Abstract
Production of lettuce (Lactuca sativa) within vertical farms is an expanding segment of controlled environment agriculture-precise manipulation of environmental parameters including mean daily temperature (MDT) and carbon dioxide (CO2) concentration enables year-round production, alongside color, yield, and crop size regulation. Our objectives included 1) quantify how MDT and CO2 interact to influence lettuce growth, development, and quality; 2) model lettuce growth under several MDTs and CO2 concentrations. Green butterhead 'Rex' and red oakleaf 'Rouxaï RZ' seedlings were transplanted into hydroponic tanks under a photosynthetic photon flux density of 300 μmol·m‒2·s‒1 for 17-h·d‒1. CO2 concentrations of 500, 800, or 1200 μmol·mol-1 and day/night and MDT setpoints of 22/15°C (MDT 20°C), 25/18°C (23°C), or 28/21°C (26°C) were maintained within growth chambers. 'Rex' fresh mass increased linearly with MDT, increasing by 18% from 20 to 26°C. 'Rouxaï RZ' fresh mass increased quadratically with MDT, with a 32% increase from 20 to 23°C, then a 7% increase from 23 to 26°C. Elevating CO2 concentrations from 500 to 800 μmol·mol-1 increased 'Rouxaï RZ' and 'Rex' fresh mass by 33 and 16%, respectively, while fresh mass did not increase from 800 to 1200 μmol·mol-1. Both cultivars had the greatest dry mass at 800 μmol·mol-1 CO2 across temperatures. At a high MDT, 'Rouxaï RZ' foliage color became more light, vibrant, and green, while a low MDT induced darker, grayer, and redder foliage. Tipburn occurred on 'Rex' across treatments, while 25% of 'Rouxaï RZ' were afflicted at 500 μmol·mol-1 CO2 and 67% at 1200 μmol·mol-1. At the light intensity studied, we recommend growing 'Rex' and 'Rouxaï RZ' at an 800 μmol·mol-1 CO2 concentration and MDT of 23°C for greatest biomass and leaf number, and slightly redder foliage in 'Rouxaï RZ' than at a 26°C MDT.