Water deficit is one of the major limitations on plant growth and as a result of anthropogenic disturbance and predicted global climate changes, drought conditions are expected to increase in the near future (Solomon et al. 2007). During the last few decades, a large amount of studies focused on implementing and validating tools and techniques to prevent drought-mediated damages were reported (Baslam & Goicoechea 2011). In this study, we have investigated the effect of AM fungi species, R. intraradices in improving the drought tolerance mechanisms in E. maackii under gradient water deficit conditions.
Water deficit had negative effect on AM inoculation (Ryan & Ash 1996), which supported our findings that AM inoculation rates in seedlings along with increasing water deficit decreased gradually. ter deficit was the key limitation in plant growth. Zhang et al. (2013) reported that the main limiting factor in growth was the seasonal drought. Plants have evolved kinds of mechanisms avoiding damages by water deficit, including forming symbiosis with AM fungi. The improvement in plant growth in AM inoculation treatment could be attributed to the maintenance of increased mineral availability for absorption and water content in plant tissue (Ahanger et al. 2014). Many researchers have reported the positive role of AM symbiosis in plant growth under drought stress (Chitarra et al. 2016; Li et al. 2015). Among AM inoculated damask rose, significant increase in uptake of essential mineral elements were detected compared with non-mycorrhizal plants under severe water deficit, reflected in growth and drought tolerance improvement (Abdel-Salam et al. 2018). Under water deficit conditions, AM fungi could modify the root morphology, such as increased root fineness, root/shoot ratio, and the length of hair root, resulting in improvement of mineral elements uptake (Hetrick 1991; Begum et al. 2019).
Water deficit had directly negative effect on plant photosynthesis, resulting from greater water absorption pressure and reduction in coupling factors (Tezara et al. 1999). Begum (2019) suggested that the severe water deficit reduced the pigment accumulation and photosynthesis attributes, which was alleviated by AM inoculation. AM-mediated increase in chlorophyII content and photosynthesis rate in damask rose under water deficit was reported (Abdel-Salam et al. 2018). Similar to our findings, improvement in gas exchange and PSII activity in seedlings that received AM inoculation treatment positively enhanced the photosynthesis function over water deficit and non-mycorrhizal seedlings.
Under moderate water deficit conditions (40%, 60% and 80% FC), seedlings that received AM inoculation treatment showed better performance in gas exchange and chlorophyll fluorescence indexes compared to the non-mycorrhizal treatment. This confirmed the report of Li et al. (2015), suggesting the role of AM fungi in substantially increasing the host plant’s drought tolerance. Formation of AM symbiosis could increase the root hydraulic conductivity, stomatal regulation in host plant, improving contact with soil particles and water extraction, which supplied better water regime for photosynthesis (Augé 2001).
As reported, reduced water availability limited the expression of growth associated genes, like tubulin and cyclin genes, reflected in reduced cellular division and proliferation (Setter & Flannigan 2001). Similar to our results, decreased biomass accumulation along increased water deficit level is directly regulated by declined photosynthesis functioning. Besides, AM inoculation could significantly ameliorate the induced declines in growth and biomass accumulation by water deficit (Zhang et al. 2019), which supported our findings that AM inoculation alleviated negative effect by water deficit in BS, BR and TB under 20%, 40% and 60% FC conditions. Leaf was more sensitive to drought compared with root, which was in line with our results that RSR increased slightly along with increasing water deficit level.
Drought mediated unbalance in reactive oxygen species (ROS), such as H2O2 and O2−, causing severe oxidative damage, and up-regulated antioxidant enzymatic activities (Kapoor et al. 2013; Ahanger & Agarwal 2017). Similar to our results, water deficit induced SOD, POD and CAT activities of both shoot and root improvement from 100% FC treatment to 40% FC treatment. However, significant decreased in SOD, POD and CAT activities at severe water deficit condition (20% FC), which may be caused by severe drought limitation. AM inoculation showed limitation in SOD activities of both shoot and root under 20%, 40% and 60% FC conditions, which suggested less sensitive of mycorrhizal seedlings to water deficit compared with non-mycorrhizal seedlings (Begum et al. 2019). Maintaining lower antioxidant enzymatic activity suggested lower ROS concentration, which benefits seedlings in regulating development processes, such as root growth, signaling, stomatal functioning and cell senescence (Miller et al. 2010). However, roots of seedlings that had received AM inoculation treatment showed significant higher POD activity under 40% and 60% FC conditions, which was in line with research of Begum et al. (2019). Furthermore, no significant difference between mycorrhizal and non-mycorrhizal seedlings in CAT activity. Loss of antioxidant enzymatic stability intensified the reduction in photosynthesis, which resulted from up-regulating chlorophyllase activity and reducing Rubisco synthesis by environmental stress (Fatma et al. 2014; Dalal & Tripathy 2012).
Many studies suggested better performance of AM fungi under moderate drought stress and ineffective role under severe drought condition (Bryla & Duniway 1997). Severe water deficit caused the reduction of effectiveness of AM inoculation in wheat, which in turn resulted in prolonging water stress (Ryan & Ash 1996), which supported our findings that AM inoculation benefited in growth, photosynthesis and antioxidant enzymatic activity of seedlings that had been subjected moderate water deficit, especially under 40%, 60% and 80% FC conditions, whereas AM inoculation was ineffective under 20% FC condition.