Annals of Botany

Poor productivity points to pollination problems

In considering endangered species, one of the factors we need to know about is reproductive efficiency. An understanding of this will help in conservation management, informing us of the potential for an increasing or a decreasing or a static population. Thus, Hirayama et al. (Kyoto and Nagaya, Japan, pp. 1009–1015) have examined the role of pollen supply and quality in the endangered Japanese species Magnolia stellata. This insect-pollinated tree has abundant large and showy flowers; the flowers are hermaphrodites, but in an individual flower the stigma ceases to be receptive before the pollen is released. Selfing within an individual flower is thus not possible, but selfing between different flowers on the same tree certainly is. The authors ascertained, in hand-pollination experiments, that M. stellata is self-fertile but in self-pollinated flowers, seed set and ovule survival were much lower than in cross-pollinated flowers. This was not due to any self-incompatibility barriers, either early or late acting, and was therefore likely to be caused by inbreeding depression. They then investigated seed set in naturally pollinated trees; the extent of selfing being determined by investigating the inheritance of particular genetic markers. The most obvious feature of these data is that ovule survival in natural pollinations was very low, much lower even than that seen in self-pollinations performed by hand. The authors attribute this to a limited availability of pollen, which reduces the overall potential for pollination, combined with the level of inbreeding depression calculated for the rate of selfing deduced from study of the genetic markers. Breeding efficiency is thus affected both by the quantity and the quality of the pollen (self-pollen having a lower quality than non-self because of inbreeding depression). What is now needed is a study of the effects of pollinator populations and of tree density and population size on pollination efficiency in this species.