Physiological analysis of methods to relieve dormancy of lettuce seeds

English. Low maximum temperature of germination restricts the cultivation of a number of lettuce cultivars to temperate conditions. Pretreatments have been studied that increase the maximum germination temperature, which is characterized by the temperature for 50% germination (T 50 ). Pretreatment consists of pre-incubation, redesiccation and storage. Pre- incubation during 16 to 20 h at 15 °C in water increases T 50 with 4 to 10 °C, depending on cultivar. Redesiccation does not cause visible damage to seeds, as judged from the developing seedling, provided that germination has not started during pre-incubation. In cultivars with a high original T 50 redesiccation does not affect T 50 . However, if the original T 50 is low, redesiccation causes a drop of T 50 to values close to the original T 50 value. The effect of redesiccation is mainly determined by the final moisture content. Temperature and rate of drying are of minor importance in this respect. Pre-incubation in solutions of polyethylene glycol makes the seeds desiccation tolerant. Dehydration results in increased K +leakage from isolated embryos, in contrast to intact seeds that do not leak due to K +-impermeable endosperm cells. T 50 decreases during storage. The decrease of T 50 and the loss of seed viability show a similar positive relationship to temperature and moisture content during storage. Besides the rise of T 50 , pre-incubation at 15 °C also enhances the capacity of seeds to germinate at increasingly more negative osmotic potentials of the incubation medium (Ψπe). The effect is characterized by the minimal Ψπe of the incubation medium for 50% germination (Ψ 50 ). Pre-incubation at 30 °C has an opposite effect. The change of Ψ 50 does not correspond with a change in concentration of amino nitrogen compounds. Both actual water potential (Ψ) and osmotic potential (Ψπ), as determined with a psychrometer, are not correlated with changing Ψ 50 values. Calculations based on the general equation of hydraulic cell growth indicate that changes in T 50 are correlated with parallel changes in the yield threshold (Y) of turgor pressure. Changes in Y appear to occur mainly in the seed envelope.

Dutch; Flemish. Dit onderzoek stelde zich tot doel een fysiologische analyse van voorbehandeling uit te voeren, gericht op het oplossen van de hierbij optredende problemen. Hierbij verdienen zowel de factoren aandacht die tijdens de verschillende onderdelen van de voorbehandeling van invloed zijn op het kiemrustniveau, als wel de fysiologische processen die de basis vormen van de kiemrust