The objective of this work was to evaluate the effects of salt stress and competition on the development and growth of lettuce plants (Lactuca sativa var. iceberg). The plants were grown under greenhouse conditions with 50% interception of solar radiation. Iceberg lettuce seedlings were transplanted to 8-liter pots with surface area of 4.9 dm2 filled with 5 kg of a substrate composed of soil, sand, and manure at the ratio of 3:1:1 v v-1. The experiment was set up in a completely randomized

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... n with a 6 × 2 factorial arrangement (six salinity levels and one or two plants per pot), and three replications. An electrical conductivity meter was used to determine the salinity level; NaCl was added to the water until reaching 0, 2, 4, 6, 8, and 10 dS m-1. The lettuce plants were sensitive to salt stress; however, the plants can be irrigated with brackish water with electrical conductivity equal to or lower than 2 dS m-1 without significant decreases in shoot fresh weight, therefore meeting the social demand for use low quality water in agriculture. The morphophysiological plasticity of lettuce plants increases the competitive potential of plants in high density crops, with one plant per 2.45 dm2