Arbuscular mycorrhizal fungi (AMF) can enhance drought tolerance in plants, whereas little is known regarding AMF contribution to sucrose and proline metabolisms under drought stress (DS). In this study, Funneliformis mosseae and Paraglomus occultum were inoculated into trifoliate orange (Poncirus trifoliata) under well watered and DS. Although the 71-days DS notably (P < 0.05) inhibited mycorrhizal colonization, AMF seedlings showed significantly (P < 0.05) higher plant growth performance and leaf relative water content, regardless of soil water status. AMF inoculation significantly (P < 0.05) increased leaf sucrose, glucose and fructose concentration under DS, accompanied with a significant increase of leaf sucrose phosphate synthase, neutral invertase, and net activity of sucrose-metabolized enzymes and a decrease in leaf acid invertase and sucrose synthase activity. AMF inoculation produced no change in leaf ornithine-δ-aminotransferase activity, but significantly (P < 0.05) increased leaf proline dehydrogenase activity and significantly (P < 0.05) decreased leaf both Δ1-pyrroline-5-carboxylate reductase and Δ1-pyrroline-5-carboxylate synthetase activity, resulting in lower proline accumulation in AMF plants under DS. Our results therefore suggest that AMF strongly altered leaf sucrose and proline metabolism through regulating sucrose- and proline-metabolized enzyme activities, which is important for osmotic adjustment of the host plant.

Citrus is a salt-sensitive plant. In the present study, the salt stress ameliorating the effect of arbuscular mycorrhizal fungi through antioxidant defense systems was reported. Three-month-old trifoliate orange (Poncirus trifoliata) seedlings colonized by Glomus mosseae or G. versiforme were irrigated with 0 and 100 mmol NaCl solutions. After 49 days of salinity, mycorrhizal structures were obviously restrained by salt stress. Mycorrhizal inoculation especially G. mosseae significantly alleviated the growth reduction of salinity. There were notably lower malondialdehyde and hydrogen peroxide contents in the leaves of mycorrhizal seedlings than in non-mycorrhizal ones. Mycorrhizal seedlings recorded notably greater activity of catalase and contents of ascorbate, soluble protein and glutathione under salinity or non-salinity conditions. The seedlings colonized by G. mosseae showed significantly higher antioxidant defense systems response to salinity than by G. versiforme. Our data demonstrate that mycorrhizal (especially G. mosseae) citrus seedlings exhibited greater efficient antioxidant defense systems, which provide better protection against salt damage.

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Walnut, an important oil fruit tree, is dependent on arbuscular mycorrhizas, while mycorrhizal roles and efficient mycorrhizal fungus in walnuts are unknown. This study was conducted to evaluate the effect of five arbuscular mycorrhizal fungi (AMF) species, including Acaulospora scrobiculata, Diversispora spurca, Glomus etunicatum, G. mosseae, and G. versiforme on plant growth, leaf gas exchange, root morphology, and root nutrient contents of walnut (Juglans regia L. Liaohe 1) seedlings. Three months of AMF inoculations later, root mycorrhizal colonisation achieved 47.0% to 76.4%. AMF treatments increased plant growth performance, dependent on AMF species. AMF-inoculated plants with D. spurca, G. etunicatum, and G. mosseae showed higher root length, projected area, surface area, and volume than non-AMF plants. Except for G. versiforme, the other four AMF treatments almost significantly increased leaf photosynthesis rate, transpiration rate, and stomatal conductivity, while reduced intercellular CO₂ concentrations and leaf temperature. AMF affected root nutrient contents, dependent on AMF and mineral nutrient species. These results, thereby, concluded that AMF had a positive role in walnuts, dependent on AMF species, and D. spurca was the best mycorrhizal fungus for walnut. Such results provide the potential possibility of a developing consortium of AMF in walnut cultivation management.

A greenhouse experiment was conducted to investigate the dynamic trend of symbiotic development in Citrus sinensis/Poncirus trifoliata trees colonized by Glomus versiforme during 12-day lasting drought and to evaluate correlation between symbiotic development and reactive oxygen metabolism of citrus. One year after planting, water was withheld from all trees for 12 days. During the drought stress mycorrhizal colonization and arbuscles showed a durative reduction. Mycorrhizal trees maintained significantly lower contents of superoxide anion, hydrogen peroxide and malondialdehyde than non-mycorrhizal control throughout the experiment. There were significantly greater activities of superoxide dismutase, guaiacol peroxidase and catalase in mycorrhizal trees throughout the drought stress period. Ascorbate and glutathione contents of mycorrhizal trees were notably higher than those of uninoculated ones during 12 days of drought stress. Correlation analysis showed that not vesicles and entry points but mycorrhizal colonization and arbuscules had a substantive direct effect on reactive oxygen metabolism. These results suggest that mycorrhizal colonization and arbuscles play a major role in improving reactive oxygen metabolism of drought-stressed citrus, thus inducing a lower oxidative damage.

Putresince (Put) as one of the important polyamines (PAs) has been identified to regulate mycorrhizal development of citrus plants. The present study was to screen an efficient concentration of Put application at the range of 0.05-1 mM on the trifoliate orange (Poncirus trifoliata) seedlings colonized by Glomus mosseae, in terms of growth, root system architecture, and chlorophyll and carbohydrate contents. Compared to the non-Put treatment, all the Put treatments, especially 0.05 mM Put, significantly increased mycorrhizal colonization of tap root in addition to first, second, and third order lateral roots. The mycorrhizal seedlings treated by 0.05, 0.1, and 1 mM Put showed greater growth (stem diameter, height, leaf number, and fresh mass) and root morphological properties ( tap root length, projected and surface areas, and volume) and higher numbers of first, second, and third order lateral roots. Bio-molecules like chlorophyll a, total chlorophyll, and carotenoid contents of the seedlings were significantly increased by the Put treatments at 0.05-1 mM. All exogenous Put application at the range of 0.05-1 mM significantly decreased sucrose contents but increased glucose contents of leaves and roots. This study suggests that exogenous Put can significantly improve growth performance and root system architecture, besides changes in physiological traits of AMF seedlings. The 0.05 mM concentration of Put showed the best effects.

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