Photosynthesis in transition: the importance of wavelengths in the green gap for Ocimum basilicum L.

Abstract

In recent decades, the absorption spectrum as well as McCree's action spectrum (350-750 nm) have served as the primary basis for photosynthesis research. Based on these spectra, it was long assumed that blue and red light may be used more efficiently for photosynthesis than green light. However, recent studies have demonstrated that green light is more effective than previously assumed, particularly at high irradiance conditions. Nevertheless, most studies still rely on the three standard wavelengths: 450, 527, and 660 nm. In this study, the effects of additional wavelengths on photosynthesis at irradiances of up to 5000 µmol m - ² s - ¹ were investigated in Ocimum basilicum L. using chlorophyll fluorescence analysis as well as gas exchange measurements. Following preliminary experiments that revealed variations in photosynthetic performance with plant age, as well as between leaf layers and the adaxial and abaxial leaf surfaces, a standardized measurement protocol was established and subsequently applied to evaluate the photosynthetic responses to different wavelengths. These wavelengths included deep blue (430 nm), cyan (485 nm), mint green (500 nm), orange (590 nm), and orange-red (625 nm). Unexpectedly, wavelengths within the "green gap" exhibited the highest CO 2 assimilation rates, followed by deep blue and reddish wavelengths. Additionally, 4000 K neutral white light was supplemented with varying proportions of these wavelengths to determine the optimal white-light composition for photosynthesis. The most favorable results were obtained when white light was supplemented with 25% shorter wavelengths (430-527 nm).