Diop et al. Solanum cultivar responses to arbuscular mycorrhizal fungi: growth and mineral status. Afr. J. Biotechnol. 2003

A greenhouse experiment was carried out in a sandy soil with a low available phosphorus to evaluate responsiveness of four Solanum aethiopicum cultivars to indigenous arbuscular mycorrhizal fungi. Results showed clear interaction between genetic variability of cultivars and fungal isolates on shoot biomass and on mineral status. Arbuscular mycorrhizal fungi can be ranked as Glomus aggregatum > Glomus mosseae > Glomus versiforme for improving yield as well as nitrogen, phosphorus, and potassium acquisition of Solanum cultivars.

Key words: Arbuscular mycorrhizal fungi, Solanum aethiopicum, sterile soil, relative mycorrhizal dependency.

Introduction

Solanum aethiopicum is a cultivated plant much appreciated by the urban and rural African populations. In order to enhance the yield of this plant with high added value, selected cultivars of Solanum adapted to the climatic conditions of the West-African region are being investigated by the Institut Sénégalais de Recherches Agricoles (ISRA). Unfortunately, most of the soils in the region are poor in phosphorus and in nitrogen, and the weak purchasing power of the most farmers does not allow them to buy artificial fertilizers.

Arbuscular mycorrhizal (AM) fungi are known to increase yield and mineral nutrition of associated plants. Therefore, efforts must be done to optimise the beneficial effects of AM fungi in sustainable agriculture (Gerdemann, 1968; Sieverding, 1991, Diop, 1996). AM fungi show little host specificity but the nutritional, physiological and growth responses as a result of the infection can differ considerably (Plenchette et al., 1983;

Ruiz-Lozano et al., 1995). Although the most species are colonized by the AM fungi, they are not necessarily infected by the most efficient ones. The knowledge of the best symbiotic partners could be a very promising solution towards sustainaibility.

In Senegal, AM fungi occurrence is widespread, diverse and can be found even in soil depths greater than 35 metres (Diop et al., 1994; Diallo et al., 2000; Dalpé et al., 2000). Conditions to produce and to preserve in vivo and in vitro AM fungi from Sahelian zones have been described (Strullu et al., 1996). However, experiments to assay agronomic value of indigenous AM fungi are few and limited. Most studies deal with temperate AM isolates. The present study was undertaken to evaluate the responses of Solanum cultivars, commonly grown in Senegal, to inoculation with selected indigenous AM fungi from Sahelian zones of Senegal.

Materials and Methods

Fungal materials

Three AM fungi; Glomus aggregatum Schenck & Smith emend. Koske (DAOM 227 128), G. mosseae (Nicol. & Gerd.) Gerdemann & Trappe (DAOM 227 131), and G. versiforme (Karsten) Berch (DAOM 227 132) were used as inoculum. Fungal isolates were propagated in association with Zea mays in plastic pots containing 1.5 kg of sterile coarse sandy soil. After 3 months of cultivation in a glasshouse, Z. mays seedlings were harvested. For each AM fungus, inoculum consisted of a soil mixture (10 g/pouch) containing heavily colonized roots of Z. mays, spores and mycelium.

Seed germination, inoculation and growing conditions

Seeds of four cultivars of S. aethiopicum (Cabrousse, L10, Sokhna and Keur Mbir) were surface sterilized in hydrogen peroxide (15%) for 3 min., then washed in sterile distilled water. Seed germination occurred two days after incubation on water agar at 28°C in the dark. One pregerminated seed of each cultivar was planted in pot filled with 1 kg of a sterilised sandy soil collected at Sangalkam (50 Km from Dakar). Inoculation was simultaneously achieved at transplantation using 10 g of inoculum for each AM fungal isolate. The sandy soil was collected at Sangalkam, located at 50 km from Dakar. The chemical and physical characteristics of the soil were as follows: clay 5.4%; fine sand 89%; carbon 0.3%; total nitrogen 0.02%; total potassium 333.5 ppm; total phosphorus 41 ppm; avalaible phosphorus 2.1 ppm; magnesium 0.3 ppm, and pH 6.0 (H₂O).

