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Arbuscular Mycorrhizal Fungi In Western Cape Grapevine Nurseries 2. Spore Counts, Root Colonisation And Soil Factors
André Meyer and John Wooldridge
Key words: organic carbon, pH, phosphorus
INTRODUCTION
Arbuscular mycorrhizal (AM) fungi are natural soil fungi capable of interacting with the roots of most plants, trees and vines to form what is, in effect, a vastly extended root system, the fungal component exchanging minerals and water for carbohydrates. These fungal associations (mycorrhizas) are beneficial since they seem to facilitate establishment of the grafted vines in the nursery beds, and their subsequent establishment in the vineyard, mainly by reducing post-plant stress. The survey of 15 Western Cape winegrape nurseries described in the first of this series of two articles (Meyer & Wooldridge, 2008) identified 12 AM species, of which four were present at all of the participating nurseries. The occurrence of the remaining species was sporadic. Since seven of the nursery soils contained six AM species, whereas only one soil contained seven, and none contained more than seven, it is possible that there is an upper limit to the number of species or isolates that can cohabit in a given soil, although why this should be the case is not known. Neither is it known why some AM species occur widely, whilst others occur in a few nursery soils only. Of equal importance is the present lack of information concerning those factors that facilitate or inhibit root colonisation and the establishment of a mutually beneficial relationship between vine root and fungus. This article discusses the relationships between AM spore count, root AM colonisation and soil parameters in the nurseries described in the previous article.
MATERIALS AND METHODS
At each of the 15 Western Cape grapevine nurseries that participated in this survey, root and soil samples were collected from two depth intervals (0-150mm; 150-300mm) in February 2006, 16-18 weeks after planting, and again in June 2006, shortly before lifting. In all cases the vines were Chenin blanc grafted onto Richter 99. The soil samples were analysed for pH, phosphorus (P) and organic carbon (OC) by a commercial laboratory (Bemlab, Strand) using standard techniques (pH, 1M KCl; P, Bray II; organic carbon, Walkley Black). Total soil AM spore counts were carried out and the rates of mycorrhizal root colonisation were determined as described by Brundrett et al. (1994). The data were statistically analysed and presented.
RESULTS AND DISCUSSION
Root colonisation and season
In February, the vines at only nine of the 15 nurseries were colonised. Amongst these the average root colonisation rate was 56% (Figure 1). By June all of the nurseries contained colonised vines and the average all-nurseries colonisation rate had increased to 63% with a minimum colonisation rate of 37% at Patatskloof and a maximum of 89% at Nuutbegin. In those nurseries where colonisation took place early in the season (February), the June colonisation rate was 66%. This figure was 8% higher than in those nurseries where colonisation took place after February. This result implies that factors that promote early colonisation lead to higher rates of colonisation being observed at lifting. Statistically, 86% of the variability in colonisation rate in June was explained by the February colonisation data.
Fig. 1. Colonisation (%) by arbuscular mycorrhizal fungi of Chenin blanc / Richter 99 vines in Western Cape nurseries as assessed in February and June, 2006.
AM Spore count and colonisation
Spores were present in the soils of all of the participating nurseries. The February spore count averaged 520 spores /100g air dry soil, and ranged from 150 at Babylonstoren (Malmesbury) to 1000 spores /100g air dry soil at Babilonstoren (Hermanus) and Groenvlei (Figure 1). The June spore counts were appreciably higher, averaging 860 and ranging from 200 at Mervespont to 1600 spores /100g air dry soil at Groenvlei. Spore count thus increases in winegrape nurseries as the time after planting increases. At the six nurseries where the vines were not colonised in February the average February spore count was 308, as compared with 661 spores /100g air dry soil in the nurseries where colonisation had taken place. These averages suggest that the likelihood of early (pre February) AM colonisation improves where the number of spores in the soil at planting is large. However, in neither February nor June was there a statistically significant (at the 95% confidence interval) relationship between spore count and colonisation rate. Less than 1% of the variability in colonisation rate was explained by spore count. In February, spore counts at Groenvlei and Babilonstoren (Hermanus) significantly (at P = 0.05) exceeded those at Babylonstoren (Malmesbury). In June, however, spore counts at Groenvlei exceeded those at Witkop, Nabygelegen and Mervespont. Counts at Patatskloof exceeded those at Mervespont. Differences in spore count were therefore apparent between some nurseries. Precisely what factors lead to these differences could not be determined from the available survey data.
