Yield and quality of lettuce in response to the plant position in photovoltaic greenhouse

In recent years there has been an increasing spread of photovoltaic greenhouses, especially in southern European countries, due to the higher incentives recognized to solar photovoltaic (PV) panels integrated on the greenhouse roofs and/or to local laws which limit the photovoltaic systems on the ground. To maximize the income from the production of electricity, often solar panels cover 50% or more of the roof, but the shading caused by these elements on the growing surface seriously limits the productivity and affects crop development. In order to assess the effects of the spatial distribution of the solar radiation inside these structures on yield and quality of leaf vegetable crops, the response of lettuce, grown in two cycles (autumn and winter-spring), in an east-west oriented photovoltaic greenhouse with the 50% of the roof covered by PV modules was analysed. The influence of span orientation, plant position (under plastic or PV roof) and cultivar were analysed as experimental treatments in a split-split plot design with two replications. Total and marketable yield of the lettuce heads and some quality parameters (dry matter and nitrate content) were evaluated. A significant variability of the total and marketable yield due to the plant position and as consequence to the solar radiation distribution inside the structure during the growing cycle was observed. Furthermore, ranges and variability of the nitrate content of lettuce (expressed as NO3 mg kg-1 of fresh weight) were affected by the plant position and harvest season. In order to maximize yield and quality of the crops, the arrangement of plant rows and transit areas, as well as the management of nutrition, should be optimized in relation to the shading caused by the PV roof during the growing cycle.