Mechanisms of chloride partitioning in the leaves of salt‐stressed Sorghum bicolor L.

Chloride transport in sheath and blade tissue and the cellular distribution of Cl‐ were investigated in an attempt to determine the physiological basis of the preferential accumulation of Cl‐ in sheaths of salt‐stressed sorghum (Sorghum bicolor L.). Import and export of 36Cl‐ in leaf sheaths and blades of intact sorghum were followed over a 2 week period. X‐ray microanalysis of frozen‐hydrated bulk tissue samples was used to determine the accumulation of Cl‐ and other elements in the vacuoles of sheath and blade cells. Sheath tissue accumulated Cl‐ despite a relatively high Cl‐ turnover rate. Chloride was shown to accumulate in most cell types of the sheath, particularly in adaxial epidermal cells. After an initial increase in the concentration of Cl‐, blade tissue regulated Cl‐ levels within certain limits. Chloride levels in blades were greater in the abaxial and adaxial epidermal cells than in other cell types. The epidermal cells of blades accumulated Cl‐ to approximately the same concentration as sheath epidermal cells. The Cl‐ concentration in the photosynthetically active mesophyll and bundle sheath cells, however, remained low. Thus, the partitioning of Cl‐ previously observed in the leaves of salinized sorghum apparently results from the ability of bundle sheath and mesophyll cells to maintain concentrations of Cl‐ at lower levels than do epidermal cells. In addition, the relatively large sheath parenchyma cells tend to serve as reservoirs for the storage of Cl‐.