SummaryConsiderable diversity in genes controlling leaf orientation in P. lanceolata has been demonstrated in a population growing in the Sydney area. The expression of these genes is greatly influenced by both light intensity and temperature; low light intensity and high temperature induces erect leaf orientation, while conversely high light intensity and low temperature favours the formation of prostrate leaves. Gibberellin apparently blocks the expression of the genes for prostrate leaf orientation. It is suggested that the seasonal variation in air temperature may be the climatic factor chiefly responsible for the disappearance of prostrate leaf orientation in midsummer and for the reappearance of this character in autumn and winter.

Wild Callery pear (Pyrus calleryana Decne.) results from a cross between various cultivars of P. calleryana and any other Pyrus individual. While many cultivars of this species are still commercially produced and sold for horticultural purposes in the United States, Callery pear is a detrimental invasive species that encroaches on many managed and natural areas, damages equipment and injures people, pets, and livestock with its thorny branches, and likely causes detrimental ecological impacts. Despite its importance as an invasive species, the mechanisms behind Callery pear’s invasion and spread are unclear. To identify potential drivers of invasion, we quantified feeding of generalist and specialist herbivores on Callery pear and four native tree species, based on insect host ranges, with choice and no-choice experiments followed by field surveys of herbivory on these same tree species. Feeding by all herbivores was lower on Callery pear than on native tree species in no-choice assays. Specifically, feeding on Callery pear was moderate by generalists and very low by specialists. Specialist feeding on Callery pear was comparable to native species in choice assays but was significantly reduced in no-choice assays. Reduced specialist feeding along with moderate generalist feeding on Callery pear in the field provides evidence for the Enemy Release Hypothesis as a potential driving mechanism behind its invasion success.