The importance of Olneya tesota as a nurse plant in the Sonoran Desert

Abstract. The function of Olneya tesota (ironwood) as a nurse plant and habitat modifier species in the Sonoran Desert was evaluated at five study sites (using 75 250-m² sample plots) from Bahia Kino, Sonora to Organ Pipe Cactus National Monument, Arizona. Beneath the canopy of O. tesota trees 75 perennial plant species were found. A principal component ordination of the plots created three groups: southern, perturbed and protected sites. A strong triple association of columnar cacti, big shrubs with berry type fruits and O. tesota was detected. The relationships of Lophocereus schottii and Peniocereus striatus with O. tesota were studied in more detail. Significant differences in soil surface temperatures and stem temperatures of P. striatus were detected beneath the shade of O. tesota trees compared with soils and plants in open spaces. Olneya tesota must be considered as a habitat modifier species with ecological and conservational importance to the plant communities in the Sonoran Desert.     

Variation in mistletoe seed deposition: effects of intra- and interspecific host characteristics

We investigated differences in host infection by a desert mistletoe, Phoradendron californicum, and examined one of the processes that contributes to these differences: variation in seed deposition among host individuals and species. In the Sonoran Desert, P. californicum parasitizes the sympatric leguminous trees Olneya tesota, Cercidium microphyllum, Prosopis velutina, Acacia constricta, and Acacia greggii. We hypothesized that seed deposition depends on host height and crown architecture. At a site in Arizona, frequency of infection did not reflect host relative abundance. Olneya tesota was parasitized at a higher frequency than expected from its abundance and maintained the highest mistletoe loads per individual host. In contrast, P. velutina was infected less frequently than expected. Infection frequency increased with host tree height for all hosts. Mistletoe seed deposition by avian dispersers differed among host species and was disproportionately high in O. tesota and P. velutina. Seed deposition was higher in infected than in non‐infected host trees, and increased with tree height in O. tesota but not in C. microphyllum. We suspect that increased seed deposition with height in O. tesota may be due to the preference of seed‐dispersing birds for higher perches. Some host tree species, such as C. microphyllum and A. constricta, probably received fewer mistletoe seeds because birds avoid hosts with dense and spiny crowns. Mistletoe populations are plant metapopulations in which host trees are patches and the frequency of infection in each host species/patch type is the result of interspecific differences in the balance between mistletoe colonization and extinction. From this perspective, our study of host use and seed dispersal is a metapopulation study of patch occupancy and propagule distribution among available patch types. Our seed‐dispersal study demonstrates that the mechanisms that create pattern in patchy plant populations can be investigated in mistletoe systems.     

Nitrogen fixation (acetylene reduction) and cross inoculation in 12 Prosopis (mesquite) species

The leguminous tree mesquite (Prosopis spp) exists on millions of hectares of semi-arid regions of the world. No whole plant acetylene reductions for mesquite have been reported in the literature and nodulation has only been reported for three of the forty-four species. We report greenhouse studies in which 12Prosopis species representing African and North and South American germplasm (1) became nodulated when inoculated with rhizobia strain isolated from a North American mesquite, (2) grew on a nitrogen free nutrient media, (3) reduced acetylene to ethylene, and (4) had a positive significant correlation between the acetylene reduction rates and above ground dry matter. The capability of mesquite to fix nitrogen must now be considered firmly established.     

Comparison of acetylene-reduction and nitrogen-15 techniques for the determination of nitrogen fixation by field bean (Vicia faba) nodules

Field bean (Vicia faba L.) cv. Maris Bead seeds were inoculated with Rhizobium Catalogue No. 1001, supplied by Rothamsted Experimental Station, and grown in sand culture supplied with a complete nutrient solution which included nitrate at either 1.5 or 6.0 mM. Nodules were detached from the roots at intervals during plant development and their rates of nitrogen fixation estimated by both acetylene reduction and ¹⁵N gas technique. There was a constant relationship, independent of nitrate supply, between the results obtained by these two methods at all samplings. The amounts of acetylene reduced divided by a factor of 5.75 gave the amount of true nitrogen fixation; this factor is about twice the theoretical value. It is suggested that this discrepancy arose because, with acetylene, all the electrons available to the nitrogenase were used to form ethylene, whereas during normal fixation only about half the electron supply was used to fix nitrogen, the remainder having been consumed in the production of hydrogen gas.     

