Nonhydraulic signalling of soil drying in mycorrhizal maize

Our objectives were to (1) verify that nonhydraulic signalling of soil drying can reduce leaf growth of maize, (2) determine if a mycorrhizal influence on such signalling can occur independently of a mycorrhizal effect on leaf phosphorus concentration, plant size or soil drying rate, and (3) determine if leaf phosphorus concentration can affect response to the signalling process. Maize (Zea mays L. Pioneer 3147) seedlings were grown in a glasshouse with root systems split between two pots. The 2 x 3 x 2 experimental design included two levels of mycorrhizal colonization (presence or absence of Glomus intraradices Schenck & Smith), three levels of phosphorus fertilization within each mycorrhizal treatment and two levels of water (both pots watered or one pot watered, one pot allowed to dry). Fully watered mycorrhizal and nonmycorrhizal control plants had similar total leaf lengths throughout the experiment, and similar final shoot dry weights, root dry weights and leaf length/root dry weight ratios. Leaf growth of mycorrhizal plants was not affected by partial soil drying, but final plant leaf length and shoot dry weight were reduced in half-dried nonmycorrhizal plants. At low P fertilization, effects of nonhydraulic signalling were not evident. At medium and high P fertilization, final total plant leaf length of nonmycorrhizal plants was reduced by 9% and 10%, respectively. These growth reductions preceded restriction of stomatal conductance by 7 d. This and the fact that leaf water potentials were unaffected by partial soil drying suggested that leaf growth reductions were nonhydraulically induced. Stomatal conductance of plants given low phosphorus was less influenced by nonhydraulic signalling of soil drying than plants given higher phosphorus. Soil drying was not affected by mycorrhizal colonization, and reductions in leaf growth were not related to soil drying rate (characterized by time required for soil matric potential to drop below control levels and by time roots were exposed to soil matric potential below typical leaf water potential). We conclude that mycorrhizal symbiosis acted independently of phosphorus nutrition, plant size or soil drying rate in eliminating leaf growth response to nonhydraulic root-to-shoot communication of soil drying.Abbreviations and Symbols ANOVA analysis of variance - Cs stomatal conductance(s) - med medium - P probability - matric potential(s) - water potential(s) This work was supported by the U.S. Department of Agriculture grant No. 91-37100-6723 and a University of Tennessee Professional Development Research Award to R.M.A. We thank Angela Berry for the graphics.     

Leaf elongation and water relations of mycorrhizal sorghum in response to partial soil drying: two Glomus species at varying phosphorus fertilization

Arbuscular mycorrhizal symbiosis has previously been shown toalter the response of sorghum leaves to probable non-hydraulicsignals of soil drying. Our objectives here were to determine:(1) how changes in phosphorus nutrition affect this root-to-shootsignalling in sorghum, (2) if mycorrhizal symbiosis can affectthe signalling process independently of effects on host P nutrition,and (3) how two Glomus species compare in their influence onsignalling. Sorghum bicolor (L.) Moench ‘G1990A’ plants weregrown with root systems split between two pots. The 332 experimentaldesign included three levels of mycorrhizae (Glomus intraradicesSchenck & Smith, Glomus etunicatum Becker & Gerd., non-mycorrhizal),three levels of phosphorus fertilization and two levels of water(fully watered, half-dried). Declines in leaf elongation with soil drying were more consistentin non-mycorrhizal than mycorrhizal plants. Relative growthrate (RGR) of both mycorrhizal and non-mycorrhizal plants initiallydeclined when water was withheld from about half of the rootsystem. With further soil drying, RGR of mycorrhizal plantseventually returned to control levels, while RGR of non-mycorrhizalplants remained depressed throughout the drying episode. Bythe end of the drying episode, mycorrhizal symbiosis had eliminateddrying-induced declines in total plant leaf length. Shoot androot dry weight declines of half-dried plants were not affectedby mycorrhizae. Declines in stomatal conductance with soil dryingwere larger and more frequent in non-mycorrhizal than mycorrhizalplants. Leaf osmotic potential and relative water content remainedsimilar in control and half-dried plants during drying, suggestingthat altered leaf behaviour of half-dried plants was due tosome non-hydraulic factor. The two fungi did not differ substantiallyin their influence on leaf behaviour. The applied phosphorustreatments did not affect either growth or stomatal responseof halfdried plants to the root-to-shoot signal, but lengthdeclines were related to actual leaf phosphorus concentrations.Rate of soil drying did not appear to influence ultimate growthreductions. We conclude that mycorrhizal fungi can modify leaf growth responseto the root-to-shoot signal of soil drying, and that this mycorrhizaleffect can occur independently of mycorrhizal effects on plantsize or phosphorus nutrition. However, plant size and nutrition,which are commonly affected by mycorrhizal symbiosis, can alsomodify the signalling process. Key words: Drought, nutrition, root signal, Sorghum bicolor, vesicular-arbuscular     

