Abstract
This study analyzed interactions among photon flux density (PPFD), air temperature, root-zone temperature for growth of lettuce with non-limiting water, nutrient, and CO2 concentration. We measured growth parameters in 48 combinations of a PPFD of 200, 400, and 750 μmol m–2 s–1 (16 h daylength), with air and root-zone temperatures of 20, 24, 28, and 32°C. Lettuce (Lactuca sativa cv. Batavia Othilie) was grown for four cycles (29 days after transplanting). Eight combinations with low root-zone (20 and 24°C), high air temperature (28 and 32°C) and high PPFD (400 and 750 μmol m–2 s–1) resulted in an excessive incidence of tip-burn and were not included in further analysis. Dry mass increased with increasing photon flux to a PPFD of 750 μmol m–2 s–1. The photon conversion efficiency (both dry and fresh weight) decreased with increasing photon flux: 29, 27, and 21 g FW shoot and 1.01, 0.87, and 0.76 g DW shoot per mol incident light at 200, 400, and 750 μmol m–2 s–1, respectively, averaged over all temperature combinations, following a concurrent decrease in specific leaf area (SLA). The highest efficiency was achieved at 200 μmol m–2 s–1, 24°C air temperature and 28°C root-zone temperature: 44 g FW and 1.23 g DW per mol incident light. The effect of air temperature on fresh yield was linked to all leaf expansion processes. SLA, shoot mass allocation and water content of leaves showed the same trend for air temperature with a maximum around 24°C. The effect of root temperature was less prominent with an optimum around 28°C in nearly all conditions. With this combination of temperatures, market size (fresh weight shoot = 250 g) was achieved in 26, 20, and 18 days, at 200, 400, and 750 μmol m–2 s–1, respectively, with a corresponding shoot dry matter content of 2.6, 3.8, and 4.2%. In conclusion, three factors determine the “optimal” PPFD: capital and operational costs of light intensity vs the value of reducing cropping time, and the market value of higher dry matter contents.
Highlights
Recent social developments have increased the allure of locally produced food and urban horticulture is increasingly seen as an option to produce locally (Benke and Tomkins, 2017; Shamshiri et al, 2018)
The realized climate conditions were maintained within 3% and 6% of the desired setpoints (Table 1) and the standard deviation was never more than 5%; 2%; and 6%
Yield increased with light intensity as expected, and there was an obvious effect of air temperature, as 24◦C resulted in the highest yield and 32◦C the lowest at all light intensities
Summary
Recent social developments have increased the allure of locally produced food and urban horticulture is increasingly seen as an option to produce locally (Benke and Tomkins, 2017; Shamshiri et al, 2018). Known as vertical farms, are capable of cultivating crops on multiple layers and achieving high crop productivity and uniformity, without any need for crop protection chemicals (Graamans et al, 2018; SharathKumar et al, 2020). Such production systems are completely insulated from the exterior climate and control light (spectrum, intensity, and photoperiod), temperature, relative humidity, and CO2 concentration. They are typically used to produce small, “stackable” plants with a short production cycle, such as leafy vegetables and herbs, seedlings and high-value medicinal crops (Kozai, 2013). Systems with full climate control, such as plant factories, allow for the optimization of the production climate when the crop response to different climate factors is known
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