Antiproliferative Potential of Olneya tesota PF2 Lectin in Human Acute Monocytic Leukemia Cells

Acute monocytic leukemia is a type of myeloid leukemia that develops in monocytes. The current clinical therapies for leukemia are unsatisfactory due to their side effects and nonspecificity toward target cells. Some lectins display antitumor activity and may specifically recognize cancer cells by binding to carbohydrate structures on their surface. Therefore, this study evaluated the response of the human monocytic leukemia cell lines THP-1 to the Olneya tesota PF2 lectin. The induction of apoptosis and reactive oxygen species production in PF2-treated cells was evaluated by flow cytometry, and the lectin-THP-1 cell interaction and mitochondrial membrane potential were evaluated by confocal fluorescence microscopy. PF2 genotoxicity was evaluated by DNA fragmentation analysis via gel electrophoresis. The results showed that PF2 binds to THP-1 cells, triggers apoptosis and DNA degradation, changes the mitochondrial membrane potential, and increases reactive oxygen species levels in PF2-treated THP-1 cells. These results suggest the potential use of PF2 for developing alternative anticancer treatments with enhanced specificity.     

Ecophysiological studies of Sonoran Desert plants

Summary The gas exchange of two Sonoran Desert plants was measured near optimum soil water conditions occurring in the summer and winter. Photosynthesis and stomatal conductance rates of a drought-deciduous shrub, Ambrosia deltoidea, and an evergreen non-riparian tree, Olneya tesota, are mainly affected by plant water potential. During such periods the diurnal gas exchange patterns are characterized by maximum rates of photosynthesis and stomatal conductance occurring early in the day, which decrease progressively thereafter. The effect of plant water potential on gas exchange is both direct and indirect. Decreasing plant water potential indirectly affects ¹⁴CO₂ assimilation by closure of stomata, and the effect is similar in both species. However, the direct effects of decreasing plant water potential are dissimilar in the two species. Plants of the shrub species have a higher potential maximum photosynthesis, but are more sensitive to plant water stress than are plants of the tree species. Both species respond to favorable growth conditions in the summer and winter, and have the potential for rapid carbon input into the Sonoran Desert ecosystem.This research was supported by National Science Foundation Grant BMS 74-02671-A04, through the U.S./I.B.P. Desert Biome     

Ecophysiological studies of Sonoran Desert plants

Summary The gas exchange and water relations of two Sonoran Desert plants was measured throughout a 12-month period. Seasonal photosynthesis patterns of both plants followed the seasonal variation in plant water potential. Ambrosia deltoidea, a drought-deciduous shrub, is mainly winter-spring active since maximum photosynthesis rates of 38 mg CO₂ dm^-2 h^-1 were measured at this time. This plant is characterized by marked seasonal variations in plant water potential, and was deciduous for approximately 120 days when plant water potential was below-50 bars. Olneya tesota, a non-riparian microphyllous tree, is evergreen and photosynthetically active throughout the entire year, although demonstrating maximum photosynthesis rates of 12 mg CO₂ dm^-2 h^-1 in spring and summer. The deep-rooted tree species maintains a favorable year-round water balance since minimum plant water potentials were seldom below-33 bars. The two species maintain a relatively high water use efficiency throughout the year, despite the high evaporative gradient characteristic of the Sonoran Desert.The leaves are the major site for carbon assimilation, contributing 87 and 81% of the annual carbon gain for the shrub and tree species, respectively. Above-ground gross primary production throughout the 12-month period was estimated solely from the leaf ¹⁴CO₂ assimilation studies. This production estimate was compared to above-ground net primary production determined by the harvest method. For both plant species gross production was interpreted to exceed net production by nearly a three-fold difference. On a per plant basis gross production was estimated to be 1.14 and 7.42 kg dry wt plant^-1 yr^-1 for A. deltoidea and O. tesota. The large difference between net and gross production is probably related to year-round utilization of carbon.This research was supported by National Science Foundation Grant BMS 74-02671-A04 through the U.S./I.B.P. Desert Biome at Utah State University     