Plants were grown in a glasshouse under a day/night cycle of 12/12h, 30/25°C, and 60% relative humidity. During the experiment, the pots were weighed daily and water loss was compensated for by top watering.

Experimental design and assessments

The experiment was arranged as a 4 x 4 random factorial design (4 Solanum cultivars, 4 inoculations (3 AM fungi plus a control) with 5 replicates. Harvest was made after 4 months and the roots washed clean with tap water. Root mycorrhizal colonization was estimated using the gridline-intersect method (Giovanetti and Mosse, 1980) after staining with trypan blue (Phillips and Hayman, 1970). Shoot dry weight was also recorded after oven-drying at 65°C for three days. Shoot mineral content (N, P, K) was determined using standard methods: colorimetry for nitrogen (after Kjeldahl digestion), phosphorus by the molybdenum blue procedure, and flame photometry for potassium. Relative mycorrhizal dependency (RMD) of Solanum cultivars was calculated by expressing the difference between shoot dry weight of the mycorrhizal plant and the shoot dry weight of the non mycorrhizal plant as a percentage of the shoot dry weight of the mycorrhizal plant (Plenchette et al., 1983).

The Newman-Keuls multiple range test (P<0.05) was used for statistical analysis. Data given at percentage values were first subjected to arcsine square root transformation.

Results

Click here to view Table 3

Main factor effects

All three AM fungal isolates increased shoot dry weight and nutritional contents of Solanum cultivars (Table 1). G. versiforme isolate stimulated fewer growth parameter and did not influence phosphorus uptake compared to uninoculated plants. G. aggregatum and G. mosseae isolates were the most efficient fungi, and had the same stimulatory pattern, except for phosphorus content where G. aggregatum was more efficient than G. mosseae.

Table 1. Effects of main factors on shoot dry matter (SDW) and mineral contents of Solanum cultivars after four months cultivation.

F1: “inoculation” factor	SDW (g)	N (mg)	P (mg)	K (mg)
Control	3.35c	63.37b	16.42c	130.56c
G. aggregatum	5.19a	115.22a	23.35a	210.51a
G. mosseae	4.78a	104.28a	19.60b	193.53a
G. versiforme	4.06b	73.95b	13.37d	154.55b

F2: “Cultivars” factor
Cabrousse	3.18c	64.19d	13.52c	127.14c
L10	5.33a	110.64a	22.08a	210.76a
Sokhna	4.90a	98.26b	20.32a	192.46a
Keur Mbir	3.98b	83.72c	16.81b	158.78b

Genetic variability between Solanum cultivars was also observed (Table 1). The L10 and Sokhna cultivars showed higher shoot dry weight and mineral content. The Cabrousse cultivar had the lowest yield and nutritional status.

Mycorrhizal root colonization and relative mycorrhizal dependency

Establishment of AM fungi was effective within roots of all Solanum cultivars (Table 2). Typical AM fungal structures were regularly found in stained roots. More than 60% AM root colonization was estimated in cortical roots of all cultivars. Among the cultivars, L10 showed the highest percentage of root colonization with 90% after inoculation with G. aggregatum, 85% with G. mosseae and 75% with G. versiforme.

Table 2. Percentage of root colonization of 4 S. aethiopicum cultivars inoculated with arbuscular mycorrhizal fungi after 4 months cultivation.

Cultivars	*G. aggregatum*	*G. mosseae*	*G. versiforme*
Cabrouse	75.50c	67.50b	62.00b
L10	90.00a	85.00a	75.00a
Sokhna	84.75b	80.00a	70.00a
Keur Mbir	75.00c	70.00b	65.00b

The relative mycorrhizal dependency (RMD) ranged widely among the cultivars tested (Figure 1). However, the calculated RMD was lower than 50% for all cultivars. The Keur Mbir and L10 cultivars showed the greatest dependency with G. mossae and G. aggregatum. The Cabrousse cultivar showed an intermediate dependency with only G. aggregatum, and Sokhna was the least dependent cultivar to all three AM fungal isolates.

Figure 1. Relative mycorrhizal dependency (RMD) of four Solanum cultivars inoculated with three arbuscular mycorrhizal fungi (G. aggregatum, G. mosseae, andG. versiforme).

