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Arbuscular Mycorrhizal Fungi In Western Cape Grapevine Nurseries (Part 1): Varieties And Distribution
André Meyer and John Wooldridge
Key words: grapevine, nursery soils
INTRODUCTION
It is essential that young grafted vines recover quickly after being planted in the nursery and, after subsequent transplanting, in the vineyard. However, recovery after replanting is often hampered by stress. Stress, the symptoms of which may resemble nutrient deficiencies or drought, is often caused by poor root establishment and function, a condition that may be induced, or exacerbated, by microbiological factors. Since grapevines are naturally dependent upon mycorrhizal associations for their normal growth and development, it is possible that stress symptoms could be alleviated by treating young vines with AM fungi, preferably of varieties that are known to be beneficial to grapevines. In practice, AM fungi are already commercially available and are sometimes marketed as bioregulators, biofertilisers or bioprotectors (Lovato et al., 1996). Arbuscular mycorrhizal fungi are widely acknowledged as being helpful, mainly because of the interchange of water and nutrients that takes place between the vine root and the fungus, which acts as a greatly extended root system (Figure 1). Nevertheless, little is known about how AM fungi function under field conditions, or how they are naturally distributed across soils and landscapes. What has been established is that the diversity of naturally-occurring AM species is considerable. Approximately 160 AM fungal taxa of the order Glomales (Glomeromycota) have been described to date, and molecular analysis indicates that the actual number of AM taxa is even higher. It is also reasonably well accepted that AM fungi compete for colonisation sites on the roots. If true, this implies that the common practice of adding commercial AM inoculants to soils that already contain populations of indigenous AM species, or to plants that became colonised with AM fungi in the nursery, is likely to be of debatable value. This article presents the results of a survey of the AM fungal populations in the soils of Western Cape grapevine nurseries. Relationships between the AM fungi, colonisation rates and soil characteristics will be discussed in the second of this series of two articles.
Fig. 1. Root colonised by the AM fungus Scutellospora calospora. External hypha with auxiliary cell arrowed.
MATERIALS AND METHODS
Fifteen grapevine nurseries in various parts of the Western Cape were selected for this survey (Figure 2). Each of these nurseries produced Chenin blanc on Richter 99, though of differing clones (Table 1). Root and soil samples were collected from two depth intervals (0-150 mm; 150-300 mm) in February 2006, 16 -18 weeks after planting, and again in June 2006, shortly before lifting. Each sample was examined and the AM fungi were identified, mainly on the basis of spore morphology (Brundrett et al., 1996).
Fig. 2. Locations of participating Western Cape nurseries.
Table 1. Participating Western Cape grapevine nurseries. In all cases soil containing Chenin blanc / Richter 99 was sampled.
RESULTS AND DISCUSSION
A total of 12 AM species were identified at the 15 nurseries. These are listed in Table 2. Four of these species: (Glomus mossae, Scutellospora calospore, Glomus spp. and Acaulaspora spp.) were present in all of the nursery soils, regardless of geographic location or soil conditions. The widespread occurrence of these AM fungi suggests that they are ubiquitous in the Western Cape or, at least to those parts of it that have similar soils, climatic conditions and cultural histories to those included in this survey.
The occurrence of the remaining eight AM species was sporadic. Of these, Gigaspora decipens, Scutellospora fulgida, Scutellospora persica, Scutellospore spp., Gigaspora albida, Glomus tortuosum. Gigaspora spp. and Gigaspora gigantea were identified at six, four, three, three, two, one, one and one nurseries, respectively. Whether or not the more sparsely represented AM species were more widely spread in earlier times could not be established. That Glomus tortuosumm, Gigaspora spp. and Gigaspora gigantea occurred at single nurseries, whereas Glomus mossae, Scutellospora calospore, Glomus spp. and Acaulaspora spp. were present in all nurseries suggests that the latter group is more tolerant of factors that might lead to loss of species diversity than the former. It is conceivable that the species list might have been longer had molecular techniques, in addition to morphological identification criteria, been employed. Arbuscular mycorrhizal species that were in the resting stage at the time of sampling could have been overlooked during the present investigation (Vimard et al., 1999).
Table 2. Presence (denoted by x) of arbuscular mycorrhizal fungal species at participating Western Cape grapevine nurseries.
The finding that three nurseries harboured four species of AM, four harboured five, seven harboured six and one nursery harboured seven may indicate that six AM species is not only the most commonly observed number, but is also the optimum number that can co-exist in Western Cape nursery soils producing Chenin blanc on Richter 99. Wider testing is required to find out if this is the case, and if there is an upper limit to the number of AM species that can co-exist in any given soil / plant combination.
That eight of the nurseries (53% of total) were hosts to either six or seven AM species (50% and 58% of total species, respectively) bodes well for the state of AM species diversity in Western Cape grapevine nurseries, and for their future sustainability (Baumgartner, 2003). Precisely how replanted grafted rootstocks from sites containing different numbers and combinations of indigenous AM fungi would react to inoculant AM species is unclear, since much would depend on the vigour and effectiveness of the different indigenous and applied AM species, and the receptivity of the rootstock to colonisation by those species.
CONCLUSIONS
The survey data presented in this first article on the natural occurrence of AM fungi in Western Cape grapevine nurseries confirms that the nursery soils contain AM fungi from several genera. Four of these were present in all 15 nurseries, implying wide distribution and perhaps indigenous status. Others were represented at only one, or a very few nurseries. Whether or not the absence of specific AM fungi represents loss of mycorrhizal diversity under the conditions which prevail at specific nurseries, is unclear.
ACKNOWLEDGEMENTS
The authors wish to thank Winetech and the ARC for jointly funding this research, and Ansie du Toit for preparing Figure 2.
For further information contact André Meyer. E-mail: meyera@arc.agric.za
REFERENCES
Baumgartner, K., 2003. Why and How? Encouraging beneficial AM fungi in vineyard soil. Practical Winery and Vineyard 14, 57-60.
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.
Lovato, P.E., Gianinazzi-Pearson, V., Trouvelot, A. & Gianinazzi, S., 1996. The state of art of mycorrhizas and micropropagation. Advances in Horticultural Science 10, 46-52.
Vimard, B., St-Arnaud, M., Furlan, V. & Fortin, J.A., 1999. Colonization potential of in vitro-produced arbuscular mycorrhizal fungus spores compared with a root-segment inoculum from open pot culture. Mycorrhiza 8, 335-338.
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