Light has many effects on plant development. For the most part, plants must make do with the natural sunlight received daily. However, by manipulating light intensity, duration, colour spectrum and so on, plant growth can be modified for specific needs. This may be necessary to prolong flowering, to allow year-round rearing and assessment of flower, pod and seed feeding insect agents, or to prolong or promote vegetative growth by inhibiting flowering or leaf-fall for young shoot feeders. For example, with the shortening day length as winter approaches, plants of the pantropical
Sida acuta are stimulated into flower and fruit production with such an intensity that vegetative growth ceases. Leaf size decreases to less than half that produced during summer months. This presented a problem when rearing a biological control agent, the voracious leaf feeding beetle
Calligrapha pantherina. By supplementing artificial lighting from the beginning of autumn to maintain 14 hours of light each day, flowering was delayed, plants continued to grow
vegetatively, and there was little reduction in leaf size until after mid winter.
Shading may be important. For example, young new growth of Theobroma cacao (cocoa) develops brown burn lesions along the margins when exposed to full sunlight. Sprouting seed and emerging seedlings of many plant species are damaged by direct sunlight, so shading is required for establishment. Conversely, shading of species requiring strong light causes etiolation (unnatural stem elongation) and increased susceptibility to
breaking and disease.
