Lettuce cultivars selected for slow bolting and unselected ones of the same groups were analyzed for morphological variations, to quantify possible adaptative modifications for the cultivation conditions in heat seasons. Cultivars of the groups "butterhead" (Regina, Glória, IAC 303, IAC 202, Sem Rival and Luciana), "leaf-lettuce" (Grand Rapids and Brisa), and "crisphead" (Great Lakes and Mesa 659) were compared in two summer seasons, in Piracicaba, Brazil, in randomized blocks design. In the first (November/1990) were analyzed the characteristics of stomata number, leaf thickness and amount of chlorophyl of the whole plants. In the second (September/1991), were analyzed the characteristics of number of leaves, and dry and fresh weight of the whole plants. Cultivars selected for heat tolerance in the "butterhead" group (Gloria and Regina), presented larger stomata number (respectively 9,487/cm² and 7,973/cm²) and thicker leaves (respectively 556 mm e 439 mm) and, also, they accumulated larger amount of dry matter (respectively 24.55 g and 25.50 g), Cultivars selected for heat of the group "leaf-lettuce" (Brisa), accumulated larger amount of green matter (446.77g) and dry ones (22.40 g), in the same way that presented larger stomata number (7,279/cm²). For the cultivars of the group "crisphead", significant difference was just observed for leaf thickness; cv. Mesa 659 presented thicker leaves (589 mm). Significant increases of vegetal biomass were verified in cultivars selected for heat in relation to the unselected ones. Cultivars that reached larger dry matter yield were Mesa 659 (28.74 g), Great Lakes (27.17 g) and Regina. The results checked that the slow bolting selection indirectly produced adaptative variations in the lettuce plants.
Lactuca sativa L.; breeding; stomata; chlorophyll; dry weight; fresh weight; variety groups; slow bolting
