Impact of Arbuscular-Mycorrhizal Fungi on Phosphorus ... - Krishikosh

Journal of Plant Nutrition, 28: 1867–1876, 2005 Copyright © Taylor & Francis Inc. ISSN: 0190-4167 print / 1532-4087 online DOI: 10.1080/01904160500251381

Impact of Arbuscular-Mycorrhizal Fungi on Phosphorus Efficiency of Wheat, Rye, and Triticale Renu Pandey,1 Bhupinder Singh,2 and T. V. R. Nair2 1

Division of Plant Physiology and 2 Nuclear Research Laboratory, Indian Agricultural Research Institute, New Delhi, India

ABSTRACT Genotypic variation and mycorrhiza play an important role in plant uptake of phosphorus (P). A pot experiment was conducted with three cereals, wheat (Triticum aestivum L. cv. PBW-34), rye (Secale cereale L. cv. R-308), and triticale (Triticale octoploide L. cv. DT-46), a hybrid of wheat and rye, to examine the genetic variation in the degree of arbuscular-mycorrhizal (AM) infection and its inheritability from parents (wheat and rye) to their progeny (triticale). The soil used for pot culture was low in available P (7.8 mg P kg−1 soil). Inoculation with AM fungi showed a significant increase in extent of root colonization for all three cereals (average 70%) compared with their performance without AM (average 19.1%). However, among the three cereals, this increase was significantly greater in rye than in the other two crops, while wheat and triticale did not differ significantly. Mycorrhizal infection resulted in 1.6, 1.7, and 1.8-fold increases in shoot, root, and total plant dry matter, respectively, compared with the un-inoculated treatment. Among the three cereals, rye recorded maximum shoot, root, and total plant dry mass and P content with AM inoculation. The P uptake by wheat, rye, and triticale was 10%, 64%, and 35%, respectively, higher with rather than without mycorrhizal infection. Rye was most responsive to AM inoculation, with mycorrhizal dependency of 193%; here again, triticale followed wheat, with similar mycorrhizal dependency. Rye showed an increase in P utilization efficiency (PUE) without AM inoculation while the PUE of triticale was intermediate between wheat and rye. High efficiency of AM symbiosis in terms of P uptake exists in rye and most of these traits in triticale seem to be inherited from wheat rather than rye. Keywords: arbuscular-mycorrhiza, inheritance, phosphorus use efficiency, rye, triticale, wheat

Received 23 December 2003; accepted 2 December 2004. Address correspondence to Renu Pandey, Division of Plant Physiology, Indian Agricultural Research Institute, New Delhi 110012, India. E-mail: renu [email protected] 1867

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INTRODUCTION Root colonization with mycorrhiza plays an important role in plant uptake of phosphorus (P) (Azcon and Ocampo, 1980; Clark, 1990; Kapulnik and Kuchnir, 1991; Hetrick et al., 1993a, 1993b). An array of plant traits contribute to high Puptake efficiencies, among which mycorrhizal (arbuscular-mycorrhizal fungi: AMF) symbiosis is one of the more important. There is much evidence to show that arbuscular-mycorrhizal (AM) symbiosis can increase both the content and concentration of P in cereal plants (Thompson, 1990; Ryan and Graham, 2002). Cereal crops respond in terms of shoot growth or grain yield to inoculation with AM fungi in P-deficient soils (Thompson, 1990; Baon et al., 1992). Mycorrhizal infection has been positively correlated with efficiency of P uptake in barley, but not in wheat or rye (Baon et al., 1992). Triticale, a hybrid of wheat and rye, is efficient in P uptake (Baier, 1990; Osborne and Rengel, 2002). It has been reported that rye and triticale were more efficient in P uptake and utilization than wheat at low rates of P supply. High nutrient [nitrogen (N), copper (Cu), zinc (Zn), and manganese (Mn)] efficiencies have been reported to exist in rye (Harry and Graham, 1981; Harry, 1982). Little reference, however, has ever been made (Krishna et al., 1985) to the inheritability of the phenotypic or genotypic traits in the progenies of the parents used as AMF hosts. In this paper, we examine the genetic variation in the degree of AM colonization and its inheritability from parents—wheat and rye—to their progeny—triticale. The effects of AMF inoculation on acquisition, allocation, and utilization efficiency of P in three cereal species, i.e., wheat, rye, and their hybrid, triticale, were also examined.

