Function Of Extrafloral Nectaries Research Articles

A novel indirect defence in Brassicaceae: Structure and function of extrafloral nectaries in Brassica juncea

While nectaries are commonly found in flowers, some plants also form extrafloral nectaries on stems or leaves. For the first time in the family Brassicaceae, here we report extrafloral nectaries in Brassica juncea. The extrafloral nectar (EFN) was secreted from previously amorphic sites on stems, flowering stalks and leaf axils from the onset of flowering until silique formation. Transverse sections at the point of nectar secretion revealed a pocket-like structure whose opening was surrounded by modified stomatal guard cells. The EFN droplets were viscous and up to 50% of the total weight was sugars, 97% of which was sucrose in the five varieties of B. juncea examined. Threonine, glutamine, arginine and glutamate were the most abundant amino acids. EFN droplets also contained glucosinolates, mainly gluconapin and sinigrin. Nectar secretion was increased when the plants were damaged by chewing above- and belowground herbivores and sap-sucking aphids. Parasitoids of each herbivore species were tested for their preference, of which three parasitoids preferred EFN and sucrose solutions over water. Moreover, the survival and fecundity of parasitoids were positively affected by feeding on EFN. We conclude that EFN production in B. juncea may contribute to the indirect defence of this plant species.

The effect of wasps and ants on the reproductive success of the extrafloral nectaried plant Turnera ulmifolia (Turneraceae)

Summary 1. To assess the effectiveness of extrafloral nectaries (EFN) as a defensive mechanism of plants it is vital to use a multiple interactions approach and assess the contribution of all visiting species and their interactions. 2. The effect of EFN‐visiting ants (Camponotus planatus, C. abdominalis, Conomyrma sp., Crematogaster brevispinosa, Forelius sp., Pseudomyrmex sp.) and wasps (Polistes instabilis, Polybia occidentalis) on the reproductive success (estimated as the number of reproductive structures) of Turnera ulmifolia (Turneraceae) was experimentally evaluated. Herbivory effects were tested using Euptoieta hegesia larvae (caterpillars), which is the main herbivore of this plant. The study was done in a coastal sand dune scrub in Veracruz, México. 3. Wasps and ants were selectively excluded using a two‐factor design (Wasps, Ants) block design, both factors with two levels (absent, present). The response variables were an index of herbivory per branch and the number of buds, flowers, ripe and unripe fruit per plant, and the seed/fruit ratio per branch. 4. After a week of placing the larvae on experimental plants, they were significantly more frequent on plants where wasps and ants had been experimentally excluded. 5. Wasp presence was associated significantly with greater numbers of buds, flowers, ripe fruit and seeds. When acting separately, wasps and ants exerted a positive effect in decreasing herbivory levels and increasing the number of unripe fruit; when acting together, however, their effect was not additive. 6. This is the first demonstration of a positive effect on the plant by wasps associated with EFN. The ecological implication of this finding is that the function of EFN and the ultimate effects on a plant will probably depend on the array of organisms visiting its EFN.

Patterns of nectar secretion in five Nepenthes species from Brunei Darussalam, Northwest Borneo, and implications for ant-plant relationships

Summary The distribution of nectaries and the temporal and spatial pattern of nectar secretion are compared for five Bornean lowland species of Nepenthes (N. albomarginata, N. bicalcarata, N. gracilis, N. mirabilis var. echinostoma, N. rafflesiana) based on results from field studies conducted in Brunei Darussalam, Northwest Borneo. Leaf development together with temporal and spatial pattern of nectar secretion is also given for cultivated Nepenthes bicalcarata, a myrmecophytic species that, in terms of nectar production, appears to be exceptional within the genus. The number of ants trapped in frequently visited pitchers of N. bicalcarata and N. mirabilis var. echinostoma was counted over a set period. The percentage of trapped individuals was remarkably small. It ranged from 0.75 to 1.59% of present ants in N. bicalcarata and from 0.34 to 0.42% in N. mirabilis. The function of extrafloral nectaries in Nepenthes with special reference to N. bicalcarata as well as implications for ant-plant relationships are discussed.

The role of ant-tended extrafloral nectaries in the protection and benefit of a Neotropical rainforest tree.

