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Index >> Rhizobium and Legume Root Nodulation >>Nodulated Leguminous Trees

Rhizobium and Legume Root Nodulation
Leguminous Plants
The Discovery of Symbiotic Nitrogen Fixation
Rhizobium Classification
Cultural Characteristics
Genetics
Flavonoids Induce Nodualation
Novel Approaches to Extend Root Nodulation
Ecology
Infection
Lectins
Structure of the Nodule
Function of the Nodule
Nitrogenase Hydrogenase Interrelationship
Stem Nodulating Legumes
Factors Affecting Nodulation
Temperature and Light
Hydrogen Ion Concentration
Mineral Nutrition

Nodulated Leguminous Trees

	Nodulated Leguminous Trees The bulk of tree species come from the sub-families caesalpinioideae and Mimosoideae of the family leguminoseae. The other sub-family papilionoideae has about 1000 tree species. Uprooting trees for studying nodulation is a difficult task and in adult trees nodules are often scarce and get dislodged from the main root. These nodules are often black in colour, resembling callus growth or tumours.
They have various shapes because they are often indeterminate in growth. The mode of entry of Rhizobium into roots is generally through root epidermis at the point of origin of lateral roots. Not all tree legumes are nodulated and if nodulated many do not possess extensive root nodules. Non-nodulated leguminous trees have extensive root system that are often deep rooted to tap nutrients. Non-nodulating Acacia albida and Cercidium sp. grow excellently in dry areas of Africa. Trees such as Tamarindus indica and Ceratonia siliqua grow and survive for over 100 years inspite of the fact they remain non-nodulalted, apparently deriving nitrogen from soil. Measurement of actual amounts of N₂ fixed by nodulating tree legumes is a tricky proposition. Eventhough acetylene reduction measurements are often unreliable, they indicate low fixation in situ for many trees except for Leucaena leucocephala where fixation of 110 kg N /ha^-1 in a low rainfall area in Tanzania has been reported.
Only few reports on natural abundance of ¹⁵N method which is more reliable than acetylene reduction method, are available. In Tanzania for instance Prosopis sp. was found to fix 25-30 kg N/ha^-1/year^-1, which had nodules on roots at a depth 4-5 m. Rhizobium isolates from tree species are generally slow growing and have been assigned to the genus Bradyrhizobium. Wide variations in Rhizobium specificity have been observed in isolates from tree species when examined by conventional cross-inoculation grouping or by numerical taxomony.
There have been reports. of nodulation of non-legumes by rhizobia. These include the well substantiated root nodulation of Parasponia spp. and the not so well authenticated reports of nodulation of Zygophyllum and Tribulus spp. In agroforestry, co-cultivation of leguminous trees with other plants have been found to be beneficial. In Nigeria and other African countries, it has been observed that maize contained significantly more nitrogen in plots where Rhizobium inoculated Leuceana plants were also grown when compared to plots where maize was grown along with Leuceana plants which were not inoculated with Rhizobium. Superior growth of non-leguminous trees such as Quercus rubra, Liquidambar styraciflua, Liriodendron tulipifera (yellow poplar) and Juglans regia (black walnut) was observed in areas previously planted with nodulated leguminous tree Robinia pseudo-acacia (black locust) for 23 years, thereby indicating the ability of this leguminous tree to leave behind nitrogenous rich manure behind in soil.
	Many more examples of this kind are available in literature mostly from temperate forests. In developing tropical countries, nodulated leguminous trees such as Prosopis spp. Leucaena leucocephala (Su-babul), Acacia nilotica, Dalbergia sissoo, Pongamia glabra, Pterocarpus indicus, Acacia auriculiformis, Acacia mangium, Calliandra calothyrsus, Sesbania bispinosa, Albizzia spp. arid others are planted to reclaim marginal denuded land.

In mixed cropping or as ground cover, herbaceous legumes such as Stybosanthes, Calapagonium and Pueraria have been planted in tree plantations to replenish soil nitrogen.

Very few reports are available on mycorrhizal habit among trees. Many species of trees from ceasalpinioideae have been shown to be ectomycorrhizal. There appears to be widespread occurrence of AM fungi among tree species. Studies have been made on the interaction of AM fungi (Glomus, Acaulospora, Gigaspora) with Rhizobium isolates from Leucaena leucocephala and benefits due to the associative effects to these two symbionts on growth and phosphorus nutrition of the plant recorded.

Similar results have been obtained with Stylosanthes sp. Since many studies have been carried out mainly from temperate regions, it is necessary to obtain factual data on tree nodulation and benefits accured by tree plantations in single or mixed cropping under tropical conditions to facilitate generalizations on the benefits of nodulated trees to the ecosystem.

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