Influence of temperature on the Pfr level required for germination in lettuce cv. Grand Rapids

The effect of temperature on the Pfr level required to induce germination in lettuce cv Grand Rapids was investigated Various Pfr levels were established in seeds, by irradiation with different fluences of red light, or by saturating irradiations with narrow bandwidth light Both approaches indicated that increasing Pfr levels caused a progressive increase in the upper temperature limit for germination The Pfr level required to induce 50% germination after a single light pulse increased from about 11% at 15 degrees and 20 degrees C to 86% at 30 5 degrees C Repeated red light irradiations raised the GT50 (temperature giving 50% germination) by about 2 degrees C, suggesting that Pfr reversion limited germination at high temperatures However, a large increase in Pfr level required to induce 50% germination was still apparent at high temperatures, even when relatively constant levels of Pfr were maintained by repeated irradiations with narrow bandwidth light These results indicate that following a single light pulse, germination at high temperatures is limited solely by the availability of Pfr The precise position of the upper temperature limit for germination is influenced by reversion, however, the underlying cause of thermoinhibition relates to an increasing requirement for Pfr at elevated temperatures