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• W1858503496 abstract "LEDs may be a suitable light source for future use as assimilation lighting in protected greenhouse cultivation. LEDs have properties which offer advantages compared to other light sources, but which also raise specific research questions. The narrow band spectrum of LEDs enables manufacturers to produce LED based lightsources specifically suitable for photosynthesis and other horticulturally relevant plant properties. The low radiated heat also makes LEDs suitable for interlighting (i.e. lighting from within the canopy), for which high pressure sodium lamps are not suitable. However, when using LEDs, crops must be able to acclimate their photosynthetic functioning to narrow band lighting (NBL) to efficiently use this light. Also, daylight-adapted leaves must be able to re-acclimate to NBL if LEDs would be used for interlighting in a high-wire grown crop. If low photosynthesis rates in older, lower leaves of the crop are also due to leaf age, besides low light, interlighting would be less effective. For investigating the intrinsic effect of NBL, we used 9 different arrays comprised of a single LED type (peak wavelengths in the range 460-668 nm) at lightlimited irradiance (50 μmol m s). Spirodela polyrrhiza (Lemnaceae) was cultivated as its leaves can not change in distance or orientation towards the light source. This enabled us to compare the effects of the different light sources on parameters such as growth rate and photosynthetic pigment composition. In order to separate the effect of light intensity and leaf age on photosynthesis, tomato plants were grown horizontally, so that older leaves were not shaded by younger leaves. Re-acclimation of leaves to NBL was investigated by illuminating older leaves (low in the canopy) using different LED arrays in a high-wire grown tomato crop. The light-harvesting apparatus of Spirodela polyrrhiza acclimated to the different NBL regimes within 6 days. Leaf age proved to be an irrelevant factor for photosynthetic capacity (Pmax) of greenhouse grown tomato plants. Pmax of leaves at a low position in a high-wire grown tomato crop, with a low Pmax, did re-acclimate to the higher light intensities supplied by the supplemental NBL by progressively increasing their Pmax. However, as it took 14 days for Pmax to increase from 5.6 to 12.4 μmol CO2 m s, maintaining a continuously higher light level within the canopy would be more effective. INTRODUCTION Recently, light emitting diodes (LEDs) are subject of debate and research with respect to their possible future use for assimilation lighting in protected plant cultivation. At this moment LED lighting systems are too expensive and, more important, not efficient enough in terms of light output per Watt energy use, for large scale replacement of high pressure sodium (HPS) assimilation lamps for commercial greenhouse horticulture. However, the light-output efficiency of LEDs increased dramatically in recent years and production costs of LED systems aimed at the horticulture will decrease Proc. XXVII IHC-S6 High-Qual. Crop Prod. under Protect. Cultiv. Ed.-in-Chief: D.J. Cantliffe Acta Hort. 761, ISHS 2007 184 if demand would be sufficient. So as well as yielding new, fundamental knowledge on plant functioning, research on plant responses to LED lighting is highly relevant for the horticultural industry. Compared to other light sources used for plant production, LEDs have several properties which are potentially useful in relation to horticulture. LEDs emit light in a relatively narrow band spectrum, so they can be manufactured with a wavelength output tailored to the plant responses desired by growers. In order to enhance production, LED arrays with wavelengths favourable for photosynthesis would be desired. However, wavelengths influencing other plant properties such as morphology (blue light enhances compactness of plants) could be desirable for ornamental horticulture. Also, nutritional qualities of food crops may be influenced by the illumination spectrum used (e.g. antioxidant content; see e.g. Oelmuller and Mohr, 1985). At this moment, LEDs produce more heat than HPS lamps per Watt energy input, due to their lower energy/light conversion efficiency. However, the radiated heat of LEDs is very low compared to HPS lamps. This affords the opportunity to cool LED lighting systems and to reuse the (low caloric) heat for e.g. greenhouse heating. The low radiated heat output also makes LEDs suitable for interlighting, i.e. lighting inside the canopy, instead of or in combination with lighting above the canopy. Previous research on interlighting with HPS lamps (Hovi et al., 2004) indicated that interlighting may be more efficient than lighting from above. Interlighting could contribute to a more effective use of light by crops, as inside the canopy light levels are low, whereas above the canopy solar radiation already provides a considerable photosynthetic photon flux (PPF) during daytime. Especially in high-wire cultivated crops (e.g. tomato, cucumber and sweet pepper) with low photosynthesis rates lower in the canopy (see e.g. Xu et al., 1997), interlighting could offer a means to enhance production. The considerable percentage of radiated heat HPS lamps produce makes it impossible to situate HPS lamps close to the leaves of a crop. LEDs, on the other hand, with their low radiated heat output and overall lower temperature, can be in contact with the leaves. Though this may not be good for photosynthesis, the leaf will not be burned. LED light output is proportional to electric current through the device, so light intensity can be controlled in a versatile way. Therefore, the light output of LED lighting systems can be easily, rapidly and efficiently adjusted to match environmental conditions or production requirements within the greenhouse, such as natural light or the stage of the crop in the production cycle. Also if lighting systems would be used comprised of different LED types, emitting light with different spectra, light quality could be adjusted to the needs of the crop. For example, the use of near far red light (690 nm) was found to increase the leaf area of developing leaves (Goins, 2002), which would result in a more effective use of the available light in early stages of the production cycle. A socially relevant issue concerning assimilation lighting, called “light pollution” (public agitation and the disturbance of the natural rhythm of birds etc. due to lighted greenhouses at night) may be reduced by the replacement of HPS lamps by LEDS for assimilation lighting. The human eye is much more sensitive to yellow/orange light (the dominant colour of HPS lamps) than it is to red light (a potentially useful colour for LED lighting systems), so an equal amount of μmols PPF can give a very different perception of light intensity for the (human) eye (Fig. 1). Mammals and birds are also more sensitive to yellow/orange light than to red (Goldsmith, 2006). Furthermore, interlighting would reduce radiation loss from the greenhouse considerably due to increased light interception within the canopy. Nonetheless, several questions need to be addressed, concerning the possible use of LEDs for assimilation lighting. In order to grow crops efficiently using narrow band lighting (NBL), plants have to be able to acclimate their photosynthetic apparatus to NBL. Any imbalance in excitation of the photosystems I and II would lead to a loss of quantum yield for CO2 fixation in a leaf. Also, the dynamics of re-acclimation to NBL of daylightadapted leaves in a crop are essential for the efficiency of interlighting in a canopy. As the production time of a leaf in a high-wire grown crop is relatively short (approximately 8-" @default.
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• W1858503496 title "PLANT PHYSIOLOGICAL ACCLIMATION TO IRRADIATION BY LIGHT-EMITTING DIODES (LEDS)" @default.
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