The sulphur plasma consists mainly of dimer molecules (S2), which generate the light through molecular emission.

Because this, instead of atomic emission, is the mechanism of light generation, the emission spectrum is continuous throughout the visible spectrum.

The lamp's output is low in infrared energy, and less than 1% is ultraviolet light. As much as 75% of the emitted radiation is in the visible spectrum, far more than other types of lamps.

The visible light output mimics sunlight better than any other artificial light source, and the lack of harmful ultraviolet radiation can be especially beneficial to museums and displays of art.

The spectral output peaks at 520 nanometres and the correlated colour temperature (CCT) is approximately 6000 kelvins with a colour rendering index (CRI) of 86. The lamp can be dimmed to 15% without affecting the light quality, and light output remains constant over the life of the bulb.

Sander W. Hogewoning*, Peter Douwstra, Govert Trouwborst, Wim van Ieperen and Jeremy Harbinson

Wageningen University, Department of Plant Sciences, Horticultural Supply Chains Group, Wageningen, The Netherlands.

Plant responses to the light spectrum under which plants are grown affect their developmental characteristics in a complicated manner. Lamps widely used to provide growth irradiance emit spectra which are very different from natural daylight spectra. Whereas specific responses of plants to a spectrum differing from natural daylight may sometimes be predictable, the overall plant response is generally difficult to predict due to the complicated interaction of the many different responses. So far studies on plant responses to spectra either use no daylight control or, if a natural daylight control is used, it will fluctuate in intensity and spectrum. An artificial solar (AS) spectrum which closely resembles a sunlight spectrum has been engineered, and growth, morphogenesis, and photosynthetic characteristics of cucumber plants grown for 13 d under this spectrum have been compared with their performance under fluorescent tubes (FTs) and a high pressure sodium lamp (HPS).

The total dry weight of the AS-grown plants was 2.3 and 1.6 times greater than that of the FT and HPS plants, respectively, and the height of the AS plants was 4–5 times greater. This striking difference appeared to be related to a more efficient light interception by the AS plants, characterized by longer petioles, a greater leaf unfolding rate, and a lower investment in leaf mass relative to leaf area. Photosynthesis per leaf area was not greater for the AS plants. The extreme differences in plant response to the AS spectrum compared with the widely used protected cultivation light sources tested highlights the importance of a more natural spectrum, such as the AS spectrum, if the aim is to produce plants representative of field conditions.

Key words: Artificial solar spectrum, blue light, growth rate, leaf mass per area (LMA), light absorptance, light interception, light quality, photomorphogenesis, photosynthetic capacity

Received 20 October 2009; Revised 5 January 2010 Accepted 8 January 2010

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Note space between MH grown plants and the absence of space between MPS grown plants and the fact that pot labels are still visible among the MH grown plants but not the MPS grown plants.

BIOTRONICS 27. 81-92, 1998

UNIFORMITY OF PHOTOSYNTHETIC PHOTON FLUX AND GROWTH OF 'POINSETT' CUCUMBER PLANTS UNDER METAL HALIDE AND MICROWAVE-POWERED SULFUR LAMPS

D. T. KRIZEKR~., M . MIRECKaI nd W. A. BAILEY

Climate Stress Laboratory, Natural Resources Institute, Agricultural Research Service, U.S. Department of Agriculture, Beltsville, MD 20705-2350, USA

(Received September 11, 1998; Accepted October 1, 1998)

KRIZEKD, . T.. MIRECKIR. . M. and BAILEYW, . A. Uniformity of photosynthetic photon flux and growth of 'Poinsett' cucumber plants under metal halide and microwavepowered sulfur lamps. BIOTRONICS 27, 81-92. The uniformity of photosynthetic photon flux (PPF) and vegetative growth of Cucumis sativus L. ('Poinsett' cucumber) were examined using growth chambers equipped with either six 400 W metal halide (MH) lamps or with a single 1000 W microwave -powered sulfur (MPS) (LIGHTDRIVETM 1000) lamp mounted on a polished stainless steel reflector with secondary screening for microwave protection.

PPF levels in each growth chamber were set initially at 500pmol m-2 s-I. Pots were placed at equal distance from one another in ten columns of six rows each (n=60). Growth measurements were only taken on the center six columns of plants (n=36). The uniformity of PPF was greater in the MPS than in the MH chamber for both the 36 and the 60 pot arrangement. However, growth measurements showed similar variance in the MH as in the MPS chamber. Plants grown for 14 days under MPS lamps had significantly greater growth than those under MH lamps. Petiole length, total stem length, and leaf enlargement were 90%, 44%. and 34% greater, respectively. in plants grown under MPS lamps than under MH lamps. Similar differences were obtained in biomass; dry weights of tops and roots of MPS grown plants were 28% and 36%, greater, respectively, than those of MH grown plants. These findings demonstrate the potential of using sulfur lamps for accelerating seedling production under controlled environments and validate the concept that sulfur lamps have a better spectral quality for plant growth than metal halide lamps. These results should be of interest to growers and researchers involved in protected cultivation. The LIGHTDRIVETM 1000 sulfur lamp should also provide a useful tool for studying the photocontrol of shoot development...