Abstract
In Nordic countries during the winter months supplemental lighting is essential for year-round cucumber production. In this research the effects of full HPS (top and interlights) illumination is compared to hybrid (HPS top, LED interlights) and full LED (top and interlights). The results showed that fruit yield was highest in the HPS-LED treatment whereas the electrical use efficiency (kg yield J-1) increased when HPS was replaced with LED. In LED-LED the light use efficiency (g fruit FW mol-1 PAR) was highest but resulted in a fewer number of fruits in mid-winter particularly and the lowest yield potential. The lower temperature and lower light level due to LED-LED lighting lead to reduced photosynthesis capacity, flower initiation rate and water use efficiency whereas increased stem elongation and leaf expansion compared to other treatments. Differences in light quality of LED and HPS are also discussed in terms of cucumber yield formation.
Highlights
Energy efficiency in greenhouse cultivation in Nordic countries is important for economically feasible production and to lower the industry’s carbon footprint
In our experiment, where solar light was virtually absent in December and January (DLI 1.3-2.6 mol d-1), it was observed that net photosynthesis rate (Pn) per unit area of leaf at equal light level (PPF) was higher in high pressure sodium (HPS)-LED than in the other treatments, which may have been a result of a broader light spectrum from a mixture of lamp types, and due to the higher leaf temperatures in the upper canopy
The light use efficiency expressed in grams fruit fresh weight per mol-1 PAR was highest in the LED-LED lighting treatment but due to fewer fruits in mid-winter in the LED-LED treatment compared to the other treatments, the yield and yield potential were lowest in LED-LED
Summary
Energy efficiency in greenhouse cultivation in Nordic countries is important for economically feasible production and to lower the industry’s carbon footprint. Greenhouse cucumber is commonly cultivated at northern latitudes under high pressure sodium (HPS) supplemental lighting year-round when the solar light level (Daily light integral [DLI]) is low during fall and winter and even on cloudy summer days Luminaire installations, both as top and intercrop lights, are frequently utilized in cucumber and tomato cultivation because of the higher yields associated with these lighting systems (Hovi et al 2004, Hovi-Pekkanen and Tahvonen 2008). Heat output relative to light output is higher in HPS than in LED lights and while HPS lights strongly radiate heat downward, most LED armatures lose convective heat upwards even though the light is focused on the plants This results in lower leaf temperatures and in some cases requires more pipe borne heating, reducing the energy savings. Aisle width can be reduced and rows increased for enhanced cultivation if HPS interlights are replaced with
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