Legumes are different: Leaf nitrogen, photosynthesis, and water use efficiency

Using robust, pairwise comparisons and a global dataset, we show that nitrogen concentration per unit leaf mass for nitrogen-fixing plants (N2FP; mainly legumes plus some actinorhizal species) in nonagricultural ecosystems is universally greater (43–100%) than that for other plants (OP). This difference is maintained across Koppen climate zones and growth forms and strongest in the wet tropics and within deciduous angiosperms. N2FP mostly show a similar advantage over OP in nitrogen per leaf area (Narea), even in arid climates, despite diazotrophy being sensitive to drought. We also show that, for most N2FP, carbon fixation by photosynthesis (Asat) and stomatal conductance (gs) are not related to Narea—in distinct challenge to current theories that place the leaf nitrogen–Asat relationship at the center of explanations of plant fitness and competitive ability. Among N2FP, only forbs displayed an Narea–gs relationship similar to that for OP, whereas intrinsic water use efficiency (WUEi; Asat/gs) was positively related to Narea for woody N2FP. Enhanced foliar nitrogen (relative to OP) contributes strongly to other evolutionarily advantageous attributes of legumes, such as seed nitrogen and herbivore defense. These alternate explanations of clear differences in leaf N between N2FP and OP have significant implications (e.g., for global models of carbon fluxes based on relationships between leaf N and Asat). Combined, greater WUE and leaf nitrogen—in a variety of forms—enhance fitness and survival of genomes of N2FP, particularly in arid and semiarid climates.

Use ofFrankiaand Actinorhizal Plants for Degraded Lands Reclamation

Degraded lands are defined by soils that have lost primary productivity due to abiotic or biotic stresses. Among the abiotic stresses, drought, salinity, and heavy metals are the main threats in tropical areas. These stresses affect plant growth and reduce their productivity. Nitrogen-fixing plants such as actinorhizal species that are able to grow in poor and disturbed soils are widely planted for the reclamation of such degraded lands. It has been reported that association of soil microbes especially the nitrogen-fixing bacteriaFrankiawith these actinorhizal plants can mitigate the adverse effects of abiotic and biotic stresses. Inoculation of actinorhizal plants withFrankiasignificantly improves plant growth, biomass, shoot and root N content, and survival rate after transplanting in fields. However, the success of establishment of actinorhizal plantation in degraded sites depends upon the choice of effective strains ofFrankia. Studies related to the beneficial role ofFrankiaon the establishment of actinorhizal plants in degraded soils are scarce. In this review, we describe some examples of the use ofFrankiainoculation to improve actinorhizal plant performances in harsh conditions for reclamation of degraded lands.

An overview of actinorhizal plants in Africa

A compilation and synthesis of information derived from plant databases and other sources on the occurrence, diversity and geographic distribution of actinorhizal plants in Africa is presented in this review. Actinorhizal plants are a specific group of non-leguminous, woody dicots having symbiotic, nitrogen-fixing root nodules that are induced on roots of actinorhizal plant species by soil actinomycetes of the genus Frankia. There is a lack of basic information on actinorhizal plants in Africa compared with other major land masses in the world. Results indicate that most, if not all, African countries and climatic regions have native or introduced actinorhizal species. A synthesis of available information indicates that there are six families, nine genera and 38 reported species of actinorhizal plants in Africa. Of these, 21 species are native and 17 are exotic. The families and corresponding number of species in each genus are: Betulaceae (native Alnus glutinosa (1), exotic Alnus (2)); Casuarinaceae (exotic Casuarina (5), exotic Allocasuarina (3), exotic Gymnostoma deplancheana (1)); Coriariaceae (native Coriaria myrtifolia (1)); Myricaceae (native Morella (19), exotic Morella cerifera (1)); Rhamnaceae (exotic Ceanothus caeruleus (1), exotic Colletia paradoxa (1)); and Elaeagnaceae (exotic Eleaegnus angustifolia (1)). Four reports of native, actinorhizal Ceanothus species in Africa found in the database were determined to be false, instead, being non-actinorhizal species. Widespread plantings of exotic Casuarinaceae have been introduced into tropical and arid zones of Africa as multipurpose trees, especially in arid regions where native species do not occur. There is a diverse assemblage of native species of Morella in Africa, mostly shrubs or small trees, which provide medicine, other useful chemicals and wildlife habitat. Many native Morella species are isolated in montane islands, apparently leading to greater speciation than in Eurasia from where the genus migrated into Africa. The current status and knowledge of African actinorhizal plants indicates a need to focus research on their biogeography, biology, ecology, genetics and use.

