Sodium concentrations in the xylem sap of rice growing in saline solution culture

It has previously been observed that old leaves of Oryza sativa growing in the presence of NaCl have higher concentrations of Na than young leaves, an explanation for which has remained elusive. We have investigated the distribution of sodium with a nondestructive method, using the spittlebug Philaenus spumarius. This insect feeds directly from the transpiration stream, utilising amino acids in xylem sap and excreting other constituents. The poor nutritive value of the sap means that a substantial quantity (200– 300 times their body weight) must pass through an insect each day for survival. Therefore, they produce a large amount of excreta that can be collected and analysed. Insects were caged on young and old leaves of O. sativa (IR36), which had been treated with NaCl (50 mM) solution. Excreta were collected at regular intervals and analysed to determine the concentration of cations present in the xylem sap supplying different leaves. Analysis of excreta showed that old leaves receive higher concentrations of Na in the xylem sap than young leaves. In a second experiment, old roots were removed from plants that were exposed to NaCl (50 mM). Na transport to old leaves was found to decrease on removal of the oldest root suggesting that old leaves are supplied by old roots, which may have become more permeable to ions with age. Therefore, the discontinuous distribution of Na ions between young and old leaves is not simply due to a different period of exposure to Na but increased transport to old leaves.