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.

Cucumber plants grown under a high pressure sodium lamp (left), fluorescent tubes (middle), and an artificial solar spectrum (right) 13d after planting the seedlings. The upper image was made before the plants were dissected for growth and morphology analysis (bar=10cm). 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

more info »

(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).