A Study On The Distribution Pattern Of Copper And The Responses Of Ultrastructure To Excessive Copper In Citrus Seedlings

Copper（Cu）is one of the essential micronutrients for higher plants.Over application of Cu-plus fungicides and pesticides,and less organic fertilization is common in the main citrus production region of Fujian Province,which resulted in continuous soil acidification and excessive Cu in the soil of citrus orchards.However,the Cu distribution pattern and the responses of ultrastructure of citrus seedlings to excessive Cu are less discussed.In the study,seedlings of Citrus sinensis（C.sinensis）and Citrus grandis（C.grandis）were treated with 0.5μmol·L^-1（as Control）or 300μmol·L^-1（excessive Cu）for 18 weeks continuously in sandy pottery experiments under p H 4.3 to 4.5.The study investigated（1）the effects of excessive Cu on the growth and photosynthetic performance of two citrus species,（2）the Cu concentration and the distribution pattern in two citrus species and（3）the effects of excessive Cu on the ultrastructure of leaves and roots.The results including:（1）Compared to Control,excessive Cu treatment inhibited seedling growth,plant height and root length of two citrus species significantly.Excessive Cu treatment decreased the net photosynthetic rate of C.sinensis leaves,however,no significant inhibition was found in C.grandis leaves.The comparison between two citrus species implied a much stable PS II structure and a higher electron transferring rate of C.grandis contributed to a better photosynthetic performance than C.sinensis under excessive Cu.（2）At the tissue level,the lateral roots of citrus seedlings are the primary Cu target sites of citrus seedlings under Control and excessive Cu treatment.The excessive Cu treatment increased the distribution rate of Cu in the lateral roots of two citrus species.However,the Cu distribution ratio decreased significantly in the stems and roots of two citrus species by Cu treatment compared to Control.At the subcellular level of Control seedlings,the Cu was mainly distributed in the organells fractions of the lateral roots.However,the excessive Cu treatment decreased the Cu ratio in the organells fractions while increased the Cu ratio in the cell wall and soluble fractions significantly in the lateral roots of two citrus species.Among the polysaccharides of lateral root cell wall,pectin is the primary site for Cu binding.Compared to C.sinensis,C.grandis had a significantly higher Cu concentration in the pectin of lateral roots under excessive Cu treatment.The chemical groups of hydroxy,carboxyl and amidogen of cell wall from C.grandis lateral roots contributed to Cu absorption and immobilization based on FTIR analysis.（3）The alteration of ultrastructure indicated that the organells,such as chlorophyll,mitochondria and vacuole were damage to different extent,besides,starch granule and crystal were deposited in the xylem of lateral roots by excessive Cu in two citrus species.Compared to C.sinensis,C.grandis had a relatively integrated structure of leaf chloroplasts and mitochondria,an thicken root cell wall and strengthen xylem blocking contributed to its higher Cu-tolerance under excessive Cu.Conclusively,the study revealed the responsive mechanisms of citrus species under excessive Cu from plant photosynthetic and physiological performance,Cu translocation and redistribution,and ultrastructure of leaves and roots of two citrus species.Compared to C.sinensis,a relatively higher Cu tolerance of C.grandis is attributed to its increased cell wall thickness,and promoted cell wall suberin deposition,which finally enhanced Cu absorption by the lateral cell wall and decreased Cu translocation from roots to shoots.The results of present study will provide theoretical evidences for copper toxicity prevention in acidified citrus orchard and provide scientific instructions for optimized nutrient management on citrus orchard.