Table_1_Plant Factories Are Heating Up: Hunting for the Best Combination of Light Intensity, Air Temperature and Root-Zone Temperature in Lettuce Production.docx

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.

本研究分析了在水分、养分与CO₂浓度均无限制的条件下，光量子通量密度（photon flux density, PPFD）、空气温度与根区温度对生菜生长的交互影响。我们设置了3个PPFD水平（200、400、750 μmol·m⁻²·s⁻¹，每日光照时长16 h），搭配20、24、28、32℃共4个空气温度与4个根区温度，形成48组试验组合，并测定了各组的生长参数。本试验供试生菜为*Lactuca sativa* cv. Batavia Othilie，共开展4个生长周期（移栽后培养29天）。其中根区温度为20、24℃且空气温度为28、32℃、PPFD为400、750 μmol·m⁻²·s⁻¹的8组组合，焦尖病（tip-burn）发病率过高，未纳入后续分析。

地上部干物质重量随PPFD升高而增加，直至750 μmol·m⁻²·s⁻¹。光子转化效率（以鲜重、干重计）则随PPFD升高而降低：在所有温度组合的平均值下，200、400、750 μmol·m⁻²·s⁻¹对应的地上部鲜重分别为29、27、21 g·mol⁻¹入射光，地上部干重分别为1.01、0.87、0.76 g·mol⁻¹入射光，同时伴随比叶面积（specific leaf area, SLA）的持续下降。当PPFD为200 μmol·m⁻²·s⁻¹、空气温度24℃、根区温度28℃时，光子转化效率达到最高，对应地上部鲜重与干重分别为44 g·mol⁻¹入射光与1.23 g·mol⁻¹入射光。

空气温度对地上部鲜产量的影响与所有叶片扩展过程相关。比叶面积、地上部生物量分配比例与叶片含水量均随空气温度变化呈现相同趋势，在24℃左右达到峰值。根区温度的影响相对较弱，在几乎所有试验条件下均在28℃左右达到最优值。在该最优温度组合下，当生菜达到商品采收规格（地上部鲜重=250 g）时，200、400、750 μmol·m⁻²·s⁻¹对应的收获天数分别为26、20、18天，对应的地上部干物质含量分别为2.6%、3.8%与4.2%。

综上，三大因素决定了“最优”PPFD：光照强度的资本与运营成本、缩短种植周期的收益价值，以及更高干物质含量的市场价值。