Abstract
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 13âd 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 4â5 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.
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Artificial solar spectrum
It has been possibble to construct a light source which, except for a deficiency in the blue, produces a spectrum that closely resembles that of a standard sunlight spectrum (Fig. 1B). The reference spectrum for the purposes of this exercise was the ASTM G173-03 direct and circumsolar spectrum; thus it excludes skylight and takes no account of cloudlight. This is a calculated, representative direct and circumsolar irradiance spectrum for 48 contiguous states of the USA, which is available for download in a tabular form (ASTM, 2003). Cloudlight spectra are not very different from direct sunlight spectra, whereas skylight spectra are conspicuously different (e.g. Endler, 1993). The total solar irradiance is comprised of skylight, direct sunlight, and cloudlight in various proportions depending on, amongst others, the height of the sun above the horizon and weather conditions. In the absence of clouds, the total irradiance is largely dominated by direct sunlight and, under these conditions, plants will experience a predominantly direct sunlight spectrum, except under a low sun angle or when the direct sunlight is filtered by other leaves. To the best of our knowledge no comparable typical spectrum exists for other regions and therefore the ASTM spectrum is a reasonable model to use, until a better catalogue of natural spectral irradiances becomes available.
The AS spectrum was provided using a 1300âW microwave-driven sulphur plasma lamp (PI-VL1, Plasma International GmbH, Offenbach am Main, Germany), which was filtered using a colour correction filter (Gamcolor filter 1581, Los Angeles, CA, USA) in order to reduce the intensity of the green wavelengths. The resulting irradiance spectrum, lacking sufficient near-infrared wavelengths, was projected onto the plants via reflection by aluminium foil on the ceiling of the climate chamber, so that the light was well distributed over the plants. Additional quartzâhalogen lamps were used to provide more near-infrared irradiance. The light output of both the plasma lamp and the quartzâhalogen lamps could be adjusted without any large changes in spectral output. The desired spectrum was obtained by adjusting the light output such that 72% of the PAR was provided by the filtered plasma lamp and 28% by the quartzâhalogen lamps. The spectrum and intensity of the three light sources used as growth treatment were measured using a spectroradiometer (USB2000 spectrometer, Ocean Optics, Duiven, The Netherlands, calibrated against a standard light source; Fig. 1B, C) and the spectra are also provided as Supplementary Table S1 available at JXB online. Light intensity was routinely measured using a quantum sensor (LI-COR Lincoln, NE, USA). The two devices produced comparable results. Additionally the natural spectrum of cloudlight in fully overcast conditions, direct sunlight, and skylight was measured at midday in Wageningen (52â°N 5.5â°E, The Netherlands) around the autumn equinox 2009 on the roof of a tall building (Fig. 1A and in tabular form as Supplementary Table S1).
Fig. 1.
(A) Relative spectra of direct sunlight (solid line), cloudlight (thick dotted line), and skylight (thin dotted line) measured around the autumn equinox (2009) at noon in Wageningen, The Netherlands. (B) Relative spectra of the artificial solar spectrum (dotted line) and a standard solar spectrum (solid line; ASTM, 2003). (C) Relative spectra of the high pressure sodium lamp (dotted line) and the fluorescent tubes (solid line).
Plant morphology
The difference in visual appearance of the plants growing under the three different spectra was striking (Fig. 2). The plants grown under HPS had a slightly bigger appearance than the plants grown under FTs. The AS-grown plants, however, developed considerably faster than those grown under HPS and FTs.
Fig. 2.
Cucumber plants grown under a high pressure sodium lamp (left), fluorescent tubes (middle), and an artificial solar spectrum (right) 13âd after planting the seedlings. The upper image was made before the plants were dissected for growth and morphology analysis (bar=10âcm). The lower three images were made before harvest and are of plants different from those on the upper image. These three images are not scaled; the leaf colour appears unnatural due to the growth light environment.
The differences in plant morphology are shown quantitatively in Table 1. Leaf 1, which was fully expanded on all plants when harvested, was smaller in the FT treatment than in the HPS and AS treatments. Leaf 2 of the AS-grown plants had twice the area of that grown under HPS and four times the area of that grown under FTs. This leaf was, however, not completely expanded on all plants at the time of harvest. The number of leaves was also significantly greater for the AS plants compared with the other two treatments, and the HPS plants had a slightly, but significantly, greater number of leaves than the FT plants. Leaf number, therefore, also contributed to the significant differences in total leaf area between the treatments; the AS-grown plants had a total leaf area which was 2.5 and 1.7 times greater than that of FT and HPS plants, respectively. The petioles of leaf 1 and 2 were approximately three times longer for the AS plants than those of the other two treatments, whereas the petioles of HPS plants were slightly, but significantly, longer than those of FT plants. Due to their long petioles the leaves of individual AS-grown plants did not shade each other, whereas from leaf 3 of plants in the other treatments there was leaf shading in individual plants. Also leaf 1 and 2 of the FT and HPS plants partially shaded the cotelydons, whereas the cotyledons of the AS plants were not shaded (Fig. 2). Leaves of the FT and HPS plants were not completely horizontal and also not oriented towards the incident irradiance such that light interception would be optimal. The leaves of the AS plants were fully horizontal and better orientated for light interception. The hypocotyl was over three times longer for the AS plants than it was for the other treatments. A similar trend was found for total plant length, which was four and five times greater for the AS-grown plants than the HPS and FT plants, respectively. The total plant length was only slightly greater than the hypocotyl length for HPS and FT plants, whereas the total length of the AS plants was much greater than that of the hypocotyl. This is due to differences in internode length between the treatments. The cotyledon area of the FT plants was smaller than that of the HPS and the AS plants, despite having already been partly developed when the plants were transferred to the spectrally different irradiances, implying that the cotyledons were affected by the growth light treatment.
Fig. 4.
IrradianceâCO2 fixation response curves for leaves grown under 100âÎźmol mâ2 sâ1 incident irradiance provided by fluorescent tubes (circles), a high pressure sodium lamp (squares), and an artificial solar spectrum (triangles). Lines through the data points represent the fit to the non-rectangular hyperbola. Error bars represent the SEM.
Lampy się nie nadają mają za mało lumenów a i box o piz*e można rozbić... Wyrzuć te siewki na dwór już gdzieś na spota...
