Yield and quality of greenhouse multi-leaf lettuce cultivars grown in soil and soilless culture under Mediterranean conditions

Multi-leaf lettuce has been proposed as a new type of product to be grown in open field or protected cultivation, especially for ready-to-eat salads. Like Batavia, oak leaf or lollo, multi-leaf lettuce is more attractive in characteristics such as size, colour, texture, but has smaller, more uniform leaves attached in a single point at the base. In the current research, we evaluated yield and quality of three multi-leaf lettuce cultivars (Ezra, Ezabel and Eztoril), in both autumn-winter and winter-spring cycles, by comparing soilless versus soil cultivation, and within soilless by comparing the supply of 50% nitrogen as ammonium form instead of sole nitrate fertilization. Soilless cultivation improved crop yield by about 20%, but only with the limiting environmental conditions of the first cycle. Among cultivars, Ezra always presented taller leaves than the others. Multi-leaf lettuce had good ammonium tolerance, never showing symptoms of toxicity. Ammonium nitrogen supply caused a slight increase in dry matter content, but only at the spring harvests. Simultaneously, it was responsible for a lower nitrate content, compared to nitrate-fed plants (-11 and -30%, respectively in first and second cycle). Generally, the nitrate content was quite low (2,470 and 1,000 mg kg-1 fresh weight, respectively in the two cycles), considering that it was a protected cultivation in winter or winter-spring. Under the operating conditions of our experiment, the influence of soil cultivation on dry matter and nitrate content was variable in relation to the cultivars. Mixed ammonium nutrition tended to increase the chlorophyll content in soilless-grown lettuce, only in the autumn-winter cycle, but this did not cause colour changes.