High photosynthetic photon flux density can attenuate effects of light quality

Light quantity can be expressed on an instantaneous basis as photosynthetic photon flux density (in µmol m^-2 s^-1) or integrated as daily light integral (DLI, in mol m^-2 d^-1). It regulates the rate of photosynthesis and over time, plant biomass. In contrast, light quality refers to the spectral distribution of incident radiation, which primarily regulates plant morphology and flowering. However, these two dimensions of radiation interact to influence plant growth and development throughout a plant’s life cycle. Numerous studies have reported the effects of light quantity on net assimilation as well as growth and yield responses such as fresh and dry mass of shoots or roots, or yield of fruits or flowers. More recently, light-emitting diodes have enabled manipulation of light quality to produce crops with desired morphological and quality characteristics. In both greenhouse and indoor plant lighting applications, the effects of light quality can be attenuated by increases in light quantity. For example, the quality of night-interruption greenhouse lighting can influence flowering of long-day plants when the DLI is low, but has less or no effect when the DLI is high. Similarly, the effect of the red-to-far-red ratio of sole-source (indoor) lighting on extension growth can be more pronounced when the DLI is low than when it is high. Here, based in part on our research with ornamentals and leafy greens, we discuss how effects of light quality can often (but not always) diminish with increasing light quantity in sole-source, supplemental, or photoperiodic lighting applications.