Some plants grow faster when given low-intensity long-day lighting and various reasons have been given for this. The experiments described here were aimed at establishing why plants grow faster and what features their responses have in common.
Petunia, Impatiens and tomato seedlings were grown in daylight for eight hours each day and in light-tight compartments for 16 hours.
The daylight was extended for either four or eight hours using light from a combination of tungsten and fluorescent lamps.
A night-interruption treatment was also given, using tungsten lamps for two hours around midnight.
Growth of all three species was promoted by all treatments, especially when natural light levels were low. Some of the extra growth of Petunia was probably attributable to its upright habit and larger leaves in long days. Long days produced no differences in leaf thickness in tomato and although the long-day leaves were a darker green, their photosynthesis was not promoted.
Impatiens showed no change in either leaf thickness or greenness in long days. It is suggested that the extra hours of low-intensity light generated enough photosynthesis to offset some respiration and hence promote growth in all three species.
If correct, long-day lighting should promote growth in most plants.
Why Does Low-intensity, Long-day Lighting Promote Growth in Petunia, Impatiens and Tomato? by Adams, Valdes and Langton (2008). Journal of Horticultural Science & Biotechnology 83 (5): 609-615.
The authors' abstract of their manuscript can be seen in full at www.jhortscib.com.