One possible function of extrafloral nectaries is to attract insects, particularly ants, which defend plants from herbivores. We determined whether ants visiting saplings of the tree Stryphnodendronmicrostachyum (Leguminosae) provide protection (decreased plant damage due to ant molestation or killing of herbivores) and benefit (increased plant growth and reproduction associated with ant presence) to the plant. We compared ant and herbivore abundance, herbivore damage and growth of ant-visited plants and ant-excluded plants grown in sun and shade microhabitats of a 6-ha plantation in Costa Rica over a 7-month period. Results show that ants provided protection to plants not by reducing herbivore numbers but by molesting herbivores. Ants also reduced the incidence of pathogen attack on leaves. Protection was greater in the shade than in the sun, probably due to lower herbivore attack in the sun. Protection was also variable within sun and shade habitats, and this variability appeared to be related to variable ant visitation. Results also indicate that ant presence benefits the plant: ant-visited plants grew significantly more in height than ant-excluded plants. The cultivation of ants may serve as an important natural biological control in tropical forestry and agroforestry systems, where increased plant density can otherwise lead to increased herbivore attack.

Extrafloral Nectaries vs Ant-Homoptera Mutualisms: A Comment on Becerra and Venable

Becerra and Venable (1989) proposed that the function of extrafloral nectaries (EFNs) may not exclusively be to attract ants which defend plants against herbivores. They questioned the protection hypothesis and proposed that EFNs may also function to defend plants from ant-Homoptera mutualisms. Arguments for their hypothesis were e.g. the lacking universality of ant protection for plants and the negative impact of ant-tended homopterans on their host plants. In general, effects of homopterans on plants are negative but the damages of ant-homopteran associations are difficult to assess. Becerra and Venable cited a few examples of beneficial by-products of these associations in form of protection for the plants by the visiting ants. However, they considered only facultative interactions. In most of the highly developed ant-plant mutualisms ants cultivate homopterans while EFNs are absent. Among plant species of 36 genera, for example, which possess permanent domatia formed by hollow stems, only four have EFNs, but in 18 of them ants were recorded to keep homopterans (Huxley 1986). In the eight best-studied obligate myrmecophytic associations (Table 1) most plant partners offer nesting space but no food in form of nectar. The only two exceptions are Acacia spp. and Leonardoxa africana. In this respect the genus Macaranga (Euphorbiaceae) is especially interesting. In SE Asia it comprises the full range of species which are obligatorily inhabited by ants to those which have only weak and unspecific associations with ants (Fiala et al. 1989, 1990). Most of the facultatively associated Macaranga species possess EFNs on the leaf margins and in addition on the lamina. Obligate myrmecophytes in this genus do not secrete sugar-containing fluids (Fiala and Maschwitz, in press) and colonizing ants keep scale insects inside the hollow stems of their Macaranga host plants. Contrary to Becerra's and Venable's hypothesis in this case a reduction of nectary production appears to be favoured. The hypothesis of superiority of extrafloral nectar to honeydew remains to be tested in facultative associations, but my preliminary observations in Malaysia do not support this view. Becerra and Venable list a number of questions which should be addressed experimentally. I will briefly comment here on the first one: Is it generally true that ants neglect Homoptera when an alternative sugar source is available? During studies on the tree genus Macaranga in Peninsular Malaysia I looked for plants with EFNs and the occurrence of homopterans on these plants. The main study sites were secondary forests and disturbed habitat which were rich in light-demanding, fast growing plants. Six common species which possess EFNs were studied: Macaranga gigantea, M. tanarius, Mallotus paniculatus, M. macrostachys, Endospermum diadenum and Croton argyratus (all Euphorbiaceae). I checked 10 specimens each for occurrence of homopterans. Two to five plants of each species were infested by Homoptera (mostly scales and membracids) despite the presence of EFNs. Other plant species without EFNs in the vicinity were not found with homopterans more often than the above mentioned species (random samples of up to five nearest neighbouring plants; giving a total of over 200 controls). All homopterans on leaves and stems of the plants were visited by various ant species (Anaplolepis longipes and three Crematogaster spp. were dominant). The ants also visited the extrafloral nectaries on these plants. However, often the ants seemed to prefer honeydew from the homopterans. On a leaf of Croton argyratus, for example, almost 2/3 of all ants visited the scale insects on the leaf lamina and only few took notice of the secreted extrafloral nectar. On Mallotus ants generally favoured membracids despite