Red to far-red ratio correction in plant growth chambers - growth responses and influence of thermal load on garden pea

Plant growth chambers are commonly used to minimize environmental variation but the light sources used vary considerably from natural light and from each other. Incandescent globes are often used to add more far-red light, with the aim of producing a more natural red to far-red ratio (R:FR), but also add to thermal load. High-intensity discharge lamps are often used to produce higher irradiances, more akin to natural light, but the thermal implications are rarely considered because air temperature is controlled. This paper examines the spectral properties and thermal implications of growth chamber light sources and takes a whole-plant physiology approach, by examining growth responses of a photoperiodic pea line (Pisum sativum L. cv. Torsdag) in the same growth chamber type under different light sources - in essence using plants to study the controlled environments rather than vice-versa. High R:FR delayed flowering and inhibited internode extension in pea. However, the addition of far-red-rich incandescent globes in the proportions provided in the growth chambers (400-500 W) did little to reduce R:FR, did not induce earlier flowering and actually further inhibited internode length. Leaflet size and yield were significantly reduced. While air temperature was maintained at 20°C in all experiments, radiant temperature was significantly higher under high irradiance and/or with incandescent added, and soil temperatures were elevated. Growth responses under these lights were similar to the effect caused by elevating the air temperature. An alternative method of controlling R:FR, without thermal load implications, using light-emitting diodes is described. © Physiologia Plantarum 2007.