Germination control by a hard seed coat: insights from a tropical legume

Classifying seed dormancy is an essential task for plant propagation; however, several plant species lack information about the kind of dormancy the seeds have or inaccurate reports are passed on without an in-depth investigation. We investigated Copaifera langsdorffii, a widespread tropical species with several contrasting reports about seed dormancy in the literature, particularly on the role of their hard seed coat on germination control. The effect of aril (seed appendage related to the prevention of germination) and dormancy-break treatments on germination were evaluated. Seed coat permeability and the role of seed size and aril on imbibition have been investigated. Seed drying and storage were carried out to investigate a possible acquisition of dormancy. The influence of aril and seed scarification on seedling emergence was also investigated. The hard seed coat has juxtaposed palisade cells, a similar feature found in seeds with physical dormancy (PY). However, intact seeds had high germination (> 70%). Seeds had a slow imbibition pattern but did not prevent it. The aril hastens imbibition, but the seed size did not affect water uptake. Hilar region is the main permeable part of the seed coat, since the dye only enters the seed in this region. Reducing seed water content or storage did not make the seeds water-impermeable. The presence of aril or scarification decreased seedling emergence. Although PY is common in leguminous trees from seasonal tropical areas, it is not present in this species. This non-dormant seed has a main permeable area in the hilar region, which controls imbibition but does not prevent it. Low germination in arillated C. langsdorffii seeds is due to high seed death caused by fungi, not an imposed dormancy. The hard coat controls water imbibition and regulate germination timing in this tropical species.