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Early or late: defending their differences
Heterophylly, the possession of different types of leaf in one plant, is widespread within the angiosperms, and is frequently seen in the differences between juvenile and adult phases of plant growth. Sometimes the differences in leaf morphology relate to changing needs in respect of defence against herbivory, for example in holly (Ilex aquifolium). It is in relation to defence that Matsuki and colleagues (Sapporo, Japan, pp. 141-147) have examined the differences between early season and late season leaves in three species of birch, Betula ermanii, B. maximowicziana and B. platyphylla. They were interested especially in the competing needs in young leaves for defence against herbivory and for growth -- provision of defence mechanisms will use photosynthate that might otherwise contribute to tree growth. They therefore assayed both total phenolics and tannin (chemical defences) and counted trichomes and measured leaf toughness (mechanical defences) in early and late leaves of all three species. As indicated by phenolic and tannin content and by trichome numbers, early leaves of B. ermanii and B. platyphylla were more defended than later leaves. However, this did not seem to occur at the expense of growth since relative growth rates (RGR) were at their highest during the time of the early-season leaves. Indeed, in B. platyphylla removal of these early leaves restricted the overall growth that year. B. maximowicziana was different; its late leaves were more defended than its early leaves, both chemically and mechanically but especially the latter. Interestingly, RGR in this species remains more or less constant through the growing season leading the authors to suggest that there is no selective advantage in investing in defence of the early leaves. These differences in defence strategy may be related to differences in ecology because although all three species can act as pioneers, B. maximowicziana is also a species of mature mixed forest.
Professor J. A. BryantUniversity of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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