Nev; Phytol. (1991), ^69

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1 Nev; Phytol. (1991), ^69 Soil mediated effects of phosphorus supply on the formation of mycorrhizas by Scutellispora calospora (Nicol. & Gerd.) Walker & Sanders on subterranean clover Y. D. THOMSON*,. D. ROSON ND L. K. OTT Soil Science and Plant Nutrition, School of griculture, University of Western ustralia, Nedlands, W.. 69, ustralia {Received 1 November 199 ; accepted 4 pril 1991) SUMMRY split-root technique was used to separate plant and soil mediated effects of phosphorus (P) supply on the formation of mycorrhizas by Scutellispora calospnra {Nicol. & Gerd.) Walker & Sanders on subterranean clover (Trifolium suhterraneum L.)- There were 1 split-root treatments, mcorporating four concentrations of P (,, and //g P g"^ soil) ranging from severely deficient for plant growth to luxurious. Increasing the supply of P to plants decreased the percentage of root length infected and the total length of roots infected by,s. cahspora. pplications of and /(g P g"' soil to half of the root system of plants reduced the percentage of root length infected in that half of the root system and in the other half of the root system to the same extent. The effects of severely deficient and moderately deficient rates of P on infection by 5. calospora were therefore mediated through the plant. These effects could be related directly to effects of P supply in decreasing the concentrations of soluble carbohydrates in roots because carbohydrate concentrations in roots did not differ between each half of the root system. In contrast to the concentrations of soluble carbohydrates in roots, the length of roots and the concentrations of P in roots were generally greater in the half of the root system that received the highest application of P. pplications of //g P g"' soil to half of the root system of plants also reduced the percentage of root length infected by S. calospora in both halves of the root system but to a greater extent in the half that received /^g P g"^ soil. This effect was associated with greater length of roots, higher concentrations of P in roots and higher concentrations of P in the soil in the half of the root system that received fi^ P g"-* soil. We concluded that the effects of luxury amounts of P on infection by S. calospora could be mediated through the soil. Key words: Vesicular-arbuscular subterranean clover. mycorrhizas, soluble carbohydrates, split-root, Scutellispora calospora, INTRODirCTION t concentrations of soi) P ranging from those that are moderately deficient to those that are luxurious for the growth of plants, increasing the supply of P has been associated with reductions in the percentage of root length infected by vesicular-arbuscular (V) mycorrhizal fungi (Daft & Nicolson, 1969; Mosse. 1973; Sanders & Tinker, 1973; mijee, Tinker & Stribley, 1989). In many cases, this reduction in the proportion of mycorrhizal root in response to added P has occurred because increased P supply has stimulated root growth more than it has stimulated the growth of the mycorrhizal fungus (Daft & Nicolson, 1969; Sanders & Tinker, 1973). In other cases, both the percentage of root length infected and Present address: Division of Forestry CSIRO, Private ag, P.O Wembley, W.. 6U, ustralia. the total length of mycorrhizal roots have been reduced by applications of P (Mosse, 1973; mijee et al. 1989). y contrast, when the supply of P is severely deficient for the growth of plants, the proportion of root length infected and the weight or length of roots infected by V mycorrhizal fungi have initially increased as the supply of P increases (Same, Robson & bbott, 1983; olan, Robson & arrow, 1984; mijee et al., 1989; Koide & Li, 199). The mechanisms involved in each of these effects are not yet fully understood. Increasing the supply of P to plants will potentially affect a number of soil and plant factors, all of which could directly or indirectly affect the formation of V mycorrhizas. t concentrations of soil P ranging from those that are moderately deficient to those that are adequate for plant growth, the P status of the plant rather than the P status of the soil is thought to

