MATURATION AND DESICCATION SENSITIVITY IN DEVELOPING SEEDS OF LETTUCE (LACTUCA SATIVA L.)

Onsets of germination and desiccation tolerance in developing seeds were determined in greenhouse-grown lettuce plants. Lettuce ‘Tango’ were planted and maintained in the greenhouse throughout its growing period. Flowers were sampled every day starting at 4 days after flowering (DAF). Seeds were manually extracted immediately from the collected flower heads after harvest and standard germination test was done. Moisture content (MC) determination was done on the freshly harvested seeds. Sample seeds from the freshly harvested flowers were also dried for 2 d in an activated desiccant (fast-drying) inside a glass desiccator at room temperature and inside a plastic germination box with saturated salt solution, sodium bromide (54% RH, slow-drying) at 25°C, to determine desiccation tolerance of the developing seeds at different stages of development. Rate of water loss in seeds during drying was also determined by measuring the seed weight loss each day during the 2 d period of drying. During drying, most water was lost (91-100% of the final seed moisture content) at first day of drying both in fast- and slow-dried seeds. Seeds dried in slow drying method lost 46-89% of its initial MC at harvest while fast-dried seeds lost 81-95% of its initial MC. The MC of seeds at different stages of development when dried over a desiccant ranged from 2.2-6.9% fresh weight basis. In slow drying, the seeds achieved equilibrium, that is, there is no change in seed weight, in the second day of drying. The moisture content of seeds after 2 d slow drying ranged between 5.7-13.9%. Seed sampling was stopped at 14 DAF wherein the flower heads at this stage are already dried and totally open. The onset of germination of freshly harvested was observed as early as 4 DAF with 98% total germination (45% normal germination). A 100-percent germination of normal seedlings was only achieved at 9 DAF, coinciding with the start of maturation drying in seeds. Maturation drying was observed at 9 DAF as indicated by the decrease in fresh weight of the seeds. The physiological maturity was achieved at 10 DAF. Tolerance to desiccation was only observed starting at 6 DAF for slow-dried seeds (3% germination of normal seedlings) and 7 DAF in fast-dried seeds (13% germination). A 100-percent germination of normal seedlings was achieved at 8 DAF for both drying methods. Slow drying of seeds in salt solutions has improved the vigor of seeds based on the speed of germination. However, drying in a dry desiccant has resulted to decrease in vigor. This could be due to rapid water loss during drying as indicated by low moisture content in seeds dried in fast drying than in slow drying method.