Nitrogen fixation (acetylene reduction) associated with communities of heterocystous and non-heterocystous blue-green algae in mangrove forests of Sinai

Summary High rates of nitrogen fixation (acetylene reduction) are associated with communities of heterocystous and non-heterocystous blue-green algae, which are widespread and abundant in the coastal mangrove forests of the Sinai Peninsula.Heterocystous forms, particularly representatives of the Rivulariaceae, grow in aerobic environments, where nitrogenase activity may be limited by the availability of nutrients such as Fe and PO₄–P. Desiccated communities of Scytonema sp. reduce acetylene within ten minutes of wetting by tidal sea water. Communities dominated by the non-heterocystous Hydrocoleus sp., Hyella balani, Lyngbya aestuarii, Phormidium sp. and Schizothrix sp., occur in close contact with anaerobic sediments and reduce acetylene in the dark as well as in the light.Nitrogen fixation in all these communities is light dependant and may be supplemented by an alternative source of reductant in the dark. The indications are that nitrogen fixation by these communities of blue-green algae, makes a significant contribution to the overall nitrogen input of the mangrove ecosystem.     

Seasonal variation in rates of heterotrophic nitrogen fixation (acetylene reduction) in Zostera noltii meadows and uncolonised sediments of the Bassin d'Arcachon, south-west France

Nitrogen fixation (acetylene reduction) rates were measured over an annual cycle in meadows of the seagrass Z. noltii and uncolonised sediments of the Bassin d'Arcachon, south-west France, using both slurry and whole core techniques. Measured rates using the slurry technique in Z. noltii colonised sediments were consistently higher than those determined in isolated cores. This was probably due to the release of labile organic carbon sources during preparation of the slurries. Thus, in colonised sediments the whole core technique may provide a more accurate estimate of in situ activity. Acetylene reduction rates measured by the whole core technique in colonised sediments were 1.8 to 4-fold greater, dependent upon the season, in the light compared with those measured in the dark, indicating that organic carbon released by the plant roots during photosynthesis was an important factor regulating nitrogen fixation. In contrast acetylene reduction rates in uncolonised sediments were independent of light.Addition of sodium molybdate, a specific inhibitor of sulphate reduction inhibited acetylene reduction activity in Z. noltii colonised sediments by > 80% as measured by both slurry and whole core techniques irrespective of the light regime, throughout the year inferring that sulphate reducing bacteria (SRB) were the dominant component of the nitrogen fixing microflora. A mutualistic relationship between Z. noltii and nitrogen fixing SRB in the rhizosphere, based on the exchange of organic carbon and fixed nitrogen is proposed. In uncolonised sediments sodium molybdate initially severely inhibited acetylene reduction rates, but the level of this inhibition declined over the course of the year. These data indicate that the nitrogen fixing SRB associated with the Zostera roots and rhizomes were progressively replaced by an aerobic population of nitrogen fixers associated with the decomposition of this recalcitrant high C:N ratio organic matter.Acetylene and sulphate reduction rates in the seagrass beds showed distinct summer maxima which correlated with a reduced availability of NH ₄ ⁺ in the sediment and the growth cycle of Z. noltii in the Bassin. Overall, these data indicate that acetylene reduction (nitrogen fixation) activity in the rhizosphere of Z. noltii was regulated both by release of organic carbon from the plant roots and maintenance of low ammonium concentrations in the root zone due to efficient ammonium assimilation.Nitrogen fixation rates determined from acetylene reduction rates measured by the whole core technique ranged from 0.1 to 7.3 mg N m^–2 d^–1 in the Z. noltii beds and between 0.02 and 3.7 mg N m^–2 d^–1 in uncolonised sediments, dependent upon the season. Nitrogen fixation in the rhizosphere of Z. noltii was calculated to contribute between 0.4 and 1.1 g N m^–2 y^–1 or between 6.3 and 12% of the annual fixed nitrogen requirement of the plants. Heterotrophic nitrogen fixation therefore represents a substantial local input of fixed nitrogen to the sediments of this shallow coastal lagoon and contributes to the overall productivity of Z. noltii in this ecosystem.     

