IRON REQUIREMENTS OF C3 AND C4 PLANTS

SUMMARY Plants having the C4 photosynthetic pathway required higher concentrations of iron in the nutrient solution for maximum growth when grown in sand culture than those with the C3 pathway. There were only small differences among species in the concentration of iron required in the leaves to achieve near maximum dry matter yield. This suggests that C3 and C4 plants differ in their ability to absorb iron from the root zone. Higher concentrations of iron were required in the nutrient solution for maximum growth of both the C3 and C4 plants when nitrate, rather than ammonium nitrate, was the sole source of nitrogen. It seems likely that when nitrate was used, there was a decrease in availability of iron to the plant as a result of the increased alkalinity in the root zone. By contrast, the root zone was acidified when ammonium nitrate was applied. Iron concentrations greater than 2 to 5 4ag Fe ml-' in the nutrient solution reduced the dry matter yields of a number of plants grown with ammonium nitrate as the nitrogen source, probably because of a decrease in the absorption of phosphorus by the roots.