Mycorrhizal fungi and nonhydraulic root signals of soil drying

We propose that mycorrhizal colonization of roots alters nonhydraulic root to shoot communication of soil drying. Split-root rose (Rosa hybrida L. cv Samantha) plants—one side of the root system colonized by Glomus intraradices Schenck & Smith, the other side nonmycorrhizal—displayed different stomatal conductances upon partial drying, depending upon whether mycorrhizal or nonmycorrhizal roots were dried. No differences in leaf water status were observed among control plants and those whose mycorrhizal or nonmycorrhizal roots were dried.     

Leaf water and carbohydrate status of VA mycorrhizal rose exposed to drought stress

Summary Shoot water relations and carbohydrate levels were compared for droughted nonmycorrhizal and vesicular-arbuscular (VA) mycorrhizalRosa hybrida L. cv ‘Samantha’ plants grown with high and low phosphorus fertilization. Leaf diffusive conductance (g _i ) of plants colonized byGlomus intraradices Schenk and Smith andGlomus deserticola Trappe, Bloss and Menge were 2 × and 1.5× greater, respectively, than in nonmycorrhizal plants. Regardless of P fertilization, leaf osmotic and bulk water potentials were 0.5 to 1.1 MPa higher in mycorrhizal than in nonmycorrhizal plants. Leaf starch, chlorophyll and water contents while fructose, glucose and total soluble carbohydrates were lower. Level of P fertilization had no effect on water relations or soluble carbohydrate content of nonmycorrhizal roses. The water status of droughted rose was impoved more byG. intraradices than byG. deserticola. Washington State University College of Agriculture and Home Economics Research Center Scientific Paper No. 7375.     

Moisture retention properties of a mycorrhizal soil

The water relations of arbuscular mycorrhizal plants have been compared often, but virtually nothing is known about the comparative water relations of mycorrhizal and nonmycorrhizal soils. Mycorrhizal symbiosis typically affects soil structure, and soil structure affects water retention properties; therefore, it seems likely that mycorrhizal symbiosis may affect soil water relations. We examined the water retention properties of a Sequatchie fine sandy loam subjected to three treatments: seven months of root growth by (1) nonmycorrhizal Vigna unguiculata given low phosphorus fertilization, (2) nonmycorrhizal Vigna unguiculata given high phosphorus fertilization, (3) Vigna unguiculata colonized by Glomus intraradices and given low phosphorus fertilization. Mycorrhization of soil had a slight but significant effect on the soil moisture characteristic curve. Once soil matric potential (_m) began to decline, changes in _m per unit change in soil water content were smaller in mycorrhizal than in the two nonmycorrhizal soils. Within the range of about –1 to –5 MPa, the mycorrhizal soil had to dry more than the nonmycorrhizal soils to reach the same _m. Soil characteristic curves of nonmycorrhizal soils were similar, whether they contained roots of plants fed high or low phosphorus. The mycorrhizal soil had significantly more water stable aggregates and substantially higher extraradical hyphal densities than the nonmycorrhizal soils. Importantly, we were able to factor out the possibly confounding influence of differential root growth among mycorrhizal and nonmycorrhizal soils. Mycorrhizal symbiosis affected the soil moisture characteristic and soil structure, even though root mass, root length, root surface area and root volume densities were similar in mycorrhizal and nonmycorrhizal soils.     

