Hormonal profile and the role of cell expansion in the germination control of Cerrado biome palm seeds.

Little information is currently available concerning the mechanisms controlling palm seed germination. We compared the anatomical and physiological aspects of seeds of two neotropical palm species showing different levels of dormancy. The seeds of Attalea vitrivir and Butia capitata were evaluated for the endogenous contents of hormones (ABA, GAs, CKs, BRs, IAA, JA, SA and the ethylene precursor ACC) in their cotyledonary petiole and operculum (structures involved in germination control), the force necessary to displace the operculum, endo-β-mannanase activities, and embryo cell elongation. The analyses were carried out on with intact dry and imbibed seeds as well as with seeds with the operculum mechanically removed, 2, 5 and 10 days after sowing. The germinabilities of the intact seeds of A. vitrivir and B. capitata were 68% and 3%, respectively; the removal of the operculum increased germination to more than 90% in both species. Reductions of ABA and increases in GAs contents coincided with cell elongation, although there is no evidence that hormonal balance and endo-β-mannanase activity are involved in operculum weakening. The ratio between the embryo length and the force required for operculum displacement (EL/OF) was found to be 1.9 times greater in A. vitrivir than in B. capitata, which means that very small elongations in each cell would be sufficient to promote germination, resulting in a lower level of dormancy in the former species. EL/OF and cell growth control are therefore important for defining dormancy level in palm seeds.