AM Spore count and species diversity
Spore count did not reflect AM species diversity (refer to Part One). Only 3% of the variability in species was accounted for by spore count in February, and 5% in June.
Effects of soil P on AM spore count and root colonisation
At the February sampling, soil P concentrations ranged from 28 to 226 mg/kg (0-150 mm, data not shown) and from 24 to 215 mg/kg (150-300 mm, Table 1). In the June samples, concentrations ranged from 60 to 196 mg P/kg (0-150 mm, data not shown) and from 17 to 197 mg P/kg (150-300 mm, Table 1). Since fruiting perennials such as grapevines perform well at soil P concentrations around 30 mg/kg, or as low as 20 mg/kg on low-clay soils (Conradie, 1994) both horizons were, with one exception, adequately to abundantly supplied with P. The four highest, and four lowest, 150-300 mm soil P concentrations were associated with average counts of 438 and 613 spores /100g air dry soil in February, and with 975 and 725 spores /100g in June, which was inconsistent. Neither was there a significant relationship between soil P concentration and spore count in either month, soil P explaining 9% of the variability in spore count in February and 13% in June.
Table 1. Arbuscular mycorrhizal spore counts, soil pH, phosphorus and organic carbon in the rootzone (150-300 mm) of Chenin blanc / Richter 99 winegrape vines in Western Cape nurseries, assessed in February and June, 2006.
At Patatskloof and Nuutbegin, where the lowest (37%) and highest (89%) June colonisation rates were respectively observed, the 150-300 mm soil P concentrations were both considerably higher than the requirement of about 30 mg P/kg, but not greatly different from each other (137 and 153 mg P/kg, respectively, in June). This finding supports Meyer et al. (2005) who found that AM fungi showed tolerance to P in vineyards, but is counter to Menge et al. (1978) and Brundrett et al. (1996) who maintain that root colonisation is inhibited by high soil P concentrations.
Although the spore counts were unaffected by the generally high (relative to the requirements of deciduous fruit trees and vines) soil P concentrations, the AM spore counts in this survey were lower than those that are commonly observed in vineyard soils, where spore concentrations range between 1000 and 3779 spores /100g air dry soil (Meyer et al., 2005).
Effects of soil pH on AM spore count and root colonisation
Soil pH’s in February ranged from 4.1 to 6.4 (0-150 mm, data not shown) and from 4.2 to 6.3 (150-300 mm, Table 1). June soil pH’s ranged from 5.3 to 6.9 (0-150 mm, data not shown) and from 4.5 to 6.5 (150-300 mm). Since fruiting perennials perform best at around pH 5.5, the pH’s observed in this trial were generally reasonable. Spore counts were not significantly related to pH in either February or June, in which months pH explained around 24% of the variability in spore count. Only 7% of the variability in colonisation in June could be explained by pH.
Effects of OC on AM spore count and root colonisation
In both February and June the OC contents of the nursery soils were below 1% at all except two nurseries (Keerweder and Babilonstoren (Hermanus)). However, no significant relationship could be established between OC and spore count; OC accounting for only 16% of the variability in spore count in February and 10% in June. This result does not support the view that adding organic amendments promotes the production of AM spores (Douds et al., 1997; Muthukumar & Udaiyan, 2000). Colonisation rate in June was only 21% explained by the OC content. This relationship was not significant.
