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exuding a drop of nectar |
collecting extrafloral nectar |
On this page (which is divided into two parts to speed up downloading) I will discuss in turn:
In the Sonoran Desert certain ant species regularly visit cacti for a reason: they come to collect the nectar that these plants produce. This nectar, though, is not the nectar produced by flowers to attract pollinators; in fact, it probably doesn't play any role at all in cactus pollination biology. Instead, it oozes out of various parts of these plants and may play a number of roles in their biology.
Plant structures that produce nectar for reasons other than attracting pollinators are called extrafloral nectaries (hereafter referred to as EFNs). EFNs are found in at least 68 families of flowering plants (Elias 1983). But like many things defined by exclusion, EFNs are a varied lot. Structurally diverse, they range from barely discernible forms to highly vascularized cup-like organs and occur on many different plant parts including leaves, stems, petioles, stipules, flowers stalks, and even on the outside of flowers. Even within the family Cactaceae there is variation in the kinds of EFNs you find and where you find them (Blom and Clark 1980; Elias 1983). However, with the exception of saguaros (Cereus giganteus), all EFN-bearing cacti at my study site had the same type of EFN even though this "type" has probably evolved more than once. Called thorn nectaries, these EFNs are modified spines (or glochids) produced by specialized axillary buds known as areoles. Below are two photographs of the very conspicuous EFNs of barrel cacti. Note the distinct areoles from which both spines and thorn nectaries arise.
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exuding a drop of nectar |
collecting extrafloral nectar |
While the majority of EFN-bearing cactus species at my study site had thorn nectaries, these species differed in the particulars of extrafloral nectar production. Among both staghorn chollas (Opuntia versicolor ) and prickly pears (Opuntia phaecantha ), extrafloral nectar secretion was seasonal and only occurred on actively growing tissue. In contrast, chainfruit chollas (Opuntia fulgida) produced it continuously on most parts of the plant although secretion was highest on new growth. Finally, barrel cacti (Ferocactus wislizenii) also produced extrafloral nectar throughout the year. But unlike chainfruit chollas, barrel cactus EFNs appeared to require continual attendance by ants in order to remain active. Several times during my study ant colonies stopped visiting one of these plants for an extended period of time. When this happened, large droplets of viscous nectar accumulated and eventually solidified. This desiccated nectar eventually disappeared but wasn't replaced by any new droplets.
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Staghorn cholla (O. versicolor) |
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Because extrafloral nectar contains sugars and other substances that plants could presumably put to other uses, secreting it would appear to be a costly process. In support of this, most plants do not secrete extrafloral nectar and those that do are are often found in high light environments where the production of carbon compounds is relatively cheap (Schupp and Feener 1991). Most cacti certainly live in light-rich environments and could afford better than most plants to give up some sugars. But cacti also have to deal with an additional cost of extrafloral nectar secretion -- one that is likely trivial for most other plants -- a loss of water. After all, most of the distinctive features that characterize cacti are evolved responses to xeric environments -- they've undergone strong selection for not giving up water, in any form, easily.
To balance these costs there must be something favoring extrafloral nectar secretion. While there may be special reasons for why cacti secrete extrafloral nectar, its likely that they do it for some of the same reasons that other plants do. Currently, there are three major hypotheses for why plants secrete extrafloral nectar. These are: (1) to excrete excess sugars; (2) to defend themselves against their herbivorous enemies; and (3) to procure additional nutrients.
The first hypothesis is that plants secrete extrafloral nectar to eliminate excess sugars that accumulate when various non-sugar materials in the phloem are directed toward developing plant organs (Frey-Wyssling 1955; Helder 1958; Mound 1962; Milburn 1975). To my knowledge, there has been only one solid attempt to test this hypothesis and that one did not support it (Baker et al. 1978). Still, the idea that extrafloral nectar secretion may occasionally play a role in plant physiology doesn't seem entirely implausible. Nevertheless, very little research continues in this area.
The second major hypothesis for why plants secrete extrafloral nectar is that they do it to attract organisms that remove, attack, prey upon, or parasitize plant herbivores. In most cases the attracted organisms are believed to be ants. These insects are frequent visitors of extrafloral nectaries and many species remove, attack, or prey upon insect herbivores and seed predators. A number of studies do indicate that ants can reduce vegetative damage or decrease seed predation on plants with extrafloral nectaries (Janzen 1966; Elias and Gelband 1975; Bentley 1977; Deuth 1977; Keeler 1977, 1980, 1981; Tilman 1978; Koptur 1979, 1984; O'Dowd 1979; Schemske 1980; Beckman and Stucky 1981; Stephenson 1982; Horvitz and Schemske 1984; McKey 1984; Barton 1986; Kelly 1986; Smiley 1986, Del-Claro et al. 1996).
Not all plants with extrafloral nectaries are visited by ants, however (Keeler 1985). Also, even when ants do visit, they don't always provide protection against herbivores (reviewed in Becerra and Venable, 1989; Rashbrook et al. 1992). Because of these two facts, people have recently begun to explore the role that extrafloral nectar plays in attracting parasitoids to plants (Treacy et al., 1987; Pemberton and Lee 1996; Stapel et al., 1997). As with ants, it is easy to understand how extrafloral nectar secretion could evolve to attract parasitoids who directly reduce herbivory on individual plants. But many parasitoids kill their host after it has finished feeding. In these situations selection is much less likely to favor extrafloral nectar secretion (Koptur 1991).