5.3. Nutrient-uptake strategy

The performance of different species in nutrient-limited ecosystems is undoubtedly affected by their inherent interspecific differences in nutrient-uptake capacity. One factor affecting this is SRL (see Section 5.1); however, quantitative differences in the affinities of the ion-transport carrier and in the capacities of the roots among different species are superimposed on this. Furthermore, many plants exploit symbiotic associations with bacteria or fungi to enhance their nutrient-competitive ability, which is the subject of the present Section.

Symbioses with N2-fixing bacteria or mycorrhizal fungi have been found in many plant species. However, literature is strongly biased towards temperate species of Europe and North America. There is still much to be learned about nutrient-uptake strategies of species in less studied regions, as exemplified by the recent discovery of specialised N-foraging snow roots in the Caucasus. We provide specific protocols in Material S2 that deal with the following strategies:

(1) N2-fixing bacteria – association with bacteria in nodules to fix atmospheric N2;

(2) arbuscular mycorrhizae – symbiosis with arbuscular mycorrhizal fungi to aid in acquisition of nutrients and water;

(3) ecto-mycorrhizae – symbiosis with ecto-mycorrhizal fungi to aid in uptake of inorganic nutrients and organic forms of N;

(4) ericoid-mycorrhizae – symbiosis with ericoid mycorrhizal fungi to aid in uptake of organic forms of N;

(5) orchids – symbiosis with orchid mycorrhizal fungi for acquiring nutrients from litter;

(6) rarer types of mycorrhizae, e.g. arbutoid mycorrhizae (Arbutus, Arctostaphylos), ecto-endo-mycorrhizae (certain gymnosperms) and pyroloid mycorrhizae (Pyrolaceae);

(7) myco-heterotrophic plants without chlorophyll that extract C and probably most nutrients from dead organic matter via mycorrhizal fungi;

(8) root- or stem-hemiparasitic green plants, such as mistletoes (Loranthaceae), that extract nutrients such as N and P from the roots or stems of a host plant;

(9) holoparasitic plants without chlorophyll that extract C and nutrients directly from a host plant;

(10) carnivorous plants that capture organic forms of N and P from animals;

(11) hairy root clusters (proteoid roots), dauciform roots in sedges and capillaroid roots in rushes, that aid in P uptake;

(12) other specialised strategies (mostly in epiphytes), including

(a) tank plants (ponds) – nutrient and water capture and storage;

(b) baskets – nutrient and water capture and storage;

(c) ant nests – nutrient uptake and storage,

(d) trichomes – nutrient and water capture through bromeliad leaves, and

(e) root velamen radiculum – nutrient and water capture and storage; and

(13) none – no obvious specialised N- ort P-uptake mechanism; uptake presumably directly through root hairs (or through leaves, e.g. in the case of certain ferns with very thin fronds).
 
Although most of the experimental work on the active uptake of nutrient ions by roots has been carried out on agricultural plants, some information on that of wild plants is available, but again relatively little for tropical and subtropical species. The radioisotope techniques for characterisation of ion-transport mechanisms are beyond the scope of the present handbook, but can be readily ascertained by consulting the ion-transport literature.

References on theory and significance: see Material S2; in addition, for snow roots, see Onipchenko et al. (2009).