| Citrus is mainly cultivated in acid soil in south China.Aluminum toxicity(Al-toxicity)induced by soil acidification limited Citrus growth.A massive study indicated the feature of plant root cell wall is related to the Al tolerance.The Citrus species is superbly adapted to Al stress,while the Al storage and distribution as well as the role of root cell wall in Al toxicity regulation are yet to be illustrated.In the study,seedlings of two Citrus species [Citrus sinensis(C.sinensis)and Citrus grandis(C.grandis)] with varied Al tolerance were used to investigate:(1)the responses of biomass allocation and root parameters to Al-toxicity.(2)the features of leaf photosynthetic performance to Al toxicity(3)the Al distribution in Citrus seedlings.(4)the mechanism of root cell wall conferring Al toxicity.The results indicated:(1)The decrease of biomass induced by Al toxicity is the combined effects of Al concentrations and stress duration.Compared to control,0.5 and 1.0 m M Al stress decreased the biomass of C.grandis seedlings significantly,while no obvious inhibition on biomass of C.sinensis seedlings was found.The remarkable decrease of biomass was existed when the treatment lasted for 44 weeks.Compared to control,0.5 m M Al stress promoted the root development of C.sinensis while decreased the root development of C.grandis seedlings.The inhibition on root length,root surface area,root volume,root projection area and average diameter was found with increasing Al levels.(2)For the photosynthetic performance,no inhibition of Pn was found in C.sinensis seedlings by 0.5 m M Al treated for 15 weeks,whereas significant decrease of Pn was found in C.grandis seedlings.With increase of Al levels and stress duration,the photosynthetic inhibition of two Citrus species was resulted by both of stomatal and non-stomatal limitation.C.sinensis seedlings performed a better chlorophyll fluorescence dynamics than C.grandis seedlings is related to the decrease inhibition of electron transfer and maintenance of higher photosynthetic pigment.(3)At tissue level,Al was mainly distributed in the lateral roots,followed by main roots,leaves and stems.At the organelle level,Al was mainly accumulated on the cell wall,followed by organelles and cytoplasmic supernatant.At the cell wall component,Al was mainly found on the pectin,followed by hemicellulose and cellulose.(4)Compared to control,Al stress increased the ratio of root cell wall content to root dry biomass while decreased the the pectin content of tow Citrus species.The results of FTIR demonstrated a much obvious peak strength fluctuation of cell wall from C.grandis roots.The isolated cell wall dynamics indicated higher absorption rate and lower desorption rate of C.sinensis root cell wall compared to that of C.grandis.Based on EST sequence blast,4 potential pectin methyl esterase(PME)genes that might participate in regulation of Al binding was found.The results of RT-q PCR indicated 4 putative PME was significantly activated in root of C.grandis seedlings,of which,no significant difference was found in C.sinensis.In conclusion,the study investigated the different responses of Citrus species with varied Al tolerance at the levels of plant physiology and molecular biology.Compared to C.grandis seedlings,the higher Al tolerance of C.sinensis seedling is related to decrease electron transfer inhibition,maintenance of higher photosynthetic pigment content and higher electron transfer efficiency of PSII under Al stress,and also,stronger absorption of Al on pectin of cell wall which contributed to less translocation Al to shoots.The results increased our understanding on Aladaption of Citrus species,which will provide theoretical evidence on the revealing of responsive strategy of Citrus species under stress. |
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