Effects of arbuscular mycorrhizal fungi on tree growth, leaf water potential, and levels of 1-aminocyclopropane-1-carboxylic acid and ethylene in the roots of papaya under water-stress conditions

Seedlings of papaya (Carica papaya L. var. Solo)were transplanted to pots with or without an arbuscular mycorrhizal (AM)fungus (Gigaspora margarita Becker and Hall). After 3 months, half the plants were subjected to water stress by withdrawing irrigation. The leaf water potential (LWP)was measured during 20 days of water-stress treatment and then the plants were harvested. Root ethylene and 1-aminocyclopropane-1-carboxylic acid (ACC)concentrations were measured and plant fresh weight determined. The LWP decreased during the water-stress treatment and this decrease was more severe in the non-AM plants. Plant fresh weight was higher for AM than non-AM plants under both conditions. Under well-irrigated conditions, the ethylene concentration in the roots was increased by the presence of AM, although there was no significant difference between AM and non-AM roots in ACC levels. ACC increased in both AM and non-AM roots under water-stress conditions. The waterstress treatment resulted in a marked increase in ethylene concentration in non-AM roots but the concentration in AM roots was slightly lower than under normal conditions.