Growth and Morphology of Greenhouse Bell Pepper Transplants Grown Under Supplemental LEDs and HPS Lighting

High pressure sodium lamps (HPS) have been the standard supplemental lighting technology used by nursery greenhouses to improved transplant growth and quality. Growers have observed that bell pepper transplants develop undesirable leaf curling if supplemented with HPS. Light emitting diodes (LEDs) are a promising supplemental lighting technology since they are reported to have higher electrical efficiency and the spectrum can be customized for specific plant applications. At the University of Arizona, an experiment was conducted to evaluate pepper morphological and growth rate responses when grown under LEDs and HPS supplemental lighting side-by-side. Four greenhouse bell pepper (Capsicum annuum) cultivars (Orangela, Viper, Fascinato, and PP0710) were grown in a greenhouse under low daily solar-light integral of 5.2 ± 0.8 mol m^-2 d^-1 for 56 days. The supplemental lighting treatments provided 55 μmol m^-2 s^-1 PPF for 18 hours (2 AM – 8 PM) for an additional 3.5 mol m^-2 d^-1 over the plant canopy surface. The treatment consisted of a red-LED (peak wavelength 633 nm, full width at half maximum (FWHM): 16 nm), a blue-LED (peak wavelength 443 nm, FWHM: 19 nm), and a 600W HPS. The growth of pepper transplants was variety dependent, ‘Orangela’ and ‘Viper’ did not show any statistical significant differences in shoot dry mass between the three treatments; the shoot dry mass of ‘Fascinato’ was 19 % greater in the HPS treatment compared to the LED treatments; and the shoot dry mass of ‘PP0710’ in the HPS treatment was 17 % and 30 % greater than the red and blue LED treatments, respectively. There was no significant difference in leaf net photosynthetic rate (Pn) between treatments or varieties. In order to evaluate leaf morphology (curling) under the different treatments, the midrib vein was cut from a mature leaf to compare the extent of leaf curling geometrically, using the ratio of sagitta (height of the arc from the midpoint) and the half-chord-length (the length between the two end points of the arc). Results showed that the HPS and red-LED treatments had 57 % greater sagitta:half-chord-length ratio (leaf curling index) than the blue-LED treatment, regardless of cultivar. From these preliminary results it is evident that bell pepper plant responses to supplemental light quality are cultivar specific, and that blue-LED supplemental lighting can be an alternative to conventional HPS supplemental lighting to prevent leaf curling on pepper transplants.