David P. Janos

Plant responsiveness to mycorrhizas differs from dependence upon mycorrhizas

Soil phosphorus response curves of plants with and without mycorrhizas reflect two different, but complementary, phenomena. The first, plant responsiveness to mycorrhizas, is represented by the difference in growth between plants with and without mycorrhizas at any designated level of phosphorus availability. This is also a measure of mycorrhizal fungus effectiveness. The second, the lowest level of phosphorus availability at which plants can grow without mycorrhizas, is here termed dependence upon mycorrhizas. The latter definition differs from conventional usage which fails to distinguish dependence from responsiveness. Sigmoid curves generated by the three-parameter, logistic equation generally can model the responses of plants to mycorrhizas and phosphorus addition and can be used to assess responsiveness, effectiveness, and dependence. Such curves reveal that plant responsiveness or fungus effectiveness determined at a single level of phosphorus availability may be misleading when used to compare different host species’ intrinsic capacities to respond to different mycorrhizal fungus species. Instead, the same relative position should be evaluated among phosphorus response curves for different species combinations. Dependence of a plant species known to benefit from mycorrhizas can be assessed with reference to only the phosphorus response curve of plants without mycorrhizas. Dependence is a constitutive property of plant species that can be used to classify them as facultatively or obligately mycotrophic. Dependence is a plant attribute upon which natural selection can act, but responsiveness and effectiveness cannot be selected directly because they are emergent properties of the interaction between plant and fungus species.

Phosphorus and intraspecific density alter plant responses to arbuscular mycorrhizas

We investigated how phosphorus availability, intraspecific density, and their interaction affect plant responses to arbuscular mycorrhizas. Four facultatively mycotrophic species: chile, cilantro, tomato, and corn were examined separately in pot experiments that employed a tropical phosphorus-immobilizing soil. Each experiment comprised nine soluble phosphorus additions, two levels of intraspecific plant density, and inoculation with arbuscular mycorrhizal fungi or not. High phosphorus signi- ficantly diminished mycorrhizal colonization of corn, cilantro, and tomato, but not chile, which was highly variably colonized. Corn roots were colonized by other root-inhabiting fungi, and mycorrhizas significantly reduced colonization by these potential root parasites. High phosphorus significantly increased relative growth rates (RGR) of all species, and high density significantly reduced RGR of cilantro, tomato, and corn. Chile showed little growth at any but the highest phosphorus additions, and consequently had no RGR response to density or mycorrhizas. Mycorrhiza inoculation caused transient depression of corn growth during the first month, but mycorrhizas increased corn RGR during the second month of growth. Both RGR and dry weights at harvest, cilantro, tomato, and corn benefited from mycorrhizas at low phosphorus availability, but this benefit diminished or changed to disadvantage as phosphorus availability increased. At low phosphorus availability, high density increased the dry weight of mycorrhizal cilantro and thereby amplified the benefit of mycorrhizas. At high phosphorus availability, increased density diminished the effects of mycorrhizas on dry weight, reducing mycorrhiza benefit to tomato and chile and reducing mycorrhiza detriment to cilantro. This study demonstrates that for three of the four plant species examined, phosphorus availability, intraspecific density, and their interaction significantly modify plant responses to arbuscular mycorrhizas.

Vesicular-arbuscular mycorrhizae of epiphytes

This article introduces reports concerning the occurrence of mycorrhizae on epiphytes in Costa Rica, Ethiopia, Venezuela, Malaysia, and Mexico. Association of vesicular-arbuscular mycorrhizal fungi with the roots of epiphytes is not well known. Vesicular-arbuscular mycorrhizal fungi (VAM) do occur in the canopy, but are uncommon except in certain sites and host taxa. Occurrence of VAM on epiphytes may be constrained by mineral nutrient availability and spatial heterogeneity in the canopy. Nevertheless, epiphytes present unique opportunities to study influences of mycorrhizae on vascular plant community composition and on the evolution of mycorrhizal associations.

Arbuscular mycorrhizal fungi colonize decomposing leaves of Myrica parvifolia, M. pubescens and Paepalanthus sp.