MATERIALS AND METHODS Seeds of triticale (Triticale octoploide cv. DT-46) and its parents wheat (Triticum aestivum L. cv. PBW-34) as well as rye (Secale cereale L. cv. R-308), as well as the AMF inoculum, containing propagules of Glomus macrocarpum, were procured from the Division of Genetics and Division of Microbiology, respectively, at the Indian Agricultural Research Institute in New Delhi. In the present experiment, pot culture was preferred over field studies in order to differentiate between the effects due to native and applied AMF strains. The field soil used for pot culture was low in available P (7.8 mg P kg−1 soil) and was sterilized for +AM treatment alone. A basal application of the recommended dose of N (120 kg N ha−1 ) and potash (40 kg K2 O ha−1 ) fertilizer was made. The soil (about 7 kg per pot) was mixed thoroughly with the fertilizers before filling of the earthen pots. Wheat, rye, and triticale were raised in pots with (+AM) and without (−AM) AMF inoculation. The mixed inoculum, which included colonized roots, soils, etc., and contained an average of 10 spores per plant, was applied. Seeds were sown by dibbling after placing the inoculum in the

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hole under the seeds and then covering them with soil. The experiment was conducted under natural conditions in the net house; the mean temperature during the growth period was 28◦ C/12◦ C. The moisture content of the soil was maintained at field capacity (17%) during the course of the experiment. A total of three plants per pot were maintained for each cultivar. The treatments were triplicated and the complete experiment was terminated at 60 d after sowing. Freshly harvested and thoroughly washed root samples were stained with trypan blue (Phillips and Hayman, 1970) and examined under a stereoscopic microscope for AM fungal infection. Percentage root colonization was calculated as follows (Singh et al., 1990): % Root colonization =

Number of AM positive segments × 100 Total number of segments scored

The mycorrhizal dependency was determined by expressing the dry weight of the plants concerned as a percentage of the dry weight of the control plants (Estaun et al., 1987). Phosphorus content in plant tissues was determined by the method of AOAC (1998) following tri-acid digestion and expressed on a dry-weight basis (mg P g−1 dry wt.). The statistical significance of difference of between means was assessed by two-way ANOVA (Gomez and Gomez, 1984). The experiment was laid out in a completely randomized design (CRD). Comparison of means was performed by calculating least significant difference (LSD) at the 5% probability level.

RESULTS AND DISCUSSION Root colonization varied significantly among species and AM fungi inoculation treatments (Table 1). With AM inoculation, rye showed a significantly higher root-colonization percentage (81.5%) while the other two species had almost similar degrees of infection (average 65.3%). Rye (23.7%) compared with wheat and triticale (15.2% and 18.4%, respectively) also showed a significantly higher degree of root infection in soils without AM application. The strain of AM fungi used in this study, G. macrocarpum, was found to be most efficient compared with other mycorrhizal strains in wheat (Rathi, 1999). However, this result contradicts the results of a pot culture (Baon et al., 1992), where a significantly lower proportion of mycorrhizal colonization occurred in rye than in wheat. Nevertheless, the mycorrhizal dependency (MD), expressed as the percentage of dry weight of inoculated plants to un-inoculated plants,[16] was found to be very high in rye (193%), which again argues against the results of Baon et al. (1992). They suggested that this difference in MD could be due to the differences in the rate of root growth as well as rates of fungal penetration of the roots and spread within the roots, or to the susceptibility of the species to fungal infection. Genetic variation in any of these parameters could influence

