Bundle sheath tissues of legume leaves as a site of recovery of solutes from the transpiration stream

The earlier observation that the fluorochrome sulphorhodamine G selectively enters cells of the extended bundle sheath system (paraveinal mesophyll) of soybean leaves from the transpiration stream, is extended to all the 26 species of legumes so far tested. The species examined were selected to include the three types of the system previously identified: Type 1 with complete systems, Type 2 with attenuated systems, and Type 3 with ordinary bundle sheaths. In Type 3 the dye selectively entered the bundle sheath cells. The hypothesis that the entry of the dye to a cell is caused by local low external pH, which suppresses its ionization, is confirmed by tests of the response of sulphorhodamine uptake to changing external pH, and of the inhibition of uptake by the ionophore dinitrophenol. An extension of this hypothesis identifies the local pH gradient as an energy source driving influx pumps to scavenge solutes from the transpiration stream and store them in special cells. It is proposed that this may be broadened to include many legumes. The activity of the proton extrusion pumps in this storage network is shown to be correlated with the flowering and fruiting of soybean, showing high activity before flowering, and disappearing during pod formation when nitrogenous materials are exported from the network.