Physiological Response of Citrus macrophylla Inoculated with Arbuscular Mycorrhizal Fungi under Salt Stress

Seedlings of Citrus macrophylla were inoculated with arbuscular mycorrhizal (AM) fungi (+AM) or not (-AM). Forty-five days after inoculation onwards, half of the +AM and -AM plants were irrigated for three months with a nutrient solution containing 50 mM NaCl. Due to the high sensitivity of C. macrophylla to salinity, AM inoculation only partly compensated the growth limitations imposed by salinity. The high intercellular CO2 levels and oxidative stress suffered by -AM plants decreased with AM inoculation. Although salinity strongly decreased photosynthesis in -AM plants, AM colonization alleviated this decrease by increasing total chlorophyll levels. However, the chlorophyll and photosynthesis levels found in +AM plants exposed to salinity were still lower than in control plants, meaning that AM did not completely compensate for the negative effect of salinity on the photosynthetic response. Although AM significantly increased the water percentage in leaves, it did not modify the water potential in plants. Mycorrhizal inoculation increased the osmotic potential and, consequently, decreased turgor, although this decrease did not affect the physiological processes that depend on leaf turgor, such as photosynthesis, which even increased in +AM plants. Given that +AM plants had higher leaf Cl levels than -AM plants but showed better growth and a better physiological response, leaf Cl concentration was not related with salt tolerance in the saline conditions used in this experiment.