Polyphenols leaching and seed dormancy in carob (Ceratonia siliqua L.) in relation to hot water treatment

Carob (Ceratonia siliqua L.) is a relevant element of the Mediterranean spontaneous vegetation. Moreover, it is useful in reforestation, and it is currently re-valued for sustainable agriculture in dryland areas. However, the difficulties tied to carob propagation (mainly seed dormancy) hamper its large-scale cultivation. In this paper, the effects of four seed treatments (no treatment [control], soaking at 70 °C and 90 °C in water, or in 96% sulphuric acid) on five carob genotypes germination were studied. As compared to the very low germination of untreated seeds (0–13% germination), sulphuric acid (93–100% germination) and 90 °C water soaking (from 72 to > 90% germination in four out the five genotypes) were effective in promoting germination. Soaking at 90 °C resulted in the leaching of a higher amount of total polyphenols from the genotypes seed coat as compared to soaking at 70 °C. A significant correlation (0.75**) was ascertained between polyphenol leaching of the different genotypes and germination. These results suggest that dormancy in this species is not primarily associated with seed coat hardness, as it is generally thought, but also with the release of polyphenols. Polyphenols determination of the dormant and the few non-dormant seeds of the different genotypes also seem to confirm this hypothesis since these last showed an almost halved total polyphenols content (on average 17.0) as compared to dormant ones (34.8 mg g−1 of seed FW). Further studies may determine the polyphenols involved, but also assess new, easier to carry out, seed treatments. The important role of the galactomannans on seed germination of carob is also discussed. Finally, similar studies may enhance the knowledge of dormancy processes in other Fabaceae species whose germination is positively affected by hot water treatments.