sign in sign up
<<
Home , Actinorhizal plant, Frankia, Nitrogen fixation, Purshia

A Fixation: The Story of the

Peter Del Tredici

Some of the we scorn as weeds perform important biological functions. By adding nitrogen to impoverished , nonleguminous nitrogen-fixing and play a key role in the process of forest succession.

It’s a great irony of the botanical world that growth is often limited by the availability of nitrogen in the when almost eighty percent of the is composed of dinitrogen gas /NZ~. The explanation lies in the chemical stability of nitrogen gas. Before atmo- spheric nitrogen can be used by plants, it must be "fixed," that is, split and combined with other chemical elements. This process requires a large input of energy and can occur either bio- logically, within the cells of various , or chemically, in factories or during storms. Among all living organisms, only bacteria have evolved the complex biochemical mecha- nisms required for . All "higher" plants and animals that are said to fix nitrogen are really only the symbiotic partners of the bacteria that do the actual work. Among plants, the cultivated (, , , etc.) are the best-known nitrogen- fixers, but many plant families besides the Leguminosae can also fix On a world- nitrogen. This flat of southern bayberry seedhngs (Mynca wide these as are some- scale, nonlegumes, they cenfera) was supposedly grown both without what negatively called, fix as much nitrogen mtrogen and without the Frankia bactena. One as legumes, but for a variety of historical of the seedhngs, however, did manage to form reasons they have been relatively neglected Frankla-induced nodules, producmg a clearcut by scientists. advantage over its sibhngs. Quite a few of these nitrogen-fixing non- legumes are native to North America and in the South) on the seashore and on exposed are mostly found in impoverished, sandy soils sandy soils back from the coast; sweet low in nitrogen. The most common are ( peregrina) on exposed, dry, sandy (Alnus sp.) in wet, open land; bayberry soils; sweet gale ( gale) in swamps; (Myrica pensylvanica in the North, M. cerifera and New Jersey tea ( americanus) 27

in open, wooded sites. On the west coast of North America, various Ceanothus and Alnus species are the most common nonlegume nitrogen-fixers. In the arid mountains of the West, buffalo berry ( canadensis), bitterbrush ( tridentata), and the mountam mahogames ( spp. ~ are important. In southern Flomda, the introduced and somewhat weedy Australian pine (Casuanna spp.) is important for stabilizing beaches, and throughout the Mid- west and East Coast the autumn and Russian olives ( umbellata and E. angustifolia) have been widely planted along embankments. All of highway A Frankra-mduced nodule on a root of sweet fern, Comptonia these plants thrive in poor soils peregnna. Note the upwardly growmg emanatmg from the where little else grows. Their abil- lobes of the nodule. ity to fix nitrogen is a significant factor in their survival under conditions that the bacteria, by producing large quantities of the would be inhospitable to ordmary plants. hormones that normally occur in very small In legumes and nonlegumes alike, the actual concentrations in the plant, force the root cells fixation of nitrogen is done by bacteria living to proliferate much faster than normal, causing inside the roots of the host plant. It is a classic multi-lobed swellings, or nodules, to form example of a mutually beneficial symbiosis: the wherever the bacteria have penetrated. Among plant provides the bacteria with sugars and a the actinomycete-nodulated plants, properly re- variety of minerals, and the bacteria provide the ferred to as actinorhizal plants, these nodules host with a usable supply of nitrogen. In the have a well-defined structure and a rather strik- case of the legumes, the bacterium may be one ing appearance, being densely branched and of several different species of the Rhizo- more or less spherical. The nodules are peren- bium, a rod-shaped bacterium found naturally nial and increase in size each year, eventually in most soils and available commercially in becoming over an inch in diameter. In some most seed catalogs as inoculant. In the plants, such as alder, the individual lobes that nonlegumes, the bacterium involved is an acti- make up the nodule are very tight and compact. nomycete, or filamentous bacterium, in the In others, such as sweet fern, each lobe of the genus Frankia. Unlike the , which exist nodule grows out into an upwardly growmg root as discrete cells, the actinomycetes grow in long creating a sort of witch’s broom effect. chains of cells similar to fungal hyphae, but These nodules are the site of nitrogen fixa- much smaller. All of the plants infected by tion. Both and Frankia bacteria pos- Frankia, with one exception, are trees and sess special enzymes, , that allow shrubs, whereas among the legumes both them to transform the nitrogen gas in the air annual herbs and trees may be infected by into which, in turn, is converted rhizobia. into amino acids. Because the reaction can only The enter the plant through occur in a low oxygen environment, the process the root hairs and grow in the cells of the roots, is often dependent on hemoglobin compounds stimulating them to grow and divide. Basically, found in the nodules, which are virtually iden- 28