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Table 1 Effect of arbuscular-mycorrhizal (AM) inoculation on percent root colonization and mycorrhizal dependency of wheat, rye, and triticale grown in low-P soil (7.8 mg P kg−1 soil) without external application of P fertilizer in a pot experiment and sampled at 60 d after sowing Root colonization (%) Crop Wheat Rye Triticale Mean LSD (P 0.05%)

+AM

−AM

15.2 ± 0.56 64.4 ± 2.39 23.7 ± 0.87 81.5 ± 3.69 18.4 ± 0.83 66.2 ± 2.45 19.1 70.8 Inoculation: 2.7 Genotype: 3.3 Inoculation × Genotype: 4.6

Mycorrhizal dependency (%) 143 ± 1.53 193 ± 1.04 142 ± 0.50 159

the degree of infection. Moreover, the present experiment was conducted under conditions of fairly high temperature compared with those used by Baon et al. (1992). Recent research suggests that in more temperate climates, such as that of southern Australia, AMF may not enhance P-uptake of winter-grown cereals (Ryan et al., 2002). Therefore, breeding cereals to have enhanced colonization by AMF under such circumstances may be detrimental. In terms of shoot, root, and total plant dry weight, plants inoculated with AM showed 1.6, 1.7, and 1.8-fold increases, respectively, compared with uninoculated plants (Table 2). Significant differences among the cereals were also observed for these traits. Rye recorded the largest increase in shoot dry matter (126%) with AM treatment compared to without, while for wheat and triticale the corresponding increases were only 25% and 27%, respectively. The increase in root dry weight due to AM inoculation was maximum in wheat (79%) followed by triticale (71%) and rye (50%). However, here again within AM treatment, root weight was highest (1.75 g) for rye while wheat (1.65 g) and triticale (1.68 g) did not differ significantly. The highest increment in total plant dry weight was observed in rye (93%) while wheat and triticale produced similar amounts of dry matter. Singh and Singh (2001) also reported highest biomass production in rye, followed by wheat and triticale. This observation also finds support in Winzeler et al. (1987), who showed a consistently greater dry matter accumulation in rye and triticale than in wheat. However, root to shoot ratio increased significantly (about 30%) in wheat and triticale due to AM inoculation, while rye had the highest root to shoot ratio (about 32% more than wheat and triticale) under the un-inoculated condition. This result suggests that there is a preferential partitioning of dry matter to the shoot rather than the roots in rye, which is translated into a higher shoot mass when rye is inoculated with AM fungi.

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−AM

1.81 ± 0.05 2.26 ± 0.06 1.51 ± 0.04 3.42 ± 0.92 1.84 ± 0.05 2.33 ± 0.06 1.72 2.67 Inoculation: 0.08 Genotype: 0.09 Ino × G: 0.13

+AM

Ino: inoculation; G: genotype.

Wheat Rye Triticale Mean LSD (P 0.05%)

Crop

Shoot dry weight −AM


+AM

Root dry weight

(g plant−1 )

−AM


+AM

Total plant dry weight

Table 2 Effect of arbuscular-mycorrhizal (AM) inoculation on shoot, root, and total plant dry weight of wheat, rye, and triticale grown in low-P soil (7.8 mg P kg−1 soil) without external application of P fertilizer in a pot experiment and sampled at 60 d after sowing

