Intermorph structural differences between stigmatic papillae and pollen grains in relation to incompatibility in Plumbaginaceae

Abstract

In dimorphic Plumbaginaceae the floral morphs differ in the wall structure at the apical region of the stigmatic papilla. In Ceratostigma willmottianum , Plumbago europaea and P. capensis , papillae of the long-styled and short-styled morphs differ in the way the cuticle is attached to the cellulose layer. In the short-styled morph of P. capensis , the cuticle of the papillae has minute protuberances. The ‘cob’ and ‘papillate’ stigmas of dimorphic Limonium species and of Armeria maritima are distinguished by the thickness of the cuticle layer at the papilla apex. In all species examined, intramorph pollinations are incompatible and inhibition of the pollen occurs at the stigma surface. Structural stigma dimorphism is probably involved in the incompatibility mechanism. In Limonium meyeri the stigma dimorphism controls the incompatibility at least at the stage of adherence of the pollen to the stigmas. Pollen grains of type A adhere to both types of stigmas; pollen of type B adheres to cob stigmas, but usually does not adhere to papillate stigmas. It is possible that a chemical ‘recognition’ is reinforced by a topographical complementarity between the outline of the stigma surface and of the pollen exine. It is suggested that the exine dimorphism may play a role in recognition by means of a different distribution of incompatibility substances held in the pollen wall. The view that the stigma and pollen dimorphism reflects the incompatibility mechanism is considered in relation to various breeding systems known in the Staticeae.