Mycorrhizal promotion of host stomatal conductance in relation to irradiance and temperature

Colonization of roots and soil by arbuscular mycorrhizal (AM) fungi sometimes promotes stomatal conductance (g _s) of the host plant, but scientists have had difficulty predicting or manipulating the response. Our objective was to test whether the magnitude of AM influence on g _s is related to environmental conditions: irradiance, air temperature or leaf temperature. Stomatal conductances of two groups of uncolonized sorghum plants were compared to g _s of plants colonized by Glomus intraradices (Gi) or Gigaspora margarita (Gm) in 31 morning and afternoon periods under naturally varying greenhouse conditions. Stomatal conductance of Gi and Gm plants was often markedly higher than g _s of similarly sized nonAM plants. AM promotion of g _s was minimal at the lowest irradiances and lowest air and leaf temperatures, but was substantial at intermediate irradiance and temperatures. AM promotion was again low or absent at the highest irradiances and temperatures. Magnitude of AM promotion of g _s was not a function of absolute g _s. Promotion of g _s by Gi and Gm was remarkably similar. Differing phosphorus fertilization did not affect g _s.     

Manganese reduction in the rhizosphere of mycorrhizal and nonmycorrhizal maize

The influence of rhizosphere microorganisms and vesicular-arbuscular (VA) mycorrhiza on manganese (Mn) uptake in maize (Zea mays L. cv. Tau) plants was studied in pot experiments under controlled environmental conditions. The plants were grown for 7 weeks in sterilized calcareous soil in pots having separate compartments for growth of roots and of VA mycorrhizal fungal hyphae. The soil was left either uninoculated (control) or prior to planting was inoculated with rhizosphere microorganisms only (MO-VA) or with rhizosphere microorganisms together with a VA mycorrhizal fungus [Glomus mosseae (Nicol and Gerd.) Gerdemann and Trappe] (MO+VA). Mycorrhiza treatment did not affect shoot dry weight, but root dry weight was slightly inhibited in the MO+VA and MO-VA treatments compared with the uninoculated control. Concentrations of Mn in shoots decreased in the order MO-VA > MO+VA > control. In the rhizosphere soil, the total microbial population was higher in mycorrhizal (MO+VA) than nonmycorrhizal (MO-VA) treatments, but the proportion of Mn-reducing microbial populations was fivefold higher in the nonmycorrhizal treatment, suggesting substantial qualitative changes in rhizosphere microbial populations upon root infection with the mycorrhizal fungi. The most important microbial group taking part in the reduction of Mn was fluorescent Pseudomonas. Mycorrhizal treatment decreased not only the number of Mn reducers but also the release of Mn-solubilizing root exudates, which were collected by percolation from maize plants cultivated in plastic tubes filled with gravel quartz sand. Compared with mycorrhizal plants, the root exudates of nonmycorrhizal plants had two fold higher capacity for reduction of Mn. Therefore, changes in both rhizosphere microbial population and root exudation are probably responsible for the lower acquisition of Mn in mycorrhizal plants.     

A Positive Root-sourced Signal as an Indicator of Soil Drying in Apple, Malus x domestica Borkh.

Plants raised from shoot-tip cultures of apple (Malus x domesticaBorkh ) were grown in divided pots so that approximately halfthe root system could be exposed to soil drying whilst the remainderwas well watered. Water was withheld from half the root systemin this way for 24 d. During this time, the daily incrementin leaf area declined to 65% of that of control plants, whichhad all their roots in well watered soil Both leaf expansionand, more particularly, leaf initiation were inhibited. Waterloss per plant also fell to 70%% of the rate exhibited by controlplants. No significant differences were detected between theleaf water status of the partially dried plants and that ofthe well watered ones After this drying period, the half-dried plants were treatedduring the night of the 24th day as follows; one group had itsroots in dry soil rewatered, another had them excised, and athird continued half dry Both the rewatered group and the groupwith roots excised showed significant recoveries in leaf growthrates compared with the group kept half dry. The inhibition of leaf growth by drying soil is discussed withregard to root-to-shoot signalling The alleviation of this inhibitionby the excision of roots in dry soil, is taken as evidence ofa positive inhibitor, produced by drying roots, influencingshoot growth. Key words: Leaf growth, soil drying, root signals, Malus x domestica     

Extraradical development and contribution to plant performance of an arbuscular mycorrhizal symbiosis exposed to complete or partial rootzone drying

