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Pollen participates in fruity features
Several ‘vine-cacti’ are now grown in different parts of the world as commercial crops. In Israel, vine cacti are cropped despite the absence of natural pollinators and the need for hand pollination since the species in question is an obligate outbreeder. This inconvenience has spurred Mizrahi et al. (Ben-Gurion University, Beer-Sheva, Israel; pp. 469-472) to study the effects of pollen source on fruit development. The major vine-cactus genera grown in Israel are Hylocereus and Selenicereus. Amongst the cultivated species, there are no cross-fertilization barriers within each genus, or between the two genera. Taking pollination of H. polyrhizus as an example, the authors’ data clearly show that other Hylocereus species and S. grandiflorus are equally effective pollinators: stigmatic receptivity and pollen tube growth rates are very similar. However, there are obvious effects of the source of pollen on fruit development. This is particularly apparent in the different growth and ripening rates of fruit following fertilization by pollen of different species, even when the fruit are growing on the same plant. Thus, for both H. polyrhizus and S. grandiflorus, pollination by H. undatus leads to earlier ripening but smaller final fruit size than pollination by S. grandiflorus; the fruit ripening characteristics tending to mimic those of the pollen parent. However, these effects of pollen source are manifest in tissues that are totally maternal in origin: the male parent does not contribute any genetic material to parts of the fruit that show these effects. This is therefore an example of the phenomenon known as metaxenia. The mechanisms involved in metaxenia, which has been described only for a handful of other genera, are unknown. Nevertheless, the authors note that this phenomenon may be commercially useful since it makes it the possible deliberately to manipulate the time of fruit ripening as a mean of regulating the supply of fruit for the market.
Professor J. A. BryantUniversity of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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