Evaluation of photosynthetic photon flux in lettuce cultivation at different shading levels

ABSTRACT Protected cultivation has grown in Brazil. Generally, greenhouses are covered with transparent plastic film and shading screen. The plastic, over time, loses its transparency due to pollution residues, dust and other debris. The loss of transparency reduces lightness, photosynthesis and leads to losses of productivity and product quality. The losses are not always detectable by the farmer. Additionally, internal shading screens are used to reduce heating transmission to the ground. The objective of this study was to evaluate the impact of shading on lettuce crop productivity and to determine the optimum shading to reach the highest productivity. Plots were set up inside and outside the greenhouse, with four shading levels with black screens (0, 35, 50 and 75%). The treatments were converted to real shading from the photosynthetic photon flux measurement. The results of fresh and dry phytomass were treated and analyzed by regression as a function of the real shading. In ambient conditions of photosynthetic photon fluxes around 1000 μmol m-2 s-1, reaching up to 2000 μmol m-2 s-1 at some hours of the day, typical of tropical environment, lettuce may support a shading of up to 50% without risk of productivity reduction; under these conditions, shading between 20 and 35% is beneficial, and can guarantee its maximum productivity in lettuce cultivation. It is recommended that the lettuce producer in protected cultivation monitors the shelf life of the plastic, avoiding that the shading exceeds 50%. In order to compare shading experiments, one should use the incident photon flux (FFI) for the whole crop cycle, indicating the minimum limit value of FFI = 600 mol m-2 cycle-1 for the crispy lettuce at an average temperature close to 21oC.

摘要 巴西设施栽培产业呈增长态势。通常温室多以透明塑料薄膜与遮阳网作为覆盖材料。随着使用时长增加，塑料薄膜会因污染物残留、粉尘及其他杂物逐渐丧失透光性，进而降低光照强度、抑制光合作用，最终导致作物产量与品质下降，且这类损失往往难以被种植户及时察觉。此外，温室内常设置内部遮阳网以减少热辐射向地面传递。本研究旨在评估遮阳处理对生菜产量的影响，并确定实现最高产量的最优遮阳率。试验在温室内外设置了多个试验小区，采用黑色遮阳网设置4种遮阳梯度（0%、35%、50%、75%），并通过光合光子通量（photosynthetic photon flux, PPF）测定结果将各处理换算为实际遮阳率。研究对生菜鲜、干生物量的测定结果以实际遮阳率为自变量开展回归分析与统计处理。结果表明：在热带典型环境中，光合光子通量日均约为1000 μmol·m⁻²·s⁻¹，单日部分时段峰值可达2000 μmol·m⁻²·s⁻¹，此时生菜可承受最高50%的遮阳率而不会出现产量损失风险；在此类条件下，20%~35%的遮阳处理可实现最佳效益，保障生菜栽培获得最高产量。据此建议设施生菜种植户定期监测塑料薄膜的透光性能有效使用期限，避免实际遮阳率超过50%。为实现不同遮阳试验的横向对比，应采用全作物周期内的入射光子通量（incident photon flux, FFI）作为评价指标；在平均温度约21℃的条件下，脆叶生菜（crispy lettuce）的全周期入射光子通量最低限值为FFI=600 mol·m⁻²·cycle⁻¹。