2 464. D. Thomson,. D. Robson and L. K. bbott determine the extent of mycorrhizal development (Sanders, 1975; Menge et al., 1978; Jasper, Robson & bbott, 1979). Several hypotheses have been proposed for this effect. The most widely accepted of these hypotheses is that an increase in the P status of the plant is associated with a decrease in the availability of fungal substrates from the host root. Increasing the supply of P to citrus (Ratnayake, Leonard & Menge, 1978) and sudangrass (Ratnayake et al., 1978; Graham, Leonard & Menge, 1981) decreased the permeability of root membranes which decreased the concentrations of soluble carbohydrates and free amino-nitrogen compounds in root exudates. Similarly, increasing the supply of P to subterranean clover (the host plant used in the present study) decreased the concentrations of soluble carbohydrates in roots (Jasper et al ; Same et al., 1983; Thomson, Robson & bbott, 1986) also affecting the concentrations of soluble carbohydrates in root exudates (Thomson et al., 1986). n alternative hypothesis for plant mediated effects of increasing P supply on V mycorrhiza] infection is that the increased concentrations of P in the plant have a direct inhibitory effect on the growth of the mycorrhizal fungus (Mosse, 1973). Relatively less is known about the effects of concentrations of soil P that are either severely deficient or luxurious for plant growth on the formation of V mycorrhizas. Same et al. (1983) proposed that when P is severely deficient for plant growth, the grow-th of the mycorrhizal fungus might be directly limited by the supply of P. Recent work by de Miranda, Harris & Wild (1989) and Koide & Li (199) with split-root techniques suggests that w'here V mycorrhizal infection is reduced at low rates of applied P, infection is limited directly by the concentration of P in the soil. t concentrations of soil P above those required for maximum plant growth, additions of P to subterranean clover reduced V mycorrhizal infection on roots even though there was little or no effect of P supply on either the concentrations of soluble carbohydrates in roots or on the length of roots at these concentrations of P (Jasper f/«/., 1979; Same c/a/., 1983; Thomson et a!., 1986). In an attempt to understand the mechanisms by which low and high rates of applied P affect the development of V mycorrhizal infection on subterranean clover, we used a split-root technique to determine whether the effects are mediated indirectly through the plant or directly in the soil. MTER1.\LS ND METHODS Experimental design There were 1 split-root treatments, incorporating four rates of P application (Table 1). Two rates that were severely deficient for plant growth (, IS fig P g ^ soil), one rate that was moderately deficient for plant growth (/(gpg~^ soil) and one rate that was luxurious for plant growth ( fig P g~^ soil) (Thomson et al., 1986), Each of these treatments was replicated three times. Experimental procedure Development of split-root systems prior to transplanting. Seeds of subterranean clover (Trifolium subterraneum L. cv, Seaton Park) were placed between two layers of muslin, on stainless steel grids, over pots (2-6 1 capacity) containing an aerated solution of 2 fim CaSO4 and 3 /im H3O3. These solutions were maintained at 2±2 C in a water bath. fter 3 d, the top layer of muslin was removed and split-root systems w-ere developed on individual subterranean clover seedlings by cutting 3 mm off the tips of 1-15 mm long radicles to induce the formation of lateral roots (Nable & Loneragan, 1984). t this stage the CaSO^/HaOg solution was replaced with a basal solution lacking P and containing the following concentrations of nutrients ifi.m): K2SO4, 65; CaCl^, 54; MgSO,.7H,O, 2; FeEDT, 1; H3O3, 8; MnCl2.4H,O, 2; ZnSO,.7H2O, -16; (NH,)^ Mo,.,4.4H26, -14; and CuSO^.SH^O, 6. For the first 8 days this basal solution was changed every second day. t each change, 1 ml of a dense suspension of Rhizobium trifolii Tl was added to each pot. For the rest of the growing period, the solutions were changed every day and were not inoculated with Rhizobium. fter 14 d (first trifoliate leaves present), the subterranean clover plants were large enough to transfer to the split-root units (Fig, 1). Transplanting procedure. t transplanting, the roots of each plant were divided evenly into two separate root systems, each comprising three first order laterals. oth of these root systems were then transplanted into fibreglass mesh cylinders (to separate root growth occurring after transplanting from roots present at the time of transplanting), into two different pots separated by a 'T' junction (Fig. 1). Each pot contained 1-5 kg of soil that had been collected from Mt. arker, Western ustralia (Thomson et al., 1986). fter the soil had been sieved through a 4 mm sieve, it was steamed for 2 h at 1 C and air dried. asal nutrients lacking P (K2SO,-7]; CaC!3-21; MgSO4.7H2O-2; ZnSO,, 7H2O-5O; CuSO4.5H,O-21; CoSO,, 7H5,O- 4; Na^MoO^. 2H2O--2 fig g~^ soil) were added to each pot in solution. Phosphorus was then applied as KHgPO^ in solution at rates equivalent to either,, or //gpg^^ soil [3, 9, 72 and 165//g P g"^ soil (bicarbonate extractable P-Colweil, 1963) after the pots had been watered to field capacity for 3 d]. These nutrients were allowed to dry and were then thoroughly mixed through the soil.

