Improvement of germinability of lettuce seeds with drum-priming under high temperature condition

Abstract

Drum-priming is an attractive technique for improving seed germination and can be performed on a bulk scale owing to its simplicity. This study investigated the cause of germination inhibition at high temperature and identified the optimal drum-priming conditions of lettuce seeds for uniform seedling growth in high temperature conditions. Using seeds of 'Hwahong,' the most thermosensitive lettuce cultivar, we demonstrated that endosperm acts as a mechanical barrier to inhibit germination at high temperature (30 degrees C). Furthermore, we provided evidence for the beneficial effect of the combination of plant growth regulators (PGRs) in the drum-priming solution on seed germination. Treatment of seeds with the PGRs gibberellic acid (GA(3)) and benzylaminopurine (BA) enhanced the germination of the lettuce seeds with longitudinal cut, which reached the maximum at 30 degrees C, and the combination of GA(3) (1 mg.L-1), BA (1 mg.L-1), and ethylene (390 mg.L-1) at 8 degrees C for 3 days was the optimal condition for drum-priming. We also explored the effect of drum-priming in the presence of a combination of PGRs on endo-beta-mannanase (EBM) production and on the structure of the lettuce seeds at high temperature and showed that in contrast to non-primed seeds, drum-priming with a combination of PGRs promoted EBM production even before germination and increased the EBM activity in primed seeds. The results indicated that drum-priming with a combination of PGRs resulted in condensation of the cytoplasm of the endosperm cells and weakening of the structure of the micropylar endosperm. In conclusion, drum-priming with a combination of PGRs led to high and rapid germination of lettuce seeds under high-temperature conditions by increasing metabolic enzyme production and decreasing mechanical resistance.