Ryan Bracewell
booger1@holly.colostate.edu
Carnivorous Plants of The Sarracenia sp.: Attraction and Absorption of Insect Prey
Abstract:
Pitcher plants appear to be some of the most specialized plants in the plant kingdom. These plants attract, catch and consume insects to supplement their diet due to the limited nutrients available in their habitat (Lloyd, 1942). Pitcher plants digest their prey with a combination of enzymes and acids, which are secreted by the plant (Lloyd, 1942). The initial attraction of insects is with lures, such as the attraction of colors, nectars and odors bacteria that help aid in the digestion of the arthropod prey. Many of the bacterium exist in a symbiotic/commensalistc relationship with the pitcher plant. The pitcher of these plants may also contain chemicals that will paralyze insects upon exposure. Volatiles in the genus Sarracenia have shown high rates of mortality on insects that come into contact with the chemical (Mody et al., 1976). The pitcher plants also release a variety of kairomones that will attract prey victims from many different orders of insects, ants being the most popular (Juniper, 1989). Different compounds such as sarraccenin and erucimide have been discovered in plants that have attractive nectaries (Jaffe et al., 1995). Many pitcher plants also have mechanical features that assist in the overall capture of prey. Pitcher plants inhabit a special niche in the environment and their carnivory on species of insects involves chemical cues as well as mechanical features. The features of carnivorous plants have amazed scientists for hundreds of years, because plants are seen as defenseless to most insects, yet these plants attract and capture potential herbivores.
Introduction:
The genus Sarracenia is represented by 8 species in
Nectar production:
Insects are lured to pitcher plants due to extra-floral nectar (Deppe et al., 2000). Glands that surround the hood and veins of the pitcher produce this nectar. The amount of nectar produced varies from plant to plant and also with age. The attractiveness of the nectar comes from the constituents that include amino acids and also carbohydrates (Cipollini et al.,1994). This insect attractant provides prey in the form of ants, but also reduces herbivory because the ants remove other insect eggs and larvae (Deppe et al., 2000). This idea of reduced herbivory is in question with Dress et al. (1997) who believes that the nectar is directly released to attract prey items. In either case, the most apparent benefit from these secretions is the attraction and capture of food. No studies have been done to quantify the amino acid secretion versus the amount of visitation by insects. Nectar from sarracenia plants have been isolated and identified and many of them are specifically attractive to insects. The nectar secretion is also filled with chemical compounds which attract insects (Jaffe et al., 1995). In studies done on heliamphora sp. Jaffe et al. (1995) isolated an iridoid rightly named Sarracenine. This enol diacetal monoterpene was originally identified in sarracenia flava and is a major volatile in the nectar. Another compound that was isolated from attractive structures of sarracenia was erucamide (Mody et al., 1976). This compound may act as a sticky component leading to the insect’s entanglement and eventual demise (Jaffe et al., 1995). These two compounds were found to be attractive to insects (Jaffe et al., 1995). Another volatile in sarracenia is the compound Coniine
H
Coniine (2 –n-propylpiperdine)
which was originally discovered in Sarracenia flava (Mody et al., 1976). This compound was also shown to cause paralysis in fire ants and eventual death at higher concentrations. All of these factors put together act as an attractant to the pitcher plant.
Amino acids and plant carnivory:
Pitcher nectar contains carbohydrates and usually amino acids (Juniper 1989). The nectar and amino acids that are secreted raise questions about the cost of carnivory. Dress et al. (1997) states that pitcher plants are associated with nutrient poor soils, so why use limited resources in making amino acid full attractants? The role of carnivory in these plants is based on the assumption that net gain of nitrogen will occur. Studies conducted by Dress et al. (1997) showed that nectar amino acids varied, but generally were made up of aspartic acid, cysteine, glutamic acid, glycine, histidine, hydroxyproline, methionine, serine, and valine. The use of available nitrogen to make these amino acids seems to contradict the nitrogen need of the plant. Newell and Nastase (1998) showed that overall insect capture in sarracenia plants is very poor. With ants being the major food source for sarracenia pupurea, these plants showed a very low capture rate. In their study it was shown that only 2.1% of visiting insects were actually captured in the sarcophagus. This would show that the pitcher plant is losing considerable amounts of nectar and amino acids to insects that are getting away. Ants are the most common visitor to the pitcher plants, and the average capture rate was half of the rate for all other insects. When pitcher plants do catch and digest prey, the amount of nitrogen and phosphorous in plant tissues increases (Christensen, 1976). Since the actual capture rates are calculated singularly, and many pitchers may be from one plant, it is hard to quantify overall nutrient acquisition for the plant. Some pitchers are efficient traps while others might be considered a burden. The question of whether the benefit from insect digestion outweighs the cost in amino acids and lost nectar is still up for debate.
