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^-.