Contribution from nitrogen fixation (acetylene reduction) to the nitrogen budget of Lake Tohopekaliga (Florida)

Nitrogen fixation, as assayed by the acetylene reduction technique, provided 44% of the input of nitrogen to a lake in central Florida (Lake Tohopekaliga) during 1984. Ninety-four percent of the lake total fixation was found in the water column and associated with Anabaena spp. The lake-wide average nitrogen fixation rate of 5.7 g N/m²-yr amounted to a mass loading of 497 metric tons of nitrogen for the year, and is one of the highest nitrogen fixation rates reported.     

Nurse Plants, Mycorrhizae, and Plant Establishment in a Disturbed Area of the Sonoran Desert

Arbuscular-mycorrhizal (AM) fungi stabilize the soil and enhance plant growth by alleviating nutrient and drought stress. Their contributions to agriculture are well known, but their role in desert ecosystems has received less attention. The AM status of perennial plants in disturbed and undisturbed plots were investigated in the Sonoran Desert near La Paz, Baja California Sur, Mexico to determine if AM fungi contribute to resource-island stability and plant establishment. All perennial plants (46 species) in the study plots were AM, but root colonization varied widely (<10 to> 70%). Roots of plants that established in greatest numbers in plant-free zones (colonizers) of disturbed areas were highly AM. Plants with trace (<10%) root colonization (cacti of the tribe Pachycereae: Pachycereus pringlei, Machaerocereus gummosus, and Lemaireocereus thurberi; and Agave datilyo) established preferentially in association with nurse trees. The pachycereid cacti grew under Prosopis articulata and A. datilyo under Olneya tesota canopies. Of the nine species of trees and arborescent shrubs in the area, the mature (>20 yr) nurse-legumes P. articulata and O. tesota supported the largest number of under-story plants. Younger plants had only occasional associates. AM propagule densities in plant-free areas were lower than under plant canopies (40 vs. 280 propagules/kg soil). Occurrence of soil mounds (islands) under plants owing to soil deposition was related to the nature of the canopies and to the AM status of the roots. Island soils were enmeshed with AM-fungal hyphae, especially in the upper layer (approximately 10 cm). Seedlings of P. pringlei, growing in a screenhouse for six months in soil collected under P. articulata, had a biomass ten times greater than plants growing in bare-area soil. The results are consistent with the proposition that AM fungi contributed to the plant-soil system of our study area by: (1) helping to stabilize windborne soil that settles under dense plant canopies; (2) enhancing the establishment of colonizer plants in bare soils of disturbed areas; and (3) influencing plant associations through differences in the mycotrophic status of the associates.     

Diurnal variation in algal acetylene reduction (nitrogen fixation) in situ

Diurnal variation in algal nitrogen fixation was studied in Lake Mendota, Wisconsin, during the summers of 1971 to 1973. Approximately two-thirds of the daily acetylene reduction in the surface decimeter occurred before noon. The decline in acetylene reduction (nmoles/liter·hr) near midday was partially because the algae relocated themselves at greater depths. However, acetylene reducing activity (nmoles per A₆₆₃ unit chlorophyll a per hour) also decreased as midday approached. Occasionally algae would resurface near the end of the day. On average, acetylene reduction (nmoles per liter per hour) was maximum at about 0900 Central standard time in the top decimeter, and acetylene reduction between 0830 and 0930 Central standard time represented 13% of the total daily acetylene reduction. Furthermore, acetylene reduction in the top decimeter, on average, represented 3.6% of the total acetylene reduction in the column. Calculation of the contribution by nitrogen fixation to a lake's fixed nitrogen budget is discussed.     

