Contents lists available at ScienceDirect Scientia Horticulturae journal homepage: www.elsevier.com/locate/scihorti Effects of daytime intra-canopy LED illumination on photosynthesis and productivity of bell pepper grown in protected cultivation Naveen C. Joshi a,1 , Kira Ratner a,1 , Orly Eidelman a , Dominika Bednarczyk a , Naftali Zur a , Yair Many a , Yosepha Shahak a , Elinor Aviv-Sharon b , Meir Achiam c , Ziva Gilad c , Dana Charuvi a, ⁎ a Institute of Plant Sciences, Agricultural Research Organization (ARO) - Volcani Center, Rishon LeZion, 7505101, Israel b The Robert H. Smith Institute of Plant Sciences and Genetics in Agriculture, Hebrew University of Jerusalem, Rehovot, 7610001, Israel c Jordan Valley Research and Development Authority, Mobile Post, 91906, Israel ARTICLE INFO Keywords: Light-emitting diodes (LEDs) Intra-canopy illumination Interlighting Bell pepper Photosynthesis Fruit-yield ABSTRACT In the past decade, light-emitting diodes (LEDs) have been replacing most other types of light sources. One emerging use for LEDs in horticulture is ‘intra-canopy illumination’ or ‘LED-interlighting’, feasible owing to their low heat output and small physical size. The interlighting technique, typically implemented in environmentally- controlled greenhouses also supplemented with overhead lighting, is mostly relevant for high-wire vegetable cultivation, in which self-shading results in light limitations for a large fraction of the canopy. Interlighting has been shown to increase the yield and/or improve fruit quality in all-year round greenhouse crops such as tomato, cucumber and sweet pepper. In this work, we utilized daytime supplemental intra-canopy LED illumination for sweet pepper grown in high-density ‘Spanish’ trellis systems within passive high tunnels in the Jordan Valley, Israel (latitude ˜32 °N). While canopy top at these conditions is not light-limited, extensive deep shading of the inner canopy is a disadvantage. In two experiments carried out in two separate seasons, the supplemental lighting, which enhanced the photosynthetic rates of the inner canopy foliage by 3.5- to 5.7-fold, resulted in significant increase (˜30%) of the fruit yield during the spring season. The added yield was attained by higher fruit numbers, with no notable effects on fruit size or weight. Our results raise the prospect that LED-interlighting may be a useful practical tool for maximizing fruit production, even in geographical regions of ample sunlight. 1. Introduction In recent years, with the advancements in the light-emitting diode (LED) technology, the use of LED lighting in horticulture has been steeply rising (Bantis et al., 2018; Mitchell, 2015; Mitchell et al., 2015; Morrow, 2008; Nelson and Bugbee, 2014). As LEDs are essentially solid- state semiconductor devices, they are able to exhibit very cool photon- emitting surfaces that generate low heat outputs (Singh et al., 2015). This characteristic, coupled with the fact that LED chips are quite small in size, allows using them as supplemental lighting within plants’ ca- nopies without overheating or burning the plant tissues (Gómez et al., 2013). Application of supplemental illumination within the canopy is coined ‘(LED)-interlighting’ or ‘intra-canopy illumination’ (Mitchell et al., 2015), used here interchangeably. The effects of LEDs, as top- or inter-lighting, on greenhouse crops can be numerous. Some of the beneficial ones include added crop-yield, higher quality yield, manip- ulation of harvest dates, less yield fluctuations during the growth cycle, as well as enhanced nutritional value (Carvalho et al., 2016; Chen et al., 2016; D’Souza et al., 2015; Folta and Childers, 2008; Li and Kubota, 2009; Massa et al., 2008; Olle and Virsile, 2013; Ouzounis et al., 2015; Singh et al., 2015). LED-interlighting has been mostly studied and utilized in northern countries (e.g., the Netherlands, Finland, Canada and northern areas of the USA) in combination with overhead illumination, required due to the low irradiance and short day length, especially in winter (Dorais, 2003; Hemming, 2011). In these regions, illumination is typically ap- plied in the context of actively-controlled greenhouses (e.g., Gómez and Mitchell, 2014; Hao et al., 2012). Intra-canopy LED illumination is most commonly implemented for year-round high-wire greenhouse vege- table cultivation, i.e., for cucumber, tomato and pepper (Gómez and Mitchell, 2014; Jokinen et al., 2012; Trouwborst et al., 2010) [for earlier interlight studies performed with high-pressure sodium (HPS) lamps see Gunnlaugsson and Adalsteinsson, 2006; Hovi et al., 2004]. Insufficient light within the canopy of such intensively-grown crops https://doi.org/10.1016/j.scienta.2019.02.039 Received 21 September 2018; Received in revised form 7 February 2019; Accepted 13 February 2019 ⁎ Corresponding author. E-mail address: charuvi@volcani.agri.gov.il (D. Charuvi). 1 These authors contributed equally to the work. Scientia Horticulturae 250 (2019) 81–88 0304-4238/ © 2019 Elsevier B.V. All rights reserved. T