SUMMARY The effect of reduction in incident solar radiation on the lettuce growth in the Subtropical Central Region of the Rio Grande do Sul State, Brazil, was evaluated during the summer. The study was carried out in the Experiment Field of the Crop Production Department of the Federal University of Santa Maria. Four 12.0m x 1.2m plots were used. Three were covered by tunnels of plastic screen with 18%, 30%, and 50% of solar radiation reduction and one was left uncovered as a control. The experiment was carried out during three periods: from 11.30.90 to 01.17.91 (period I), from 02.06.91 to 03.25.91 (period II), and from 12.10.91 to 03.01.92 (period III). Samples of lettuce plants were taken every seven days to evaluate stem and leaf dry matter, and leaf number throughout the three experimental periods. Stem length were measured only during period III. All growth parameters evaluated were higher inside the tunnels than outside. The treatment with 30% of solar radiation reduction showed the highest lettuce growth rate.
plastic screen; shading; lettuce
