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Considerazioni generali sullo stato attuale delle aree verdi storiche della provincia di Siena.
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New perspectives in nectar evolution and ecology: simple alimentary reward or a complex multiorganism interaction?
Full text linkFloral and extra-floral nectars are secretions elaborated by specific organs (nectaries) that can be associated with plant reproductive structures (the so-called floral nectaries found only in angiosperms) or vegetative parts (extrafloral nectaries). These secretions are common in terrestrial vascular plants, especially angiosperms. Although gymnosperms do not seem to have true nectar, their ovular secretions may share evolutionary links with angiosperm nectar. Nectar is generally involved in interactions with animals and by virtue of its sugar and amino acid content, it has been considered a reward offered by plants to animals in exchange for benefits, mainly pollination and indirect defense against herbivores. These relationships are often cited as examples of classical mutualistic interactions. Nonetheless, recent studies dealing with compounds less abundant than sugars and amino acids challenge this view and suggest that nectar is much more complex than simply a reward in the form of food. Nectar proteins (nectarins) and nectar secondary compounds have no primary nutritious function but are involved in plant-animal relationships in other ways. Nectarins protect against proliferation of microorganisms and infection of plant tissues by pathogens. Nectar secondary compounds can be involved in modulating the behavior of nectar feeders, maximizing benefits for the plant. Nectar-dwelling microorganisms (mainly yeasts) were recently revealed to be a third partner in the scenario of plant-animal interactions mediated by nectar. There is evidence that yeast has a remarkable impact on nectar feeder behavior, although the effects on plant fitness have not yet been clearly assessed
Pollination, pollen viability and pistil receptivity in Cucurbita pepo
No full textCucurbita pepo carries male and female flowers on the same plant, and is pollinated by nectar-collecting bees. The nectaries are dimorphic in the two sexes and pollen is loaded and unloaded as the bees gain access to the nectar. Both types of flower are open for only 6 h (from 0600 h to 1200 h); male flowers open and close half an hour earlier than female flowers. The latter produce more nectar and are visited more often by the bees than the male flowers. Pollen viability determined by fluorescein diacetate (fluorochromatic reaction) decreases by 20% during anthesis and more rapidly after the flower closes. This decrease is due to dehydration of the grain, especially around the pore where the intine is exposed. An unusual feature of this species is that the grains do not dehydrate before anther dehiscence. Female receptivity has two aspects, that of the stigma lasting 4 d, and that of the ovules lasting 2 d. The receptivity of the two sexes and the short period of anthesis are discussed in the light of the reproductive ecology of the species
Nectar: plant interface for complex interaction with biotic environment
No full textAngiosperms’ nectar, fl oral and extrafl oral, is a valuable energetic alimentary resource for a large variety of animals from insects to small mammals, birds, marsupials and reptiles. It frequently mediates mutualistic relationships between the two partners. In recent years it was clearly demonstrated that this relationships actually often involves other partners: nectar dwelling micro-organisms. Yeasts and bacteria
may alter considerably the nectar composition and can be the causal agents of some plant diseases. Nectar has biochemical defences to inhibit micro-organism proliferation, i.e., a heterogeneous arsenal of proteins with anti-fungal and anti-microbial activity that has just been discovered in the last years. Nonetheless yeasts are almost ubiquitous in
nectar and their direct and indirect interactions with nectar foragers are almost unknown as well as the consequences for plant reproduction. A recent advance in nectar biology was the recognition of some secondary compounds, especially alkaloids and non-protein amino acids, in modulating the foraging behaviour of nectar feeders through several
effects on insect neurophysiology. All these studies demonstrate that nectar has a wider range of more complex interactions than previously thought
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