Effects of AMF inoculation on growth and photosynthetic physiological characteristics of Sinocalycanthus chinensis under conditions of simulated warming
The global climate warming is affecting ecosystems,especially endangered plant species whose distribution is mostly restricted to few areas. Arbuscular mycorrhizal fungi (AMF) form symbiotic associations with many plant species, supplying mineral nutrients to their host plants. Sinocalycanthus chinensis is a national second-class protected plant species,distributed in only a few areas in Zhejiang and Anhui provinces in China. Preliminary investigation has revealed higher infection rate of natural populations of S. chinensis by AMF. The role of AMF in S. chinensis under global warming has remained unknown. To reveal the effects of AMF on S. chinensis under the conditions of global climate warming,the pot experiment was conducted with one-year-old S. chinensis seedlings inoculated with AMF (inoculation of rhizosphere soil at a rate of 5%,v /v; soil without AMF was used as control) and temperature increase was simulated by an electric radiant infrared heater (+ 2℃ measured around the leaves; normal temperature was used as control) . Growth rate, morphogenesis,photosynthetic physiological characteristics,antioxidant activities,membrane lipid peroxide level,nutrient content of S. chinensis seedlings were compared under four treatments: AMF,simulated warming (SW) ,both AMF inoculation and simulated warming (AMF + SW) ,and control (CK) . The results showed that: (1) AMF inoculation had a significant positive effect on plant height and average root diameter,and a negative effect on the width-to-length ratio of mature leaves,total root length,total number of root tips,special root length (root length /root biomass) ,and special root surface area (root surface area /root biomass) . Double-factor variance analysis showed that the average root diameter and total number of root tips was significantly correlated with AMF and SW. This showed that symbiotic association between AMF and S. chinensis may modify the nutrient absorption strategy of S. chinensis through modification in root morphology.(2) The diurnal change in net photosynthetic rate (P_n) of leaves in S. chinensis seedlings under AMF + SW treatment had a typical bimodal curve that was determinately regulated by stomatal conductance. AMF affected significantly the daily mean P_n . After fitting the photosynthesis light response using modified models of rectangular hyperbola,we found that Pn in plants with AMF + SW and AMF treatments were distinctly higher than that of CK when light intensity was more than 50 "mol m~(-2) s~(-1) . The double-factor variance analysis showed that AMF significantly affected the daily mean P_n,maximum net photosynthetic rate,light saturation point (LSP) ,and dark respiration rate,whereas SW had a significant effect on LSP. Fitting of the CO_2 response by using modified models of rectangular hyperbola revealed that Pn in plants under AMF + SW and AMF treatments was significantly higher than that in the CK with increasing the concentration of CO_2 . Initial carboxylation efficiency under AMF + SW treatment and photosynthetic capacity under AMF treatment were significantly higher than that of the CK. The CO_2 compensation point after the treatment by AMF + SW or AMF was significantly lower than that of the CK. (3) The content of soluble sugars in leaves after the treatment with AMF was significantly higher than that in other treatments. The content of soluble proteins in leaves of plants treated with AMF + SW or AMF was distinctly higher than that of the CK. The double-factor variance analysis showed that SW and AMF + SW interaction had significant effect on the content of soluble sugars in leaves,and both AMF and SW had significant effect on the content of soluble proteins in leaves. The results provided the theoretical foundation for conservation and artificial cultivation of S. chinensis under conditions of global climate change.