pH Changes Associated with Iron‐Stress Response

When Fe‐inefficient T3238fer and Fe‐efficient T3238FER tomatoes were supplied iron, and nitrogen as nitrate, they increased the pH of the nutrient culture. When they were supplied nitrogen as ammonium, they decreased the pH. When Fe supply was limited, Fe‐stress response developed in T3238FER that opposed the usual nitrate response and decreased, rather than increased, the pH. A “reductant” which reduced Fe3+ to Fe2+ was released from the roots of these plants and lowered the pH; and there was a tremendous increase in the uptake of Fe. T3238fer did not produce “reductant” in response to Fe‐stress; the pH increased, and the plants developed Fe‐deficiency when nitrogen was supplied as nitrate. Nitrogen nutrition and iron‐stress response are important factors associated with iron chlorosis in plants. Release of hydrogen ions from roots of Fe‐stressed plants is caused by more than response to imbalanced uptake of cations and anions.