Differences in N response among corn (Zea mays L.) genotypes reflect variation in numerous processes involved in N use efficiency. In order to facilitate the study of such variation, we develop and demonstrate a concept for evaluating the contribution of N uptake and utilization processes to variation in N use efficiency. Eight hybrids were grown in a replicated field experiment at two levels of N fertilizer on a Dothan loamy sand (Typic Plinthic Paleudult). Differences among the hybrids for components of N use efficiency were evaluated from measurements of grain yield, N accumulation in the plant at silking, and N accumulation in the grain and stover at harvest. Significant differences were found among hybrids and between N levels for all traits. Interactions among hybrids and N levels were significant for all traits except grain yield. At low N supply, differences among hybrids for N use efficiency were due largely to variation in utilization of accumulated N, but with high N they were due largely to variation in uptake efficiency. Variation in proportion of N translocated to grain was also important at the low N supply. Variation in N accumulated after silking was not important at either level of N supply. Variation in N remobilization from vegetative tissue to grain was moderately important at the low N supply. Hybrids with similar levels of N use efficiency showed marked differences in component traits which contribute to efficiency.

Estimates of genetic variance and heritability provide useful guidelines for answering many questions which arise in a plant breeding program. In this paper we have attempted to (a) delineate appropriate questions which can be answered by estimation of genetic variance components and heritabilities (b) examine the assumptions necessary for estimation and (c) clarify the limitations imposed on interpretation of the estimates by the assumptions.

No abstract

A population of switchgrass, Panicum virgatum L., consisting of 33 half‐sib families was investigated over 2 years to obtain estimates of genetic parameters for several agronomic and nutritive traits, including height, maturity, dry weight, in vitro dry matter disappearance (IVDMD), and percent N. Dry weight was less heritable than other traits, with an individual narrow‐sense heritability of only 0.25. Narrow‐sense heritability for dry weight calculated on a family mean basis was 0.59. Dry weight and IVDMD were not significantly correlated while the correlation between dry weight and percent N was highly negative. Maturity was also negatively correlated with dry weight. The IVDMD and percent N, both measures of forage quality, had family narrow‐sense heritabilities averaged over three measurements per year of 0.79 and 0.74, respectively. Mean values for both traits were highest early in the season and declined markedly with maturity. The IVDMD and percent N were not significantly correlated. The estimated genetic parameters were used to form several selection indices involving three traits: dry weight, IVDMD, and percent N.

No abstract

Improved corn (Zea mays L.) hybrids are generally developed at relatively high N fertility levels. Previous studies have shown that hybrid populations developed by different selection methods vary in their responsiveness to environments. Because N supply is an important factor of the environment which affects yield a field study was conducted to determine the N response of population hybrids improved by intra (full‐sib family) and inter (reciprocal recurrent) selection methods. The responses of the original population hybrid and the two improved population hybrids to N rates of 56, 168, and 280 kg N/ha were studied on an Aquic Paleudult. The improved populations produced more total dry matter and grain at each N level than did the original hybrid. This was associated with an increase in the ear number per plant as the N rate increased. The N concentration of the improved population hybrids at silking was correlated with ear number per plant. Average N use efficiency (grain per unit of N applied) for the improved population hybrids was greater than for the original hybrid. The higher yield of the reciprocal recurrent selection at 56 and 168 kg N/ha as compared with the full‐sib family selection was associated with a higher average N uptake efficiency.

Root growth and development can affect N uptake. In turn, N additions may affect corn (Zea mays L.) root growth. Prolific (multiple ear) hybrids may need a more active root system to satisfy ear demand. To determine the extent of the effect of N fertilization on root growth in the field, three corn hybrids (Pioneer 3320, and prolific hybrids I202×Mol7 and I117×B73) were grown under fertilizer application rates of 56, 140, and 224 kg N ha−1 to determine (i) whether application of N fertilizer stimulated root growth of corn in the zone of application in the field 20 d before silking, at silking, and at physiological maturity and (ii) whether prolific hybrids have enhanced root development before silking compared with nonprolific hybrids. All three hybrids were grown at a uniformly low plant‐population density. Root length density to a depth of 60 cm, averaged over hybrids, was 1302, 2238, and 1184 m plant−1 in 1987, and 2195, 2833, and 3317 m plant−1 in 1988 for the three sampling dates. There were few genotypic differences in root length density. Root weight averaged over hybrids was 20.4, 40.5, and 21.5 g plant−1 in 1987, and 44.6, 61.7, and 58.9 g plant−1 in 1988 for the three sampling times, respectively. Root weight at harvest declined as N application increased, but increased in response to N at two earlier sampling times. The second year of the study, when rainfall was more plentiful than in the first year, root growth continued after silking for all hybrids. This coincided with higher yields. The data suggest that N stimulated root length in the area of application, without affecting total root length.

Genetic and biochemical analyses suggest that the developmental program of catalase (H 2 O 2 :H 2 O 2 oxidoreductase, EC 1.11.1.6) activity in maize scutella is controlled by a temporal regulatory gene ( Car1 ) that is distinct from the structural genes thus far identified. Recombination data show that Car1 is located about 37 map units from the Cat2 structural gene on the chromosome 1S. Turnover studies indicate that Car1 may act by regulating the rate of catalase synthesis.