Thermotolerance in lettuce seeds:: Association with ethylene and endo-β-mannanase

Weakening of the endosperm tissue around the radicle tip before radicle protrusion and a potential role of endo-beta-mannanase during germination of lettuce seeds (Lactuca sativa L.) at high temperature (35 degrees C) were investigated. Seeds from the thermotolerant genotypes 'Everglades' and PI 251245 had greater endo-beta-mannanase activity before radicle protrusion at 35 degrees C than the thermosensitive genotypes 'Dark Green Boston', 'Valmaine' and 'Floricos 83', Thermotolerant genotypes also generated more ethylene at high temperature. At 35 degrees C, germination of 'Dark Green Boston' and 'Everglades' seeds produced at days/nights of 20/10 degrees C was 10 % and 32 %, respectively, whereas germination of seeds produced at days/nights of 30/20 degrees C was 67% and 83%, respectively. Higher endo-beta-mannanase activity was observed before radicle protrusion in 'Dark Green Boston' seeds produced at 30/20 degrees C compared with those produced at 20/10 degrees C. A relationship between seed germination at high temperature, ethylene production, and an increase in endo-beta-mannanase activity before radicle protrusion was confirmed.