Nectaries

It Is generally conceded that floral nectaries evolved as an attractant to pollinating Insects. The function of extrafloral nectaries Is not so obvious. Some suggest that the secretion of sugars Is associated with a shift from a "sink" to a "source of carbohydrates during development (, . Others propose that... 

Attack by sucking insects often increases the rate of secretion in a positively correlated manner with Increased infestation levels. Bentley argues that nectaries frequently remain a carbohydrate source until maturity, while developing buds, flowers, and fruit never become a carbohydrate source sucking Insects reduce the carbohydrate source pressure of an organ and yet nectar secretions Increase with increasing infestations. Thus, the source/slnk hypothesis does not seem to apply. 

Extrafloral nectaries (Fig. 2.1) are probably the most frequently described adaptations believed to serve as indirect defenses. They have been described in approximately 1000 species from 93 plant families including numerous dicotyledonous species, ferns, and such diverse monocotyledonous taxa as lilies, orchids, sedges, and grasses (Koptur, 1992). They are found in virtually all plant types including herbs, vines, shrubs and trees, annuals as well as perennials, and successional as well as climax species. 

Defense is a further category in which plants employ food rewards to acquire protection by arthropod mutualists. The provision of food sources allows plants to recruit or sustain predators or parasitoids, which, in turn, can provide protection against herbivory. The plant-derived food structures involved in indirect defensive interaction can be divided in two main groups food bodies and extrafloral nectaries. 

Extrafloral nectaries include a wide range of nectar-excreting structures, which are distinguished from their floral counterparts by the fact that they are not involved in pollination. Extrafloral nectar is typically dominated by sucrose and its hexose components glucose and fructose. The fact that these common sugars are acceptable to the majority of insects, combined with the exposed nature of extrafloral nectaries, makes them suitable food sources for abroad range of insects. Compared with floral nectar, extrafloral nectar often has increased fructose and glucose levels (Tanowitz... 

Extrafloral nectar production can be raised in response to both nectar removal (Koptur, 1992 Heil et al, 2000) and tissue damage. Stephenson (1982), using Catalpa speciosa, was the first to investigate the latter mechanism. He diluted the nectar of individual nectaries with water and demonstrated that the diluted nectar collected from sphingid-damaged leaves was richer in solutes compared with nectar collected from undamaged leaves. Smith et al (1990) point out that this does not resolve whether C. speciosa actually increased its nectar volume or whether it produced the same volume with an increased solute concentration. 

All these examples focus on the temporal aspect of nectar induction. In addition, extrafloral nectaries are also especially suited for the study of spatial dynamics following induction. This aspect can be easily assessed because of the discrete distribution of nectaries, the possibility of non-destructive sampling, as well as the ease of nectar collection. With respect to the spatial pattern of induction, Wackers et al. (2001) showed that the impact of herbivory on extrafloral nectar induction is primarily localized (i.e., restricted to the damaged leaf). This local increase in nectar production can help in actively guiding ants to the site of attack. In addition, a weaker systemic response was found. This systemic induction was restricted to the younger leaves. 

Direct costs further include the costs involved in active nectar sequestration, as well as the cost involved in producing the nectary. This latter cost is probably low, as nectaries are often simple and small, showing little differentiation. In other types of defense, costs relating to biosynthesis, transport, and storage (i.e., autotoxicity) can... 

The sophisticated equipment required for volatile identification has long confined the topic of herbivore-induced volatiles to the laboratory, but extrafloral nectaries have traditionally been studied in the field. Moreover, the work on extrafloral nectaries has mainly addressed wild plant species within their natural habitat, whereas the study of plant volatiles has long focussed on agricultural crops. As a result, we have a relative wealth of held evidence for the defensive function of extrafloral nectaries. 

Using exclusion experiments, several studies were able to demonstrate that ants effectively protect the plant against herbivory (O Dowd and Catchpole, 1983 Wagner, 1997 but see O Dowd and Catchpole, 1983 Rico-Gray and Thien, 1989). In the same way, reduction of herbivory has recently been demonstrated in mutualisms between extrafloral nectaries and spiders (Ruhren and Handel, 1999), as well as predatory wasps (V. Rico-Gray, personal communication). 

