The present invention relates to lettuce plants (Lactuca sativa L.) comprising a genetic determinant, which when homozygously present in a seed, provides the seed in an unprimed state with the capability to germinate at a high temperature, which genetic determinant is obtainable from a lettuce plant comprising said genetic determinant, representative seed which was deposited with the NCIMB under accession number NCIMB 41914, NCIMB 41915, NCIMB 41916, NCIMB 41917, NCIMB 41918, NCIMB 41919, NCIMB 41922, NCIMB 41923, NCIMB 41926, and NCIMB 42216.