Stigma and style anatomy and ultrastructure in Italian Oxalis pes-caprae L. and their possible connection with self-incompatibility

The South-African Oxalis pes-caprae with trimorphic flowers is naturalised in many Mediterranean countries. In Italy only short-styled (S) populations are known for certain and the plant is believed to reproduce only asexually, due to self- and intramorph-incompatibility. This study aims to clarify anatomical, biochemical and ultrastructural features of the stigma and style of an Italian S population, also to define their possible role in incompatibility. Additional observations were also carried out on other Italian population and on short-, long- (L) and mid-styled (M) flowers from plants of South African origin. Morphological and biochemical features of flowers collected in different phenological stages during the whole flowering season were observed both under LM and TEM. In S flowers, three different zones could be distinguished in each stigma-style complex: zone I (stigmatic), zone II (substigmatic), zone III (stylar). The main differences concern the transmitting tissue: in zones I and III this is composed of loosely arranged cells with thick walls, with an abundant soft matrix which at anthesis is rich in mucopolysaccharides and lacks pectins. In zone II, it is more compact, with a less abundant wall matrix, at anthesis containing both mucopolysaccharides and pectins. In S flowers, subjected to illegitimate pollination, many pollen tubes penetrate the stigmatic papillae but apparently are arrested in zone II; only few—and mostly at the end of flowering period—succeed reaching zone III, where they encounter no further resistance to growth. Differently, after legitimate pollinations, pollen tubes succeed in crossing transmitting tissue of zone II, where cell walls of cells lying close to pollen tubes show a considerably reduced pectin content. In L and M flowers of South African origin, no peculiar transmitting tissue could be noticed in the substigmatic zone. In such flowers, pollen tubes seemingly grow easily from stigma to style both after legitimate and illegitimate pollinations. Results suggest that in S flowers the rigid transmitting tissue in zone II acts as a mechanical barrier for illegitimate pollen tubes, as the spaces between cells are narrower than the tube diameter and pectins maintain the rigidity of the cell walls, preventing cells from separating from one another. This obstacle can be overcome by legitimate pollen tubes, which make their way between cells, possibly releasing or activating specific pectinases. However, the blocking of illegitimate tubes is not absolute: a few of such tubes grow beyond zone II and reach the ovules, so that occasional fertilisation and embryo formation can be observed. In M and L flowers, different self-incompatibility mechanisms can be hypothesised.