The effect of nitrate nutrition on iron chlorosis and leaf growth in sunflower (Helianthus annuus L.)

The objective of this work was to investigate the effects of nutrient solution pH, nitrogen form (NO₃, NH₄NO₃), bicarbonate and different Fe concentrations in the nutrient solution on the Fe concentration in roots and on the development of Fe deficiency symptoms in sunflower plants (Helianthus annuus L.). High pH in the nutrient solution induced by nitrate supply or by a pH-stat device led to increased Fe concentrations in roots and low leaf Fe concentrations associated with a significant decrease in leaf chlorophyll concentration manifested by yellow leaves. Plants of the nitrate fed treatments with 1 μM Fe in the nutrient solution were also characterized by reduced leaf growth and by the suppression of new leaf formation. The reduced leaf growth and the suppression of new leaves only occurred with nitrate and not with NH₄NO₃ in all treatments with 1 μM Fe in the nutrient solution. All symptoms were removed by a high Fe concentration in the nutrient solution (100 μM) at low external pH proving that suppression of leaf formation, reduced leaf growth and low chlorophyll concentration were caused by Fe deficiency. In the nitrate treatment with a low Fe supply (1 μM Fe) and pH 4 in the nutrient solution leaf chlorophyll concentrations similar to the controls were found. In comparison to control plants (NH₄NO₃, 1 μM Fe), leaf growth was still significantly reduced, and new leaf formation was suppressed. The chlorophyll concentration and CO₂ assimilation rate did not differ from those of the control plants. These results show that Fe deficiency is also characterized by small green leaves and the suppression of leaf formation. At the onset of leaf development, leaf growth and new leaf formation may respond more sensitively to poor Fe efficiency than chlorophyll concentration. In experiments with NO₃ plus HCO₃, simulating soil solution conditions prevailing in calcareous soils, the Fe efficiency of the youngest leaves was poor, showing retarded leaf growth and low chlorophyll concentration.