Abstract
Glasshouse experiments were conducted in winter and spring growing seasons in order to evaluate the effects of shading on production and nutritional quality of lettuce (Lactuca sativa L. cv. Parris Island), under Mediterranean climate conditions. In both seasons, plants were cultivated hydroponically under four different levels of photosynthetically active radiation intensities (26, 47, 73 and 100 % of incident light intensity). The results showed that stomatal conductance and photosynthetic rate significantly decreased in shaded plants. This strong negative correlation of leaf physiological parameters with light deficiency resulted in lower biomass yield production in both growing seasons. Moreover, the nutritional value (ascorbic acid concentration) was also significantly decreased in relation to incident light intensity decrease. In contrary, a strong positive correlation of leaf total chlorophyll content and nitrate content with light deficiency was detected. However, nitrate concentration in all treatments remained within the European Union's permissible levels being significantly lower in plants produced in spring compared to winter.
Highlights
Lettuce consists one of the most important cultivated vegetable in Greece, contributing significantly to national economy
Nitrate concentration was significantly higher in plants grew during the winter period compared to those in spring (Table 1) irrespective the light intensity applied, indicating that different growing seasons affect nitrate content
These higher values of nitrate concentration during the winter season could be attributed to lower photosynthetic active radiation (PAR) intensity exhibited during that period and/or to differences in other microclimatic parameters
Summary
Lettuce consists one of the most important cultivated vegetable in Greece, contributing significantly to national economy. Lettuce leaves are usually consumed raw and without any restriction to daily intake. Lettuce is characterized by its great ability to accumulate nitrate in leaves which can be harmful to human health (Cometti et al, 2011). Nitrate concentration in lettuce is considered one of the more important quality parameters. Nitrate accumulation in plants is quite complex since it is influenced by both genetic and environmental factors (Novo et al, 2008). Among environmental factors light intensity is reported to strongly affect nitrate accumulation in plants
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