Several old nodules of the root system of the southern bayberry, , growing on the sand dunes along the outer banks of North Carolma. The root, with its attached nodules, was exposed by the wmd, which m this area never seems to stop blowmg. tical to those found in the red blood cells of ani- of nearly seventy years of unsuccessful attempts mals. By binding with oxygen, the hemoglobin to isolate the orgamsm responsible for nitrogen in the nodule helps to create the microenviron- fixation from an actinomycete-nodulated plant. ment that the enzyme requires. It In contrast, the faster-growing Rhizobium is interesting to note that when actinorhizal bacterium was first isolated in pure culture plants are grown in water culture, the young, ninety years earlier, in 1888. This disparity is succulent nodules are often pink in color, due to the main reason why the actinorhizal symbiosis the presence of hemoglobin. is not nearly as well understood as the Rhizo- Work on actinorhizal plants took a giant step bium association. forward in 1978 when a research team that in- In general, actinorhizal plants are sunloving cluded Dale Callaham, currently of the Univer- pioneers in early successional stages of revegeta- sity of Massachusetts at Amherst, the late tion of the north and south temperate regions Professor J. G. Torrey of the Harvard Forest (with the exception of and Myrica in Petersham, Massachusetts, and the present species in the tropics). They do best on sandy or author, successfully isolated and grew, inde- swampy soils where nitrogen is scarce and their pendent of its host plant, the slow-growing ability to extract it from the air is a distinct ad- Frankia bacteria responsible for fixing nitrogen vantage. Usually they are not found in shady, in sweet fern (Comptonia peregrina). This was forested situations or on rich farmland, where the first time that the bacteria from any they would lose their competitive advantage. of the nonlegumes had been Most nitrogen-fixing legumes, on the other cultivated independently, and it marked the end hand, are tropical and subtropical herbs that 29

