Fine Structure and Cytochemistry of the Stigma Surface and Incompatibility in some Distylous Linum Species

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Abstract
The receptive surface of the stigma in distylous Linum grandiflorum and L. pubescens was studied by electron microscopy and cytochemical techniques. In both floral morphs a proteinaceous-lipoidal coating is present on the papilla surface. In thrum stigmas the cuticle is highly irregular and pitted at the papilla tip. The cuticle is dislodged and torn at anthesis and an osmiophilic secretion product is released within a pectinaceous matrix. The secretion product stains for proteins and lipids and contributes to adhesion of pollen. In the larger pin papillae the cuticle is wavy, continuous, thicker than in thrum papillae and adjoins the cell wall. In both species the surface of the two types of pollen grains is coated with lipids and protein. A similar behaviour of the male gametophyte is observed in incompatible pollinations of L. pubescens, L. mucronatum and L. grandiflorum. In intramorph thrum pollinations pollen tubes are arrested within the stigma. In intramorph pin pollinations the majority of pollen grains fail to adhere to the stigma. Low permeability to water in pin papillae, as determined by the neutral red test, may be a factor preventing imbibition of the few adhering grains. Tubes of the few germinated grains are inhibited inside the stigma. On the part of the stigma, the difference in the major site of inhibition in the two intramorph incompatible combinations may be accounted for by the dissimilar properties of the papillae, i.e. the occurrence of wet thrum stigmas and dry pin stigmas. Functionally, the unusual association of sporophytic incompatibility with wet thrum stigmas is attributed to retention of the secretory material on individual papillae.