In a breakthrough research, scientists in India have discovered a high concentration of carbon dioxide (CO2), being used to trap insects and small mammals, in a genus of carnivorous plants
Researchers found the leaf-evolved biological traps of the plant Nepenthes to be CO2-enriched cavities, and that CO2 emission from pitchers act as sensory cues attracting insects towards these traps. This is the first time that high CO2 content has been detected in carnivorous plants, though CO2 emitting devices are widely used by man as traps against mosquitoes, flies and other insects.
Most of the characteristic features of Nepenthes pitchers are influenced by the high content of CO2 entrapped within them. This study also hypothesizes Nepenthes pitchers as natural model systems mimicking an anticipated elevated CO2 scenario on Earth. With their adaptations, Nepenthes (and other carnivorous plants) survive and they remain as one of a unique life forms on the surface of Earth.
The research was conducted by India’s Jawaharlal Nehru Tropical Botanic Garden and Research Institute and National Centre for Earth Sciences. The findings have been published in the journal Scientific Reports.
In the study, CO2 was demonstrated as an insect attractant emitted by Nepenthes, genus of carnivorous plants, which consists of approximately 160 species distributed in the Madagascar-South East Asia-North Australia-New Guinea region.
Nepenthes and most other carnivorous plants grow in wet, sunny and nutrient-poor habitats. In order to supplement this nutrient deficiency, the species evolved mechanisms to capture arthropods (insects, ants etc.) and involve in mutuality interactions with small mammals (tree shrews, rats, bats etc.), through their leaf-evolved prey traps called pitchers.
CO2 contents in near mature, unopened Nepenthes pitchers were found to be in the range 2500-5000 ppm. This elevated CO2 atmosphere is approximately ten times the Earth’s atmosphere viz. 400 ppm. But, the gas composition within Nepenthes pitchers has not been studied so far.
How pitchers use CO2
Nepenthes pitchers are modified leaves in which their upper surface curls around and fuses to form the inner side of the pitcher. The tendrils of aerial pitchers are usually coiled in the middle, and once in contact with other objects for long enough, they curl around them, forming anchor points for pitchers. In this way, Nepenthes tendrils help to support the growing stem of the plant. As it matures, the pitcher ‘inflates’ and gets partially filled with an acidic enzymatic fluid. Pitchers also have a flap (operculum), which initially ‘hermetically seals’ the growing trap, and once mature breaks open for prey capture. In most Nepenthes species, a lid covers the pitcher opening and protects it from rain, preventing dilution of the pitcher fluid.
Large Nepenthes pitchers are capable of trapping rodents, lizards and birds. Once open, Nepenthes pitchers involve in prey capture from a few weeks up to nine months depending on the species.
The study showed that Indian pitcher plant, Nepenthes khasiana, as more CO2-enriched air-streamed through the pitchers, they attracted (captured) substantially higher number of insects.
CO2 is a sensory cue and most insects pay special attention to ‘subtle variations’ or ‘gradients’ of CO2 in the form of plumes arising from individual point sources. Insects have well developed CO2 receptors, which can detect these variations (even small variations) as a means of locating their food. The study demonstrated CO2 as an insect attractant emitted by Nepenthes prey traps and revealed a new prey capture mechanism within them.
In an earlier study published in Plant Biology in 2013, Dr Baby and his colleagues reported strong blue fluorescence emissions in ultraviolet primarily from the prey trapping regions of Nepenthes and other carnivorous plants. These blue emissions gradually developed with the growth of Nepenthes’ prey traps and diminished towards their death. These fluorescence emissions are one of the factors luring insects towards the passive Nepenthes traps.
Higher respiration rate
Unlike most plant leaf structures, high growth rate and unique physiological functions (prey attraction, capture, digestion, absorption of nutrients) of Nepenthes pitchers demand more energy, prompting higher respiration rates in the trap tissues, resulting in the release of more of CO2. “Carnivorous plants follow a photosynthetic pathway, with high CO2 levels which enhance respiration rates. The study demonstrated respiration of pitcher tissues as the factor contributing to the high CO2 within the ‘closed cavities’ of Nepenthes traps,” Dr Baby told Asia Times.
Most Nepenthes species secrete pitcher fluids with viscous-elastic properties. Fluids in unopened pitchers are sterile, and once open microbes and inquilines invade them. “Pitcher fluid plays a critical role in the enzymatic digestion of trapped preys and absorption of their nutrients into the plant system,” he said.
Dr Baby and his colleagues reported high levels of CO2 dissolved in Nepenthes pitcher fluids. They also observed very low levels of dissolved oxygen within pitcher fluids, particularly in prey captured pitchers. CO2 dissolved in pitcher fluids plays a key role in maintaining their acidic pH, and thus ensuring optimum activities of the digestive enzymes secreted by the specialized glands within the inner surface of Nepenthes pitchers. This acidic pH could also be controlling the growth of pitcher inhabitants (microbes, mosquito larvae, small aquatic organisms etc.). CO2 dissolved in the pitcher fluid is one of the factors making it acidic and it also acts as a preservative to the pitcher fluid. Moreover, these unique plants sense the sequential events of lid opening, CO2 release and prey capture, and they release antifungal naphthoquinones (droserone, 5-O-methyl droserone) into their pitcher fluid, preventing infections from incoming preys. The high CO2 atmosphere at the interior of Nepenthes pitchers and the dissolved CO2 in the pitcher fluids might also act as an intoxicant to the trapped preys.
Nepenthes tendrils and pitchers show high growth rates compared to their lamina. More investigations, in the light of the discovery of CO2 within, could possibly unravel the tissue growth patterns and provide further evidence towards faster growth rates in Nepenthes pitchers.
Link with climate change
CO2 (high) and CO, CH4 and N2O (ambient) found in Nepenthes pitchers are greenhouse gases. Global CO2 levels are predicted to go up to 800 ppm by 2100 and further onto even higher levels. This elevation of CO2 levels on Earth surface has prompted several simulation studies towards understanding its effects on various features of plants. Nepenthes prey traps with elevated CO2 contents (3000-5000 ppm) are simulating this futuristic scenario in their ‘closed cavities’ (before trap opening). As in other CO2-enrichment experiments, high carbohydrate and low protein contents were detected in Nepenthes pitchers. Carbohydrate accumulation is a major acclimation response to elevated CO2. High carbohydrate contents in pitchers, transformed into nectar by nectaries, act as a major ‘lure’ in prey capture.
A recent burst of growth in Nepenthes pitchers, enhanced carbohydrate levels, declined protein levels, drop in photosynthetic capacity, high respiration rate evolved stomata and acidic pitcher fluids, are probable manifestations of the enhanced CO2 atmosphere within them. This evidence also infer Nepenthes pitchers as ideal examples reflecting the effects of an anticipated high CO2 level on Earth’s surface on the characteristic features of plants.