CONCLUSIONS
Results from a survey of AM populations in Western Cape grapevine nurseries producing Chenin blanc on Richter 99 indicate that AM fungal spore counts vary from nursery to nursery, but are generally lower after planting (February) than before lifting in June. High spore counts in the nursery soils tend to promote early colonisation which, in turn, is associated with higher spore counts and higher colonisation rates at lifting. This, positive, effect of high spore count, allied with the observation that AM spore counts in nurseries are lower than in vineyards, suggests that supplementation of spore numbers with commercial AM inocula at planting could facilitate early colonisation. By implication, a threshold spore count may exist below which supplementation is considered advisable. If so, this value has yet to be determined. Such a value would be likely to depend not only on the total soil spore count, but also on the indigenous species present, their relative abundance, the species present in the inoculum and the relative effectiveness of all the species concerned. Conceivably, the effectiveness of different species of AM fungi could vary with crop type and soil conditions. Spore count was not related to AM species diversity. Neither were spore count and AM colonisation affected by soil P concentrations above those needed by vines, pH, or by the organic carbon content of the nursery soil.
ACKNOWLEDGEMENTS
The authors wish to thank the ARC and Winetech for funding this research, and Isabella van Huyssteen for carrying out the statistical analysis.
For further information contact André Meyer. E-mail: meyera@arc.agric.za
REFERENCES
Brundrett, M., Bougher, N., Dell, B., Groove, T. & Malajczuk, N., 1996. Working with mycorrhizas in forestry and agriculture. ACIAR Monograph 32. Australian Centre for International Agricultural Research, Canberra.
Brundrett, M., Melville, L. & Peterson, L., 1994. Practical Methods in mycorrhiza research. Mycologue Publications, Guelph.
Conradie, W.J., 1994. Vineyard fertilisation. ARC Fruit, Vine and Wine Research Institute, Nietvoorbij. Private Bag X5026, Stellenbosch, 7599.
Douds, D.D., Galvez, L., Franke-Snyder, M., Reider, C. & Drinkwater, L.E., 1997. Effect of compost addition and crop rotation point upon VAM fungi. Agric. Ecosyst. Environ. 65, 257-266.
Menge, J.A., Steirle, D., Bagyaraj, D.J., Johnson, E.L.V. & Leonard, R.T., 1978. Phosphorus concentrations in plants responsible for inhibition of mycorrhizal infection. New Phytol. 80, 575-578.
Meyer, A.H., Valentine, A.J., Botha, A., Archer, E. & Louw, P.J.E., 2005. The occurrence and infectivity of arbuscular mycorrhizal fungi in inoculated and uninoculated rhizosphere soils of two-year-old commercial grapevines. S. Afr. J. Enol. Vitic. 25, 90-94.
Meyer, A.H. & Wooldridge, J., 2008. Arbuscular mycorrhizal fungi in Western Cape grapevine nurseries 1. Varieties and their distribution. Wineland (submitted for publication).
Muthukumar, T. & Udaiyan, K., 2000. Influence of organic manures on arbuscular mycorrhizal fungi associated with Vigna unguiculata (L.) Walp. in relation to tissue nutrients and soluble carbohydrate in roots under field conditions. Biol. Fertil. Soils. 31, 114-120.
ABSTRACT
Surveys of naturally-occurring arbuscular mycorrhizal (AM) fungi were carried out during February and June 2006 in 15 Western Cape grapevine nurseries containing Chenin blanc on Richter 99. Spore counts varied between nurseries but were generally lower in February than before lifting in June. Most of the February spore counts were lower than those that commonly occur in vineyard soils. High February spore counts were associated with early colonisation, which was, in turn, associated with higher spore counts, and higher colonisation rates at lifting. Spore counts and colonisation rates were not significantly affected by soil pH, or by the organic carbon content of the nursery soil. Neither was the AM fungi inhibited by soil P concentrations in excess of those which are regarded as adequate for grapevines, or by other soil factors.
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