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Symbiotic N<inf>2</inf>-fixation in tropical legume species growing at high geographic elevation

Abstract:

Thirteen nodulated suffructicose tropical legume species were collected during the rainy season in full sun-exposed areas of a tropical cloud forest, between 1,400 and 1,650 m above sea level. Annual rain fall averaged 1,080 mm, and the soil and air temperature were 25°C and 22°C, respectively. The soils were clay-loan with poor drainage. According to vegetation inventories carried out in highland and lowland areas the tropical legumes were considered as opportunistic ecological generalist species capable to colonize areas of contrasting geographic elevation. Based on the values of the relative abundance of ureides (RAU), the highland species Chamaecrista patellaria, Aeschynomene brasiliana, Calopogonium mucunoides, Crotalaria anagiroides, Desmodium barbatum, Indigofera lespedezioides, Stylosanthes leiocarpa, Vigna vexillata and Mimosa orthocarpa were categorized as good N2-fixers (RAU>60%). Whereas, Calopogonium sp., Desmodium intortum and Indigofera suffructicosa behaved as intermediate N2-fixers (RAU 30-59%). Low rates of N2-fixation (RAU<30%) were detected only in individuals of Mimosa albida. These differences in RAU among the highland species were not correlated with any of the ultrastructural characteristics of nodule infected cells such as the presence of fibre-like structures in the peribacteroid space, number of peribacteroid units per 100 mm, shape of bacteroids, number of bacteroids per peribacteroid unit, or number of poly-β-hydroxybutyrate grains per bacteroid. For all highland legume species the mean RAU value was significantly higher than the mean value reported for all lowland legumes. The enhanced symbiotic efficiency in highland species was a consequence of higher leaf non-structural carbohydrate content and a 2.2 fold increase and 1.7 fold reduction in the shoot ureide and nitrate content, respectively, when compared to values of lowland legumes. The higher ureide:nitrate ratio in highland legumes allowed the assumption that symbiotic N2-fixation prevails above nitrate uptake under the favorable climatic conditions encountered by legumes at high geographic elevation.