Germination constraints of dicarpic cypselae of Bidens pilosa L.

Abstract Bidens pilosa L. is a heterocarpic weed species with two cypselae types that present morpho-physiological differences, being the peripheral type smaller and slower to germinate than the central one. We aimed to verify how the germination mechanism varied between types. We focused on two mechanisms: (1) pericarp constraints (physical and chemical) and (2) hormonal stimulation (Abcisic acid [ABA] and Gibberellin [GA]). Both cypselae types are physically constrained by the pericarp, for when it is excised both seed types increase their germination, but behavioral differences still remain. The pericarp of the peripheral type also has chemical inhibitors that effectively inhibited germination of the intact central cypsela. To test the hormonal effects, we focused on the ABA:GA control. Both cypselae responded to an exogenous ABA concentration gradient, however there is no variation between types on the sensitivity to it. Also, both cypselae types were indifferent to Fluridone (ABA inhibitor), which indicates that the dormancy is not maintained by de novo ABA synthesis. Cypselae types had different sensitivity to an exogenous GA3 gradient, the central type being more sensitive to the treatment than the peripheral one. But when the endogenous GA synthesis was blocked by Paclobutrazol, both types responded equally to same GA3 concentrations. This indicates that endogenous GA synthesis may be related to differences observed on germination of cypselae types. To conclude, seed types differ on their growth potential to overcome the pericarp resistance: while the inhibitor in the peripheral pericarp reduces growth potential, GA increases it.

摘要 三叶鬼针草（Bidens pilosa L.）是一种具异形瘦果（cypselae）的杂草物种，其两类瘦果存在形态与生理差异：外围型瘦果（peripheral type）相较于中央型瘦果（central type）体积更小且萌发速率更缓。本研究旨在明确两类瘦果的萌发机制差异，重点探讨两大调控途径：（1）果皮的物理与化学约束效应；（2）激素调控（脱落酸[Abscisic acid, ABA]与赤霉素[Gibberellin, GA]）。研究发现，两类瘦果均受到果皮的物理约束：剥离果皮后，两类瘦果的萌发率均显著提升，但二者的萌发行为差异仍持续存在。此外，外围型瘦果的果皮中含有化学抑制物质，可有效抑制完整中央型瘦果的萌发。为验证激素调控效应，本研究聚焦于ABA与GA的浓度平衡。两类瘦果均对外源施加的ABA浓度梯度产生响应，但二者对ABA的敏感性并无显著差异。同时，两类瘦果均对氟啶酮（Fluridone，ABA合成抑制剂）无响应，这提示种子休眠并非由从头合成的ABA所维持。两类瘦果对外源GA₃浓度梯度的敏感性存在显著差异：中央型瘦果对该处理的敏感性高于外围型瘦果。但当内源GA合成被多效唑（Paclobutrazol）阻断后，两类瘦果对相同浓度GA₃的响应趋于一致。上述结果表明，内源GA合成可能与两类瘦果萌发的表型差异相关。综上，两类瘦果在突破果皮阻力的生长潜能上存在差异：外围型瘦果果皮中的抑制物质会降低其生长潜能，而GA则可提升该潜能。