"Carnivorous" or insectivorous plant species are found in areas which are particularly deficient in nutrients. They have adapted by changing their leaves in various ways to organs which act as traps and are suitable for trapping insects and at the same time excrete proteases and chitinases that digest them (Fig. 2.3.12 A). They are differentiated into:
These types have evolved in very different taxonomic groups (Fig. 2.3.12B). Often the same trap types occur in geographically and taxonomically distinct regions (pitfall traps in Nepenthes: Indonesian islands; Sarracenia: eastern USA, Darlingtonia: western USA; Ce-phalotus: SW Australia).
The effectiveness of insect capture can be measured using stable 1SN isotopes, as insects have a higher 1SN value, depending on their trophic level, than soil N (Table 2.3.3; Schulze et al. 1991; W. Schulze et al. 1997).
Roridula gorgonias (South Africa) is a special case as it does not excrete proteases. The sticky leaves catch insects, but they are harvested by bugs (Hemiptera) which are adapted to life on the sticky trap and move slowly and carefully. The excrement of the bugs is used by the Roridula as N source (Ellis and Midgley 1996).
Nepenthes takes nitrogen from the trapped insects in the form of ammonium which is absorbed via glands (W. Schulze et al. 1997) where amino acids are formed before transport to the bundle sheath. There polypeptides are formed (a N concentration mechanism), and then loaded into the phloem. In young Nepenthes shoots 100% of the N content originates from trapped insects. In Venus flytraps (Dionaea) survival depends on the seedlings being able to trap insects (W. Schulze et al. 2001). Only after a significant catch can the larger spring traps develop that are suited to catch larger insects; only the rosette is capable of flowering.
I Table 2.3.3. The contribution of insects N to the N nutrition of insectivorous plants. (W. Schulze et al. 2001) Trap type Genus Growth form Proportion of insect in plant N (%)
Sticky trap Drosera Rosette 20 (e.g. D. erythrorhiza, Western Australia)
Climber 53 (e.g. D. macrantha, Western Australia)
Upright, low 48 (e.g. D. stolonifera, Western Australia)
Upright, high 54, (e.g. D. gigantea, Western Australia)
Roridula Shrub 70
Pitfall trap Cephalotus Rosette 26
Nepenthes Climber 62 (100% in buds)
Darlingtonia Rhizome 76
Heliamphora Rhizome 79
Brocchinia Erect rosette 59
stem. In the xylem of spruce, amino acid concentrations are greater than 100mmol/l (Kummetz 1996) during the emergence of new needles. • as polypeptides and proteins. They are formed when the osmotic activity of amino acids may interfere with metabolism, for example, during the loading of phloem the number of molecules must be decreased and N must be concentrated in a few molecules (Schulze et al. 1999). Important storage proteins, e.g. prolamine, glutelin, and albumin, accumulate in seeds but these proteins are also some of the proteins required in metabolism and have additional storage and protective functions. The C02-reducing enzyme Rubisco (ribulose-bis-phosphate-carbox-ylase/oxygenase) forms about 30% of the N content of a leaf. Only a proportion of this pro-
Pinguicula Utricularia Genlisea Polypompholyx
Sarracenium Heliamphora Prose ra ceae
| Fig. 2.3.12. A Different forms of leaf used to capture Insects. Spring traps: Dionaea muscipuia, pit-fall trap: Darlingtonia californica and Nepenthes spp., sticky traps: Drosera spp., suction traps: Utricularia spp. B Distribution of Insectivorous plants In anglosperms. (From W. Schulze 1991)
tein is normally active, the excess serving to safeguard the photosynthetic apparatus at high photon flux, or a proportion may be degraded during N deficiency and used for growth (Stitt and Schulze 1994). Before leaves are shed, part of this nitrogen is degraded and the amino acids re-translocated back to the plant. This remobilisation of nitrogen from leaves comprises about 20% of the leaf N (Chapin and Kedrovski 1983; Koch et al. 1988).
• as glucosides. These plant poisons serve as protection against herbivory and may be transported to sites of growth or metabolised, serving as N storage.
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