Reproductive ecology of Ochetophila trinervis in Northwest Patagonia

Native actinorhizal species Ochetophila trinervis (Gillies ex Hook. & Arn.) Poepp. ex Miers, also known as Discaria trinervis (Kellermann et al. 2005), grows along watercourses and distant from them, along a rainfall gradient in north-west Patagonia. We studied the reproductive ecology of this species under different macro- and micro-environmental conditions, in three zones in a rainfall gradient (western, intermediate and eastern) with two sites for each one, near and distant to a watercourse (riparian and dry-land). We performed field studies and germination trials. Plant size, reproductive effort (seed abundance per branch) and seed bank size of O. trinervis, were favoured by the proximity of streams in the drier environments of the gradient. The abundance of seedlings and saplings in the field was very low, which was in agreement with a lack of germination in the field, despite the good germination capacity of seeds. Sexual reproduction of O. trinervis was affected by the low abundance of seedlings. Lower rainfall, higher temperatures and a longer growing season along the gradient favour the potential regeneration of the species. Because of its nitrogen-fixing capacity and other features, O. trinervis has potential for reclamation of eroded lands in Patagonia. Patterns of seed biology and regeneration presented in this study will aid in the use of O. trinervis in the reclamation of disturbed lands.

Nodulation and Growth of Alnus nitida and Alnus maritima Inoculated with Species-specific and Nonspecific Frankia

Actinorhizal plants form N2-fixing symbioses with soil-borne bacteria of the genus Frankia. Potential exists for development of sustainable, actinorhizal nursery crops that obtain most of their required N through N2 fixation, but information on host-symbiont specificity, presence of compatible Frankia in soils, and techniques to inoculate during plant production is lacking. Our objectives were to determine the effect of inoculum type and source and the effect of supplemental N on nodulation, growth, and N content of two actinorhizal species, Alnus nitida (Spach) Endl. and Alnus maritima (Marsh.) Muhl. ex Nutt. Plants of both species were subjected to one of four inoculum treatments (two crushed-nodule inocula: species-specific and cross inoculation, and two soil inocula: soil collected beneath native Alnus rubra Bong. in Washington state and native prairie soil from Iowa), were supplied fertilizer with or without N, and were grown in a greenhouse for 22 weeks. Inoculated plants nodulated, grew larger and faster, and accrued greater N content than uninoculated controls in both fertilizer treatments. Plants that received species-specific inoculum grew larger, acquired more dry weight from symbioses, and accumulated higher N content than cross-inoculated plants. Plants of A. nitida inoculated with soil from Washington state grew larger and accumulated more dry weight from symbioses than those inoculated with prairie soil, but A. maritima grew similarly with soil inoculum from both sources. Our results demonstrate that A. nitida and A. maritima can benefit from N2-fixing symbiosis during production and that potential exists for development of superior inocula and inoculation techniques.