have migrated north and become important in agricultural environments. It is primarily because the legumes are involved with food production that they have attracted the lion’s share of scientific attention, but this situation is changing rapidly. People are becoming aware that the potential value of actinorhizal plants is of equal importance, if not equal conspicuousness, to the legumes. Experi- ments have been conducted by forestry manag- ers in which actinorhizal plants are grown in conjunction with various economically desir- able trees: red alder and Ceanothus with Dou- glas fir on the West Coast, alder with poplar for pulp on the East Coast, and Elaeagnus with black walnut in the Midwest. In all cases, the experiments resulted in richer soil and faster growth rates in the desired species. In the Northeast, sweet fern, bayberry, and Elaeagnus are used extensively for stabilizing roadside bankings and revegetating traumatically dis- turbed ground. Actinorhizal plants have a much longer his- tory of human use in Europe and Asia than they do in North America. In Japan, the Asian species of Myrica and Alnus are grown in association with various conifers to improve the soil and The root system of Comptonia peregnna four weeks stop erosion, while along the northern coast of after mnoculation with a pure culture of Frankia and the British the Europe Isles, buckthorn, bactena. The bactema stimulated the development is cultivated for the rhamnoides, of over fifty-five nodules on this ten-week-old purpose of stabilizing shorelines, as well as for seedling. its edible . Throughout the tropics, the genus Casuarina is not only important in stop- Bayberry is an attractive midsize that ping seashore erosion, but is also an important keeps its longer than most plants and is source of fuel and timber in areas that otherwise covered for most of the winter with waxy gray produce very little. The future looks bright berries (the source of bayberry candles). In for the actinorhizal plants, especially in the addition, bayberry is highly salt tolerant and context of forestry and habitat restoration, as performs equally well near the seashore or on land managers move from experimentation highway embankments where road salt applied into implementation. during the winter tends to accumulate. The The fact that actinorhizal plants grow where arborescent have traditionally been little else can makes them particularly useful thought of only in terms of land reclamation, for covering bare ground. Apart from this func- but recently commercial nurseries have begun tional recommendation, however, many of to recognize their ornamental potential. Finally, these plants are aesthetically pleasing as well. the autumn and Russian olives, which were Various Ceanothus species, which are widely widely planted in the 1970s, have a beautiful grown in mild climates, are covered in spring- silver-gray foliage and that birds love time with blue, pink, or white . Sweet to feed on. Unfortunately, the birds love the fern, which is fast growing and small, is perfect fruits so much they have dispersed the plant for any situation with full sun and sandy soil. well beyond its initial area of cultivation. As 30

The Australian pme, Casuanna equisetifolia, growmg on the beach at Haena Point on the north coast of Kaual, Hawam. Incessant wave action has exposed the massme, layered root system of the tree. No doubt this species’ability to fix atmospheric mtrogen is pmmamly responsible for its ability to surmve the harsh conditions. 31

a result of this weedy tendency, the olives are no longer recom- mended for large-scale erosion- control plantings. Measurements of nitrogen fixation in actinorhizal plants taken over ex- tended periods of time have shown that pure stands of alder bush are capable of fixing up to 280 pounds of nitrogen per acre per year. This is much greater than the amount of ni- trogen fixed by (90 pounds per acre per year) or peas (66 pounds per acre per year), but comparable to that fixed by . Most of the nitrogen fixed by the actinorhizal plants enters the through the decomposition of fallen leaves, twigs, branches, and fine roots. This process is much slower than that which occurs in agricul- tural situations, where leguminous cover crops are plowed into the soil at the end of a single growing season. It is worth keeping in mind that the greatest degree of nitrogen fixa- tion, in legumes and actinorhizal plants alike, occurs when soil levels of nitrogen are relatively low. High levels of nitrogen, applied as fertil- izer, tend to reduce bacterial activity. What this means is that the plant and the bacteria work best together The the Russian olme, when conditions are worst: the silvery gray fohage of Elaeagnus angus- tifolia, can be very stmkmg m the landscape, as shown here at the symbiosis is most effective when it Montreal Botanical Garden. becomes most necessary. To put it another way, the nitrogen-fixation symbiosis is Torrey, J. G., and J. D. Tjepkema, eds. 1979. Symbiotic fixation in a dynamic interaction between two indepen- nitrogen actmomycete-nodulated plants. Botamcal Gazette Special Supplement dent that is entered into when eco- organisms 140.SI-SI26. logical conditions are such that neither partner could survive long without it. Acknowledgments The author thanks Dr. of the De- References John Tjepkema partment of Plant Biology and Pathology, University Callaham, D., P. Del Tredici, and J. G Torrey. 1978. of Mame at Orono, for reviewing this article, and Isolation and cultivation in vitro of the acknowledges the late Dr. John Torrey for his actinomycete causing root nodulation in mspiration. Comptonia Science 199: 899-902. Schwmtzer, C R , and J. D. Tjepkema. 1990. The Biology of Frankia and Actmorhizal Plants. San Diego Peter Del Tredici is Director of Living Collections at the Academic Press. Arnold Arboretum.