Hyphae and vesicles of arbuscular mycorrhizal fungi (AMF) were found within the decomposing leaves of Myrica parvifolia, M. pubescens and Paepalanthus sp. at three montane sites in Colombia. Hyphae, vesicles, and arbuscule-like structures were also found within scale-like leaves of the rhizomes of Paepalanthus sp. The litter found in the vicinity of the roots was divided into three decomposition layers. The highest AMF colonization occurred in the most decomposed leaves, which were in close association with roots. In contrast, there were no differences in AMF colonization of roots present in the different decomposition layers. Colonization of decomposing leaves by AMF did not differ between the two closely related species M. parvifolia and M. pubescens, nor between two sites (Guatavita and Zipacón, Colombia) differing in soil fertility. Occurrence of vesicles in decomposing leaves was correlated with abundant AMF extraradical hyphae among the leaves. We propose that AMF enter decomposing leaves mechanically through vascular tissue. As a consequence, AMF are well positioned to obtain and efficiently recycle mineral nutrients released by decomposer microorganisms before their loss by leaching or immobilization in soil.

Common mycorrhizal networks amplify size inequality in Andropogon gerardii monocultures

Arbuscular mycorrhizal fungi can interconnect plant root systems through hyphal common mycorrhizal networks, which may influence the distribution of limiting mineral nutrients among interconnected individuals, potentially affecting competition and consequent size inequality. Using a microcosm model system, we investigated whether the members of Andropogon gerardii monocultures compete via common mycorrhizal networks. We grew A. gerardii seedlings with isolated root systems in individual, adjacent containers while preventing, disrupting or allowing common mycorrhizal networks among them. Fertile soil was placed within the containers, which were embedded within infertile sand. We assessed mycorrhizas, leaf tissue mineral nutrient concentrations, size hierarchies and the growth of nearest neighbors. Plants interconnected by common mycorrhizal networks had 8% greater colonized root length, 12% higher phosphorus and 35% higher manganese concentrations than plants severed from common mycorrhizal networks. Interconnected plants were, on average, 15% larger and had 32% greater size inequality, as reflected by Gini coefficients, than those with severed connections. Only with intact common mycorrhizal networks were whole-plant dry weights negatively associated with those of their neighbors. In the absence of root system overlap, common mycorrhizal networks likely promote asymmetric competition below ground, thereby exaggerating size inequality within A. gerardii populations.

VESICULAR-ARBUSCULAR MYCORRHIZAE IN TWO TROPICAL MONODOMINANT TREES

Mycorrhizae, symbioses between plant roots and fungi, are found in virtually all terrestrial plant ecosystems. Substantial evidence indicates that mycorrhizal associations are beneficial to both partners. In most systems, the fungus benefits the host plant by effectively scavenging nutrients and in return, utilizes carbohydrates produced by the plant (Allen 1991, Harley & Smith 1983). Moreover, both major types of mycorrhizae, vesicular-arbuscular mycorrhizae (VAM) and ectomycorrhizae (EM), can alter the outcome of competitive interactions between host plants (Allen & Allen 1984, Hartnett et al. 1993, Hetrick et al. 1989, Newman et al. 1992, Perry et al. 1989). Early studies focused primarily on temperate mycorrhizae, but more recently, ecologists have turned toward the tropics. In contrast to the temperate zone, where trees tend to be EM, the majority of tropical tree species surveyed thus far form VAM (Janos 1983). Some notable exceptions of tropical trees forming EM occur in the plant families Dipterocarpaceae, Caesalpiniaceae, Euphorbiaceae, Fagaceae and Myrtaceae. Although tropical forests are typically diverse, many tree species in these five families grow in persistent, monodominant stands which can extend for hundreds of hectares (Connell & Lowman 1989). Janos (1983, 1985) has suggested that under low nutrient or seasonal conditions, EM may be more efficient than VAM at scavenging nutrients. Furthermore, EM may be able to absorb organic nitrogen (Alexander 1983), decompose organic litter (Trojanowski et al. 1984) and/or protect the hosts roots from herbivory or pathogen attack (Marx 1972). Hence, the ability of some tropical trees to associate with EM has been hypothesized to give them a competitive advantage by which they can achieve local or regional dominance

Vesicular-arbuscular mycorrhizae of epiphytic and terrestrial Piperaceae under field and greenhouse conditions

