Incompatibility in forest trees

Abstract

Despite the great importance for forest tree breeding, very limited knowledge is yet available about the breeding systems of forest trees. Where incompatibility has been studied in the hardwoods; patterns have been observed which confirm the general rules detected for other angiosperms. Self- and interspecific incompatibility at the level of pollen tube growth has been reported for example in Betula and Alnus . In Alnus one case of unilateral interspecific incompatibility has been found. Self-incompatibility has, so far, not been reported from the conifers. Inter-specific incompatibility in the form of the arrestment of the pollen tube growth in the nucellus tissue has been observed in Picea and is particularly clear in Pinus crosses between the subgenera and Haploxylon and Diploxylon , but also within the Diploxylon -group. The nature of the incompatibility mechanism is still unknown, but serological differences related to the behaviour in the crosses has been detected in birch and pine pollen. It is suggested that the complex polysaccharidic composition of the cell walls and membranes might form a specific stereochemical basis for the incompatibility reaction. The presence of a combination of self-pollination, polyembryony and genetic load is discussed as an alternative mechanism favouring outbreeding in the Gymnosperms.