Gnzman-Plazola. R.A.. R. Ferrera

Gnzman-Plazola. R.A.. R. Ferrera-Cerrato and JJX Etchevers.
Centro de Edafologia, Colegio de Postgraduados, Montecillo, Mexico.
LEUCAENA LEUCOCEPHALA, A PLANT OF HIGH MYCORRHIZAL
DEPENDENCE IN ACID SOILS
In order to obtain significant growth and fixation of atmospheric nitrogen in Leucaena
leucocephala in acid soils it is necessary to use high rates of phosphate fertilizer
(Moreno, 1981; Guzman-Plazola et al., 1983). One possible, less-expensive alternative
is to inoculate leucaena with vesicular-arbuscular mycorrhizal (VAM) fungi, which can
improve phosphorus uptake, particularly when this nutrient is found in small amounts or
is barely assimilable (Munns and Mosse, 1980; Lara-Fernandez y Ferrera-Cerrato, 1986;
Gardezi et al., 1988). It would be most beneficial to combine the benefits of
mycorrhizal and Rhkobium symbiosis. In this work we have assessed the response of
Leucaena leucocephala to Rhkobium and VAM inoculation. Our main purpose was to
estimate the potential of this double inoculation to substitute for nitrogen and
phosphate fertilizers in acid soils.
Materials and Methods: An experiment was carried out under greenhouse conditions in a
complete randomized block design. The principal effects and interactions of the
following factors were evaluated: nitrogen (0 ppm N, 20 ppm N and Rhkobium loti),
mycorrhiza (Glomus intraradices, and no inoculation), phosphorus source (phosphate rock
and ordinary superphosphate) and P rate (0,50,100 and 150 ppm). The experiment
consisted of 48 treatments, each repeated three times, with a duration of 130 days.
Plastic pots were used as experimental units and were filled with 4 kg of soil dried in
the open air. This soil was sandy-loam in texture, with 8.7 ppm P (Bray-1), 0.05 % N,
pH = 5.3, and had been previously fumigated with methyl-bromide. Two seedlings of
Leucaena leucocephala cv Peruana were transplanted to each pot. In each pot, as
mycorrhizal inoculant, 50 g of soil with spores of Glomus intraradices plus 1 g of corn
roots with 80% of colonization were applied. The units inoculated with R. loti
received one milliliter of a broth that had approximately 10° rhizobia of the strain
CIAT-1920, previously mixed with 1 g of CaCO/25 ml of broth. In the plants that were
not inoculated a solution of CaCO of the same concentration was applied.
Ammonium nitrate and phosphate rock from San Luis Potosi (15.3% P) or ordinary
superphosphate were used as N and P sources. Irrigations were carried out with
distilled water. A 150 ml of Long Ashton nutrient solution with no nitrogen nor
phosphorus were applied every two weeks.
The following variables were evaluated: (a) dry weight of aerial part, (b) diy weight
of root, (c) height of plant, (d) dry weight of nodules, (e) nitrogen content in the
aerial part, (f) phosphorus content in the aerial part and (g) mycorrhizal
colonization.
Results
Growth and Nodulation. Inoculation of Glomus intraradices produced substantial
increases in the leucaena growth. When the phosphorus rate was zero, the fungus
induced increases over the control of 380, 2,280,1,790 and 460% in the height, above
ground dry weights, and root and nodules of the plants, respectively (Figure 1). These
effects were equivalent to application of 150 ppm P. Plants that did not receive
mycorrhizal treatment showed a strong response to phosphate fertilizer. In general
this effect was even more pronounced with ordinary superphosphate than with phosphate
rock, but the differences, except the dry weight of nodules, were only significant in
the treatment with 100 ppm P. Mycorrhizal plants presented a small response to
phosphate fertilizer application, with the exception of the root dry weight in the
Figure 1.
Effect of the interaction of Mycorrhiza x Rate of P2 O5 x P source over (a)
height of-L. leucocephala, (b) diy weight of above ground biomass, (c) diy
weight of roots, (d) dry weight of nodules. (E) Effect of the interaction of
Mycorrhiza x N source on the dry weight of above ground biomass. +M =
Glomus intraradices; -M = without endomycorrhiza. OS = ordinary
superphosphate; PR = phosphate rock.
150 ppm P treatment applied as ordinary superphosphate. None of the other rates or
sources of this element produced major effects other than those obtained with the
mycorrhizal fungus in the absence of fertilization. Rhizobium loti produced
significant effects only in plants inoculated with Mycorrhiza. The above ground dry
weight in the plants inoculated with Rhizobium was higher than that of those inoculated
only with the mycorrhizal fungus (Figure le). The application of 20 ppm N in all cases
did not affect the behavior of these variables.
Phosphorus and nitrogen content in above-ground biomass: Inoculation with G.
intraradices produced increases over the control of 3,500 and 850% in the phosphorus
and nitrogen content of the above ground biomass, respectively (Figure 2a and b). The
effect of this fungus on the first variable was larger than the one produced by the
application of 150 ppm P, and the fungal effect on the nitrogen content was equivalent
to that of this rate. The P and N content in the above ground plant parts increased in
the plants not treated with Mycorrhiza as the rate of phosphate fertilizer increased.
As expected, ordinary superphosphate produced a larger effect than phosphate rock
(Figure 2a and c). In plants treated with Mycorrhiza, the application of P did not
produce a significant effect on N content, but considerable increases did occur in the
content of phosphorus. Within the group of plants fertilized with phosphate rock, the
fungal activity and the phosphate fertilizer had additional effects over phosphorus
content in the above ground biomass. In the experimental units treated with
superphosphate, the 50 ppm application in the presence of fungus had a synergistic
effect on this parameter. However, when the phosphorus level increased there was
negative interaction, similar to that observed in the previous variables.
