Growers of greenhouse-grown products focus to a high degree on reducing the use of energy. The economic aspect of reducing energy for heating has already induced growers to use dynamic temperature control to a greater extent. Danish pot-plant producers reduced their use of energy for heating per square meter greenhouses by 23% from 2000 to 2006, whereas their use of electricity per square meter greenhouses, mainly used as supplemental growth light, increased by 30% in the same period (Møllenberg, 2003, 2007). It has been reported that supplemental growth light has increased in Denmark although the overall energy use per unit has decreased (Gartnerirådgivningen/AgroTech, 2008). Supplemental growth light is important for plant production during the darkest period of the year, but knowledge of its optimal use for plant production is lacking. Present greenhouse installations typically have limited supplemental light control options with a fixed intensity, so the main question is when to use supplemental light.
As a result of the increasing cost of electricity, one option is to use the supplementary light during periods of low electricity price, because, for many growers in Denmark and other places, it varies on an hourly basis. Correct matching of the plant circadian clock with the environmental period does influence the net photosynthesis positively (Dodd et al., 2005; Hotta et al., 2007). However, supplemental light during the night period might reset the circadian rhythms of the crop, and it is not known to what degree night interruption might affect the net canopy carbon gain and subsequent plant growth.
Extending the length of the day to obtain a certain light integral might result in greater net photosynthesis as a result of a reduction in dark respiration (Nemali and van Iersel, 2004; Xu et al., 2004). However, supplemental light is mainly used during the dark period of the year where light conditions during the day and nights are low and the increase in photosynthesis with increasing light intensity is more or less linear. The light level at which linearity ends depends on the plant species and the capacity of the plant for light acclimation of photosynthesis to the prevailing light level (Rosenqvist, 2001). At light levels below a photosynthetic photon flux (PPF) of 200 μmol·m−2·s−1, the leaf assimilation of chrysanthemum can be considered as responding close to linearly to irradiance levels. Supplemental light levels differ but are often lower than 200 μmol·m−2·s−1 at the canopy level. Pot plants grown at a typical production facility during winter in Denmark will be exposed to irradiance levels below 500 μmol·m−2·s−1.
The aim of the study was to evaluate the efficiency of photosynthesis of an intact crop on a short-term basis and its growth during a 4-week period with supplementary light at different periods of the day at identical light integrals. Short-term evaluation time of the four supplemental light strategies was aimed to get the most value for the money out of the installation. The results are evaluated for their importance in typical greenhouse production practice.
Albrechtová, J.T.P., Vervliet-Scheebaum, M., Normann, J., Veit, J. & Wagner, E. 2006 Metabolic control of transcriptional–translational control loops (TTCL) by circadian oscillations in the redox- and phosphorylation state of cells Biol. Rhythm Res. 37 381 389
Brodersen, C.R. & Vogelmann, T.C. 2007 Do epidermal lens cells facilitate the absorptance of diffuse light? Amer. J. Bot. 94 1061 1066
Brodersen, C.R., Vogelmann, T.C., Williams, W.E. & Gorton, H.L. 2008 A new paradigm in leaf-level photosynthesis: Direct and diffuse lights are not equal Plant Cell Environ. 31 159 164
DeLucia, E.H., Nelson, K., Vogelmann, T.C. & Smith, W.K. 1996 Contribution of intercellular reflectance to photosynthesis in shade leaves Plant Cell Environ. 19 159 170
Dodd, A.N., Salathia, N., Hall, A., Kevei, E., Toth, R., Nagy, F., Hibberd, J.M., Millar, A.J. & Webb, A.A.R. 2005 Plant circadian clocks increase photosynthesis, growth, survival, and competitive advantage Science 309 630 633
Hollinger, D.Y., Kelliher, F.M., Byers, J.N., Hunt, J.E., Mcseveny, T.M. & Weir, P.L. 1994 Carbon-dioxide exchange between an undisturbed old-growth temperate forest and the atmosphere Ecology 75 134 150
Hotta, C.T., Gardner, M.J., Hubbard, K.E., Baek, S.J., Dalchau, N., Suhita, D., Dodd, A.N. & Webb, A.A.R. 2007 Modulation of environmental responses of plants by circadian clocks Plant Cell Environ. 30 333 349
Jensen, L.T., Rosenqvist, E. & Aaslyng, J.M. 2006 A daylight climate chamber for testing greenhouse climate control strategies and calculating canopy carbon dioxide exchange HortTechnology 16 191 198
Møllenberg, S. 2003 Gartneriregnskabsstatistik 2002: Materialeudvælgelse og definitioner Fødevareøkonomisk Institut, Rolighedsvej 25, 1958 Frederiksberg C Denmark
Møllenberg, S. 2007 Gartneriregnskabsstatistik 2006: Materialeudvælgelse og definitioner Fødevareøkonomisk Institut, Rolighedsvej 25, 1958 Frederiksberg C Denmark
Nemali, K.S. & van Iersel, M.W. 2004 Light effects on wax begonia: Photosynthesis, growth respiration, maintenance respiration, and carbon use efficiency J. Amer. Soc. Hort. Sci. 129 416 424
Roderick, M.L., Farquhar, G.D., Berry, S.L. & Noble, I.R. 2001 On the direct effect of clouds and atmospheric particles on the productivity and structure of vegetation Oecologia 129 21 30
Rosenqvist, E. 2001 Light acclimation maintains the redox state of the PSII electron acceptor Q(2) within a narrow range over a broad range of light intensities Photosynth. Res. 70 299 310
Thornley, J.H.M. 2002 Instantaneous canopy photosynthesis: Analytical expressions for sun and shade leaves based on exponential light decay down the canopy and an acclimated non-rectangular hyperbola for leaf photosynthesis Ann. Bot. (Lond.) 89 451 458
Tibbitts, T.W., McSparron, D.A. & Krizek, D.T. 1986 Spectral effects on the use of photon flux sensors for measurement of photosynthetic photon flux in controlled environments Biotronics 15 31 36
Vogelmann, T.C. & Martin, G. 1993 The functional-significance of palisade tissue: Penetration of directional versus diffuse light Plant Cell Environ. 16 65 72
Wulfsohn, D., Sciortino, M., Aaslyng, J.M. & García-Fiñana, M. 2009 Non-destructive, stereological estimation of canopy surface area Biometrics
Xu, Q.Z., Huang, B.R. & Wang, Z.L. 2004 Effects of extended daylength on shoot growth and carbohydrate metabolism for creeping bentgrass exposed to heat stress J. Amer. Soc. Hort. Sci. 129 193 197