2018 ASHS Annual Conference

Variety testing of vegetables is an important step for beginning businesses to narrow down product focus, generate example product for potential customers, and determine performance of varieties under one’s own environmental conditions. Greenhouse grown lettuce has a reputation of being more tender, having grown with fewer environmental stressors, than field grown counterparts. Greenhouse lettuce receives a shifted spectrum of light than outdoor plans due to solar interception by greenhouse glazing materials, which can lead to differences in plant morphology. High pressure sodium (HPS) fixtures are currently the most commonly used supplemental lighting source in greenhouse lettuce production. The introduction of light emitting diodes (LEDs) into the greenhouse industry can allow growers to adjust light spectrum and potentially alter plant morphology, biomass, pigmentation, and development of the physiological disorder, leaf tip burn. The objective of this experiment was to determine response of 13 varieties of head lettuce to greenhouse performance under HPS or LED light.

Lettuce was grown from seed in a common area until seedlings had 3-4 true leaves. Seedlings were then transplanted into a split nutrient film technique (NFT) system with two lighting arrays established above each half of the NFT system. The NFT system shared a common nutrient solution reservoir the location of the channels was separated by 3 m so that the two lighting arrays could share the same greenhouse but avoid light pollution between treatments. One lighting array used HPS fixtures while the other array had LEDs set to a fixed red:blue ratio (80% red : 20% blue). Both arrays were adjusted to provide a similar light intensity of about 200 µmol·m^-2·s^-1. which had been split in half. Quantum sensors were placed under each lighting array and were connected to a microprocessor which used the LASSI (Light and Shade System Implementation, Albright et al., 2010) control algorithm to adjust the daily supplemental lighting period to complement ambient light to achieve a fixed daily light integral (17 mol·m^-2·d^-1). Data were collected on plant fresh weight and tip burn index. Qualitative data were collected on taste, color, and texture.

Four cultivars exhibited a significant response (p ≤ 0.05) to light source: ‘Lotus’, ‘Seurat’, ‘Teodore’, and Xandra. Three additional cultivars had a moderate response (p ≤ 0.10) to light source: ‘Crunchita’, ‘Greenstar’, and ‘Rex’. For these seven cultivars fresh weight under LED was less than HPS. Qualitatively, several varieties were found to have an unacceptable bitter taste. In some cases, taste varied between the HPS and LED grown plants. Regarding leaf color, red leaf varieties showed greater red pigmentation when grown under the LED array.