SYMBIOTIC NITROGEN-FIXATION IN DIOCLEA-GUIANENSIS BENTH, A SHADE-TOLERANT AND ALUMINUM-ACCUMULATOR LEGUME SPECIES NATIVE OF TROPICAL CLOUD FORESTS

Understorey seedlings and juveniles as well as adult plants of Dioclea guianensis were analyzed for the content of ureide, alpha-amino-N, ni trate and nonstructural carbohydrate in the different plant compartmen ts. Plants were harvested during the rainy season in a tropical cloud forest located at 1,747 m above sea level. At this time of the year th e photosynthetically active radiation at the understorey stratum was 9 2% lower than that recorded at the forest canopy, and the clay loam so ils had a pH of 4.1, 91% Al saturation and a 3:1 NH4+:NO3- ratio. In t his forest, all individuals of D. guianensis with stem height higher t han 17 cm were nodulated. Nodules were first observed on seedlings wit h stem heights of 10 cm and there was a synchronization between the de pletion of the cotyledon reserves and the onset of the symbiotic nitro gen fixation. For all individuals, the determinate nodules formed by a Bradyrhizobium strain were located mainly at the 0-20 cm uppermost so il horizon. The ultrastructural analysis of nodules revealed bacteroid units with 4+/-1 rod-shaped bacteroids containing a dense fibrillar n uclear material and scarce poly-3-hydroxybutyric acid grains. The nodu le mass increased with plant height and nodulated juveniles showed sig nificantly lower shoot nitrate and higher shoot/root ratios than non-n odulated individuals. In seedlings and juvenile plants the nodulation process induced a significant increase in the ureide content in leaves , roots and shoots. In addition, a significant decrease was observed i n the alpha-amino-N and non-structural carbohydrate content in leaves and roots, when compared to values in non-nodulated individuals of sim ilar stem heights. The rates of nitrogen fixation, estimated from meas urements of the relative abundance of ureides (RAU) in shoots, ranged from 90% in seedlings and juveniles to 60% in adult plants with leaves exposed to full sunlight. Such ontogenetic changes in RAU appear to b e related to a significant progressive increase in the shoot nitrate c ontent rather than to alterations in the plant ureide content. Based o n present results D. guianensis was categorized as a shade-tolerant, g ood N-2-fixer and Al-accumulator species. The tolerance to shade, the accumulation of aluminium in roots and leaves and the partial exclusio n of this ion from nodule tissues were considered to be the probable m echanisms underlying the efficiency of the symbiotic nitrogen fixation under the tropical cloud forest conditions.