When sunlight levels are low, supplemental lighting (SL) can accelerate rooting and improve liner quality. In the past, SL was commonly provided by high-pressure sodium (HPS) lamps. Today, light-emitting diode (LED) fixtures are becoming the standard because they offer longer lifespans, lower heat output, lower energy use, and greater overall efficiency.
Even so, crops may respond differently to the light emitted by LEDs, depending on the fixture's spectral output. As LED technology advances and fixtures deliver higher light intensities, supplemental lighting can sometimes produce unwanted responses in sensitive cultivars of petunia, Rieger begonia, phlox, and other species when they are rooted as cuttings.
One common issue is purple foliage developing during propagation in plants with red, purple, or dark pink flowers. This purpling is caused by the accumulation of red and blue pigments known as anthocyanins. These pigments can build up for several reasons, including low temperatures, high light intensity, exposure to short-wavelength light such as blue or ultraviolet light, or nutrient deficiencies.
During propagation of unrooted cuttings, it is easy to assume that foliage purpling is the result of a nutrient deficiency, most likely phosphorus (P). However, phosphorus deficiency typically appears as purpling on lower leaves. In contrast, purpling caused by high-intensity LED supplemental lighting shows up as splotchy or mottled discoloration on leaves directly exposed to the light, regardless of leaf age.
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