Autumn leaves on
hybrid trees
What happens when a deciduous tree is crossed with an
evergreen? Do half the leaves fall off, do all the leaves
half-fall off, or does the habit of one parent predominate? A hybridization between two species with contrasting leaf behaviours has been described by Stecconi et
al. (Bariloche and Buenos Aires, Argentina, pp.
775–786) in the genus Nothofagus. Indeed,
the way by which the hybrids were discovered helps to answer our opening questions. The authors describe two species that occur in the Nahuel Huapi National Park. In the habitat under study, the
deciduous N. antarctica is a dominant species but there are some individuals of the evergreen N. dombeyi. The authors’ attention was caught by the occurrence during autumn of some individual trees showing a complete range of leaf behaviour: from complete abscission through partial senescence (indicated by autumn colouration without abscission), to leaves that remained green and attached. The authors thus suspected that the trees were hybrids. They have investigated this further by examining a range of features from leaf and floral morphology to isoenzyme patterns and by subjecting the data to
rigorous statistical analysis. In some of the characters investigated, the hybrids exhibit a complete range between the two parents. These include leaf behaviour (as already mentioned), certain aspects of leaf morphology and the number of flowers in staminate inflorescences. In other morphological traits, the hybrids resemble one or other parent, whilst for the informative isozymes, the putative hybrids are clearly heterozygous. The authors conclude that these individuals are indeed hybrids, probably F1. The distribution of the hybrids in relation to individual N. dombeyi trees suggests that the latter is the male parent. The authors discuss their results in relation to the evolution of Nothofagus. For my part, I would love
to know the mechanisms that enable different leaves on the same tree to respond differently to the same environmental cues.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
