| It is well known that arbuscular mycorrhizal fungi (AMF) widly distribute inheavy metal contaminated soil. Heavy metal-adapted AMF provide direct linksbetween soil and roots, and consequently may have an essential contribution tophytoremediatiom by influencing heavy metal availability and enhancing planttolerance. In order to provide scientific references for the phytoremediation of theheavy metal contaminated soil, we explored the effects of AMF on the tolarnce ofplant to heavy metal stress and its mechanisms. After treated with differentcombination of Cu and AMF, the photosynthetic characteristics and chlorophyllfluorescence parameters of a non-metallophyte Elsholtzia splendens collected in cleansoil were measured on a sunny day in early August (vegetative stage), middle October(flowering stage), early December2013(fruiting stage), by using a portable Li-6400photosynthesis measuring system. Four treatments imposed in the experiment were:control (CK), copper stress, inoculated with AMF, copper stress and inoculated withAMF. We discussed the roles and mechanisms of AMF in improving E. splendens’scopper tolerance ability in the aspects of copper content in plant, AMF colonization,mineral nutrition uptake and plant flowering phenology, using the technologies ofinductively coupled plasma-atomic emission spectroscopy (ICP-AES), WinRhizoimage analysis system and binocular microscope (Leica L2). The main results are asfollows:1. Effects of AMF inoculation on the growth and biomass allocation of E.splendens under copper stressThe plant height, basal diameter, biomass, branch number, root total surface areaand root total volume of E. splendens under copper stress were significantly lower than those of CK. Inoculation with AMF significantly increased the plant number,plant height and biomass of E. splendens under copper stress, indicating an alleviationeffect on the inhibition of copper on the growth of E. splendens. In the vegetativestage, inoculation with AMF significantly increased the root average diameter, totallength, total surface area, total volume and root/shoot ratio, but decreased the specialroot length than those of E. splendens under copper stress. In the flowering stage,inoculation with AMF significantly increased the special leaf area and significantlydecreased the root shoot ratio of E. splendens under copper stress. Thes resultssuggested that inoculation with AMF could enhance the Cu tolerance ability of E.splendens by effectively adjusting the allocation between below-ground andabove-ground biomass, and consequently the ability of E. splendens in response to theenvironmental changes.2. Effects of AMF inoculation on the copper accumulation and mineral nutritionuptake of E. splendens under copper stressUnder copper stress, AMF inoculation significantly decreased the copper contentin the root, stem and leaf of E. splendens at different life history stages, especially forthe copper content in the above-ground part. Copper content in different organs of E.splendens randed in the orders: root>stem>leaf. There was a significant impact ofcopper stress on the content of S, K, Mg, Ca in different organds of E. splendens. Inthe vegetative stage, inoculation with AMF increased the content of Mg in leaf andthe content of S and K in stem of E. splendens under copper stress by26.7%,28.2%and63.7%, respectively. In the flowering stage, the content of K in shoot and thcontent of Ca and K in root of E. splendens inoculated with the AMF weresignificantly higher than those of E. splendens treated with only copper by24.5%,45.2%,17.9%and65.5%, respectively. In the fruiting stage, the content of K in leafand root of E. splendens inoculation with AMF were significantly higher than those ofE. splendens under copper stress without inoculation by53.6%and92.8%,respectively. These results suggested that AMF could enhance the heavy metaltolerance ability of E. splendens after alleviating the damage of copper stress and the nutrition shortage by enhancing the uptake of mineral nutrition including K, Ca andMg, inhibiting the transfer of Cu from th root to the above-ground part.3. Effects of AMF inoculation on the photosynthetic physiology of E. splendensunder copper stressIn whole growth period, the diurnal variation in net photosynthetic rate (Pn) inthe leaves of E. splendens under all treatments took on a one-peak curve. The dailymean Pnin the leaves of E. splendens ranked in the orders: vegetataivestage>flowering stage>fruiting stage, which was in accord with the plant growth rule.The decreasing of the daily mean Pnof E. splendens under copper stress in thevegetative stage and fruiting stage were mainly due to a non-stomatal limitation, whilethat of of E. splendens in the fruiting stage was non-stomatal limitation. In thevegetative stage and fruiting stage, inoculation with AMF significantly increased thedaily mean Pn, and significantly decreased the daily mean stomatal conductance anddaily mean intercellular CO2concentration (Ci) of E. splendens under copper stress. Inthe vegetative stage, inoculation with AMF significantly increased the maximum netphotosynthetic rate (Pnmax), light saturation point (LSP), apparent quantum yield(AQY), but decreased the CO2compensation point (CCP). In the flowering stage,inoculation with AMF significantly increased the Pnmax, LSP, CO2saturation point(CSP), carboxylation efficiency (CE), while the light compensation point (LCP)significantly decreased. In the fruiting stage, inoculation with AMF significantlyincreased the LSP, but decreased the CCP, LCP.4. Effects of AMF inoculation on the relative chlorophyll cotent and chlorophyllfluorescence parameters of E. splendens under copper stressThe relative chlorophyll cotent of E. splendens in the vegetative stage variedsignificantly among different treatments. Compared with CK, the relative chlorophyllcontent, the maximal efficiency of PSII photochemistry (Fv/Fm), maximalfluorescence (Fm) of E. splendens under copper stress significantly decreased, whilethe minimal fluorescence (Fo) significantly increased. Without the stress of copper,inoculated with AMF had no significant effect on the relative chlorophyll content of E.splendens. Compared to only copper treatment, the relative chlorophyll content of E. splendens inoculated with AMF increased by77.8%and27.0%, respectively, in thevegetative stage and flowering stage. Under copper stress,,E. splendens inoculatedwith AMF had higher Fv/Fmin the. However, in the fruiting stage, no significantdifferences were found for the relative chlorophyll cotent and chlorophyllfluorescence parameters of E. splendens with different treatments. These resultssuggested that inoculation with AMF could alleviate the damage of copper stress on E.splendens via inceasing the relative chlorophyll content in the leaves and improvingthe light envery conversion efficiency.5. Effects of AMF inoculation on phenology and fitness of E. splendens undercopper stressA certain plasiticy was found for the phenology of E. splendens with differenttreatments. Under copper stress, the onset, peak flowering date and last flowering datewere delayed and flowering duration shortened, while the inflorescence width,inflorescence biomass, seed weight and seed number significantly decreased and theseed1000-kernel mass significantly increased. There were no significant differencesof the flowering phenology between AM plants and non-AM plants without copperstress. Inoculation with AMF shift the the onset, peak flowering date, last floweringdate to earlier date by6d,6d,5d,3d, respectively, while increased the florescencewidth, seed number, seed weight by13.5%,49.8%and58.9%, respectively. Theseed1000-kernel mass decreased without significance. There were significantnegative correlations between onset and duration and significant positive correlationsbetween duration and seed number. These results suggested that inoculation withAMF could effectively contrubitue to fitness of E. splendens by increasing theflowering duration, enhancing the seed weight and having the erly onset, peakflowering date. |
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