3 Phosphorus and mycorrhizal effects in clover 465 lkathene beads T' junction Soil level Plastic support 13 cm diameter pot Fibreglass mesh cylinder Figure 1. Diagram (to scale) of a split-root unit with the roots of transplanted plants of subterranean clover growing into two separate pots. One hundred grams of dry soil inoculum of Scutellispora calospora (Nicol. & Gerd.) Walker & Sanders [referred to in previous work (Thomson et al., 1986; Thomson et al., 199) as Gigaspora calospora (Nicol. & Gerd.) Gerd.] was mixed through the top 6 cm of each pot. S. calospora was chosen because, in the work of Thomson et al. (1986), it had been the mycorrhiza] fungus most sensitive to increases in the supply of P. Pots were maintained at C in a water bath and were watered to field capacity each day with de-ionized water (12*^ w/w). Harvest. Forty-two days after sowing, the tops of all treatments were harvested and weighed. The roots in each pot were then washed free of soil and weighed. Those roots that had grown out of the fibreglass mesh cylinder and were in the top 6 cm of the pot were cut into 1 cm segments, mixed thoroughly and sub-sampled. The rest of the roots were discarded. Samples of tops and roots were digested in 4:1 nitric:perchloric acid (Johnson & Ulrich, 1959) and then the P concentrations in these digests were determined by the molybdovanado-phosphoric acid method ( oltz & Lueck, 1958). Samples of root were also extracted twice for 3 min in SO % ethanol in a water bath running at 9 C and the concentrations of soluble carbohydrates in these extracts were determined by the phenol-sulphuric acid method (Montgomery, 1961). Other root samples were cleared and stained for V mycorrhizal infection as described by bbott & Robson (1981). Total length of roots, total length of mycorrhizal roots and the percentage of root length infected in the top 6 cm of pots were assessed on these samples using a line intercept method (Newman, 1966). Stained roots were spread evenly over a Petri dish and were examined under a binocular dissecting microscope. t 15 randomly-selected fields of view, roots were examined for the presence of infection at points of intersection with a hair-line. RESULTS s the supply of P to the plant increased (with additions of P to pot and/or to pot of the sphtroot units), the fresh weight of tops increased (Table 1). The fresh weight of tops in the - treatment is unusually high and does not appear to be related to higher concentrations of P in these plants (Table 1). ll replicates of this treatment recovered faster from the transplanting procedure. Increasing the supply of P to the plant increased the total fresh weight of roots (Table 1) and the total length of roots (top 6 cm of pot) (Table 2) from pots and. The fresh weight of roots and the length of roots (with the exception of the - treatment) were greater in those pots that received higher rates of applied P (Tables 1 and 2). This appeared to be at the expense of root growth in those pots that received lower rates of applied P. Where pot and pot received the same amount of P, root growth was generally the sanne in each pot (Tables 1 and 2). The concentrations of P in tops (Table I) and in

4 466. D- Thomson,. D- Robson and L. K. bbott Table 1. Ejfect of the split-root treatments on the fresh weight of tops, concentrations of P in tops and fresh weight of roots from pot and pot P applied C«8 6r ' soil) Fresh wt of tops (g plant"^) -22'' -74" 1 55^ 2-5='' '59'' 1-72^ 6'28'' 2-65" 2-87" 2-8'' P in tops (% of dry wt) -9'' -4'"' -33^ -48'' Q.TQllf 38**" -41' -4^'' -45 < -64" Fresh wt of roots (g pot-') -44^ 19" ^ O^O" ^ 15" l'oo" -8'^^'' -74"^ 1-23" 34" *'57*'' *-91*"^ Ml*" *42'' *.94i><- *2'47^ Q.95b>- 1-1"'- 1-12'- Values within columns followed by the same letter are not significantly different (P < -1). sterisks indicate