Sweet potato plants were grown with or without Glomus intraradices in split-root pots with adjacent root compartments containing a soil with a low availability of phosphate. One fungal tube, from which root growth was excluded, was inserted into each root compartment. During 4 weeks before harvest, the soil moisture level in either both or only one of the two root-compartments of each pot was decreased. Controls remained well watered. Low soil moisture generally had a negative effect on the amount of extraradical mycelium of G. intraradices extracted from the fungal tubes. Sporulation in the fungal tubes was much higher compared with the soil in the root compartment, but remained unaffected by the soil moisture regime. Concentrations of P in extraradical mycelium were much lower than usually found in plants and fungi, while P concentrations in associated mycorrhizal host plant tissues were in an optimum range. This suggests efficient transfer of P from the extraradical mycelium to the host plant. Despite the negative effect of a low soil moisture regime on extraradical G. intraradices development, the symbiosis indeed contributed significantly to P uptake of plants exposed to partial rootzone drying. The possibility that extraradical arbuscular mycorrhizal fungal development was limited by P availability under dry soil conditions is discussed.     

The improvement of plant N acquisition from an ammonium-treated,drought-stressed soil by the fungal symbiont in arbuscular mycorrhizae

The ability of the external mycelium in arbuscular mycorrhiza for N uptake and transport was studied. The contribution of the fungal symbiont to N acquisition by plants was studied mainly under waterstressed conditions using ¹⁵N. Lettuce (Lactuca sativa L) was the host for two isolates of the arbuscular mycorrhizal fungi Glomus mosseae and G. fasciculatum. The experimental pots had two soil compartments separated by a fine mesh screen (60 m). The root system was restricted to one of these compartments, while the fungal mycelium was able to cross the screen and colonize the soil in the hyphal compartment. A trace amount of ¹⁵NH ₄ ⁺ was applied to the hyphal compartment 1 week before harvest. Under water-stressed conditions both endophytes increased the ¹⁵N enrichment of plant tissues; this was negligible in nonmycorrhizal control plants. This indicates a direct effect of arbuscular mycorrhizal fungi on N acquisition in relatively dry soils. G. mosseae had more effect on N uptake and G. fasciculatum on P uptake under the water-limited conditions tested, but both fungi improved plant biomass production relative to nonmycorrhizal plants to a similar extent.     

Hydraulic conductance and water potential gradients in squash leaves showing mycorrhiza-induced increases in stomatal conductance

Stomatal conductance (g _s) and transpiration rates vary widely across plant species. Leaf hydraulic conductance (k _leaf) tends to change with g _s, to maintain hydraulic homeostasis and prevent wide and potentially harmful fluctuations in transpiration-induced water potential gradients across the leaf (ΔΨ _leaf). Because arbuscular mycorrhizal (AM) symbiosis often increases g _s in the plant host, we tested whether the symbiosis affects leaf hydraulic homeostasis. Specifically, we tested whether k _leaf changes with g _s to maintain ΔΨ _leaf or whether ΔΨ _leaf differs when g _s differs in AM and non-AM plants. Colonization of squash plants with Glomus intraradices resulted in increased g _s relative to non-AM controls, by an average of 27% under amply watered, unstressed conditions. Stomatal conductance was similar in AM and non-AM plants with exposure to NaCl stress. Across all AM and NaCl treatments, k _leaf did change in synchrony with g _s (positive correlation of g _s and k _leaf), corroborating leaf tendency toward hydraulic homeostasis under varying rates of transpirational water loss. However, k _leaf did not increase in AM plants to compensate for the higher g _s of unstressed AM plants relative to non-AM plants. Consequently, ΔΨ _leaf did tend to be higher in AM leaves. A trend toward slightly higher ΔΨ _leaf has been observed recently in more highly evolved plant taxa having higher productivity. Higher ΔΨ _leaf in leaves of mycorrhizal plants would therefore be consistent with the higher rates of gas exchange that often accompany mycorrhizal symbiosis and that are presumed to be necessary to supply the carbon needs of the fungal symbiont.     

