Differences in Inhibition of Emergence by High Temperature between Lettuce Cultivars

Abstract

Seeds of 14 lettuce cultivars for spring or summer cultivation were sown in cell trays filled with seeding mixture and kept in growth cabinets at 20, 30, or 35°C for six days to study emergence inhibition at high temperatures. All cultivars commenced emergence on day three after sowing at 20°C, on day two at 30°C, and on days two to five at 35°C. Emergence rates on the day of emergence (at 35°C, on the day when many cultivars began to emerge) and the day after, and the final emergence rate were 52–97%, 77–98%, and 81–100%, respectively, at 20°C; 7–66%, 25–87%, and 61–98%, respectively, at 30°C; and 0–43%, 0–50%, and 1–63%, respectively, at 35°C. All cultivars showed emergence inhibition at 30°C or higher but the degree of inhibition varied greatly between them. The upper temperature limit for emergence, T50, depended greatly on the cultivar, ranging from 31°C for ‘Olympia’ to higher than 35°C for ‘Château’, ‘King Cisco’, and ‘Sacramento’. Comparison of the present results with those of previous reports suggests that breeding efforts over recent decades have generally improved the performance of lettuce emergence at increased temperatures.