Growth and quality of leafy vegetables in hydroponics: effect of environmental conditions and mineral nutrition

摘要

The aim of the PhD thesis was to evaluate the influence of environmental conditions and mineral nutrition on growth, mineral composition and quali-quantitative traits of Lactuca sativa L. var. acephala and two Brassica species (Brassicia juncea L. and Brassica rapa L. subsp. nipposinica var. chiloneifera). In lettuce experiments, the effects of five nutrient solution concentrations (with the same macro-nutrient proportion) and six nutrient solutions having different macro-nutrient proportions (at the same total concentration) were evaluated in two consecutive growing seasons (spring and summer season) while the effects of three NO3:NH4 ratios in the nutrient solution and three radiation levels were tested in Brassica experiments. The results showed that during the summer season the growth and yield of lettuce were increased due to the better climate conditions (mainly radiation and temperature). However, the overall quality of lettuce in the summer season was decreased (lower content of carbohydrates and higher nitrate content). The use of nutrient solution concentration of 37 and 44 mequiv L-1 gave the maximum marketable fresh yield for the spring and summer season, respectively. In the second experiment, the effects of macronutrient proportions on marketable fresh yield were not evident; the macronutrient proportions influenced only the mineral content of leafy lettuce. In Brassica experiments, the results showed that a medium PAR level improved the plant growth and reduced the leaf nitrate concentration of Brassica rapa while the use of a nutrient solution containing 100% of NO3 or 50% NO3 and 50% NH4 lead to a better plant growth but increased the leaf nitrate concentration. In Brassica juncea, an high PAR level was required to maximise plant growth; at high PAR level, the use of a nutrient solution containing 100% NH4 reduced the leaf nitrate concentration, the water requirement and increased the N e S concentration of leaves without adverse effects on plant growth. Ammonium nutrition had a positive effects on nutritional value of Brassica species by increasing glucosinolate concentration; however, a negative influence on flavonoid concentration was observed with high ammonium concentration in the nutrient solution. Flavonoid concentration increased with high radiation levels. We concluded that the management of the nutrient solution and environmental factors, together with a good knowledge of the crop responses to these factors could improve the quality of these leafy vegetables.