significant differences (P < O^IO) between paired pots of the split-root units. roots (Table 2) increased in response to increasing P applications. This effect was less pronounced for the concentrations of P in tops. Phosphorus was translocated from roots in soil containing higher concentrations of P to roots in soil containing lower concentrations of P (Table 2). Despite this effect, in all but the treatment, P concentrations were higher in those roots that received higher rates of applied P (Table 2). The concentrations of soluble carbohydrates in roots generally decreased as the supply of P to the plant increased (Table 2). This effect was most pronounced at the lowest rates of applied P, where P was limiting to plant growth (Table 2, Fig. 2ft). Carbohydrate concentrations in roots were correlated with the P status of tops, whether expressed as the concentrations of P in tops (Fig. 2 a) or as the percentage of maximum growth of tops (Fig. 26). Carbohydrate concentrations in roots did not differ between pot and pot of the split-root units (Table 2) and therefore correlated poorly with the concentrations of P in roots. Increasing the supply of P to the plant decreased the percentage of root length infected by S. calospora (Table 2). t high rates of P application, the total length of mycorrhizal roots (top 6 cm of pot) from pots and was also reduced (Table 2). The percentage of root length infected by S, calospora did not differ between pot and pot except where //gpg-^ soil was applied (Table 2). With the exception of the luxury rate of P, the effects of P supply in decreasing infection by 5. calospora were therefore correlated with the effects of P supply in decreasing the concentrations of soluble carbohydrates in roots (Fig. 3). They were not related to either the concentrations of P in roots or the length of roots (Table 2). Where /^g P g"' soil was applied to a pot, the percentage of root length infected in that pot was reduced to approximately 2 "o regardless of the rate of P applied to the other pot in the split-root unit (Table 2). This effect did not correspond with lower concentrations of soluble carbohydrates in roots in these high-p pots (Table 2). The relationship between the concentrations of soluble carbohydrates m roots and the percentage of root length infected by S- calospora was therefore non-linear (Fig. 3). Lower levels of infection in pots that received //g P g~^ soil corresponded with higher concentrations of P in roots (- and - treatments. Table 2), greater length of roots (- and - treatments. Table 2) and higher concentrations of P in the soil. In the - treatment, the length of roots Table 2. Ejfect of the split-root treatments on the length of roots, concentrations of P in roots, concentrations of soluble carbohydrates in roots and extent of mycorrhisal infection on roots in the top 6 cm of pot and pot P applied (/*gg;-^ soil) Length of roots (m in top 6 cm of pot P in roots (% of dry wt) Sotuble carbohydrates in roots {" of fresh wt) Root length infected (%) Length of mycorrhizal roots (m in top 6 cm of pot) 9.4<i^ 3-8^'' 2-9" 8-'^'' 5-3"^ 3-7*^ 21-7" 16-'^ 15-7'^' 26-3'' 7.9a lo'o"'' *2-'' *23-5'" 8-9'^ *21-4' *48-7'' 2-1'' 19.9b. 23-8" -9^ '8" -21" *O-37'"' -23" *4'" -42"' *-54'"' -28= -35'' Q.32e<i»o-44'='' -4''*' -44'''' -48' -45'"^ 43^* *-56'- -64^ '' -83^ Q.yib.- -66'''"- -75^" 57" 69"'- '69'"' -6^" -63'"' ' '71""' '63'"'" -54*" O-73'"' -65'"'*' '65^''' -77'' -5" -6T""' 73 8' 67lie 74''^ 53 "" 56'' 41 *17" (<!. 58 "" 61 '" 48^ #24^ 57.d 52*"^" 47" *16" 16 2'^ 6'9 '' 6"3 '' 2.5^1) «7.4.>d J.jn #J.2<le 3'3'"' 4-"^ 3'8'' 6-3'"' 2-r" *13-r 1-4'- ll^"" 9-1" 12-5"^ 7.4c *'^-2^ 4'Z 4"o Values within columns followed by the same letter are not significantly different (P < 1). sterisks indicate significant differences (P < -1) between paired pots of the split-root units.