Extra floral nectar production:
The nectar that is produced by the sarracenia sp. plant help attract potential prey items. Pitcher plants are attractive in physical characteristics as well as in nectar production (Deppe et al., 2000). Sugar content in nectar production does vary during a 24-hour period. Deppe et al. (2000) noticed in experiments that overall sugar volume was greater in samples taken from pitchers at night than from pitchers taken during the day. Pitcher plants were also bagged during both periods to not allow for insect visitation to disrupt the results. Newell and Nastase (1995) reported that sarracenia sp. plants have a greater diurnal attraction to insects than nocturnal attraction. These contrasting reports beg to question the reason for higher nighttime production of nectar, when less potential prey are actively searching for nectar. Actual concentrations of nectar have not been studied and this may account for the added attractiveness of sarrracenia during the day. The overall effectiveness of nectar production and insect capture in sarracenia seems poor.
Soil nutrients:
Sarracenia sp. plants are associated with swamps and bogs that are generally sandy and lacking inorganic nutrients (Plummer, 1963). These soils are usually acidic and low in nitrogen, calcium, potassium, magnesium, and phosphorous (Christensen, 1976). Because of these deficiencies it has been concluded that the benefit of carnivory from sarracenia sp. is the acquisition of these nutrients from insect prey. Nitrogen is a very important nutrient in plant growth and because of this nitrogen has generally been considered the most important benefit from carnivory. Nutrient deficient pitcher plants have been shown to acquire needed nutrients from insects (Christensen, 1976). In studies done by Christensen (1976) the extent of nutrient uptake from insects was measured and quantified. Plants were grown in nutrient deficient soils and insect feeding was prohibited. The plants that were grown in this matter were generally smaller and less viable. Plants that were grown in substrates that were high in all nutrients needed, but not allowed insects, grew rapidly and reached larger sizes. Of most interest were the plants that were grown in deficient soils but were fed a diet of insects. These plants grew as well as their counterparts that were grown in nutrient rich environments. After analysis of the components of the leaves it was discovered that nitrogen and phosphorous uptake by the plant was significant, but that calcium, magnesium and potassium were not of great importance. Christensen ‘s (1976) data shows that in low nitrogen and phosphorous environments, carnivory is an important process. These statements are a direct contradiction of work done by Plummer (1964). In his studies he too agrees with the nutrient uptake by sarracenia sp. plants, but finds that the amount of nitrogen and phosphorous elements to be negligible; whereas, the amount of metallic ion uptake is integral to the survival of the plant. Plummer (1964) showed that the low levels of potassium might be related to the uptake of asparagine, leucine, tyrosine, phenylalanine, lysine, and arganine from their insect prey. Plummer (1963) also states that the productivity of the habitat decreases through the year, which may mean that carnivorous plants may get more benefit from metallic ions, than from nitrogen. The exact quantities of nutrients that pitcher plants acquire from their feeding on insects is up for debate, but the role of carnivory is to supplement the diet and allow for competition in areas where overall nutrient composition is scarce. The nutrients that are sequestered by pitcher plants may also help the soil contents for future generations (Christensen, 1976). Speculation about the uptake of nutrients in a normal growing season may remain, but the amount of nutrients, even if not absorbed may be deposited and enrich the soil around the plants after it dies (Christensen, 1976).
Sarcophagus liquor:
The make up of the sarcophagus liquor has been under study for some time and much research has been done in this area (Darwin, 1888; Lloyd, 1942; Hepburn et al.,1927). Tests were conducted on opened and unopened pitchers. Earlier tests were done on pure pitcher fluid and not on the many bacterial components of the liquor. Pitcher plants are usually associated with acidic habitats, but some can be found in alkaline areas as well. The acidity or alkalinity of the pitchers is constantly under change and because of this different absorption rates were to be expected. Proteases in the liquor responded differently for certain species. Conclusions that were drawn by Hepburn et al. (1927) were that all of the sarracenia contain proteases that break down the insects, and that these proteases work best in an alkaline environment. These experiments were conducted on a range of species and populations and variation in sarcophagus liquor acidity and alkalinity was such that no hard line conclusions could be drawn. Since these plants are subject to a wide range of environmental factors, these experiments really showed how adaptive to variation they are. Later studies by Plummer and Kethley (1964) have also shown that plant enzymes are secreted into the sarcophagus after prey ingestion in certain species, but other species have not demonstrated the same positive results found by Hepburn et al. (1927). The different species of sarracenia all take on different forms, and through their speciation ,they have developed different sarcophagus and traps, and different nutrient needs. Some species have been shown to secrete a lot of enzymes while others secrete less, and some that don’t secrete any (Schnell, 1976). The many different species all have their own needs and ways of supplying them. Relative enzyme activity has also been questioned, when in nature most pitchers are full of a microcosm of algae, bacteria, and invertebrates, which may be aiding in the digestion along with the secretions (Schnell, 1976).
