The Glomus Mosseae (GM), was obtained from the collection preserved at the Institute of Plant Nutrition and Resources, Beijing Academy of Agriculture and Forestry Sciences, China. Fusarium oxysporum Schlecht. f.sp. fragariae Winks et Williams was provided by the laboratory of Plant Disease Ecology, Agricultural University of Hebei, China. The strawberry (F. ananassa Duch.) in this experiment is cultivar ‘Totonka’.

The seedlings of strawberry ‘Totonka’ with consistent growth were selected to be planted in nutrition bowls that were disinfected by potassium permanganate and contained sterilized matrix, which was the mixture of perlite and vermiculite in a ratio of 1:1. Fifteen-day-old strawberry was used for all the experiments. Conidia suspension of F. oxysporum f.sp.fragariae was adjusted to 3×10⁶ conidia/mL with sterile distilled water.

Thirty strawberry plants were inoculated with 30 mL G. mosseae in the soil (G.m, about 15000 inoculum potential units), the other 30 plants representing the non-inoculated controls were grown naturally. To evaluate the presence of G. mosseae in strawberry plants, roots in the pots, some days after their inoculation, were sampled from inoculum site and surveyed by means of acid fuchsin stain after the roots were lysised in alkaline solution. When infection rate in strawberry roots was more than 90% after inoculation with G. mosseae, a standard size drop (5ml) of conidial suspension of the pathogen was placed around the roots of individual plant. Control plants (CK) were inoculated only with an equal amount of sterile distilled water and each treatment was repeated twice. The status of F. oxysporum f.sp.fragariae of strawberry plants growing in the pots was also assessed by means of acid fuchsin stain, and when the infection rate of the pathogen in roots was more than 90%. The roots of strawberry plant were taken as a sample at 5 days’ intervals for further study.

Lignin was determined by the ash -free residue by the two-stage H₂SO₄ hydrolysis (Huang, Jinhua, et al., 2003). The hydroxyproline-rich glycoprotein (HRGP) was determined according to Kivirikko (Kivirikko K. I.,1967).

Peroxidase (POD) activity was determined by measuring the increase in absorption at 470 nm according to Kalir et al. (1984) with modification. Catalase (CAT) activity was determined by monitoring the decomposition of H2O2 at 240 nm following the method of Aebi (1984). Superoxide Dismutase (SOD) activity was performed according to the description by Cakamsh and Marschner (1992).

To measure the electric conductivity, 30 roots sampled from each plant were put into a tube containing 10 ml distilled water and shaken for 12 h at 180 rpm. Following this, DDS-11A conductivity meter was used to measure it.

The data were analyzed by data processing system 7.05 (DPS7.05). Means were compared with Duncan's new multiple range method test. All statistical analyses were done at p=0.05.

The change in the content of lignin is an important index to determine the structural change of root cell walls. If the content lignin of plant roots increases, it indicates that the root cell walls get thick, lignified, and form barriers to defense pathogen invasion.

The content of lignin in strawberry showed a reduction after inoculation with F. oxysporum f.sp.fragariae for 5 days in AM and non-AM plants; however, in mycorrhizal plants, higher lignin content was observed in comparison with non-AM plants (Fig. 1). The content of lignin increased gradually after inoculating the pathogen, and the maximum content of lignin was obtained on the 25^th day (149.52mg/g). The increment in the F. oxysporum f.sp.fragariae plus G. mosseae treatment was much higher than others.

Fig. (1). Effect of VAM on lignin content in strawberry roots at 5-day intervals after the inoculation of Fusarium oxysporum Schlecht. f.sp. fragariae.

HRGP is the major structural protein of plant cell walls, and is closely related with the formation of lignin. When the plant pathogen attacks or cell wall is damaged, large amounts of HRGP accumulate in plant to fix the damaged structure of the cell wall, thus enhancing disease resistance of plant.

Fig. (2) illustrates the changes in HRGP content of VAM- treated and non-treated strawberry roots at 5 days’ intervals after the inoculation of Fusarium oxyspo-rumSchl.f.sp. fragariae. The content of HRGP in strawberry roots with no-treatment was basically stable. However, the content of HRGP in strawberry roots decreased gradually after the pathogen inoculated and was 10.08 mg/g on the 250^th day after inoculation. Additionally, the application of VAM significantly increased the content of HRGP when compared with the control under both CK and FO conditions (Fig. 2).

Fig. (2). Effect of VAM on HRGP in strawberry roots at different time intervals after the inoculation of Fusarium oxysporum Schlecht. f.sp. fragariae.

Lipid peroxidation and large amounts of superoxide radicals producing in plants were usually caused by invasions by pathogens. The protective enzymes, for examples SOD, POD and CAT protect the cell through eliminating superoxide radicals. In addition, protective enzyme POD could catalyze cross-linking effects of glycoproteins which are rich in hydroxyproline of the cell wall in plants, thus cell wall can be strengthened, and blocking the intrusion of pathogens directly.

The activities of SOD, POD and CAT of strawberry roots were relatively stable under natural condition, but brought changes in the pathogen when the pathogen was attacked with varying the degrees of change. Among these changes, the maximum activity of SOD and POD was observed on the 5^th day after inoculation, and of CAT on the 10^th day.

The activity of protective enzymes SOD and POD was observed on the 10^th day after inoculated with VAM G. mosseae, which was shown later than FO treatment. The peak activities of SOD and CAT were observed on the 15^th day and 10^th day after inoculation. Additionally, the application of VAM significantly increased the activities of those protective enzymes when compared with the control under both CK and FO conditions (Table 1).

The electric conductivity of the plants is an important indicator for reflecting the reaction of peroxidation in cell membrane. Higher conductivity affects the structure of the cell membrane that would result in damaging cell membrane and interfering in normal physiological metabolism of the plant.

The electric conductivity of strawberry roots was observed to be relatively lower and stable under natural condition, but it sharply increased after inoculation with pathogens (Table 2). It was 90.9% on the 25^th day after inoculation. Inoculation with VAM fungi reduced the extent of increment in the electrical conductivity induced by pathogen infection. The conductivity of G.m plus FO treatment was higher than the CK treatment, but significantly lower than the FO treatment.