Anatomy, Ultrastructure and Assimilate Concentrations of Roots of Citrus Genotypes Differing in Ability for Salt Exclusion

Seedlings of Rangpur lime (Citrus reticulata var. austera hybrid?)and Etrog citron (C. medica) were treated in water culture with0, 25, 50 or 100 mol m^–3 NaCl, and in sand culture with0 or 100 mol m^–3 NaCl. Leaf chloride analyses indicatedthat Etrog citron accumulated the most chloride at all levelsof salinity. The structure, ultrastructure and concentrationsof chloride and reserve assimilates of the primary root up to50 mm back from the tip were compared between genotypes andbetween salt treatments. There were no differences in root anatomy in the absence ofsalt between the two genotypes. The hypodermal cells developedlignified and suberized walls which blocked the plasmodesmataand resulted in degeneration of the cell contents. Frequentthin-walled passage cells in the hypodermis had living contentsand may represent major sites of ion uptake into the symplasm,which was connected with the stele via plasmodesmatal connectionswith and between the cortical cells. The primary endodermalcells had lignified casparian strips and plasmodesmata in othercell wall areas. These connections were blocked by secondarysuberization of all except the endodermal passage cells oppositethe protoxylem arcs. Suberization of the hypodermis and endodermis and the appearanceof granular deposits in the vacuole occurred closer to the roottip of both genotypes after treatment with 100 mol m^–3NaCl. Levels of starch and triglyceride in 10 mm serial segmentswere similar between roots of control and salt-treated plantsand increased with distance back from the root tip. Chlorideconcentrations increased with salt treatment but values (ona tissue water basis) were similar between genotypes and betweenthe apical and proximal ends of the root. Reducing sugar concentrationsdecreased with salt treatment to a similar extent in roots ofboth genotypes. Key words: Citrus, Roots, Salinity