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Significant species, as well as AM treatment effects, were observed in P concentration and accumulation in plant tissues (Table 3). Phosphorus concentration mean averaged over cereal species was found to be significantly higher with AM inoculation (14%) than without AM. However, rye showed the highest P concentration under both growing conditions. Wheat had the lowest P concentration, while in triticale, the result was intermediate between wheat and rye. Similarly, the P uptake was 1.9 times higher in mycorrhizal than in nonmycorrhizal plants. The mycorrhizal impact on P uptake was almost four-fold higher (172%) in rye than in wheat (40%) and triticale (35%) when compared with P uptake under non-mycorrhizal treatment. This result finds support from Thompson (1990), who also suggested that mycorrhizal plants are more efficient in P uptake. The mean P utilization efficiency (PUE), expressed as grams dry matter produced per milligram P absorbed, was similar or higher in uninoculated plants compared to PUE of inoculated plants (Figure 1). This result is indicative of the fact that un-inoculated plants not only take up less P but also produce relatively less biomass when compared with +AM plants, which have a much higher plant P level and are equally capable of producing greater plant biomass that results in a reduction of their PUE. Among the three cereals, rye recorded the lowest PUE, indicating that a high amount of P is locked in the plant tissues and is not efficiently utilized for dry-matter production. It may also be possible that in rye, carbon costs for maintaining AM symbiosis are high and limit its PUE. However,

Figure 1. Effect of arbuscular-mycorrhizal (AM) inoculation on phosphorus utilization efficiency (PUE) of wheat, rye, and triticale grown in low P soil (7.8 mg P kg−1 soil) without external application of P fertilizer in a pot experiment and sampled at 60 d after sowing.

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−AM


+AM

Ino: inoculation; G: genotype.

Wheat Rye Triticale Mean LSD (P = 0.05%)

Crop

Root/shoot ratio −AM


+AM

P concentration (mg P g−1 dry weight)

−AM


+AM

P uptake (mg P plant−1 )

Table 3 Effect of arbuscular-mycorrhizal (AM) inoculation on root-to-shoot ratio, P concentration, and uptake of wheat, rye, and triticale grown in low P soil (7.8 mg P kg−1 ) without external application of P fertilizer in a pot experiment and sampled at 60 d after sowing

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wheat and triticale produced significantly higher dry matter per unit P absorbed in association with AM fungi. It has been reported that AM infection results in higher P concentration in plant tissues (Buwalda et al., 1985; Thompson, 1990), but it also results in less efficient utilization of P by the plants. In our study, rye showed similar behavior when inoculated with AM fungi. Baon et al. (1992) reported that in rye, AM infection was not positively correlated with efficiency of P uptake, which contradicts our findings. Most of the P-efficiency traits in triticale were found to be similar to those of wheat, which suggests their inheritance in triticale from wheat rather than from rye. These results are in principle agreement with the results of a pot experiment conducted by Singh and Singh (2001), who observed that for most of the photosynthetic parameters, triticale showed similarity with wheat rather than with rye. On the other hand, the above results contradict the reasoning of Graham (1988) who mooted that triticale inherits nutrient efficiency from rye parentage. However, our study clearly shows that rye is extremely responsive to AM, as evident from a three-fold increase in P uptake of plants inoculated with AM when compared with −AM. It has been observed (Krishna et al., 1985) that the percent AM colonization between parents and progenies is a heritable trait. This P-uptake efficiency trait linked to AM colonization could be incorporated into other cereal species through conventional plant breeding programs. CONCLUSIONS High efficiency of AM symbiosis in terms of P uptake exists in rye and most of the traits in triticale appear to be inherited from wheat rather than from rye. Future research is needed to transfer P uptake efficiency traits from rye to wheat, through either a conventional or a molecular breeding approach to breed for these traits in a single genotype of commercial importance. ACKNOWLEDGMENTS The authors are thankful to Dr. C. S. Singh, Division of Microbiology, Indian Agricultural Research Institute, New Delhi for supplying the VAM inocula. REFERENCES Association of Official Analytical Chemists (AOAC). 1998. Official methods of analysis of AOAC International, 16th edition, ed. P. Cunniff, 20. Gaithersburg, MA: AOAC International, Maryland, USA. Azcon, R., and N. A. Ocampo. 1980. Factors affecting the vesicular-arbuscular infection and mycorrhizal dependency of thirteen wheat cultivars. New Phytologist 87: 677–685.

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