Shoot weight and mineral content

The biomass of the Cabrousse cultivar was increased by inoculation with G. aggregatum and G. mosseae but not by G. versiforme. Inoculation by all three AM fungi significantly increased shoot dry weight of the L10 and Sokhna cultivars compared to control plants. G. mosseae and G. aggregatum induced the greatest biomass on L10 and Sokhna cultivars. For Keur Mbir cultivar, only G. aggregatum significantly increased biomass (Table 3).

Table 3. Effects of arbuscular mycorrhizal fungi on shoot dry matter (SDW) and shoot mineral mass of four S. aethiopicum cultivars. (Cont = Control, Ga = G. aggregatum, Gm = G. mosseae and Gv = G. versiforme).

Only G. aggregatum significantly increased shoot phosphorus content of all cultivars. G. mosseae only influenced phosphorus content of L10 compared to uninoculated control plants. G. versiforme had a negative effect on phosphorus uptake. G. aggregatum and G. mosseae increased shoot nitrogen and potassium of all cultivars, while G. versiforme only increased nitrogen and potassium uptake in the Sokhna cultivar.

Discussion

All AM fungi tested induced a higher degree of root colonization of Solanum cultivars. However, G. versiforme appeared to be less aggressive among the fungal isolates. Our results also confirm that genetic variability of plant cultivars can influence the efficiency of AM isolates (Declerck et al, 1995; Clark and Zeto, 2000).

Growth and mineral nutrition of plants are commonly enhanced by inoculation with AM fungi (Diop, 1996; Clark and Zeto, 2000). Except for G. versiforme, our results confirm positive effects of AM inoculation on growth of the cultivars. Increase in growth and biomass of inoculated plants strongly depends on their ability to access minerals from the soil. Therefore, positive effects of tested AM fungi on phosphorus content could be related to the ability of symbiotic fungi to enhance soil P depletion zones around roots (Li et al., 1991; Clark and Zeto, 2000; Smith et al., 2001). The importance and function of extra- and intra-radical forms of AM fungal hyphae could also explain differences in P acquisition among AM isolates. Similar results had been found on soybean cultivars indicating that P uptake by mycorrhizal plants fluctuates with fungal isolates and genetic variability within cultivars (Khalil et al., 1994).

Nitrogen and phosphorus uptake of inoculated Solanum cultivars confirmed the role of genetic symbiotic factors in controlling translocation of mineral elements. It has already been demonstrated that extra-radical AM fungal hyphae are able to take up and transport N from soil to plants (Bago et al., 1996). Phosphorus and nitrogen uptake in Solanum cultivars, inoculated with G. aggregatum and G. mosseae, follow the same pattern. This is particularly important in leguminous plants which require high amounts of P and N to achieve biological nitrogen fixation (Barea et al., 1992; Diop et al., 2000). Potassium acquisition by Solanum cultivars was also improved by AM fungi even though G. versiforme only had a positive effect on K uptake of the Sokhna cultivar. The acidity of the Sangalkam soil could also explain the enhancement of K translocation to plants (Clark and Zeto, 2000).

The relative mycorrhizal dependency (RMD) refers to the degree of plant responsiveness to mycorrhizal colonization by producing maximum growth or yield at a given level of soil fertility (Gerdemann, 1975). Relative mycorrhizal dependency is related to morphological and physiological properties of root systems (Mosse et al., 1973; Gianinazzi-Pearson, 1984). Our study included a wide range of tested Solanum cultivars. Nevertheless, the calculated RMD was not above 50%. It has already been demonstrated, in soil with low available phosphorus, that plants develop numerous and longer root hairs (Bhat and Nye, 1973). Solanum cultivars bored profuse root hairs in our experimental soil that had low available phosphorus (2.1 ppm). Development of such fine-rooted systems could probably help Solanum to satisfy its growth over a relative short period, even though G. aggregatum and G. mosseae had a fairly high RMD on the L10 and Keur Mbir cultivars.

Acknowledgement

This work was supported by a grant from International Foundation for Science (contract N° C/2896).

References