Beyond red and blue: Unveiling the hidden action of green wavelengths on plant physiology, metabolisms and gene regulation in horticultural crops

Light intensity, photoperiod and spectral composition drive many fundamental functions of plant life and interacts with other environmental parameters and cultivation factors in determining the crop performance. Indeed, in addition to providing energy to power photosynthesis, light imparts precise signals regulating plant growth, development, and metabolism in photomorphogenesis, through the different wavelengths, detected by specific photoreceptors also at very low fluence rate. While the efficiency of blue and red wavelengths in the photosynthetic process and their role in photomorphogenesis, as well as their absorption spectra, have been long since demonstrated, green radiation was considered useless (if not even detrimental) for plants for a long time, because of the poor action spectrum of photosynthesis and the weak absorption and high reflection in plant tissues. It is known now that instead the green light sustains photosynthesis in the deeper leaf lamina and canopy layers and participate to several photomorphogenetic processes. However, its role in the complex scenario of plant responses to light environment is still unclear and results in literature are sometimes conflicting. The aim of this review is to update the knowledge on the effects of green, as both monochromatic light and portion of multispectral radiation, on plant physiology, metabolism, and transcriptional regulation to the most recent advances, with a special focus on those underlying useful agronomic outputs in terms of plant growth and yield, and product quality in vegetable and herbaceous crops. Last findings on these aspects are summarised in order to determine if and how green light-mediated responses can contribute to boost the plant performance in greenhouse and controlled environment horticulture.