The effect of phosphorus (P), potassium (K), and magnesium (Mg) deficiency on the development of leaf symptoms (chlorosis and necrosis) and activities of ascorbate-dependent H2O2 scavenging enzymes (ascorbate peroxidase, monodehydroascorbate reductase, dehydroascorbate reductase, and glutathione reductase) was studied in bean (Phaseolus vulgans) plants over a 12 d period of growth in nutrient solution. With increasing plant age Mg- and K-deficient leaves developed severe interveinal chlorosis and, accordingly, chlorophyll concentrations were reduced. However, in P-deficient leaves neither chlorosis nor necrosis appeared; the leaves remained dark green and even at an advanced stage of P deficiency, chlorophyll concentrations were still higher than those of control plants. In K- and, particularly, Mg-deficient leaves with an increase in severity of leaf chlorosis, activity of ascorbate-dependent H2O2- scavenging enzymes was progressively increased. In contrast, in P-deficient leaves, as in leaves of the control plants, activity of H2O2-scavenging enzymes remained at a low level over the 12 d period. Accordingly, compared with P-deficient and control plants, Mg- and K-deficient leaves with elevated anti-oxidative potential showed much higher resistance to chlorophyll destruction by the herbicide paraquat. Elevated levels of H2O2-scavenging enging enzymes in Mg- and K-deficient leaves indicate a higher production of H2O2 and related toxic O2 species. It Is suggested that in Mg- and K-deficient leaves, utilization of photoreductants in CO2 fixation is restricted because of impaired export and thus accumulation of photosynthates. This disturbance might lead to enhanced photoreduction of molecular O2 to toxic O2 species causing chlorophyll destruction (chlorosis), a process which is not important in P-deficient leaves where export of sucrose is not affected.
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İsmail Çakmak (1994) studied this question.