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

Abstract

Seedlings of Rangpur lime (Citrus reticulata var. austera hybrid?) and Etrog citron (C. medica) were treated in water culture with 0, 25, 50 or 100 mol m−3 NaCl, and in sand culture with 0 or 100 mol m−3 NaCl. Leaf chloride analyses indicated that Etrog citron accumulated the most chloride at all levels of salinity. The structure, ultrastructure and concentrations of chloride and reserve assimilates of the primary root up to 50 mm back from the tip were compared between genotypes and between salt treatments. There were no differences in root anatomy in the absence of salt between the two genotypes. The hypodermal cells developed lignified and suberized walls which blocked the plasmodesmata and resulted in degeneration of the cell contents. Frequent thin-walled passage cells in the hypodermis had living contents and may represent major sites of ion uptake into the symplasm, which was connected with the stele via plasmodesmatal connections with and between the cortical cells. The primary endodermal cells had lignified casparian strips and plasmodesmata in other cell wall areas. These connections were blocked by secondary suberization of all except the endodermal passage cells opposite the protoxylem arcs. Suberization of the hypodermis and endodermis and the appearance of granular deposits in the vacuole occurred closer to the root tip of both genotypes after treatment with 100 mol m−3 NaCl. Levels of starch and triglyceride in 10 mm serial segments were similar between roots of control and salt-treated plants and increased with distance back from the root tip. Chloride concentrations increased with salt treatment but values (on a tissue water basis) were similar between genotypes and between the apical and proximal ends of the root. Reducing sugar concentrations decreased with salt treatment to a similar extent in roots of both genotypes.