Location and Anatomy of Nodules on Alnus maritima Subjected to Flooding

Although many species of Alnus Miller grow in wet soils, none is as closely associated with low-oxygen, waterlogged soils as Alnus maritima (Marsh.) Muhl. ex Nutt. (seaside alder). An actinorhizal species with promise for use in horticultural landscapes, land reclamation, and sustainable systems, A. maritima associates with Frankia Brunchorst, thereby forming root nodules in which gaseous nitrogen is fixed. Our objective was to determine how root-zone moisture conditions influence the occurrence, location, and anatomy of nodules on A. maritima. Plants of Alnus maritima subsp. maritima Schrader and Graves were established in root zones with compatible Frankia and subjected to four moisture regimens (daily watered/drained, partially flooded, totally flooded, and totally flooded with argon bubbled through the flood water) for 8 weeks. Oxygen content of the root zone, number and location of nodules on root systems, and dry weight and nitrogen content of shoots were determined. Root-zone oxygen content ranged from 17.3 kPa for daily watered/drained plants to 0.9 kPa for argon-treated plants. Across all treatments, 87% of the nodules were within the upper one-third (4 cm) of the root zone. Although shoot dry weights of daily watered/drained and partially flooded plants were not different, daily watered/drained plants had more nitrogen in their leaves (2.53 vs. 2.21 mg·g-1). Nodulation occurred in all treatments, but nodules on totally flooded roots (with or without argon) were limited to a single lobe; in contrast, multilobed nodules were prevalent on partially flooded and daily watered/drained plants. Frankia infection within submerged nodule lobes was limited to one or two layers of cortical cells. Submerged nodules developed large air spaces between cortical cells, and phenolic-containing cells appeared to inhibit Frankia expansion within the nodule. These data suggest that access to root-zone oxygen is critical to the Frankia-A. maritima subsp. maritima symbiosis, and that plants of this subspecies in the drained soils of managed landscapes may benefit more than plants in native wetland habitats from nodulation and nitrogen fixation.

Adaptations of Alnus maritima Nodules to Low Oxygen in the Root Zone

Alnus maritima (Marsh.) Muhl. ex Nutt. is unique among alders in its degree of preference for low-oxygen soils of wetlands. An actinorhizal species with promise for use in sustainable horticulture, A. maritima develops a root-nodule symbiosis with nitrogen-fixing Frankia. Nodules of other actinorhizal species that are obligate wetland natives are adapted to low oxygen, and expression of hemoglobin is common to these taxa. Our objectives were to determine the range of oxygen tension under which Alnus maritima subsp. maritima fixes nitrogen and to investigate a potential role for hemoglobin in adaptation of nodules to low oxygen. Roots of plants, cultured aeroponically, were subjected to eight oxygen tensions from 0 to 32 kPa. After four weeks, plant dry weight, nodule fresh weight, nitrogenase activity, and photosynthetic rate were measured. In addition, nodules were assayed spectrophotometrically for the presence of hemoglobin. A quadratic function best described the influence of oxygen on plant dry weight, nodule fresh weight, nitrogenase activity, and photosynthetic rate with maximal values above 20 kPa. Alnus serrulata (Ait.) Willd. is sympatric with A. maritima subsp. maritima but is not an obligate inhabitant of wetlands. In a separate experiment, we found higher nitrogenase activity in A. maritima subsp. maritima than in A. serrulata (0.74 vs. 0.26 μmol/h per plant) at hypoxic oxygen tensions. Further, optical absorption spectra of nodule extracts confirmed hemoglobin within nodules of A. maritima subsp. maritima. Our data suggest that hemoglobin contributes to oxygen regulation in nodules of A. maritima subsp. maritima.

Nodular symbionts ofShepherdia,Alnus, andMyricafrom a sand dune ecosystem: trends in occurrence of soilborneFrankiagenotypes

A successional sand dune system along the Lake Michigan shoreline was chosen to study the impact of edaphic factors, vegetation cover, and topographic position on Frankia strain distribution and infectivity. On this site, three actinorhizal species, Myrica gale L., Alnus incana (L.) Moench subsp. rugosa (Du Roi) Clausen, and Shepherdia canadensis (L.) Nutt., grew in different communities. Soil samples were collected on plots devoid of actinorhizal plants and serially diluted to inoculate the three native host plants in a greenhouse study. Strains present in the nodules formed were then genetically characterized using PCR-RFLP of the 16S–23S intergenic spacer (IGS). An additional study site was included to estimate the impact of the sympatric presence of the three host species on soil infectivity and strain diversity. On this second site, soils used as inocula were collected in the rhizosphere of M. gale and S. canadensis. The M. gale and A. incana nodular strains belonged to an homogeneous cluster, whereas the S. cana densis nodular strains were separated into two distinct genetic clusters, irrespective of edaphic conditions and proximity to the host's root systems. A χ2analysis conducted on Shepherdia-infective strains showed the dominance of two distinct genotypes, with one of them being specific to newly formed dunes lacking plant cover and the other specific to older, stable dunes with dense vegetative cover.Key words: Frankia, Myrica, Shepherdia, Alnus, IGS 16S–23S, sand dunes.