|Casuarinas: Nitrogen-Fixing Trees for Adverse Sites (BOSTID, 1984, 114 p.)|
As noted earlier, casuarina foliage consists of long "needles" with regularly spaced nodes. The needles are actually modified branchlets, termed cladodes, and differ from pine needles in that they are segmented and pull apart easily. The leaves are so reduced that they look like whorls of tiny teeth and are barely visible as small scales forming a ring around the node between each segment. The number of teeth in each whorl reflects the number of leaves and is, in part, diagnostic of the species.
The branchlets bear tiny ridges along their length formed from the fused leaves. The stomata (the pores for gas exchange) lie between the ridges, mostly in furrows, which protect them from the environment. This protection of the stomata endows casuarinas with great tolerance to drought and salt. But casuarinas have more features that help them resist stress. A thick, waxy cuticle covers the branchlets, and the roughly circular cross-section reduces the surface area exposed to the sun. Both of these features help reduce the plant's water losses. In this way, casuarinas avoid desiccation in dry areas. Furthermore, the cylindrical form and high amount of structural tissue prevents wilting of the foliage at low tissue-water potentials. Thus, in drought conditions casuarinas can survive without danger of tissues collapsing.
Casuarinas have very reduced male and female flowers, arranged in catkin-like inflorescences. Most species have separate male and female trees (dioecious), but in a few species the male and female flowers occur on the same tree (monoecious). They are wind pollinated. After pollination the female inflorescence develops into a small woody "cone," which has beak-like valves that open at maturity to release small winged seeds.
FIGURE: Casuarina equisetifolia. Although these drawings are of one species, they are generally representative of the parts of most casuarinas. (P. B. Tomlinson)
For many Australian species of casuarina, flowering takes place during a brief period of the year and is more or less consistent from year to year. Other species are less regular. Casuarina cristata, for example, flowers either in spring or following a heavy rain.
As discussed previously, the root hairs of most casuarina species are invaded by the filamentous soil actinomycete, Frankia. When the microorganism's infectious threads reach the cortical root cells they divide and expand, forming lobes on the outsides of the root. The result is a woody perennial "nodule" that is spherical and up to 10 cm in diameter.
The association with Frankia benefits the casuarina, and the resulting symbiosis provides nitrogenous compounds that fertilize the plant's growth. Only the young tips of each nodule appear to actively fix nitrogen. The woody nodules are found in large masses from the base of the trunk out to near the drip line. They are most readily visible just beneath the soil surface, but they have been located as deep as 10 m in the soil.
The woody nodules are long lived, but eventually they decay, releasing Frankia spores and particles into the soil. The survival time and mechanisms of distribution of Frankia in the soil are unknown and deserve research.
The greatest number of nodules have been found where the soil activity is close to neutral. However, natural stands of Casuarina glauca are well nodulated in soils that are quite acidic (to pH of about 4). Nevertheless, nodulation normally is most successful within the range of soil pH from 6 to 8.
Nodulation is very much influenced by the available moisture and the soil aeration. Oxygen is necessary for the fixation process, so that active nodules normally occur only on surface roots that are obtaining adequate moisture. The symbiosis cannot occur in waterlogged sites. However, nodules are abundant in natural stands of Casuarina glauca where the water table is within 30 cm of the soil surface and nodules occur on the root system of Casuarina cunninghamiana growing at sites that are periodically submerged, such as river banks.
Also necessary for nitrogen fixation are minute amounts of mineral elements such as molybdenum, cobalt, and copper. These, however, are usually provided by even the poorest of soils.
Casuarinas fix about as much nitrogen as legumes. In a study in Senegal, the amount of nitrogen in soil under casuarina trees increased annually at rates of 58 kg per hectare when compared with nearby unplanted sand dunes.
In a natural forest of Casuarina littoralis near Sydney, Australia, the annual nitrogen accumulated in the litter under the trees was 290 kg per hectare, largely because of nitrogen fixation.
In one experiment, nodulated seedlings of Casuarina glauca grown in a greenhouse increased their shoot nitrogen content about thirteenfold within 60 days of the first appearance of nodules.
The actual rate of nitrogen fixation appears to depend on environmental factors and on the casuarina species (and perhaps even on the particular strain of tree) as well as on the strain of the Frankia symbiont.
The Frankia that inhabits root nodules of actinorrhizal plants, such as casuarinas and alders, was first isolated and cultured from alders only in 1978. Since that time, microbiologists have isolated a dozen strains of Frankia from the roots of different host plants maintained in a worldwide collection in Middlebury College, Vermont (USA). Some of them have been shown to fix atmospheric nitrogen when grown in pure culture.
In 1983 pure strains of Frankia were, for the first time, isolated from casuarina nodules and cultured in artificial media. Tests have shown that these pure cultures are suitable as inoculum for casuarina seedlings and rooted cuttings.
Using such isolates and pure cultures, growers may in the future be able to match the best strain of Frankia to the appropriate casuarina species so as to maximize nitrogen fixation.
Whether a single Frankia strain will infect all of the casuarina species or whether more than one strain will be needed is not yet known, but some recent experiments suggest that different strains of Frankia are needed to infect and nodulate the different casuarina species.
Casuarina roots, in common with roots of many other tree species, have symbioses with a number of genera of soil fungi that help the trees scavenge mineral nutrients from the soil. These fungi are called mycorrhizae ("fungus-root"); two main types occur:
1. Ectomycorrhizae. These form an intricate meshwork of threads, or hyphae, clinging to the root surface and penetrating between root cells. From this mantle of fungal hyphae, thin filaments, barely visible to the naked eye, radiate out into the soil where they absorb nutrients and water and transport them back to the root. Ectomycorrhizal fungi on casuarinas include such genera as Cenococcum, Pisolithus, Hymenogaster, Thelephora, Rhizopogon, and Amanita.
2. Endomycorrhizae. The hyphae of these fungi actually invade the cortex of the root and proliferate between, and actually penetrate into, the root cells themselves. They are identified by microscopic arbuscules (hyphae-branched-like trees) and vesicles (thick-walled oval structures that develop within the cells). These fungi belong to the family Endogonaceae and are abundant in many soils. The most common genus is Glomus.
The association of casuarina roots with both types of
mycorrhizae significantly enhances the trees' adaptability as well as their
ability to grow in harsh environments. Specifically, the fungi help the trees
· Improving mineral nutrition. Mycorrhizae have been shown to be particularly important in making phosphorus available to the tree, but they also help absorb soil nitrogen and microelements.
· Increasing tolerance to drought. This is of special importance during field plantings, as it prevents or improves recovery from wilting.
· Influencing the nitrogen-fixing activity of Frankia. In phosphorus-deficient soils, nitrogen fixation in root nodules is markedly reduced in the absence of mycorrhizae.
· Improving soil structure. Hyphal mats contribute to the binding of soil particles and thus reduce the harmful effects of wind and water erosion.
· Increasing resistance to some disease infections by preventing access of the organisms to the plant root.
· Alleviating the effects of acid soils, excessive aluminum, and certain other toxic soil conditions.
Casuarina roots also interact with unidentified soil microorganisms that cause the development of "proteoid roots." These unusual structures occur as tight-packed rows of lateral rootless that form thick mats just below the soil surface. They greatly increase the surface area that casuarina roots have for nutrient absorption.
To date, proteoid roots usually have been observed in association with organic debris in the soil and in sand under the tree drip-lines. Little is known about their benefits. However, other plants that form such roots demonstrate increased absorption of phosphate from soil.