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Arbuscular Mycorrhizae (AM) in vineyards (Part 2)
Effects of AM on young grapevine performance
André Meyer & John Wooldridge, ARC Infruitec-Nietvoorbij, Stellenbosch
Key words: Arbuscular mycorrhizal fungi, grapevines, root colonisation, rootstocks, stress
INTRODUCTION
As was shown in Part 1 of this two-part series, arbuscular mycorrhizal (AM) fungi, notably of the genera Glomus, Acaulospora, Scutellospora and Gigaspora, occur naturally in some Western Cape viticultural soils (Meyer & Wooldridge, 2008). Certain species of AM fungi have been found to have positive affects on the growth and development of young, pot-grown grapevines under greenhouse conditions with sterilised soil (Menge et al., 1983). Other pot trial studies have shown that AM fungi improve uptake of nutrients, thereby stimulating vine growth (Deal et al., 1972; Karagiannidis et al., 1995; Biricolti et al., 1997). Further, AM fungi are thought to play a positive role in plant water relations, though not specifically in vines. Whether benefits similar to those cited in the literature can be obtained under commercial vineyard conditions is nevertheless uncertain. This is because the activity, and the ability of AM fungi to infect the roots of higher plants, could conceivably be inhibited at high soil phosphorus (P) concentrations (Menge et al., 1978; Brundrett et al., 1996). In comparison with AM trials, which are often carried out under suboptimal conditions, notably with regard to soil P availability, vineyard soils tend to be supplied with a relative abundance of P.
The purpose of this second article is to discuss the effects of the mycorrhizal treatments described in Part 1 on grapevines. Aspects that were specifically investigated were root colonisation, the effects of AM fungi on vine leaf and sap nutrient concentrations, cane growth and water relations.
MATERIALS AND METHODS
Since the layout and procedures used in this field trial have already been described (Meyer et al., 2004; Meyer & Wooldridge, 2008), only a brief summary will be presented here.
The study was carried out in commercial vineyards using Merlot grafted onto Richter 110 (110R), Richter 99 (99R) and 101-14 Mgt rootstocks. Five treatments were applied at planting. Three of these treatments (CAM1, G1054 and CAM2), involved the inoculation of each vine with an AM inoculum at planting. Treatment four consisted of a fungicide soil drench, applied shortly after planting. The fifth treatment (control) received neither fungicide nor AM inocula.
Over a period of two growth seasons after the treatments were applied (1998/99, 1999/2000), the vines were subjected to measurements of vigour, leaf water potentials, leaf nutrient concentrations and xylem sap composition. Percentage root colonisation by AM fungi was determined as described by Brundrett et al. (1994). The data were subjected to analysis of variance. Student’s least significant difference values were calculated at the 5% probability level to facilitate comparison between treatment means.
RESULTS AND DISCUSSION
Root colonisation
Microscopic analysis of the grapevine roots after the first grapevine growing season (1998/99) showed that the roots of the three rootstocks, in each of the five treatments, were colonised with AM fungi (Fig. 1). Colonisation rates after the first season ranged from c. 40% to 85% (Fig. 2). That the rootstocks in the control treatments were also colonised confirms the presence of infective indigenous AM species in the vineyard soils. Also, colonisation took place despite the presence of P in the soils at concentrations up to 89 mg/kg. This confirms that colonisation by AM fungi is not necessarily inhibited by P, at least, not to the extent thought possible by Menge et al. (1978) and by Brundrett et al. (1996).
Drenching with fungicide did not prevent the roots from becoming colonized (Fig. 2). This implies either that the AM fungi that colonised the roots survived the fungicide treatment, or that the fungicide drench was effective but of short term effect, following the waning of which the AM fungus-depleted zone around the roots was recolonised by AM fungi from the surrounding soil (Menge, 1982). A further trial in which the status of the soil AM fungal population is monitored before and at frequent intervals after the application of fungicide is needed to provide clarity on this issue.
Only in the case of 99R were the differences between treatments significant: The CAM1-treated vines showing a higher level of root colonisation than the control and CAM2 treatments. The pattern of responses to the treatments did not show a consistent trend in the three rootstocks, although it may be pertinent that root colonisation tended to be low in the control, relative to the remaining treatments, in the 101-14 Mgt and 99R rootstocks.
As assessed after the second grapevine growing season (1999/2000), colonisation rates were generally higher than after the first season, ranging from c. 70% to 90%. There were, however, no significant differences in colonisation rates between treatments in any of the rootstocks. Neither was there a tendency for colonisation rates to be higher in one rootstock than in another (data not shown). The combination of high colonisation rates and lack of difference between treatments strongly suggests that the relationships between soil AM populations and roots had stabilized, and that an equilibrium condition had been reached at some time after the first season but before the end of season two.
Leaf and xylem nutrient mineral concentrations
As assessed in mid season the treatments had no significant or consistent effects on the concentrations of phosphate and of amino acids in the xylem sap of the Merlot on either the 110R or the 101-14 Mgt rootstocks. Vines on 99R provided too little sap to test effectively. Neither the leaf blade potassium (K) nor the leaf-blade nitrogen (N) levels were affected by the treatments (data not shown). The only significant differences occurred in the 101-14 Mgt vines in which xylem sap nitrate concentrations were appreciably higher in the CAM1 (311 µmol/l) than in the fungicide treatment (111 µmol/l), and where leaf P concentrations were higher in the control and fungicide (both 0.25%) than in the CAM2 treatment (0.20%). It may also be pertinent that in 110R the xylem sap nitrate concentrations tended to be higher in the CAM1 and CAM2 treatments (169 and 184 µmol/l, respectively) than in the fungicide treatment (86 µmol/l) in 110R, and that in 101-14 Mgt, xylem sap phosphate, at 231 µmol/l, tended to be lower than in the G1054 and CAM1 treatments (both close to 372 µmol/l). Conceivably, the xylem nitrate and leaf P concentrations were influenced by the applied treatments to a greater extent where the Merlot was grafted onto 101-14 Mgt rather than 110R. Interaction between AM fungi and rootstock with regard to their combined effects on vine nutrition is a field which merits further research.