5 Phosphorus and mycorrhizal effects in clover / /» P in lops (% of dry wt) 4 2 / / / / J (b) 4 ^ \ \\\ \ Soluble carbohydrates in roots (% of fresh wt) Figure 3. Relationship between the concentrations of soluble carbohydrates in roots and the percentage of root length infected by Scutellispnra calospora. Those points were //g P g ' soil was applied to a pot are arrowed. The dotted line is not statistically fitted Maximum growth of tops (%) Figure 2. Relationships between (a) the concentrations of P in tops and (b) the percentage of maximum growth of tops and the concentrations of soluble carbohydrates in roots. The dotted lines are not statisticallv fitted. infected by S. calospora was decreased in the high-p pot (Table 2). DISCUSSION The percentage of root length infected and the total length of roots infected by S. calospora on subterranean clover were reduced by applications of P. t concentrations of soil P that were severely deficient for plant growth and at concentrations of P that were moderately deficient for plant growth, this decrease in infection was mediated through the plant and could be related directly to effects of P supply in decreasing the concentrations of soluble carbohydrates in roots. In other studies using subterranean clover as the host plant, reductions in the percentage of root length mycorrhizal in response to low (Thomson et al.., 1986) and intermediate (Jasper etal., 1979; Same f? a/., 1983 ; Thomson e/a/., 1986) rates of applied P have also been correlated with reductions in the supply of soluble carbohydrates in roots. Soluble carbohydrates are considered to be important substrates for mycorrhizai fungi (Ratnayakee^a/., 1978; Same et al., 1983; Thomson et al., 1986) and therefore important in regulating the formation of the mycorrhizal symbiosis. The detrimental effect of the P treatment that was luxurious for plant growth on the percentage of root length infected by S. calospora was not associated with lower concentrations of soluble carbohydrates in roots. This effect might have resulted from correlating infection with the current carbohydrate status of the plant rather than with a previous carbohydrate status, at a time when infection was initiated (Thomson e) al, 199). However, Jasper et al (1979) and Same et al (1983) also demonstrated that applications of P above those required for maximum growth of subterranean clover reduced the extent for root length infected by V mycorrhizal fungi despite no effect of P supply on the concentrations of soluble carbohydrates in roots at these rates of P addition. Factors otber than the supply of soluble carbohydrates in roots may therefore limit the development of V mycorrhizas on subterranean clover at luxury rates of P. The lower fractional infection by S. calospora in pots that received the luxury rate of P were associated with greater length of roots (- and -

6 468. D. Thomson,. D. Robson and L. K. bbott treattnents), higher concetitrations of P in roots (- and - treatments) and higher concentrations of P in the soil. Greater length of roots in pots could have indirectly reduced the proportion of mycorrhizal roots. Higher concentrations of P in roots could have had a direct inhibitory effect on infection by S. calospora [as proposed by Mosse (1973)], although we are not sure how this effect would have operated because most (85-95 %) of the inorganic P present in plant cells (which may comprise more than 7 o of "^he total P in P- adequate plants) is present in a "non-metabolic pool' in the cell vacuole (ieleski & Ferguson, 1983). Higher concentrations of P in roots could also have decreased the permeability of root membranes [as proposed for citrus (Ratnayake et al., 1978) and sudangrass (Ratnayake et al., 1978; Graham et al., 1981)], further decreasing the concentrations of soluble carbohydrates in root exudates. However, in addition to the observation made by Thotnson et al. (1986) that there appears to be little or no effect of P supply an the permeability of root membranes of subterranean clover to soluble carbohydrates and free amino-nitrogen compounds, differences in root P concentrations between paired pots of the splitroot units were observed at \ower rates of P than /ig P g"^' soil without an effect on infection by S. calospora. n alternative hypothesis for the effect of the luxury rate of P on infection by S. calospora is that the higher concentrations of P in soil had an inhibitory effect on mycorrhizal development. There is evidence in the literature to support such an effect. Spore germination and hyphal growth of V mycorrhizal fungi on agar (Hepper, 1983; Pons & Gianinazzi-Pearson, 1984) and in soil (Siqueira, Hubbell & Valle, 1984) as well as development of external hyphae in soil (bbott, Robson & De