MYCORRHIZAL COLONIZATION OF HALF-HIGH BLUEBERRY CULTIVARS INFLUENCED BY CULTURAL PRACTICES

Mycorrhizae probably play an important role in the biology of Vaccinium spp. and in the development of a sustainable agriculture, because mycorrhizae may be beneficial in reducing the utilization of agricultural chemicals, such as fertilizers and growth regulators. The objective of this study was to evaluate the effects of different cultural practices on the level of mycorrhizal colonization in field-grown half-high blueberry (Vaccinium corumbosum × Vaccinium angustifolium) cultivars Northblue and Northcountry. The root samples were taken from five-year-old blueberry plants, cleaned and stained with trypan blue. The colonization rate of roots by hyphae was studied under a light microscope. Our results indicate that mycorrhizal colonization was significantly higher on roots of cultivar Northblue. Using of plastic mulch in plantation increased mycorrhizal colonization. There was a positive correlation between mycorrhizal colonization and growth, and between mycorrhizal colonization and yield of blueberry plants. INTRODUCTION In a sustainable production system the essential problems are the increased use of pesticides and synthetic fertilizers. The addition of mulches is an effective way to reduce weed growth in cultivation of horticultural crops. Mulching is used frequently in blueberry plantations because it controls weeds, maintains appropriate soil moisture content, and prevents extreme soil temperatures. Amendment of organic material prior to establishment of blueberry generally increases growth and fruit yield (Blatt, 1992; Sanderson, Cutcliffe, 1991; Peterson, 1987; Starast et al., 2002). Mycorrhizae probably play an important role in the biology of Vaccinium spp. and in the development of a sustainable production system, because mycorrhizae may be beneficial in reducing the utilization of agricultural chemicals. Mycorrhizal fungus increase the uptake of nitrogen (Stribley and Read, 1974) and phosphorus (Pearson and Read, 1973), thereby improving nutrient levels and growth rates of calcifuges, which grow on acidic or peaty soils in which the natural rooting processes are slower. Largent et al. (1980) found not only that 61% of Vaccinium plants examined had mycorrhizae present in the root tips but also that the degree of colonization was differed. The objective of this study was to evaluate the effects of different production practices on the level of mycorrhizal colonization in field-grown half-highbush blueberry (Vaccinium corumbosum × Vaccinium angustifolium) cultivars ‘Northblue’ and ‘Northcountry’. 449 Proc. VIII IS on Vaccinium Culture Eds.: L. Lopes da Fonseca et al. Acta Hort. 715, ISHS 2006 MATERIAL AND METHODS One–year old in vitro propagated half-high blueberry plants (V. corymbosum × V. angustifolium) were established at the Horticulture Farm in South part of Estonia in a silt loam (Enti Umbric Albeluvisol, base on World Reference Base classification) soil in May 1997. The treatments for ‘Northblue’ were mineral soil with or without plastic mulch and for ‘Northcountry’ mineral soil with or without peat mulch. Plantation has not been fertilized in any year. Each treatment consisted of 5 plants, and was replicated 3 times. In August 2001, 2 root samples were taken from each plant. Blueberry plants were sampled by removing small sample with a cylindrical (29 mm diameter, 150 mm length) stainless steel corer. Roots were separated from samples by washing the soil through a coarse screen. The 10 % w/v KOH was used to clear roots, and 0.005 % trypan blue in lactoglycerol (1:1:1 lactic acid, glycerol and water) was used for staining (Brundrett et. al. 1996). From each root sample, 5 root segments of 5 mm long were separated and observed with a light microscope. Assessment of mycorrhizal colonization under the light microscope was done by randomly selecting microscope field of view and cross-hair positions. On each root segment, 3 observation fields were studied and number of hyphae which were across centre horizontal axis was counted. The diameter of root segments were also measured (mm). In August 2001 the fruits were weighed and the height and width of blueberry plants were measured. Also the number of shoots (longer than 15 cm) per plant was counted. One-way analysis of variance (ANOVA) was conducted to compare means. The possible correlations, R, and regression formulas were calculated. Significant (P<0.05) differences were marked by asterisk (*) in figures. RESULTS On the average, 2.1 to 4.5 hyphae were counted on the observation field of roots (Fig. 1). Significantly more hyphae were observed on ‘Northland’ plants, which were cultivated with plastic mulch. The hyhae number of half-high blueberry cultivar ‘Northblue’ was 3.6 and ‘Northcountry’ 2.2. The number of ‘Northblue’ hyphae was significantly higher than that of ‘Northcountry’. Approximately 3.4 hyphae on the observation field were observed for plants with mulch treatment compared to the 2.4 hyphae found with control. The number of hyphaes was significantly higher in case of both cultivars with mulch treatment. In the fourth growing year, plant height was 12 to 38 cm and width was 31 to 6 cm. Plants had 2 to 5 one–year old shoots. Plant width and height of half-high blueberry were strongly correlated with the number of hyphae (Fig. 2), with correlation coefficients of 0.703 and 0.949, respectively. There was no significant correlation between number of shoots and number of hyphae (R=0.434). The yield of half-high blueberry plants was 20 to100 g per plant. Figure 3 shows significant correlation between fruit yield and hyphae number (correlation coefficient, 0.712*). Plants that had more hyphae produced higher yields. The diameter of observed roots was 0.1 to 0.9 mm. A significant increase in the root diameter of half-high blueberry was related to an increase in the number of hyphae on the root (Fig. 4). The value of correlation coefficient was 0.512*. DISCUSSION AND CONCLUSIONS Adding peat to soil and using peat as mulch increased acidity of substrate (Starast et al., 2003). The results with pure culture of Hymenoscyfus ericae indicated that hydroxamate siderophore production by H. ericae has its optimum pH at 4.5 (between 3.5 and 5.5) (Federspiel et al., 1991). In our experiment a little more hyphae on roots of plants with peat mulch were found, but it is not significant different from the control. Using plastic mulch significantly increased mycorrhizal colonization compared to the control. The using of organic mulches also has some disadvantages. Mulching prevents the soil from emitting heat, which may lead to a lower temperature at the mulch surface.