Effect of Phosphate Application to Arbuscular Mycorrhizal Onion on the Development and Succinate Dehydrogenase Activity of Internal Hyphae

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Pot experiment was conducted to examine the effect of phosphate (P) application on the growth and metabolic activity of the internal hyphae of mycorrhizal onion roots. Onion seedlings (Allium cepa L.) inoculated with arbuscular mycorrhizal fungi, Gigaspora margarita (Becker and Hall) were grown in pots filled with sand treated with a P-free nutrient solution. At 40 d after sowing three treatments were applied : 1) injection of 1,000 mg P L^<-1> solution to shoot, 2) addition of 8 mg P L^<-1> nutrient solution to sand, and 3) addition of P-free nutrient solution to sand (control). The percentage of the length of infected root, number of appressoria per unit root length and succinate dehydrogenase (SDH) activity of internal hyphae were determined at 0,40, and 80 d after the treatments. The mycorrhizal infection and length of infected root were reduced with the P application treatments 1) and 2). Number of appressoria in relation to the length of infected root decreased with time and was not affected by the treatments 1) and 2). There was a significantly positive correlation between the number of appressoria per plant and length of infected root per plant. At 40 d after the treatment, the percentage of SDH-active hyphae did not differ among the treatments. These results suggest that P application inhibits the appressorium formation in roots and does not affect the metabolic activity of internal hyphae.
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