Activities within pitcher plant liquor:
Pitcher plant liquor will harbor bacteria, algae, and small arthropods upon opening to the surrounding environment (Plummer, 1964). These components of the liquor are in a dynamic environment and their aid in absorption has been of considerable study. The variables that play a role in proteolysis are as follows: (a.) the amount of insects in the fluid. (b.) the types of bacteria present in the fluid (c.) the optimum environment for each enzyme (d.) the secretions from the pitcher leaf to the sarcophagus (e.) acid-alkaline pH (f.) the physiological activities of the plant and (g.) climatic factors (Plummer, 1964). Newly emerging and closed pitchers do not have any bacteria in the liquor (Hepburn et al., 1927). The insects that are trapped are digested by liquor proteases, enzymes from bacteria, and the autolytic processes within insect tissues (Plummer, 1964). Unopened pitchers are free of bacteria, so any present in the pitcher liquor are brought in by the caught insects themselves. These bacteria are thought to assist in the breakdown of insect carcasses and aid in overall absorption (Plummer, 1964). These ideas are contrary to Hepburn’s et al. (1927) original conclusions that the bacteria are a sideshow to the real process of absorption. The liquor can exist in either aerobic of anaerobic conditions, which favor different types of inhabitants. In anaerobic conditions, the number of decaying insects and over abundance of bacteria can produce odors of ammonia, sulpher and amines (Llyod, 1942). Under aerobic conditions, species of mosquitoes and midges may inhabit the sarcophagus (Bradshaw, Creelman, 1984). The average pH of the liquor in “fresh” liquor was 4, but this value becomes more alkaline with the age of the plant (Plummer, 1964). In Plummer’s (1964) experiments, he found that with the introduction of microorganisms from insects, the liquor became more alkaline (except with the introduction of carpenter ants, where the pH went down due to formic acid). Plummer (1964) believes that the pitcher plants govern the amount of absorption and liquor release so that an optimal amount of bacteria are active in the liquor. In the early stages of plant growth, where the liquor is at a lower pH and is relatively clear of bacteria, protein hydrolysis would be very advantageous. When the plant matures and the liquor becomes more alkaline, the proteinases are less likely to work and the breakdown by bacteria take on a significant role (Plummer, 1964). This theory has raised questions because by the time that most pitcher plants have reached the alkaline stage, they are at the end of the growing season anyway.
Mutualism in Sarracenia:
Insects have evolved to live in the environment that is produced by pitcher plants. Many flies and moths complete their life cycle in the safety of the sarcophagus of the plant (Bradshaw, Creelman, 1984). These inquilines benefit from the host but also may add to the overall breakdown of prey material. Bradshaw and Creelman (1984) confirmed that pitcher plants take up carbon dioxide from the water that the inquilines were leaving as a by-product. The addition of midges to the liquor of the plants enhances the nitrogen production in the liquor (Bradshaw, Creelman, 1984). The insects that inhabit the pitcher plants form valuable nutrients in the forms of their by-products. Bradshaw suggests that inquiline respiration and metabolism compliments photosynthesis for pitcher plants. This study proposes that the photosynthetic efficiency increases due to carbon dioxide availability is as important as the nitrogen contribution from basic carnivory. The insects that inhabit pitchers also breakdown the chitinous exoskeleton of the insect prey allowing for the proteinases to breakdown the carcasses faster. Bradshaw states “Whatever factors led to carnivory in ancestral pitcher plants at the same time provided a food supply for bacteria, protozoa and other inquilines, resulting incidentally in enhanced carbon dioxide as well as nitrogen production.”
Conclusion:
The complexities of attraction and
absorption in the genus sarracenia
are not completely understood.
Conflicting results and hypotheses on the actual need and quantity of
absorption have not been answered. These
plants have been studied for quite some time and most data is relatively old,
with only nectar analysis being of recent interest. Undoubtly the original role of carnivory was
the supplementation of nutrients in nutrient poor environments; yet is
carnivory necessary for all plants to succeed in their respected environments,
and is nitrogen storage the only answer to the question? Work that has been done on these pitcher
plants would suggest that every species and every population that exists lives
in its own equilibrium and to try to summarize the overall relationship of
attractants and the equilibrium that exists in the liquor would be exhaustive. Questions about actual capture and exact
nutrient deficiencies may not be completely answered, but carnivory in sarracenia has led to the plants
evolution and to its infiltration of many bogs and wetlands in
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