(A) Effect of the interaction of Mycorrhiza x P source x rate of P2 O5
fertilization over P content in above ground biomass of L. leucocepnala;
(B,C, & D) Effect of interactions of Mycorrhiza x rate of P2 O5
application, Mycorrhiza x P source and Mycorrhiza x N source, respectively,
over the N content in above ground biomass. +M = with Glomus intrarudices, ■
M = without. OS = ordinary superphosphate, PR = phosphate rock.
p in aerial part (mg/2 plants)
Figure 2.
aerial part (mg/2 plants)
+M
As observed for the above-ground biomass, Rhizobium loti inoculation affected only the
nitrogen content of the VAM-treated plants (Figure 2d). The 20 ppm N application had
no effects in either of the aforementioned variables.
Mvcorrhizal colonization. With the exception of the P application levels none of the
factors studied had any effect on mycorrhizal colonization (Figure 3). As soon as the P
content of soil was increased, total colonization gradually decreased. Both this
variable and the intraradical vesicles frequency diminished by approximately 10 units
when applying 50 ppm P to soil. This behavior exhibited little change with larger
rates of P application. The percentage of arbuscules was not affected by factors under
study.
Discussion: Leucaena plants inoculated with Glomus intraradices exhibited greater
capacity to uptake phosphorus from soil than non-inoculated plants. This difference
produced large increases in the different growth parameters. The improvement in the
phosphate nutrition of the plant favored the symbiosis with Rhizobium loti and
consequently the fixation of atmospheric nitrogen. Nodulation was almost nil in the
plants where phosphorus was not applied nor fungus inoculated. Dry weight of the
nodules depend on the availability of this element for the plant. The nodules biomass
was closely correlated to the production of dty matter (r = 0.85), nitrogen (r = 0.79)
and phosphorus (r = 0.81) contents in the aerial part. This explains the positive
results obtained with double inoculation (G. intraradices + R. loti) over the dry
weight and nitrogen content in the aerial part.
Figure 3.
Effect of P7 O5 rate on Glomus intraradices root colonization of L.
leucocephala.
0
50
100
The response to phosphate fertilization in plants without endomycorrhiza showed, in
general, a linear trend. In some cases it appeared to adopt a pattern similar to a
sigmoid curve, typical of nutrient-deficient soils (Volke, 1981). Nevertheless, VAM
inoculation had the same effect as the highest rate of P fertilization (150 ppm P).
This indicates that leucaena is a species of high mycorrhizal dependence in soils as
those utilized in this research and its atmospheric nitrogen fixation potential can be
efficiently utilized by means of double symbiosis (rhizobium + glomus).
The endophyte’s effect on the plant’s nutrition and growth was reduced as the
phosphorus content in the soil increased. This was due to the depressing effect of
this element on VAM colonization. The linear correlation coefficient between this
variable and the phosphorus content in the aerial part was negative (r = -0.531;
inverse relations also were observed with the root (r = -0.56) and aerial part (r = 0.3) dry weights. This indicates that the reduction in fungal colonization was related
to a larger growth, and it occurred precisely in the plants that had the best
phosphorus nutrition. As the availability of phosphorus increased, the plant reduced
the endophyte presence in the radical system ana began substituting the biological with
the mineral source, possibly with a reduced metabolic cost.
The high response to VAM inoculation also is attributable to the initial low P content
of the soil. The synergistic effect observed in the treatment with 50 ppm P (ordinary
supeijvhosphate) shows that in P-deficient soils it is possible to reduce P
fertilization by VAM inoculation and maintain a higher productivity level than could be
obtained by applying either of factors independently. This would allow the most
efficient utilization and significant savings on fertilizer.
In P fertilization tests in acid soils or soils with high P fixing capacity, high rates
of this element are frequently required to achieve yield increases. Such results are
commonly explained in terms of the physical and chemical characteristics of the soil
environment. Our data, however, indicate that the mycorrhizal symbiosis may have a
significant influence on this type of response and the suppression of mycorrhizal fungi
may generate a need for more fertilization. As a result of our research we may infer
that whatever is the response to phosphorus application, under natural conditions the
mycorrhizal symbiosis may have an important role in leucaena growth, thereby suggesting
it should be taken into account in future studies of test for soil productivity.
References:
Gardezi, A.K., R. Ferrera-Cerrato, and V. Lara-Fernandez. 1988. Effect of the double
inoculation of Rhizobium sp and V-A endomycorrhizae on Acacia cyanophylla in an
andosol in Mexico. NFTRR 6:31-33.
Lara-Fernandez, V. and R. Ferrera-Cerrato. 1986. Study of the vesicular-arbuscular
endomycorrhizal Leucaena leucocephala symbiosis. LRR 7:94-96.
Moreno, R.A. 1981. Eficiencia de cepas de rhizobium y efecto de P, Mo, Fe, Co y
encalado en la nodulacion y produccion de biomasa de Leucaena leucocephala en
suelos acidos de Huimanguillo, Tabasco. MS Thesis, Colegio de Postgraduados,
Chapingo, Mexico.
Munns, D.N. and Mosse, B. 1980. Mineral nutrition of legume crops, p 115-125. In RJ.
Summerfield and A.H. Bunting (eds.) Advances in Legume Science. University of
Reading, England.
Volke, H.V. 1981. Estimacion de funciones de regresion en experimentos con
fertilizantes y densidad de plantas con fines de determinacion de optimos economicos.
Centro de Edafologia, Colegio de Postgraduados. Chapingo, Mexico. 65 p.