EFFECT OF VARIOUS NITROGEN FORMS ON THE PH IN LEAF APOPLAST AND ON IRON CHLOROSIS OF GLYCINE-MAX L

The effect of NH4NO3 (control) and increasing NO3- levels in nutrient solutions containing no and 100 mu M Fe respectively on iron chlorosis of Glycine max was investigated. After two weeks of growth apoplastic pH in excised leaves was measured by means of fluorescence. In plants growing without Fe supply increasing concentrations of NO3- in the nutrient solution which also was applied to the cut end of the petiole, resulted in a pH increase in the leaf apoplast from 5.34 (NH4NO3) to 5.50 (NO3-) associated with chlorosis observed with intact plants. A close negative correlations was found between chlorophyll concentration and pH in the apoplast (r = -0.97). While leaves in the treatment exclusively fed with NO3- were strongly chlorotic, those in the NH4NO3- treatment were green. With the exception of the plants only fed with NO3- the Fe concentration in the leaves was not affected by the type of N nutrition. It is therefore assumed that some Fe is immobilized in the leaf tissue by high apoplast pH induced by an increase in the proportion of nitrate in the nutrient solution. Plants fed with Fe (100 mu M) showed no chlorosis, regardless of the form of N nutrition and hence regardless of apoplast pH. The Fe concentration in leaves of Fe fed plants was approximately twice those in the leaves not supplied with Fe.