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 ( Ͻ 10 to Ͼ 70%). Roots of plants that established in greatest numbers in plant-free zones (colonizers) of disturbed areas were highly AM. Plants with trace ( Ͻ 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 ( Ͼ 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. . 1996. A framework for tackling drought and land degradation. Journal of Arid Environments 33:309-320.

The objective of the study was to determine whether nutrient fluxes mediated by hyphae of vesicular‐arbuscular mycorrhizal (VAM) fungi between the root zones of grass and legume plants differ with the legume's mode of N nutrition. The plants, nodulating or nonnodulating isolines of soybean [Glycine max (L.) Merr.], were grown in association with a dwarf maize (Zea mays L.) cultivar in containers which interposed a 6‐cm‐wide root‐free soil bridge between legume and grass container compartments. The bridge was delimited by screens (44 μm) which permitted the passage of hyphae, but not of roots and minimized non VAM interactions between the plants. All plants were colonized by the VAM fungus Glomus mosseae (Nicol. & Gerd.) Gerd. and Trappe. The effects of N input to N‐sufficient soybean plants through N2‐fixation or N‐fertilization on associated maize‐plant growth and nutrition were compared to those of an N‐deficient (nonnodulating, unfertilized) soybean control. Maize, when associated with the N‐fertilized soybean, increased 19% in biomass, 67% in N content and 77% in leaf N concentration relative to the maize plants of the N‐deficient association. When maize was grown with nodulated soybean, maize N content increased by 22%, biomass did not change, but P content declined by 16%. Spore production by the VAM fungus was greatest in the soils of both plants of the N‐fertilized treatment. The patterns of N and P distribution, as well as those of the other essential elements, indicated that association with the N‐fertilized soybean plants was more advantageous to maize than was association with the N2‐fixing ones.

Photosynthetic data coflected from Pisum sativum L. and Phaseolus vulgaris L. plants at different dages of development were related to symbiotic N2 fiation in the root nodules. The net carbon exchange rate of each leaf varied directly with carboxylation efficiency and inversely with the CO2 compenation point. Net carbon exchange of the lowest leaves reputed to spply fixed carbon to root nodules decined in parfel with H2 evolution from root nodules. The dece in H2 evolution abo coincided with the onset of flowering but preceded the pe-k in N2 fiation activity measured by acetylene-dependent ethylene production.A result of these changes was that the relative efficienc of N2 fiation in pes inreased to 0.7 from a initial value of 0.4. The data reveal that attempts to identify pbotosynthetic contributions of leaves to root noduks will require carefl tming and suggest that the relative efficiency of N2 fixation may be influenced by source-sik relationMips.The importance of photosynthesis for symbiotic N2 fixation in legumes has been inferred from various physiological experiments which altered the availability of photosynthetic products and revealed a corresponding change in symbiotic N2 fixation (6,8,17,21,24). Ontogenetic patterns of N2 fixation have been reported for various legumes, but there appear to be no data relating CO2 and N2 reduction in the same plant. The export of photosynthates to the root system has been established to be primarily a function of the lower leaves (14,22). A basis for identifying legumes with an increased potential for N2 fixation may result from understanding CO2 exchange characteristics in these leaves, particularly that of carboxylation efficiency. This latter parameter is of interest because it has been found to exhibit genotypic variation independent of diffusive resistance (1).Numerous studies on symbiotic N2 fixation have utilized the C2H2 reduction assay (5) Carbon Dioxide Fixation. Assimilation of CO2 by attached leaves was measured in a flow-through gas exchange system with apparatus and data-handling procedures as described by Augustine et al. (1). Plants were selected randomly from a uniform stand each week and assayed by inserting each leaf individually into the assimilation chamber and measuring net CO2 exchange at a light intensity of 1,500 ,uEi, with a chamber temperature of 21 C and four CO2 concentration (50, 100, 150, and 300 ,ul/l). Leaves were detached after photosynthetic measurement, and their area determined with a Lambda Instruments LI-3000 area meter. Data obtained for net CO2 uptake and the equilibrium CO2 concentrations in the chamber were computer-plotted (1) as a second order regression line through the data points corresponding to the four CO2 concentrations used. The CO2 compensation point (x intercept) and the slope of the regression line at the compensation point were determined by extrapolation. This slope, an indication of the leaf's capacity to respond to changes in ambient CO2 concentrations, was used as a measure of carboxylation efficienc...

Bean (Phaseolus vulgaris L. cv. Dwarf) roots were inoculated with Rhizobium phaseoli and colonized by the vesicular-arbuscular mycorrhizal (VAM) fungus Glomusfasciculatum Gerd. and Trappe or left uncolonized as controls. The symbiotic associations were grown in an inert substrate using 0, 25, 50, 100, or 200 milligrams hydroxyapatite (HAP) (CaioIPO4161OH12) per pot as a P amendment. Plant and nodule dry weights and nodule activity increased for both VAM and control plants with increasing P availability, but values for VAM plants were significantly lower in all parameters than for controls. Inhibition of growth and of N2 fixation in VAM plants was greatest at the lowest and highest P regimes. It was smallest at 50 milligrams HAP, where available P at harvest (7 weeks after planting) was 5 micrograms P per gram substrate. At this level of P availability, the association apparently benefited from increased P uptake by the fungal endophyte. Percent P values for shoots, roots, and nodules did not differ significantly (p > 0.05) between VAM and control plants. The extent of colonization, fungal biomass, and the fungus/association dry weight ratio increased several fold as HAP was increased from 0 to 200 milligrams. It is concluded that intersymbiont competition for P and photosynthate was the prunary cause for the inhibition of growth, nodulation, and nodule activity in VAM plants. Impaired N2 fixation resulted in N stress which contributed to inhibition of host plant growth at all levels of P availability.Enhancement of N2 fixation by root nodules as a result of improved P nutrition is well documented (12,18,20), and appears to depend on the high P requirement of the bacteriods (3). When the availability of P is low, increased P uptake in legumes colonized by VAM2 fungi enhances host plant growth and also stimulates N2 fixation (2,12,14). This has been defined as mycotrophic growth (15 significant sink for carbohydrates (6). Nodulation and N2 fixation have also been shown to depend directly on the availability of carbohydrates (9). Competition for photosynthates by the microsymbionts of the tripartite legume/Rhizobium/VAM fungal association therefore appears likely.The benefits of enhanced nutrient uptake by VAM fungi may be counteracted by the loss of carbohydrates from the host to the fungal endophyte (21). The concentration of P in the substrate appears to be crucial in determining whether VAM colonization will be beneficial, detrimental, or will occur at all. When P is extremely limiting, growth of both symbionts is inhibited (12). When P availability is low, enhanced growth of the host occurs (mycotrophy; Ref. 15). At intermediate levels of P, fungal proliferation may be at the expense of the host without enhancing P uptake ( 13), while at the high levels of P fungal growth is inhibited (21). In a nonsorbing medium, such as the sand-perlite culture with HAP as the P source used in the present investigation, P concentration at the absorbing root or fungal surface depends on the distance between absorbi...