Cane growth
As was the case for root colonisation and tissue nutrient concentrations, the applied treatments had no consistent effects on vine growth, as indicated by total cane length per vine (Fig. 3). In the case of 99R, the effects of the treatments were not significant. In the 101-14 Mgt rootstocks, however, cane growth in the fungicide treatment exceeded that where G1054 was applied. In the 110R vines, cane length in the G1054 treatment exceeded that of all other treatments. That both cane length and root colonization were unusually pronounced in the G1054-treated 110R rootstock may not be coincidental. It is also possible that the low xylem sap nitrate content in the fungicide-treated 101-14 Mgt vines may have been due to dilution resulting from cane growth which, as indicated in Fig. 3, was relatively great in this treatment.
Water relations
Between early February and late April 1999, the average levels of water stress in the vines (tension, indicated by increasingly negative leaf water potentials, Table 1) tended to be higher in the control than in the remaining treatments. This trend was consistent in all three rootstocks. Other responses to the treatments differed between rootstocks, the sequence being: Control (significantly) > fungicide > CAM2 in 101-14 Mgt; control > all other treatments in 110R, and control > CAM2 in 99R. Why higher average tensions were observed in the controls is unclear, particularly since the roots in the control treatments were well populated (> 40%, Fig. 1) with AM fungi. It is nevertheless possible that supplying AM inoculants at planting resulted in a considerably greater density of infective AM being present near the roots, even if for a limited period, than was the case for the uninoculated treatments. Such a relative abundance of AM would certainly have promoted early and extensive colonization, perhaps leading to the observed reduced water stress levels during the summer months which immediately followed planting and inoculation, a period when leaf water potentials in excess of the norm of -1 200 kPa were frequently experienced.
CONCLUSIONS
This study was of preliminary nature, and of limited scope. The results nevertheless suggest that grapevine rootstocks are highly susceptible to colonisation by AM fungi which, as was shown in Part 1 of this series, are naturally present in at least some Western Cape vineyard soils. These fungi were sufficiently abundant, and sufficiently infective that colonisation rates in the roots of two of the three rootstocks that received AM-containing inoculants did not significantly differ from those in the controls after one season. After two seasons, no differences in colonisation rate were apparent between treatments in any of the rootstocks. The principle difference between treatments was that stress, as indicated by water potential, tended to be high in the controls relative to the fungicide and AM-inoculated treatments. In view of possible reductions in rainfall associated with climate change, the possibility that AM inoculation may alleviate water stress to some degree is of considerable importance. Further work is needed to substantiate the findings especially with regard to the nature of the indigenous AM populations, and of the effects of AM on vine water relations.
For further information contact André Meyer at meyera@arc.agric.za.
REFERENCES
Biricolti, S., Ferrini, F., Rinaldelli, E., Tamantini, I. & Vignozzi, N., 1997. VAM fungi and soil lime content influence rootstock growth and nutrient content. Am. J. Enol. Vitic. 48, 93 - 99.
Brundrett, M., Melville, L. & Peterson, L., 1994. Practical methods in mycorrhiza research. Mycologue Publications.
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.
Deal, D.R., Boothroyd, C.W. & Mai, W.F., 1972. Replanting of vineyards and its relationship to vesicular-arbuscular mycorrhiza. Phytopathol. 62, 172 - 175.
Karagiannidis, N., Nikolaou, N. & Mattheou, A., 1995. Influence of three VA-mycorrhiza species on the growth and nutrient uptake of three grapevine rootstocks and one table grape cultivar. Vitis 34, 85 - 89.
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.
Menge, J.A., 1982. Effect of soil fumigants and fungicides on vesicular-arbuscular fungi. Phytopathol. 72, 1125 - 1132.
Menge, J.A., Raski, D.J., Lider, L.A., Johnson, E.L.V., Jones, N.O., Kissler, J.J. & Hemstreet, C.L., 1983. Interactions between mycorrhizal fungi, soil fumigation and growth of grapes in California. Am. J. Enol. Vitic. 34, 117 - 121.
Meyer, A.H. & Wooldridge, J., 2008. Arbuscular mycorrhizae (AM) in vineyards 1. Effect of AM inoculation, fungicide and rootstock on soil AM population. WineLand, April 2009, 67 - 69.
Meyer, A.H., Valentine, A.J., Botha, A., Archer, E. & Louw, P.J.E., 2004. Young grapevine response and root colonisation following inoculation with arbuscular mycorrhizal fungi. S. Afr. J. Enol. Vitic. 25, 26 - 32.
ABSTRACT
Grapevine rootstock cultivars 101-14 Mgt, Richter 99 and Richter 110 are readily colonised by arbuscular mycorrhizal (AM) fungal species that are native to some vineyard soils of the Western Cape. The responses of grapevines to AM fungal treatments and to a fungicide application were generally inconsistent. Evidence suggests that inoculation with some commercial AM inoculants at planting may result in less water stress being experienced than where colonisation is achieved only by the native soil AM population.
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