Effects of Light Intensity and Photoperiod on Growth, Physiology, and Morphology of Spirodela polyrhiza

Abstract

Daily light integral (DLI) is widely used in horticulture as a single metric combining photosynthetic photon flux density (PPFD) and photoperiod, but it remains unclear whether duckweeds respond to DLI as one integrated light dose or to its components separately. Spirodela polyrhiza, a fast-growing duckweed species relevant for biomass production and wastewater-linked cultivation, was used to test this question. Three clones were grown for five days under a balanced factorial design combining four PPFD levels, from 25.9 to 111.2 µmol m−2 s−1, with two photoperiods, 16 h and 24 h. Six response variables were measured, grouped into growth and energetic traits (relative growth rate, biomass productivity, light use efficiency) and density-acclimation traits (dry matter percentage, chlorophyll content index, leaf mass per area). All were tested using analysis of variance and analysis of covariance with DLI as covariate. DLI fully explained biomass production and LUE, with no remaining treatment after correction, but it did not explain RGR, which retained separate intensity- and duration-dependent responses. Pigmentation rose with instantaneous irradiance, dry matter content was contingent on the particular PPFD and photoperiod combination, and structural investment intensified under continuous light. Clone performance also differed clearly, with interactions between clone and intensity being detected for growth and morphology, and one clone leading across most conditions. DLI was therefore sufficient for predicting energetic output, but it could not replace dirent direct consideration of PPFD and photoperiod when interpreting growth-rate and acclimation responses in S. polyrhiza.