Effect of nutrient concentration and growing seasons on growth, yield and quality of leafy lettuce (Lactuca sativa L.) in a hydroponic system

Lettuce is becoming an increasingly important vegetable, both as a fresh market product and a ready-to use vegetable, especially in urban areas of South Africa. Nutrient solution concentration is one of the most practical and effective ways of controlling and improving the yield and nutritional quality of crops for human consumption. However, optimal fertilizer concentration for leafy vegetables also depends on the prevailing environmental conditions. This study was carried out to determine the effects of different nutrient solution concentrations and growing seasons on growth, yield and quality of leafy lettuce. The trial was conducted in a black and white shade net structure and the nutrient concentration treatments were 1.0, 2.0, 3.0, and 4.0 mS.cm-1. Measurements taken included: leaf number, leaf area, fresh leaf mass, dry leaf mass, dry root mass, as well as chlorophyll content. The sensory evaluation procedure was only done on plant samples grown during summer and winter seasons. The results showed that growth was less affected by nutrient concentration than by growing season. Regardless of the nutrient concentration, plants grown in summer reached maturity quicker as compared to plants grown in winter. Generally, leaf number, leaf area, leaf area index, fresh leaf mass, dry leaf mass and dry root mass did not significantly increase with increasing nutrient concentrations and therefore, yield was not influenced by nutrient concentrations. Quality was influenced by nutrient concentrations during the summer-autumn seasons where increasing nutrient concentration induced increased chlorophyll content of the leaves. During the winter-spring seasons this phenomenon was not significant. The study demonstrated that growth, yield and quality of lettuce were not significantly influenced by nutrient solution concentrations of 1.0, 2.0, 3.0 and 4.0 mS.cm-1. The sensory evaluation also showed no significant differences on the colour (quality) and flavor of the lettuce samples grown during summer and winter seasons and unfortunately it was not done during autumn and spring seasons. Irrespective of the nutrient solution concentration, growth was influenced by growing season because plants grown during summer reached maturity quicker as compared to plants grown during the other seasons.