Growth of mycorrhizal tomato and mineral acquisition under salt stress

 High salt levels in soil and water can limit agricultural production and land development in arid and semiarid regions. Arbuscular mycorrhizal fungi (AMF) have been shown to decrease plant yield losses in saline soils. The objective of this study was to examine the growth and mineral acquisition responses of greenhouse-grown tomato to colonization by the AMF Glomus mosseae [(Nicol. And Gerd.) Gerd. and Trappe] under varied levels of salt. NaCl was added to soil in the irrigation water to give an EC_e of 1.4 (control), 4.7 (medium) and 7.4 dS m^–1 (high salt stress). Plants were grown in a sterilized, low P (silty clay) soil-sand mix. Mycorrhizal colonization was higher in the control than in saline soil conditions. Shoot and root dry matter yields and leaf area were higher in mycorrhizal than in nonmycorrhizal plants. Total accumulation of P, Zn, Cu, and Fe was higher in mycorrhizal than in nonmycorrhizal plants under both control and medium salt stress conditions. Shoot Na concentrations were lower in mycorrhizal than in nonmycorrhizal plants grown under saline soil conditions. The improved growth and nutrient acquisition in tomato demonstrate the potential of AMF colonization for protecting plants against salt stress in arid and semiarid areas. Accepted: 21 February 2000     

The influence of arbuscular mycorrhizal colonization on soil–root hydraulic conductance in Agrostis stolonifera L. under two water regimes

The hypothesis that mycorrhizal colonization improves the soil–root conductance in plants was experimentally tested in a growth chamber using pot cultures of Agrostis stolonifera L. colonized by Glomus intraradices. Plants were grown in 50-l pots filled with autoclaved sand/silt soil (1:1), with and without the mycorrhizal fungus. Within the mycorrhizal treatment, half of the pots remained well watered, while the other half was subjected to a progressive water deficit. Soil water potential (estimated as plant water potential measured at the end of the dark period), xylem water potential measured at the tiller base, transpiration rate, and soil water content were monitored throughout the experiment. Soil–root hydraulic conductance was estimated as the ratio between the instantaneous transpiration rate and the soil and xylem water potential difference. To obtain cultures with similar nutritional status, the P in the modified Hoagland’s nutrient solution was withheld from the inoculated pots and applied only once a month. Even though there were no differences on growth or nutrient status for the mycorrhizal treatments, water transport was enhanced by the inoculum presence. Transpiration rate was maintained at lower xylem water potential values in the presence of mycorrhizae. The analysis of the relationship between soil–root hydraulic resistance and soil water content showed that mycorrhizal colonization increased soil–root hydraulic conductance as the soil dried. For these growing conditions, this effect was ascribed to the range of 6–10%.     

Mycorrhizal responsiveness of aerobic rice genotypes is negatively correlated with their zinc uptake when nonmycorrhizal

Plant Zn uptake from low Zn soils can be increased by Zn-mobilizing chemical rhizosphere processes. We studied whether inoculation with arbuscular mycorrhizal fungi (AMF) can be an additional or an alternative strategy. We determined the effect of AMF inoculation on growth performance and Zn uptake by rice genotypes varying in Zn uptake when nonmycorrhizal. A pot experiment was conducted with six aerobic rice genotypes inoculated with Glomus mosseae or G. etunicatum or without AMF on a low Zn soil. Plant growth, Zn uptake and mycorrhizal responsiveness were determined. AMF-inoculated plants produced more biomass and took up more Zn than nonmycorrhizal controls. Mycorrhizal inoculation, however, significantly increased Zn uptake only in genotypes that had a low Zn uptake in the nonmycorrhizal condition. We conclude that genotypes that are less efficient in Zn uptake when nonmycorrhizal are more responsive to AMF inoculation. We provide examples from literature allowing generalization of this conclusion on a trade off between mycorrhizal responsiveness and nutrient uptake efficiency.     

