Sarracenia purpurea, a phytotelm
Sarracenia purpurea is an especially remarkable species in the genus. On one hand, it is the most primitive carnivorous plant, since it does not secrete digestive enzymes, but the decomposing of prey is dependent on bacteria, some protozoa, and some mosquito larvae living as symbionts in the pitcher fluid. On the other hand, S. purpurea is advanced by establishing a symbiotic relationship to the organisms mentioned. This has only been possible by special structural and physiological adaptations in the plant.
Diagram of Sarracenia purpurea pitfall traps with symbionts and prey. The pitcher is divided into zones. The lid is zone 1. Contrary to the lid in other species, the lid is upright and the mouth of the pitcher is freely exposed for collecting rainwater. The lid is equipped with nectaries and downward directed hairs. The nectaries allure the prey. Zone 2 is a gliding zone with roofed scaly epidermal cells and nectaries making the zone shiny and moist. Zone 3 is a mirror-smooth waxed gliding zone with glands secreting water. Zone 4 is the large nutrient absorbing gland, which simultaneously performs photosynthesis. The level of the pitcher fluid varies and the upper part of zone 4 also supports up to two mm long downward pointed blocking hairs (not shown in the diagram). – Myggeæg = mosquito eeg, Bytte = prey, Fordøjelses produkter = digestive products.
The bottom zone in the pitfall trap (pitchers) functions like in other Sarracenia species as one large gland, which absorbs nutrients from the pitcher fluid, but the glandular cells have an additional function. The gland cells are specialized epidermal cells containing chloroplasts. That is not normal for leaves of terrestrial plants, but in water plants there are chloroplasts in both the mesophyll and in the epidermis. This is an adaptation to the reduced light intensity reaching submersed leaves. The inside of the pitfall traps also receives reduced light and in addition, the bottom zone is under water. This is the background for chloroplasts having developed in the epidermal glandular cells. Immunocytochemistry, fluorescent microscopy as well as measurements of oxygen production prove that the chloroplasts perform photosynthesis.
The chloroplasts must have carbon dioxide (CO2) to perform photosynthesis. A terrestrial plant usually gets CO2 through the stomata, which in Sarracenia are placed on the outside of the pitcher. However, this route is not possible in S. purpurea since the intercellular air spaces are effectively blocked by the hypodermis, the layer just below the epidermis. The hypodermal cell walls are partly corked (suberized) partly lignified, and furthermore impregnated with a substance making their cell walls completely airtight. However, the hypodermal cells are still living cells in contact with all adjacent cells through small cytoplasmic pores called plasmodesmata. Thereby, the absorbed nutrients are distributed throughout the plant. The epidermal chloroplasts are referred to receiving CO2 from the pitcher fluid, and the source is the respiration and digestive processes of the symbionts. The symbionts are bacteria and larvae of the mosquito Wyeomyia smithii (Culisidae) and the mite Metriocnemus knabi (Chironomidae) besides a number of protozoa, which all have developed immunity to the enzymes of the pitcher fluid.
A small more or less permanent water accumulation on a plant supporting living animals is called a phytotelm. We are not far from concluding that the phytothelm of Sarracenia purpurea represents an independent ecosystem. However, the system is not completely self-supporting. Besides rainwater, prey, bacteria, and mosquito eggs must be delivered from the outside. When this is met, the respiration of the symbionts will in addition to bacterial decomposition provide the chloroplasts with CO2 and the plant with nutrients, while the plant through photosynthesis provides oxygen to the symbionts.
Henning S. Heide-Jørgensen, February 2021.