Effects of four arbuscular mycorrhizal fungi on tolerance of Vaccinium corymbosum to drought stress

Blueberry (Vaccinium spp.) is well known for its rich anthocyanins and other bioactive compounds, which helps preventing from cardiovascular disease and other chronic illnesses. Over the past decades, blueberry has been cultured in many areas of China, thus became the fastest-growing species in fruit production, potentially making China one of the largest blueberry-producing countries in the world. However, blueberry growth and production were severely affected by drought events in areas of Yangtze River. Vaccinium plants preferred acidic soils and spontaneously formed mutualistic symbiotic associations mainly with soil fungi of the phylum Ascomycota, called ‘ericoid mycorrhizae’. However, some reports have indicated that the absence of ericoid mycorrhizal fungi inoculum may allow Vaccinium plants to associate with arbuscular mycorrhizal (AM) fungi, and AM fungi inoculation significantly enhanced growth of Vaccinium plants. Southern highbush blueberry (Vaccinium corymbosum) has short or even rare root hairs in field systems, and depends on fungi for optimal growth. Meanwhile, few reports were about the effects of AM fungi on osmotic adjustment and reactive oxygen metabolism of southern highbush blueberry.The purpose of this study is to evaluate the effects of Glomus mosseae, G. versiforme, G. intraradices and G. etunicatum on the southern highbush blueberry seedlings under drought stress conditions, and antioxidant and osmotic adjustment matters, mineral nutrition in blueberry plants and enzymatic activities in rhizosphere soil were investigated.Two soil water regimes (well-watered[WW] and drought stress[DS]) and five AM fungi inoculations (four Glomus and non-AM fungi inoculation [CK]) were arranged in a complete randomized block design. Each treatment (one plant per pot) was performed in three replicates. Water treatments began after 134 days (July 31, 2014) adaption of greenhouse conditions, and WW pots were maintained, but DS pots were cut off for 20 days of water supply and resumed on August 20. Meanwhile, the leaves of blueberry cultivar Misty plants were collected for determination of physiological index on 13 d drought,20 d drought and 2 d rewatering, respectively.The results showed that the leaf relative water content (RWC) and chlorophyll contents decreased in DS treatments, while the soluble sugar content, superoxide dismutase (SOD) activity and malondialdehyde (MDA) contents increased, but had no significant effects on mycorrhizal colonization. Under the DS conditions, leaf RWC, chlorophyll, soluble sugar contents and SOD activity were significantly higher in AM fungi-inoculated plants, especially in G. mosseae-inoculated plants than in non-AM fungi-inoculated plants, while the leaf MDA content was lower. In addition, AM fungi, especially G. mosseae, increased phosphorus (P) and potassium (K) contents in the leaves, stems and roots, as well as acid phosphatase (ACP), catalase (CAT) and urease activities in rhizosphere soil of blueberry plants, in comparison with the non-AM fungi-inoculated treatment (CK).These results indicate that the drought tolerance of blueberry cultivar Misty is enhanced with AM fungi inoculation through increase of antioxidant enzyme and osmotic adjustment, and the soil environment is improved, accompanied by P and K uptake increases by plants. In addition, G. mosseae is the most beneficial Glomus among the four AM fungi to improve the resistance of blueberry cultivar Misty plants against DS.