oer, 1984) have been reduced by large applications of P to the medium. In addition, high concentrations of soil P can affect a number of other chemical components in the soil, such as soil ph, which has itself been shown to affect the soil phase of certain V tnycorrhizal fungi [spore germination and hyphal growth from germinated spores (Porter, 1982); hyphal growth in soil from infected roots (bbott & Robson, 1985)]. dditions of small amounts of P to soil that is severely deficient in P for plant growth have increased the percentage of root length infected and the weight or length of roots infected by V mycorrhizal fungi (Same et al.., 1983; olan et al., 1984; mijee et al., 1989; Koide & Li, 199). This effect might not have been observed in our study because by distributing the mycorrhizal inoculum evenly through the top half of the pot rather than banding it near the soil surface or spot applying it, we removed any possible indirect effects of inoculum placement on the extent of infection (Thomson et al., 1986). lternatively, the mycorrhizal fungus used in our study might not have been directly limited by the supply of P (Same et al., 1983) at these concentrations of P. CKNOWLEDGEMENTS This research received support from the Wool Research Trust Fund on the recommendation of the ustralian Wool Corporation. One of us (.D.T.) held a Commonwealth Postgraduate Research ward during the course of the work. REFERENCES OTT, L. K. & ROSON,. D. (1981). Infectivity and effectiveness of five endomycorrhizal fungi; competition with indigenous fungi in field soils. ustralian Journal of grimtturat Research 32, OTT, L, K. & ROSON,. D. (J985). The effect of soil ph on the formation of V mycorrhizas by two species of Glomus. ustralian Journal of Soil Research 23, , OTT, L. K., RO.SON,. D. & DE OER, G, (1984). The effect of phosphorus on the formation of hyphae in soil by the vesicular'arbuscular mycorrhizal fungus, Glomus fast iculatumjvm' Phvtologist 97, 437^H6. MIJEE, F., TINKER, P.. & STRILEY. D. P. (1989). The development of endomycorrhizal root systems. VII. detailed study of effects of soil phosphorus on colonization. 'eit Phvtotogist 111, 435^M-6. IELESKI, R. L. & FERCL.SON, I,, (1983). Physiology and metabolism of phosphate and its compounds. In: Inorganic Plant Nvtrition (Ed. by. Lauchli & R. L. ieleski), pp Springer-Verlag, erlin, Germany. OLN, N. S-. ROSON,. D. & RROW, N. J. (1984). Increasing pviosphotus supply can increase the infection of plant roots by vesicular-arbuscular mycorrhizal fungi. Soit iology and iochemistrv 16, oltz, D.F. & LuECK, C. H. (1958), Phosphorus. 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7 Phosphorus and mycorrhizal effects in clover 469 mycorrhiza. IV. In soil given additional phosphate. New Phytologist 72, NI-E, R. O. & LoNEGN, ], F. (1984). Translocation of manganese in subterranean clover, II. The effects of leaf senescence and of restricting supply of manganese to part of a split-root system. ustralian Journal of Plant Physiology 11, NEWMN, E. I. (1966)..\ method for estimating; the total length of root in a sample. Journal of pplied Ecology 3, 13'^-145, PoNS, F. & GININZZI-PERSON, V. (1984), Influence du phosphore, du potassil!n:i, de I'azote et du ph sur It compoitement in vitro de champignons endomycorhizogenes a vesicuies et arbuscules, Cryptogame mycolagie 5, 87-1, PORTER, W, M, (1982), Factors affecting the distribution and abundance of i>esicular-arbuscular mycorrhizal fungi. PhD, Thesis, University of Western ustralia, RTNYKE, M,, LEONRD, R, T, & MENGE, j,.\. (1978), Root exudation in relation to supply of phosphorus and its possible relevance to mycorrhiza formation. New Phvtologist 81, SME,. I., ROSON,. D, & OTT, L, K. (1983). Phosphorus, soluble carbohydrates and endomycorrhizal infection. Soil iology and iochemistry 15, , SNDERS, F. E. (1975), The effect of foliar applied phosphate on the mycorrhizal infection of onion roots. In: Endomycorrhizas (Ed, by F. E, Sanders,. Mosse & P,, Tinker), pp, , cademic Press, London. SNDERS, F, E, & TINKER, P-, (1973), Phosphate flow mto mycorrhiza! roots. Pesticide Science 4, , SiQi-ElR, J- O,, HUELL, D, H. & VU.E, R, R. (19S4), Effects of phosphorus on formation of the vesicular-arbuscular mytorvhizal symbiosis. Pesguisa gyopecuaria fanietra 19, THOMSON,. D,, ROSON,, D. & OTT, L. K, (1986). Eff'ects of phosphorus on the formation of mycorrhizas by Gigaspora cauispora and Glomus Jaicicalatum in relation to root carbohydrates, Nev: Phytologist 13, , THOMSON,. D,, ROSON,, D, & OTT, L, K. (199), Mycorrhizas formed by Gigaspora calospora and Glomus fasciculatum on subterranean clover in relation to soluble carbohydrate concentrations in roots. Neu: Phvtologist 114,

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