Contribution of an arbuscular mycorrhizal fungus to the uptake of cadmium and nickel in bean and maize plants

Two experiments were carried out in pots with three compartments, a central one for root and hyphal growth and two outer ones which were accessible only for hyphae of the arbuscular mycorrhizal fungus, Glomus mosseae ([Nicol. and Gerd.] Gerdemann and Trappe). In the first experiment, mycorrhizal and nonmycorrhizal bean (Phaseolus vulgaris L.) plants were grown in two soils with high geogenic cadmium (Cd) or nickel (Ni) contents. In the second experiment, mycorrhizal and nonmycorrhizal maize (Zea mays L.) or bean plants were grown in a non-contaminated soil in the central compartment, and either the Cd- or Ni-rich soil in the outer compartments. In additional pots, mycorrhizal plants were grown without hyphal access to the outer compartments. Root and shoot dry weight was not influenced by mycorrhizal inoculation, but plant uptake of metals was significantly different between mycorrhizal and nonmycorrhizal plants. In the first experiment, the contribution of mycorrhizal fungi to plant uptake accounted for up to 37% of the total Cd uptake by bean plants, for up to 33% of the total copper (Cu) uptake and up to 44% of the total zinc (Zn) uptake. In contrast, Ni uptake in shoots and roots was not increased by mycorrhizal inoculation. In the second experiment, up to 24% of the total Cd uptake and also up to 24% of the total Cu uptake by bean could be attributed to mycorrhizal colonisation and delivery by hyphae from the outer compartments. In maize, the mycorrhizal colonisation and delivery by hyphae accounted for up to 41% of the total Cd uptake and 19% of the total Cu uptake. Again, mycorrhizal colonisation did not contribute to Ni uptake by bean or maize. The results demonstrate that the arbuscular mycorrhizal fungus contributed substantially not only to Cu and Zn uptake, but also to uptake of Cd (but not Ni) by plants from soils rich in these metal cations. Deceased 21 September 1996 Deceased 21 September 1996     

Field response of wheat to arbuscular mycorrhizal fungi and drought stress

Mycorrhizal plants often have greater tolerance to drought than nonmycorrhizal plants. This study was conducted to determine the effects of arbuscular mycorrhizal (AM) fungi inoculation on growth, grain yield and mineral acquisition of two winter wheat (Triticum aestivum L.) cultivars grown in the field under well-watered and water-stressed conditions. Wheat seeds were planted in furrows after treatment with or without the AM fungi Glomus mosseae or G. etunicatum. Roots were sampled at four growth stages (leaf, tillering, heading and grain-filling) to quantify AM fungi. There was negligible AM fungi colonization during winter months following seeding (leaf sampling in February), when soil temperature was low. During the spring, AM fungi colonization increased gradually. Mycorrhizal colonization was higher in well-watered plants colonized with AM fungi isolates than water-stressed plants. Plants inoculated with G. etunicatum generally had higher colonization than plants colonized with G. mosseae under both soil moisture conditions. Biomass and grain yields were higher in mycorrhizal than nonmycorrhizal plots irrespective of soil moisture, and G. etunicatum inoculated plants generally had higher biomass and grain yields than those colonized by G. mosseae under either soil moisture condition. The mycorrhizal plants had higher shoot P and Fe concentrations than nonmycorrhizal plants at all samplings regardless of soil moisture conditions. The improved growth, yield and nutrient uptake in wheat plants reported here demonstrate the potential of mycorrhizal inoculation to reduce the effects of drought stress on wheat grown under field conditions in semiarid areas of the world.     

Influence of Phosphorus and Endomycorrhiza (Glomus Intraradices) on gas exchange and plant growth of Chile Ancho Pepper (Capsicum Annuum L. cv. San Luis)

Seedlings of chile ancho pepper were grown in pots containing a pasteurized mixture of sand and a low phosphorus (P) sandy loam soil, and either inoculated (VAM) or not inoculated (NVAM) with the endomycorrhizal fungus Glomus intraradices. Long Ashton nutrient solution (LANS) was modified to supply P to the seedlings at 0, 11, and 44 g(P) m^-3 (P₀, P11, P₄₄, respectively). Low P depressed net photosynthetic rate (P_N), stomatal conductance (gs), phosphorus use efficiency (P_N/P), and internal CO₂ concentration (C_i). The mycorrhiza alleviated low P effects by increasing P_N, g_s, P_N/P, and decreasing C_i. At P₀, C_i of NVAM plants was equal to or higher than that of VAM plants, suggesting nonstomatal inhibition of photosynthesis. Gas exchange of VAM plants at P₀ was similar to that of NVAM plants at P11. Endomycorrhiza increased leaf number, leaf area, shoot, root and fruit mass at P₀ and P₁₁ compared to NVAM plants. Reproductive growth was enhanced by 450 % in mycorrhizal plants at P₄₄. Root colonization (arbuscules, vesicles, internal and extraradical hyphae development) was higher at lower P concentrations, while sporulation was unaffected. The enhanced growth and gas exchange of mycorrhizal plants was in part due to greater uptake of P and greater extraradical hyphae development. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effect of mycorrhizal inoculation and soluble phosphorus fertilizer on growth and phosphorus uptake of pearl millet

