The Use of a Photoselective Film during Lettuce Seed Production for Improving Germinability

Previous studies suggested that modifying red to far red (R:FR) ratio of maternal environment during lettuce seed development represents a practical alternative to reduce thermoinhibition (reduction of germination at high temperatures) and photoblasticity (light requirement for germination) in seeds. The objective of this work was to evaluate the effect on seed quality of using a photoselective polyethylene film (PPF) during lettuce seed production. Seed of three lettuce cultivars (Tango, Pyramid Cos, and Gallega de Invierno) was produced 1.44 m²cages representing one of these treatments: i) control, covered with conventional polyethylene film; b) PPF, covered with a PPF (Solatrol®, bpi.visqueen Horticultural Products); and c) PPF+FL, covered with the same PPF plus permanent fluorescent light. Inside the cages, the R:FR ratio during the day averaged 1.0 for the control and 1.5 for the PPF and PPF+FL treatments. Additionally, during the night the PPF+FL treatment has a R:FR ratio of 3.2. Photosynthetically active radiation varied among days and time hours, but was similar for the three treatments. Plants of each cultivar were cultivated in pots filled with a soilless media, watered daily and fertilized once per week. At bolting, three plants per cultivar were randomly assigned to a cage of one of the treatments. There were 6 replications (cages) per treatment in a completely randomized design. Seed harvest was performed manually. Seed germination was evaluated at 20, 25 and 30ºC in light and darkness. At 20ºC and 25ºC (light), seed from all cultivars germinated near 100% and there were no significant difference among treatments. However, at 20ºC (dark), seed from Tango presented a significant reduction in germination (5.4%), problem that was alleviated by PPF and PPF+FL treatments (41.9 and 67.5% germination, respectively). At 25ºC (dark), seed from all cultivars presented a significant reduction in germination (lower than 3% for control treatments), but photoblasticity was reduced in seed from PPF (10% to 21% germination) and PPF+FL (18% to 31% germination) treatments. At 30ºC (light) only Tango seed had germination over 95%, with no difference among treatments; in Gallega, seed from PPF and PPF+FL treatments germinated around 50%, while the control did 29%. Pyramid Cos germination at 30ºC (dark) was between 4.7% (control) and 13.7% (PPF+FL), but differences were not significant. At 30ºC (dark), seed from all cultivars and treatments germinated no more than 5%; the only exception was seed from the PPF+FL treatment in Tango, where germination was 24.3%.