Nitrogen Fixation Associated with Sand Grain Root Sheaths (Rhizosheaths) of Certain Xeric Grasses

Nitrogen fixation (acetylene reduction) was found to be associated with sand grain root sheaths (rhizoseaths) occurring on the following xeric grasses: Oryzopsis hymenoides (Roem. and Shult.) Ricker, Agropyron dasystachyum (Hook.) Scrib., Stipa comata Trin. and Rupr., and Aristida purpurea Nutt. Acetylene reduction rates associated with whole plant specimens of these species varied from 515 to 920 nmol C₂H₄/(g dry wt.) × (6 days). Nitrogenase activity was shown to be associated with the rhizosheaths. Bacillus polymyxa-like nitrogen fixers were isolated from the rhizosheaths of each grass. The isolates reduced acetylene and assimilated ¹⁵N₂.     

ROLE OF THE NITROGEN-FIXING BACTERIAL MICROFLORA IN THE EPIPHYTISM OF TILLANDSIA (BROMELIACEAE)

Nitrogen-fixing activity in the phyllosphere of 12 species of Tillandsia from different Mexican habitats was evaluated by the acetylene reduction assay, and nitrogen-fixing microorganisms were isolated and characterized. The leaves from eight of the 12 Tillandsia species examined exhibited nitrogenase activity in enrichment cultures. Among the microorganisms implicated—Agrobacterium, Bacillus, Erwinia, Pseudomonas, Rahnella, Vibrio, and Xanthomonas—only Bacillus megatherium reduced acetylene in pure culture. Our findings suggest that nitrogen fixation in the phyllosphere of the sampled epiphytes occurs under suitable conditions and that most of the bacteria involved are primarily soil and water inhabitants. The results also suggest a relationship between the composition of the nitrogen-fixing microbial communities grown on the leaf and the different development of the leaf area in Tillandsia due to the aerial components (wings) of the trichomes.     

Biological Nitrogen Fixation is not a Major Contributor to the Nitrogen Demand of a Commercially Grown South African Sugarcane Cultivar

It has previously been reported that endophytic diazotrophic bacteria contribute significantly to the nitrogen budgets of some graminaceous species. In this study the contribution of biological nitrogen fixation to the N-budget of a South African sugarcane cultivar was evaluated using ¹⁵N natural abundance, acetylene reduction and ¹⁵N incorporation. Plants were also screened for the presence of endophytic diazotrophic bacteria using acetylene reduction and nifH-gene targeted PCR with the pure bacterial strains. ¹⁵N natural abundance studies on field-grown sugarcane indicated that the plants did not rely extensively on biological nitrogen fixation. Furthermore, no evidence was found for significant N₂-fixation or nitrogenase activity in field-grown or glasshouse-grown plants using ¹⁵N incorporation measurements and acetylene reduction assays. Seven endophytic bacterial strains were isolated from glasshouse-grown and field-grown plants and cultured on N-free medium. The diazotrophic character of these seven strains could not be confirmed using acetylene reduction and PCR screening for nifH. Thus, although biological nitrogen fixation may occur in South African sugarcane varieties, the contribution of this N-source in the tested cultivar was not significant.     

Reassociation of a modified mycorrhiza with the host plant roots (Pinus radiata) and the transfer of acetylene reduction activity

Summary Experiments were carried out to assess the potential for reassociation of modified strains of the mycorrhizal fungus Rhizopogon sp., capable of acetylene reduction activity in vitro, with the roots of its host plant (Pinus radiata). Reassociation was effected and acetylene reduction assays indicated that nitrogenase activity was present in the reassociated whole plants. Those host plants symbiotic with the modified strains had higher levels of nitrogen than those associated with the wild type fungus under nitrogen deprived conditions. Uptake of phosphate was unimpaired in the modified mycorrhiza. Electron microscopy showed that hyphae of the modified strains as well as lying in the intercellular spaces were often found within the cells of the root cortex. This was in contrast with the wild type strains where no such intracellular growth was found. One strain was sound to be pathogenic to seedlings of Pinus radiata. re]19760603     

