Red and Far-red Light Treatments to Modify Thermoinhibition, Photoblasticity, and Longevity in Lettuce Seeds

Germination in lettuce (Lactuca sativa) seeds can be inhibited by high temperatures (thermoinhibition) or darkness (positive photoblasticity). Priming is commonly used as a seed treatment to avoid these problems. However, this treatment is complicated, expensive and has detrimental effect on seed longevity. The objectives of this study were to 1) develop a simple alternative treatment to priming, based on red light irradiations and 2) to develop a treatment to extend seed longevity. Lettuce seeds from cultivars Tango, Milanesa, Ideal Cos, and Gallega de Invierno were partially hydrated in containers with 98% relative humidity (RH) and irradiated with red (R) light for 24 hours, or far-red (FR) light for 216 hours. Throughout the treatment, seeds reached 16% water content and, once finished, they were dried with air at 30 degrees C. Untreated (control) and primed seeds were also evaluated. Germination was evaluated in light and darkness at temperatures between 12 and 36 degrees C in a thermogradient table. Effects in longevity were estimated by evaluating germination after various aging periods at 45 degrees C and 75% RH. The R-light treatment improved germination in the dark at temperatures up to 25 degrees C, but did not have a significant effect on seed germination under light. Germination after accelerated aging showed that R- and FR- light treatments extended seed longevity when compared with control seeds. Therefore, this effect would not be associated to photoblasticity alleviation or imposition. Primed seeds deteriorated faster than the control. Compared with priming, the R-light treatment was simpler and improved seed longevity. However, priming effects on alleviation of seed photoblasticity and thermoinhibition at temperatures over 25 degrees C were greater.