Abstract
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.
Highlights
O cultivo protegido tem crescido no Brasil
The treatments used in the experiment with photosynthetic photon fluxes (PPF) are shown in Figure 1 (A)
The error bars have large amplitude, especially in Evaluation of photosynthetic photon flux in lettuce cultivation at different shading levels treatments with lower shading because they contain data collected at different times
Summary
O cultivo protegido tem crescido no Brasil. Geralmente, as estufas utilizam plástico transparente e tela de sombreamento na cobertura. In addition to the use of clear plastic for crop protection, shading screens are commonly used to reduce irradiance and temperature within the greenhouses, increasing the productivity of the protected crops (Santosh et al, 2017; Lemos Neto et al, 2017). Seabra et al (2009) found similar data for lettuce in tropical environment with high temperatures In this case, the productivity was higher when shading was close to 50%, and crops were favored with a specific type of reflective screen, which promoted the reduction of temperature by 10 to 20%. The thermal accumulation, consolidated in the Degrees-Day concept, is the simplest means of productivity comparability as a function of the difference between the average temperature and the basal temperature of the crop, below which the crop stops growing (Araújo et al, 2010)
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