Nitrogen fixation and nitrate utilization by marine and freshwater Beggiatoa

Four newly isolated marine strains of Beggiatoa and five freshwater strains were tested for nitrogen fixation in slush agar medium. All strains reduced acetylene when grown microaerobically in media containing a reduced sulfur source and lacking added combined nitrogen. The addition of 2 mmol N, as nitrate or ammonium salts, completely inhibited this reduction. Although not optimized for temperature or cell density, acetylene reduction rates ranged from 3.2 to 12 nmol·mg prot^-1 min^-1. Two freshwater strains did not grow well or reduce acetylene in medium lacking combined nitrogen if sulfide was replaced by thiosulfate. Two other strains grew well in liquid media lacking both combined nitrogen and reduced sulfur compounds but only under lowered concentrations of air. All freshwater strains grew well in medium containing nitrate as the combined nitrogen source. Since they did not reduce acetylene under these conditions, we infer that they can assimilate nitrate.     

Contribution of nitrogen fixation to nitrogen nutrition in an alpine sedge community (Caricetum curvulae)

Summary In situ acetylene reduction assays (ARA) were carried out over two growing seasons at 2550 m in the upper alpine zone of the Tyrolean Central Alps of Austria. For comparative purposes, some Fabaceae species introduced into the upper alpine zone from lower elevation (2000 m) were subjected to ARA. At the end of the growing season the potted plants were transferred to the laboratory where their acetylene reducing activities were measured again. In situ nitrogenase activity is very low. The highest values were found in association with Leucanthemopsis alpina and Veronica bellidioides (150 and 217 nmol ethylene 24 h^-1 per pot respectively). Higher levels of activity were detected in pots transferred to the laboratory (maximum value 750 nmol ethylene 24 h^-1 per pot; assay temperature about 12°C higher than in the field) and in the Fabaceae transferred to the upper alpine zone (14×10³ nmol ethylene 24 h^-1 per pot of Trifolium badium and T. pallescens). Maximum nitrogen input in the field is in the range of 8 mg m^-2 a^-1. Therefore, under natural circumstances biological nitrogen fixation contributes only very small amounts of nitrogen to this alpine vegetation system, the process being inhibited by low soil temperatures. Possible alternative sources and patterns of N acquisition are discussed in relation to the overall nitrogen economics of plants of the upper alpine zone.     

Acetylene reduction by nitrogen-fixing blue-green algae

Summary Known nitrogen-fixing species of blue-green algae are capable of reducing acetylene to ethylene, but acetylene is not reduced by Anacystis nidulans, which does not fix nitrogen. Cycad root nodules which contain blue-green algae as endophytes reduce acetylene. Acetylene reduction is inhibited by carbon monoxide. Nitrate or ammonium-nitrogen has no immediate effect on algae reducing acetylene, but algae grown on nitrate-nitrogen gradually lose their capacity to reduce acetylene. Nitrate-nitrogen also inhibits heterocyst formation in these algae and there is a fairly direct correlation between the abundance of heterocysts in a particular sample and its capacity to reduce acetylene. Aphanizomenon flosaquae reduces acetylene and fixes nitrogen in unialgal culture and there is strong presumptive evidence that these reductions are carried out by the alga rather than by associated bacteria. The molar ratios of ethylene: ammonia produced vary within the range 1.4–1.8.     

Nitrogen fixation (Acetylene reduction) by free-livingRhizobium meliloti

Evidence is presented thatRhizobium meliloti is able to fix nitrogen (as scored by acetylene reduction) while growing in the absence of a plant host. The highest nitrogen fixation rate was obtained when bacteria were grown in defined liquid medium under normal atmosphere in sealed tubes. The nitrogen fixation rates obtained are of the same order as those reported for other rhizobia.