Abstract
For decisions on supplemental lighting a quantitative knowledge of the plants' responses to light under varying conditions is fundamental. In this study, we developed light dose-response curves of growth and morphological traits for Ocimum basilicum L. and examined the effects of light color (blue, red, and white plus far-red) and natural environment (season) on these curves. Four greenhouse experiments were conducted throughout the year to determine the efficiencies of the light regimes on growth and their effects on plant morphology. A special aspect was the photosynthetic efficiency of far-red light. Linear and monomolecular relationships were found for the relationships between plant traits and supplemental light dose. Traits related to biomass productivity increased linearly with light dose whereas some morphological characters showed a saturation behavior. Red light and white plus far-red light were more efficient in plant dry weight production than blue light, and the plants adapted differently to the light qualities: higher biomass under red light was related to a plant architecture more favorable for light capture, i.e., taller plants and bigger leaves. White plus far-red light, on the other hand, increased leaf mass per area (LMA) and light use efficiency (LUE). Blue light resulted in lowest plant light interception and LUE. Considering photosynthetic effects of near-infrared light (PPFD800, 400–800 nm) instead of photosynthetic photon flux density (PPFD700, 400–700 nm) led to strongly reduced efficiencies. Traits related to photosynthesis such as dry weight, LMA and LUE were particularly affected by PPFD800. There were no interactions between the efficiencies of the different light colors and the seasons. Efficiencies of all light regimes were significantly lower during summer compared to spring and winter. Higher dry weight production during summer compared to winter and spring were a consequence of increased light interception rather than changes in LUE. The observed differences in seasonal efficiencies were directly linked to the amount of natural light present as indicated by changes in the ratio of supplemental to natural light.
Highlights
Growth and morphology of plants are strongly influenced by the light environment under which they are grown
Positive linear or monomolecular relationships were found between all plant parameters and supplemental light intensity, except for light use efficiency (LUE) and energy use efficiency (EUE) which were negatively correlated with supplemental PPFD700 dose (Figures 3, 4)
Red light stimulated the elongation of the plant axes more than far-red and blue light
Summary
Growth and morphology of plants are strongly influenced by the light environment under which they are grown. Light dose, photoperiod, and the growing period are major determinants of the plant’s adaptation to the environment (Goto, 2003). Plants grow under natural daylight that strongly varies in light spectrum and intensity depending on the weather, time of day, season and atmospheric conditions. An overcast sky leads to increased proportions of blue light, whereas the ratio of red to far-red (R/FR) has been shown to vary little with weather conditions and season and is roughly 1.2 in natural daylight. Changes in R/FR occur during sunrise and sunset as well as when the light penetrates the plant canopy (Smith, 1982). A reduction in R/FR induces photomorphogenetic effects, known as shade-avoidance responses, that are usually characterized by a rapid elongation of stems and leaves (for comprehensive reviews on shade-avoidance see: Franklin and Whitelam, 2005; Vandenbussche et al, 2005; Franklin, 2008)
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