ROOT AMMONIUM ASSIMILATION ENZYME ACTIVITIES, GRAIN STORAGE PROTEIN AND GRAIN YIELD RESPONSE TO HIGH AMMONIUM FERTILIZER AMONG MAIZE HYBRIDS

TIM CLIFFORD MURPHY, Purdue University

Abstract

A whole-plant hypothesis linking genotypic differences in efficiency of ammonium assimilation and utilization to grain yield under high ammonium fertilizer levels was investigated in genetically diverse maize single cross hybrids. Four hybrids and 5 parental inbreds were grown in the growth chamber and sampled at the 3-leaf stage. Comparisons were made between hybrids and inbreds and within each group for root ammonium assimilating enzyme activity levels, root and shoot dry weights, nitrogen contents, and root protein contents with zero ammonium and 25 mM ammonium treatments. Glutamate synthase (GOGAT) activity per gram root tissue was significantly higher in inbreds than hybrids in response to 25 mM ammonium but total GOGAT per plant did not differ significantly between inbreds and hybrids. Glutamate dehydrogenase (GDH) specific activity and activity per plant was significantly higher in hybrids as was root and shoot total weight, nitrogen content, and root total protein. Root and shoot nitrogen percents were significantly higher in inbreds. Glutamine synthetase (GS) did not differ significantly on a per protein or per root tissue basis between hybrids and inbreeds. All three enzymes showed increased levels of activity in response to 25 mM ammonium. Among inbreds response to 25 mM ammonium indicated the root nitrogen percent and root free nitrogen percent correlated significantly negatively with GOGAT specific activity, activity per root tissue and activity per plant. GDH specific activity and activity per root tissue correlated significantly positively with seedling shoot growth in inbreds. Among hybrids response to 25 mM ammonium indicated both GOGAT and GDH specific activities correlated highly negatively with shoot total weight at the three leaf stage. Root nitrogen percent correlated significantly negatively with GS specific activity and root growth. Grain yield in 2 years field trials among the 4 hybrids correlated highly significantly with GOGAT specific activity, activity per gram root tissue and activity per plant in growth chamber seedlings. Grain yield correlated significantly with GDH specific activity and highly positively with GS per gram root tissue. Grain yield also correlated highly significantly negatively with shoot total nitrogen of seedlings. All three activities, GOGAT, GDH, and GS on a per root tissue basis correlated significantly with grain total protein content and highly positively with grain total zein. Grain yield and protein contents were correlated highly negatively with root free nitrogen percent. These results suggest that genotypic ammonium assimilation enzyme differences in activities accentuated under high ammonium stress may correlate with grain yield of genetically elite maize hybrids grown with high ammonium fertilizer in a nearly optimal environment. Genotypic differences in grain yield and grain total protein, albumin plus globulin, and zein protein fractions among 12 genetically diverse maize single cross hybrids were investigated as sinks for grain dry matter and assimilated organic nitrogen in each of 2 years' yield trials. Yield did not correlate consistently with kernel weight, kernel number, or any of the protein fraction contents among genotypes. However, significant weight differences from the base to the tip of the ears of 12 single cross hybrids overall did correspond to parallel significant differences in zein storage protein percents. Non-significant differences in non-zein protein percents were found. These results suggest that utilization of the assimilated nitrogen as storage proteins in the grain does not correlate with grain yield among genotypes although the importance of this effect on yield may be confounded with genotypic whole plant differences affecting supply and rate of supply of assimilated carbon and nitrogen to the grain sinks.

Degree

Ph.D.

Subject Area

Agronomy

COinS