GLUTAMATE-DEHYDROGENASE ISOMERIZATION - A SIMPLE METHOD FOR DIAGNOSING NITROGEN, PHOSPHORUS, AND POTASSIUM SUFFICIENCY IN MAIZE (ZEA-MAYS L.)

Improved nutrient-use efficiency in cropping systems is needed to incr ease farm income and to minimize possible nutrient emission to the env ironment. Plant glutamate dehydrogenase (GDH) offers a means for impro ved diagnosis of the nutrient status of crops. Zea mays L. DK-68 was f ertilized with nutrient solutions containing different ratios of nitro gen, phosphorus, and potassium (N, P, K). Maize shoot GDH was fraction ated to its population of isoenzyme by Rotoforation followed with nati ve PAGE. The GDH of the control maize had the complete set of 28 isoen zymes. Fertilization of the maize with N1P1K1 did not repress the 14 a nodal but did repress the 14 cathodal isoenzymes. As the P content of the fertilizer increased in the sequence N0P0K0, N1P0K1, N0P1K0, N1P1K 1, N0P2K0, and N0P4K0, the amination maximum velocity (V-max) of the e nzyme decreased curvilinearly in the sequence 310, 246, 140, 130, 108, and 83 mu mol min(-1) mg(-1), respectively. The highest dry matter yi eld occurred when the fertilizer contained 1-3 mM P-i, which was thus the nutrient deficiency-sufficiency interphase. In the GDH integration of the signals from the N, P, and K so as to respond with a character istic isoenzyme population pattern, the signal from N superseded that from K; in turn, the signal from P superseded those from N and K. Sign al integration was therefore based on the dominance by the most nucleo philic nutrient. In this case, the decreasing order of nucleophilic do minance was PO43- > NH4+ > K+.