Significance of Legumes for the Distribution of Plant Species in Grassland Ecosystems at Different Altitudes in the Alps

The composition of plant species in grassland communities is determined in part by negative and positive interactions among neighbouring plants; such interactions could be dispersal or - in the case of legumes - symbiotic N2 fixation or resource acquisition. We hypothesised that small scale distribution of plant species may be in part affected by (i) the plant's narrow dispersal in the close neighbourhood, (ii) by the N input into the soil through symbiotic N2 fixation by legumes, and (iii) by small scale heterogeneity of N availability in the soil. To address these hypotheses, in four species-rich semi-natural and natural grasslands along an altitudinal gradient from 900 to 2100ma.s.l., the association of species that grew within a radius of 5cm of selected plant species (central plant, CP) was studied. The distribution of the plant species within the plant community was not random and the species composition that grew within a radius of 5cm of a CP varied with altitude. The strength of the association between a CP and its neighbouring species decreased with increasing altitude. The 5cm radius was most dominated by individuals of the same species as the CP, suggesting that narrow dispersal (either through seeds or through vegetative propagates) may be most important for the species composition within the sward. δ15N values of non leguminous species growing near legumes compared to non leguminous species that grew far from legumes indicate that N from symbiotic N2 fixation may be another factor influencing the association of plant species. Effects of small scale heterogeneity in apparent N availability in the soil were detected to be the least important for the species compositio

Increased abundance of MTD1 and MTD2 mRNAs in nodules of decapitated Medicago truncatula.

To gain insight into the molecular processes occurring in root nodule metabolism after stress, we used a mRNA differential display (DDRT-PCR) approach to identify cDNAs corresponding to genes whose expression is enhanced in nodules of decapitated Medicago truncatula plants. Two full-length cDNAs of plant origin were isolated (MTD1 and MTD2). Sequence analysis revealed that MTD1 is identical to an EST clone (accession number AW559774) expressed in roots of M. truncatula upon infection with Phytophthora medicaginis, while MTD2 is highly homologous to an Arabidopsis thaliana gene (accession number AL133292) coding for a RNA binding-like protein. The two mRNAs started to accumulate in root nodules at 4 h after plant decapitation and reached even higher transcript levels at 24 h from the imposition of the treatment. MTD1 and MTD2 mRNAs were mainly induced in nodules, with very little induction in roots. The abundance of the two transcripts did not change in response to other perturbations known to decrease nitrogenase activity, such as nitrate and Ar/O2 treatments. Our results suggest that MTD1 and MTD2 represent transcripts that accumulate locally in nodules and may be involved in changes in nodule metabolism in response to decapitation

Increase in the concentrations of amino acids in the vascular tissue of white clover and white lupin after defoliation: an indication of a N feedback regulation of symbiotic N2_2 fixation

Defoliation, particularly experienced by pasture and forage legumes, is associated with a fast and dramatic decrease in the nitrogenase activity (C$_2$H$_2$ reduction, H$_2$ evolution in Ar/O$_2$). Since the plant's N demand is temporarily reduced after defoliation, the nitrogenase activity may be down-regulated through a N feedback mechanism. To test this hypothesis, the concentrations of free amino acids in the xylem of white clover and in the phloem of white lupin were measured before and after defoliation. The concentrations of free asparagine, glutamine and aspartate were the highest of all the amino acids before defoliation. The concentrations of these amino acids increased three-fold in the xylem of white clover and two-fold in the phloem of white lupin within one to three hours after defoliation. The results are consistent with the concept of a N feedback mechanism that regulates nitrogen fixation in legume plants, in which free amino acids, cycling in the vascular tissue, act as trigger molecules.Accroissement de la concentration d'acides aminés dans le tissu vasculaire du trèfle blanc et du lupin blanc après défoliation : une indication de la régulation par rétroaction de la fixation symbiotique d'azote. La défoliation subie plus particulièrement par les légumineuses fourragères pâturées ou fauchées, est suivie par une décroissance rapide et spectaculaire de l'activité de la nitrogénase (réduction de C$_2$H$_2$, évolution de H$_2$ dans Ar/O$_2$). Puisque la demande d'azote de la plante est temporairement réduite après la défoliation, l'activité de la nitogénase devrait être abaissée grâce à un mécanisme de rétroaction de l'azote. Pour tester cette hypothèse, la concentration de trois acides aminés dans la xylème du trèfle blanc et dans le phloème du lupin blanc a été mesurée avant et après la défoliation. La concentration de l'asparagine de la glutamine et de l'aspartate libres était la plus élevée de toutes celles des acides aminés avant la défoliation. Elles ont triplé dans le xylème du trèfle blanc et doublé dans le phloème du lupin blanc dans une période comprise entre une et trois heures après la défoliation. Ces résultats sont en bon accord avec le concept d'un mécanisme de rétroaction de l'azote qui régule la fixation de l'azote dans les légumineuses, dans lequel les acides aminés libres, circulant dans le tissu vasculaire, agissent comme des molécules de déclenchement