We examined the roots of 27 epiphytic and terrestrial species of Piperaceae collected in primary and secondary habitats in Monteverde, Costa Rica. Terrestrial roots of only two of the nine Peperomia species, two of eight Piper species, and of Pothomorphe umbellatum contained internal vesicles and/or arbuscules. We did not find internal vesicles and/or arbuscules in 3024 cm of fine roots of epiphytic Piperaceae, even though 15% of these root segments had associated external typical glomalean hyphae. Glomus and Acaulospora spores, and Gigaspora auxiliary cells occurred in both canopy and terrestrial habitats. After inoculation of a low nutrient substrate, the facultatively epiphytic Peperomia costaricensis averaged 23% mycorrhizal root length. Relatively high atmospheric inputs of dissolved inorganic nutrients that alleviate the requirement for mycorrhizae, and heterogeneity of mycorrhiza inocula in the canopy may explain the absence of mycorrhizae from epiphytic Piperaceae. We suggest that the Piperaceae comprises predominantly facultatively mycotrophic species, and that facultative mycotrophism facilitates their radiation to the canopy.

Characterizing variation in mycorrhiza effect among diverse plant varieties.

Exploitation of arbuscular mycorrhizal fungi may be an important approach for development of reduced-input agriculture. We discuss the use of linear models to analyze variation in mycorrhiza response among diverse plant varieties in order to assess the value of mycorrhizas. Our approach allows elimination of variation linked to differences in plant performance in the absence of mycorrhizas and the selection of plant lines that might harbor genetic variation of use to improve the mycorrhizal symbiosis in agriculture. We illustrate our approach by applying it to previously published and to novel data. We suggest that in dealing with a relative trait such as mycorrhiza effect, the choice of measure used to quantify the trait greatly affects interpretation. In the plant populations under consideration, we find evidence for a greater potential to increase mycorrhiza benefit than previously suggested.

Inoculation with arbuscular mycorrhizal fungi enhances growth of Litchi chinensis Sonn. trees after propagation by air-layering

Lychee (Litchi chinensis Sonn.) is typically propagated by air-layering mature tree branches which are potted in fertilized, soil-free media after cutting. The size of these branches, low phosphorus retention by pot substrates, and fertilization all might combine to preclude benefits of arbuscular mycorrhizas to lychee. In order to examine the potential of lychee to benefit from arbuscular mycorrhizas in an agriculturally realistic context, lychee air-layers were grown for 469 days in ca. 95-l pots of soil-free substrate inoculated with field-collected arbuscular mycorrhizal roots or not at two different levels of phosphorus fertilization. High phosphorus fertilization (a one-time addition of ca. 1.32 g l−1 slow-release triple-superphosphate) had no detectable effects on mycorrhiza formation, lychee survival, net CO2 assimilation, or growth. Inoculation with indigenous South Florida arbuscular mycorrhizal fungi improved leaflet expansion as early as 120 days after inoculation, and subsequently enhanced height growth and leaf production but did not affect stem diameter growth, net CO2 assimilation, or survival. At harvest, although mycorrhizal colonization was low (average 7.4% colonized root length), mycorrhizal plants had 39% higher above-ground dry weight than control plants. Below-ground dry weights did not differ, but inoculated plants had lower fine root to leaf dry weight ratios than control plants. Leaflets of inoculated plants had higher concentrations of P, K, Cu, and Zn, and lower concentrations of Ca, Mg, and Mn than those of control plants, but total Kjeldahl nitrogen and iron concentrations did not differ significantly 10 months after inoculation. Mycorrhiza enhancement of lychee growth occurred even though phosphorus clearly was not limiting for growth. Our observations suggest that in this soil-free medium, arbuscular mycorrhizal fungus enhancement of copper and iron nutrition improved lychee growth.

Mycorrhizal associations of tropical legume trees in Sierra Leone, West Africa

Abstract We examined for mycorrhizae the fine roots of 27 tree species which occurred in natural forest, a forestry plantation and a reforestation site in Sierra Leone, West Africa. Twenty tree species had vesicular-arbuscular mycorrhizae, nine of these in the Mimosoideae and Papilionoideae had bacterial nodules. Seven species had ectomycorrhizae, among which all six legume species are in the Caesalpinioideae. Three species of Australian Acacia used widely in reforestation in Sierra Leone had vesicular-arbuscular mycorrhizae.