Solanum cultivar responses to arbuscular mycorrhizal fungi : growth and mineral status

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.


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 *Corresponding author.E.mail: Tahir.Diop@ird.sn.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; Values within columns with the same letter are not significantly different (P = 0.05) by the Newman-keuls test.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.

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 (H 2 O).
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.

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 parameters 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.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.
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.

Shoot weight and mineral content
The biomass of the Cabrousse cultivar was increased by inoculation with G. aggregatum and G. mosseae but not  3).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)..71a 193.24b 194.83b Keur Mbir. 4.00bc 5.11a 4.44b 3.32c 14.84c 23.00a 18.21b 11.19c 59.69b 113.5a 99.82a 61.90b 120.29c 205.50a 179.92b 129.40c Means within a row followed with the same letter are not significantly different (P=0.05) by the Newman-Keuls test.
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.

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

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