A number of studies have provided the ultimate proof for the defensive function of extrafloral nectaries by demonstrating that herbivory reduction by ants actually translates to an increased reproductive fitness of nectar-providing plants (Koptur, 1979 Rico-Gray and Thien, 1989 Oliveira, 1997 Wagner, 1997). In the most... 

In addition to these empirical studies, there is indirect ecological evidence for the defensive function of extrafloral nectaries. Several studies have reported correlations between the abundance of plants with extrafloral nectaries and ant abundance (Pemberton, 1998 Rico-Gray et al, 1998). Bentley (1977) and Rickson (1977) showed that plants may lose extrafloral nectaries in ecosystems void of mutualist ant species. 

Even though this evidence supports the defensive function of extrafloral nectaries, the evidence is largely based on myrmecophilous plants. In other plant species, the benefit of ant attendance is not always as clear (O Dowd and Catchpole, 1983 Koptur and Lawton, 1988). In these species, the provision of extrafloral nectar may serve to enhance the effectiveness of other plant-predator (Ruhren and Handel, 1999) or plant-parasitoid interactions (Lingren and Lukefahr, 1977 Bugg etal, 1989 Koptur, 1994), or serve other (non-defensive) functions. 

Adjei-Maafo, I. K. and Wilson, L. T. (1983). Factors affecting the relative abundance of arthropods on nectaried and nectariless cotton. Environmental Entomology 12 349-352. 

Bentley, B. L. (1977). Extra-floral nectaries and protection by pugnacious bodyguards. Annual Review of Ecology and Systematics 8 407 127. 

Cuautle, M. and Rico-Gray, V. (2003). The effect of wasps and ants on the reproductive success of the extrafloral nectaried plant Turnera ulmifolia (Turneraceae). Functional Ecology 17 417 123. 

Frey-Wyssling, A. (1955). The phloem supply to the nectaries. Acta Botanica Neerlandica 4 358-369. 

Koptur, S. (1979). Facultative mutualism between weedy vetches bearing extrafloral nectaries and weedy ants in California. American Journal of Botany 66 1016-1020. 

Extrafloral nectary-mediated interactions between insects and plants. In Insect-Plant Interactions, vol. IV, ed. E. A. Bernays, pp. 81-129. Boca Raton, EL CRC Press. 

Koptur, S. and Lawton, J. H. (1988). Interactions among vetches bearing extrafloral nectaries, their biotic protective agents, and herbivores. Ecology 69 278-283. 

Koptur, S., Rico-Gray, V. and Palacios-Rios, M. (1998). Ant protection of the nectaried fern Polypodium plebeium in central Mexico. American Journal of Botany 85 736-739. 

O Dowd, D. J. and Catchpole, E. A. (1983). Ants and extrafloral nectaries no evidence for plant-protection in Helichrysum spp. ant interactions. Oecologia 59 191-200. 

Oliveira, P. S. (1997). The ecological function of extrafloral nectaries herbivore deterrence by visiting ants and reproductive output in Caryocar brasiliense (Caryocaraceae). Functional Ecology 11 323-330. 

Pascal, L. and Belin-Depoux, M. (1991). La correlation entre les rythmes biologiques de l assodation plante-fourmis les cas des nectaries extra-floraux de Malpighiaceae americaines. Comptes Rendus Hebdomadaires des Seances de Acad mie des Sciences, Paris series III, 312 49-54. 

Pemberton, P. W. (1998). The occurrence and abundance of plants with extrafloral nectaries, the basis for antiherbivore defensive mutualisms, along a latitudinal gradient in east Asia. Journal of Biogeography 25 661-668. 

Ruhren, S. and Handel, S. N. (1999). Jumping spiders (Salticidae) enhance the seed production of a plant with extrafloral nectaries. Oecologia 119 227-230. 

Stephenson, A. G. (1982). The role of the extrafloral nectaries of Catalpa speciosa in limiting herbivory and increasing fruit production. Ecology 63 663-669. 

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