Effects of 35 C Heat Treatments on Photosensitive Grand Rapids Lettuce Seed Germination

Abstract

Grand Rapids lettuce (Lactuca sativa L.) seeds were given 35 C heat treatments to increase photodormancy in a subsequent 20 C dark period. Short heat treatments (1-5 hours) induced a significant germination percentage increase of from 16% to over 50% depending on seed lot. With longer heat treatments dark germination percentage was gradually reduced to zero. If given at the end of 35 C, far red or red followed by far red further increased the amount of dark germination.Thermodormancy also delayed red-stimulated germination by 10 hours or more when red was given following a long 35 C treatment. The presence of Pfr was required during this time since far red light remained effective in reversing at least 50% of the red stimulation for up to 16 hours compared to only 4 hours in nonheat-treated seeds.Grand Rapids lettuce seeds (Lactuca sativa L.) are typically photosensitive and exhibit the classical phytochrome photoreversible response (4,5,11). Heat treatments (30-37 C) have been performed on imbibed seeds (7,9,10,14,16,19) inducing a thermodormancy in which dark germination percentage is reduced to zero in both photodormant and nonphotodormant seeds. This phenomenon is typically attributed to a conversion of Pfr to Pr (10,16,19). Thermodormancy may be completely eliminated by R2 at 20 C or, in some cases, by a combination of CO2 + ethylene (7,14).Heat treatments are being used routinely to drive dark germination percentage to zero in many studies of photodormant seed germination. This report examines the changes in the germination potential during heat treatments, as well as the effective response to R and FR after heat treatments. MATERIALS AND METHODSSeed Source. Grand Rapids lettuce seeds (Lactuca sativa L.)were used in all experiments. Lots 670-C, 671-C, and 672-C were obtained from Rocky Mountain Seed Co., Denver, Colo. Lot 12310-18639 was obtained from Ferry-Morse Seed Co., Mt. View, Calif. Seeds were stored at -20 C to maintain light sensitivity and viability.Imbibition. All sample sizes were about 200 seeds. Seeds were placed in moistened filter paper-lined 9-cm glass Petri dishes, wrapped in a double thickness of heavy duty aluminum foil, and placed in covered metal containers. The containers of seeds were then put into appropriate 35 C or 20 C growth chambers.Light Sylvania 150-w soft white flood lamps underlaid with a 3 mm thickness of Rohm and Haas 2444 R Plexiglas having a peak transmission at 680 nm. The FR source consisted of four General Electric 150-w clear reflector flood lamps underlaid with a 3 mm thickness of Rohm and Haas V-58015 "black" Plexiglas. Lamps and filters were separated by a water heat trap such that the temperature change within the chamber during the treatments did not exceed 1.2 C. Transmission spectra of R and FR sources were obtained by an ISCO, Model SR, spectroradiometer and calibrated against an ISCO, Model SRC, spectroradiometer calibrator standard lamp. The FR transmission spectrum of the phytochrome-sensitive range is given in Figure 1.Light treatments consisted...

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