Summary Six mycorrhizal fungi were tested as inoculants for pearl millet (Pennisetum americanum Leeke) grown in pots maintained in a greenhouse. VAM fungi varied in their ability to stimulate plant growth and phosphorus uptake. Inoculation withGigaspora margarita, G. calospora andGlomus fasciculatum increased shoot drymatter 1.3 fold over uninoculated control. In another pot trial, inoculation withGigaspora calospora andGlomus fasciculatum resulted in dry matter and phosphorus uptake equivalent to that produced by adding phosphorus at 8 kg/ha.The influence of inoculatingGigaspora calospora on pearl millet at different levels of phosphorus fertilizer (0 to 60 kg P/ha) as triple superphosphate in sterile and unsterile alfisol soil was also studied. In sterile soil, mycorrhizal inoculation increased dry matter and phosphorus uptake at levels less than 20 kg/ha. At higher P levels the mycorrhizal effect was decreased. These studies performed in sterilized soil suggest that inoculation of pearl millet with efficient VAM fungi could be extremely useful in P deficient soils. However, its practical utility depends on screening and isolation of fungal strains which perform efficiently in natural (unsterilized) field conditions.     

The impact of carbofuran soil application on growth and mycorrhizal colonization by Glomus clarum of groundnut

The influence of soil application of carbofuran on the growth response of groundnut, and both mycorrhizal colonization and sporulation of Glomus clarum was studied in a pot culture experiment. Carbofuran application with or without mycorrhizal inoculation increased the height of the potted plants measured 8 weeks after sowing. Mycorrhizal plants were significantly taller than nonmycorrhizal plants at the final harvest time (14 weeks). Carbofuran, at the recommmended field dose of up to 2 kg/ha, greatly increased shoot dry matter and pod yield in mycorrhizal groundnut. Colonization and sporulation by this VAM fungus were also enhanced significantly at these dose levels. The application of carbofuran at 5 kg/ha inhibited both growth and mycorrhizal status of groundnut.     

Studies of iron transport by arbuscular mycorrhizal hyphae from soil to peanut and sorghum plants

 The influence of an arbuscular mycorrhizal (AM) fungus on phosphorus (P) and iron (Fe) uptake of peanut (Arachis hypogea L.) and sorghum (Sorghum bicolor L.) plants was studied in a pot experiment under controlled environmental conditions. The plants were grown for 10 weeks in pots containing sterilised calcareous soil with two levels of Fe supply. The soil was inoculated with rhizosphere microorganisms only or with rhizosphere microorganisms together with an AM fungus (Glomus mosseae [Nicol. & Gerd.] Gerdemann & Trappe). An additional small soil compartment accessible to hyphae but not roots was added to each pot after 6 weeks of plant growth. Radiolabelled P and Fe were supplied to the hyphae compartment 2 weeks after addition of this compartment. After a further 2 weeks, plants were harvested and shoots were analysed for radiolabelled elements. In both plant species, P uptake from the labelled soil increased significantly more in shoots of mycorrhizal plants than non-mycorrhizal plants, thus confirming the well-known activity of the fungus in P uptake. Mycorrhizal inoculation had no significant influence on the concentration of labelled Fe in shoots of peanut plants. In contrast, ⁵⁹Fe increased in shoots of mycorrhizal sorghum plants. The uptake of Fe from labelled soil by sorghum was particularly high under conditions producing a low Fe nutritional status of the plants. These results are preliminary evidence that hyphae of an arbuscular mycorrhizal fungus can mobilise and/or take up Fe from soil and translocate it to the plant. Accepted: 6 March 1998