear to gain a functional advantage from strong spectral acclimation under low irradiance. Instead, physiological stability emerges as the prevailing strategy, with the greater plasticity of C. globularis potentially facilitating persistence near the lower macrophyte limit.