Low light often limits photosynthesis and growth and reduces plant quality and is the main limiting factor for the production of horticultural crops, such as vegetables and ornamental bedding plants, during the winter in temperate climates (Gaudreau and Chartbonneau, 1994; Lopez and Runkle, 2008; Nelson, 2012). Daily light integral [DLI (total PPF integrated over 24-h)] in greenhouses in the northern latitudes of the United States can be as low as 2.5–10 mol·m−2·d−1 from November to February (Korczynski et al., 2002). Supplemental lighting is often needed to produce high-quality crops in controlled-environment agriculture but can substantially increase production costs. For example, van Iersel and Gianino (2017) estimated that the cost of supplemental lighting provided by high-pressure sodium lamps can account for about 30% of the farm gate value for vegetable greenhouses. The high cost of supplemental lighting in controlled environments necessitates the need for more efficient use of supplemental light.
Supplemental light use efficiency can be improved by implementing energy-efficient lights, such as light-emitting diodes (LEDs) (Nelson and Bugbee, 2014). In addition to the efficiency of the lights, the overall efficiency at which the electrical energy is converted into plant biomass depends on how efficiently plants use light for photosynthesis. Not all the light absorbed by plants is used in the photochemical reactions of photosynthesis. Some of the absorbed light is dissipated as heat and a small fraction is re-emitted as chlorophyll fluorescence (Maxwell and Johnson, 2000). The quantum yield of photosystem II, the efficiency with which photosystem II (PSII) uses absorbed photons for electron transport, or the moles of electrons transported per mole of photons (typically expressed as a unitless fraction) can be easily measured using chlorophyll fluorescence (Genty et al., 1989; Maxwell and Johnson, 2000). In addition, chlorophyll fluorescence can be used to estimate the ETR, which is often closely correlated with photosynthetic rate (Beer et al., 1998; Flexas et al., 1999), and the degree of heat dissipation under various light conditions. As light intensity increases, plants use the absorbed light less efficiently for photosynthesis, while more of that absorbed light energy is dissipated as heat or through chlorophyll fluorescence (Baker, 2008; Demmig-Adams et al., 1996; van Iersel et al., 2016b). As a result, supplemental light provided at high intensities or in addition to a high ambient light intensity is expected to be used relatively inefficiently for photosynthesis (van Iersel and Gianino, 2017) and presumably growth. Clearly, it is not beneficial to provide supplemental light when plants cannot use that light efficiently for photosynthesis.
Genetic adaptation of plants to their light environment occurs over generations and at the population level and has resulted in photosynthetic differences among species. High-light-adapted species generally have greater photosynthetic capacity; i.e., a higher maximum photosynthetic rate and a higher light-saturation point than shade-adapted species (Björkman, 1981). By contrast, shade-adapted species tend to reach maximum photosynthetic capacity at much lower light intensity and are more likely to incur damage to the photosynthetic reaction centers under high light [photoinhibition (Demmig-Adams and Adams, 1992)]. Although the photosynthetic responses to PPF have been reported for many species, this information has seldom been used to examine how the supplemental light use efficiency changes with changing PPF and how supplemental lighting can be optimized for different species. van Iersel and Gianino (2017) reported that plants with different photosynthetic capacities responded differently to supplemental light. They simulated the responses of net photosynthesis (An) of two species to supplemental light provided at different ambient PPF and found that the high-light-adapted species Campanula portenschlagiana (with greater photosynthetic capacity) showed pronounced increases in An when supplemental light was provided at a relatively high ambient PPF of 250 μmol·m−2·s−1, whereas little increase in An was observed in the low-light-adapted plant Heuchera americana when supplemental light was provided at the same ambient PPF (van Iersel and Gianino, 2017).
In addition to adaptation, short-term acclimation to light, typically taking place within minutes to weeks (within the life cycle of a plant), can also induce phenotypic modifications that alter plants’ photosynthetic light use efficiency (Anderson et al., 1995; Björkman, 1981; Valladares and Niinemets, 2008). Such modifications include changes in leaf anatomical structures (Evans and Poorter, 2001; McMillen and McClendon, 1983), chlorophyll content and chlorophyll a/b ratio (Givnish, 1988; Niinemets, 2010), changes in electron transport capacity per unit chlorophyll (Anderson and Osmond, 1987), rubisco content and activity (Björkman, 1981; Seemann, 1989), xanthophyll cycle pigment pool size (involved in heat dissipation of the absorbed light) (Demmig-Adams and Adams, 1992; Logan et al., 1998), and maximum photosynthetic capacity (Oguchi et al., 2005). The importance of acclimation to different light levels for the efficiency with which plants use supplemental light for photochemistry has not been studied.
Currently, supplemental lighting in greenhouses is typically controlled by a timer or based on ambient light levels. The ability of a crop to efficiently use the supplemental light for photosynthesis is seldom considered when developing supplemental lighting strategies. Few studies have investigated the feasibility of optimizing supplemental lighting in controlled environments based on plant physiological responses to light (van Iersel et al., 2016a, 2016b) and there is a need for information on how supplemental lighting can be optimized for crops adapted and/or acclimated to different light environments. Chlorophyll fluorescence measurements are especially well-suited for this purpose because they can be used to determine how efficiently plants use the provided light in a real-time, noninvasive manner. Therefore, our objectives were the following: 1) to determine the photochemical responses of different species to a wide range of light intensities; 2) to quantify how light acclimation affects crops’ photochemistry; and 3) to examine how supplemental light use efficiency changes with changing ambient light levels, and how supplemental light can be optimized for crops adapted and/or acclimated to different light environments.
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