Plant morphological and development responses to light quality in a horticultural context

Many horticultural crops (food and ornamental) are produced year-round in greenhouses at high latitudes, where the limited availability of natural sunlight restricts plant production during large parts of the year. To enable year-round plant production supplemental light is necessary to enhance photosynthesis, the primary process that drives growth and production. It is therefore not surprising that during the last two decades most of the research effort related to light in greenhouse horticulture has been directed towards optimizing the supplemental light use efficiency for photosynthesis, with emphasis on light intensity, duration and since recently also on light quality. For a long time, high pressure sodium (HPS) lamps were the preferred lamps for supplemental lighting. Nowadays, Light Emitting Diodes (LEDs) are gaining importance, mostly because of their potentially higher energy efficiency. Another important, less-well known attribute of LEDs is the much better possibility to control light quality. Besides the effect on photosynthesis, light quality also influences plant morphological and developmental processes, mostly mediated by a set of blue, red and far-red photoreceptors (i.e., cryptochromes, phototropins and phytochromes). Several of these processes, such as for instance internode and petiole elongation growth and leaf expansion have a direct impact on productivity via plant photosynthesis as mediated by light interception. Light quality can also induce leaf deformations and epinasty, which can negatively influence biomass production via reduced light interception. In ornamental crops, such as chrysanthemum, leaf deformations can have severe negative impact on the final ornamental value. Other important effects of light quality involve the development of stomatal density and the control of stomatal aperture, which both attribute to stomatal conductance and therefore potentially influence productivity, while also the leaf hydraulic resistance is influenced by light quality. This paper will overview some plant morphological and developmental processes that are influenced by light quality and are important for plant production in protected environments.