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-. [ABSTRACT FROM AUTHOR]