Oxygen-Dependent Exclusion of Sodium Ions from Shoots by Roots of Zea mays (cv Pioneer 3906) in Relation to Salinity

Using radio-tracers, we measured Na and K' accumulation in roots and transport to shoots in Zea mays (cv Pioneer 3906) as a function of NaCI concentration and 02 partial pressure in the nutrient solution. Under fully aerobic conditions, roots partially excluded Na from the shoots over a wide range of NaCI concentration (0.2-200 millimolar). With root anoxia, the exclusion mechanism broke down so that much greater amounts of Na4 reached the shoots, with simultaneous inhibition of K' transport. The ratio Nae/K' entering the shoot consequently increased 90 to 200 times. Increases in Na' transport were rirst detected when the 02 partial pressure was reduced from ambient (21% v/v) to 15%, whereas K' transport was not inhibited until 02 concentrations were <5%. Since soil 02 deficiency can often accompany high salinity in irription agriculture, failure of the Na' exclusion mechanism may be a contributory factor in salinity damage of salt-sensitive glycophytes. Irrigation agriculture in arid and semiarid climates frequently leads to an accumulation ofsalts in the soil, restricting the growth and yield of many crops. On a global scale, approximately onethird of all irrigated land is affected by excess salinity (6), with a reduction in crop performance in the most salt-sensitive species or varieties with NaCl concentrations above 10 to 20 mm (22). Such crops are damaged when abnormally large amounts of sodium (or sometimes chloride or sulfate) are transported from the roots to the leaves (8). Salt tolerance in many nonhalophytes is associated with exclusion of sodium by the roots from the shoot, by two mechanisms: first, an outwardly directed active transport of Na+ at the plasmamembrane of root cortical cells results in extrusion of the ion towards the outer medium (14, 26) and second, resorption of Nae from the xylem sap and its accumulation by xylem parenchyma cells in the root and stem base lessens the quantity that reaches the leaves (12, 17, 29, 33). In some species, xylem parenchyma cells become specialized as transfer cells (17). Either mechanism of Na+ exclusion might in principle be impaired by interference in energy-dependent ion transport, and it is noteworthy that abnormal amounts of Na4 reach the shoots of plants when the roots are treated with an ' Supported by a grant from the Kearney Foundation of Soil Science and a Fulbright Senior Research Scholarship (M. C. D.). 2 Present address: Long Ashton Research Station, Long Ashton, Bristol BS18 9AF, United Kingdom. uncoupler such as 2,4-dinitrophenol (24) or in anaerobic (02 free) nutrient solution (4, 9). Water saturation (flooding) of the soil occurs with irrigation followed by slow drainage, or as the result ofa rising water-table. It can quickly lead to depletion ofthe soil 02(10, 23, 30) through the respiration ofroots and soil micro-organisms especially when temperatures are high, with consequent inhibition of root growth and function (5). The soil environment is particularly adverse where irrigation leads both to high water-tables and to excess salinity, as in the San Joaquin Valley, CA. Flooding the soil impairs the uptake and transport to the shoot of nutrient ions more than photosynthesis or dry matter accumulation, so that the concentrations ofN, P, and K in leaves decline (20, 21). The concentration ofNa+, however, increases under these conditions, suggesting that a breakdown in the mechanism ofNa+ exclusion could be a contributory factor in plant damage. The object of the present work was to characterize the root controlled mechanism by which Na+ is excluded from corn (Zea mays), a moderately salt-sensitive species (22), by examining the sensitivity to O2 partial pressure and to a wide range of NaCl concentrations. The movement to the shoot of an essential cation, K+, was also followed. We used an early maturing variety of corn grown in California, and show that it contrasts in its ability to exclude Na+ compared with a European variety described in earlier work (4). MATERIALS AND METHODS Plant Growth Conditions. Corn (Zea mays L. cv Pioneer 3906) was germinated in the dark on moist paper at 20 to 25C. After 2 d, germinated plants were transferred to a controlled environment room at 25°C and supported by a stainless steel mesh suspended over an aerated nutrient solution comprising (mM): KNO3, 0.1; Ca(NO3)2, 0.4; NH4H2PO4, 0.1; MgSO4, 0.05; together with Fe as Fe-EDTA (50 Mm) and FeSO4. (NH4)2SO4 (50 Mm), and micronutrients. After a day in the dark, plants were exposed to light (800 ;tmol m 2 sg', 16-h photoperiod, 70% RH). After a further 2 d, plants were supplied with nutrient solution at 10 times the stated concentration, except for Fe and micronutrients which were unchanged. Two days later (i.e. at 7 d from the start of germination), groups of four plants were placed in fresh nutrient solution in 3.5-L plastic containers fitted with airtight lids. Circular holes were cut in the lids so that each corn seedling could be supported around the stem base by a closefitting, split, rubber stopper. Glass tubes passed through the lid so that the nutrient solution could be vigorously gassed, with a volume flow of 0.5 L min-'/culture. Fresh nutrient solution was supplied on alternate days.