THE APPLICATION OF LEDS AS ASSIMILATION LIGHT SOURCE IN GREENHOUSE HORTICULTURE: A SIMULATION STUDY

Abstract

The application of light-emitting diodes (LED’s) as potential source for assimilation lighting in greenhouse production systems opens up a range of new possibilities. LED’s produce light in a very narrow wavelength range and do not directly emit heat radiation. The heat which is produced by LED’s due to their limited energy conversion efficiency, can be drawn away via convective (water)cooling. As a result, LED’s can be applied at relative dark places within the crop to increase leaf photosynthesis at locations where assimilation light normally doesn’t penetrate. In theory this type of intercrop lighting could significantly increase crop photosynthesis. Existing simulation models for greenhouse/crop systems can be used to simulate the potential effects this intercrop lighting on crop photosynthesis. It is unclear however, whether the assumptions and simplifications that are justified in present crop models cause problems in simulations of growth systems with intercrop lighting. It may be anticipated that photosynthetic capacity of leaves that are subjected to intercrop lighting adapt different than leaves that are subjected to top lighting by natural light and assimilation light only. In this simulation study we investigated the sensitivity of leaf photosynthesis to adaptation of leaf photosynthetic components at different CO2/light combinations, using the widely used steady-state model of Farquhar et al. (Planta 149: 78–90, 1980) for photosynthesis. The results are used to discuss limitations of current crop photosynthesis models to simulate production in greenhouse systems with intercrop lighting. INTRODUCTION For decades, supplemental assimilation light (AL) has been applied in greenhouses for commercial production at latitudes where natural light (intensity and/or day length) limits plant production (Heuvelink et al., 2006). Supplemental AL positively influences production via increased crop photosynthesis and in many crops also via a positive effect on product quality (Marcelis et al., 2002). The commercial application of AL started in ornamental crops, but nowadays also increasing area’s of vegetable crops are supplied with supplemental AL (Knijff et al., 2004). For long, the economic advantages of AL overruled its inherent disadvantages. Important drawbacks of the use of AL are the energy use and concomitant CO2 emission, while often supplemental AL from greenhouses significantly also adds to light pollution. With increasing energy prices and increasing societal demands to reduce energy use, CO2-emission and light pollution (Tibbitts, 2002), this situations quickly changes. Today, High Pressure Sodium (HPS) lamps are the most commonly used sources for supplemental AL in greenhouse horticulture. At the moment, HPS-lamps are still the most energy efficient AL sources available for commercial plant production, but they have certain characteristics that may limit their application in future. Due to the technology of light production (gas discharge) HPS-lamps operate at a high temperature (>200C), which results in significant radiant heat emission (infrared) towards their direct environment. HPS-lamps emit radiation in a broad band spectrum, including heat 1407 Proc. IS on Greensys2007 Eds.:S. De Pascale et al. Acta Hort. 801, ISHS 2008 radiation. As a result HPS-lamps cannot be applied at close distance from leaves, and sufficient ventilation or cooling capacity should be available to avoid too high greenhouse temperatures. This may restricts the possibilities for future use of HPS lamps in future energy saving greenhouse concepts, where cooling is a major issue (Opdam et al., 2005). The search for energy efficient alternatives for HPS-lamps led to Light-Emitting Diodes (LEDs) as possible candidate. At this moment, the most efficient LED’s are still less efficient than HPS lamps (Fig. 1), but the PAR efficiency of LEDs developed very fast last decade. HPS-lamps, on the other hand, are at the end of their developmental cycle and no significant further progress in PAR efficiency is expected (de Ruijter, 2004). The current ‘inefficiency’ of LED’s implies that a major part of the electrical energy input is still lost for photosynthesis and converted into heat. However, in contrast to heat from HPS lamps, heat from LED’s can be dissipated via convective (water)cooling systems, and therefore removed from the crop environment. As a consequence, LED’s can well be applied at relative dark places within a canopy to increase photosynthesis at locations in the canopy where supplemental AL applied at the top of the canopy normally hardly penetrates. In theory, this type of intercrop AL could significantly increase crop photosynthesis, while at the same time light pollution might be reduced due to reduced light reflection from the upper canopy surface. LED’s emit radiation in a relative narrow band of wavelengths of only several nm’s. The exact wavelength depends on the materials used in the LED. LED’s are available in a broad range of wavelengths broadly covering the PAR spectrum (400-700 nm). This in principle enables the possibility to optimize the wavelength output of future LED-based AL sources for the photosynthesis process and related physiological responses such as opening of the stomata (stimulated by blue light), by choosing the right (combination of) colored LED’s (Brazaityte et al., 2006; Hogewoning et al., 2007; Kim et al., 2006). Both, from greenhouse systems and from plant physiological point of view the application of LED’s within the crop as supplemental intercrop assimilation lighting open new and challenging opportunities, especially in rather dense canopies such as for instance ‘high wire’ tomato, sweet pepper and cucumber. Unfortunately, crop physiological consequences of intercrop AL are largely unknown. Significant changes in leaf photosynthetic capacity at different depths in a canopy can be expected (Schapendonk et al., 1999). Intercrop AL will probably interact with the normal adaptation process of mature leaves that usually move from a high light environment towards a relative low light environment when they age. The quantitative effects of the adaptation are not known yet, while disturbances in the natural light environment with canopy depth may influence the instantaneous efficiency as well as the long term efficiency of the application of AL. Simulation models are often used to predict the effect of AL scenario’s in interactions with other environmental factors (e.g. CO2 and temperature) on crop production. Until now, these simulation models do not, or hardly account for adaptations and vertical gradients in (decreasing) photosynthetic capacity within a canopy. These gradients may especially be relevant when intercrop AL is applied and may negatively influence crop photosynthesis under all light circumstances (no AL, top AL and intercrop AL). In this paper we investigate the potential effect of supplemental AL (applied either from the top (top AL) or within the canopy (intercrop AL)) on crop photosynthesis with a conventional simulation model (without a vertical gradient in photosynthetic capacity) and with a new model that includes vertical gradients in photosynthetic capacity. MATERIALS AND METHODS The simulation models used in this study are multi-layer derivates from SUCROS (Goudriaan and van Laar, 1994). Within these models, the canopy is vertically divided in 10 layers of equal leaf area (Cavazzoni et al., 2002), equally distributed over plant height. Absorbed light is calculated per individual crop layer following the method of Spitters (1986). Absorbed light is used as input for a photosynthesis module, which calculates photosynthesis per crop layer according to the biochemical model for steady state