Exogenous NO(3) Influx and Endogenous NO(3) Efflux by Two Maize (Zea mays L.) Inbreds during Nitrogen Deprivation.

The influence of nitrogen stress on net nitrate uptake resulting from concomitant (15)NO(3) (-) influx and (14)NO(3) (-) efflux was examined in two 12-day-old inbred lines of maize. Plants grown on (14)NO(3) (-) were deprived of nitrogen for up to 72 hours prior to the 12th day and then exposed for 0.5 hour to 0.15 millimolar nitrate containing 98.7 atom% (15)N. The nitrate concentration of the roots declined from approximately 100 to 5 micromolar per gram fresh weight during deprivation, and (14)NO(3) (-) efflux was linearly related to root nitrate concentration. Influx of (15)NO(3) (-) was suppressed in nitrogen-replete plants and increased with nitrogen deprivation up to 24 hours, indicating a dissipation of factors suppressing influx. Longer periods of nitrogen-deprivation resulted in a decline in (15)NO(3) (-) influx from its maximal rate. The two inbreds differed significantly in the onset and extent of this decline, although their patterns during initial release from influx suppression were similar. Except for plants of high endogenous nitrogen status, net nitrate uptake was largely attributable to influx, and genetic variation in the regulation of this process is implied.

Utilization of Previously Accumulated and Concurrently Absorbed Nitrogen during Reproductive Growth in Maize : Influence of Prolificacy and Nitrogen Source.

A prolific maize (Zea mays L.) genotype was grown to physiological maturity under greenhouse conditions to examine the effects of reproductive sink demand on (a) the remobilization of N accumulated during vegetative growth, and (b) the partitioning of N accumulated concurrent with ear development. One- and two-eared plants were treated with either a NO(3) (-) or NH(4) (+) source of (15)N-labeled N during reproductive growth. Plants with two ears enhanced grain production, N remobilization from the stalk and roots, and N translocation to the grain from concurrently assimilated N. But, remobilization of leaf-N was unaffected by ear number. In addition, N uptake and total dry matter accumulation during the reproductive period were also unaffected, although P uptake was greater in the two-eared plants. Less than 15% of the total K(+) uptake was accumulated after silking while during this time more than 40% of the total N and more than 50% of the total P were absorbed. The data also indicate that with NO(3) (-) nutrition, internal recirculation of K(+) between shoots and roots may play a prominent role in the transport of nitrogenous solutes during grain development. N source had no effect on dry matter production and N uptake of both one- and two-eared plants. However, slightly greater partitioning of labeled-N from the NH(4) (+) source to the grain was observed in the two-eared plants.

Nitrate Uptake and Partitioning by Corn Root Systems : Differential Effects of Ammonium among Genotypes and Stages of Root Development.

The relative effects of ammonium on nitrate uptake and partitioning during induction were compared among decapitated seedlings of three corn (Zea mays L.) genotypes at two developmental stages. This study tested the hypothesis that root systems efficient at translocating products of ammonium assimilation away from sites of nitrate uptake or reduction would exhibit less inhibition of nitrate uptake by ammonium compared to root systems with inefficient N translocation efficiency. Inhibition of nitrate uptake by ammonium was relatively slight at day 5 ranging from 0% to 20% among the three genotypes, as compared to greater inhibition, from 20% to 37%, at day 8. Five-day-old roots exhibited negligible xylem translocation capacity in comparison with those grown for 8 days. Thus, although the capability to translocate ammonium assimilates out of the root increased between days 5 and 8, inhibitory effects of ammonium also increased. In the absence of ammonium, nitrate uptake per unit root mass increased between days 5 and 8. This increased activity of the uptake system was proportionally more sensitive to ammonium.Partitioning of entering nitrate into the reduction process was positively correlated with lateral root development of the inbred root systems at 5 and 8 days. This is supportive of a localization of a major portion of nitrate reduction occurring in root apical regions. Nitrate reduction was the partitioning process most severely inhibited by ammonium in all cases, ranging from 39% to 55% inhibition. In contrast, ammonium-inhibition of nitrate accumulation in the root tissue and translocation via xylem vessels varied with genotype and root age.Two mechanisms of ammonium-inhibition of nitrate are implicated, one which directly affects nitrate reduction and the uptake system associated with it, and another which may involve potassium as an intermediate regulator of nitrate accumulation in the root tissue and nitrate translocation out of the root tissue.

Stability analyses for estimating relative durability of quantitative resistance.

Experiments in which a series of host cultivars are inoculated in all combinations with a series of pathogen isolates have been used to detect specificity in the host resistance. A theoretical model of polygenic resistance involving both general and specific interactions with pathogen virulence was developed to test the abilities of statistical analyses to discriminate between host genotypes with different levels of general and specific resistance. Estimates of levels of specific resistance could be obtained in regressions of disease severity scores for each host cultivar X pathogen isolate combination vs. the virulence index of each isolate. If the virulence index was based on the mean disease severity induced by the isolate over all host cultivars, the slopes of the regression lines were correlated with the levels of specific resistance in host cultivars. If the virulence index was based on the disease severity induced by the isolate on a host cultivar with a minimum of specific resistance, the mean squares for deviations from the regression were correlated with the levels of specific resistance in host cultivars. A method was developed to consistently choose host cultivars with minimum specific resistance. The two regression analyses gave estimates of specificity in randomly generated, model genotypes of approximately equal accuracy, although the second method appeared to be more accurate when the numbers of loci controlling resistance and virulence were small. The best estimates of numbers of genes for specific resistance were obtained by calculating a rating based on mean disease severity, the mean square for deviation from the regression on the virulence index based on disease severity on the cultivar with minimum specific resistance and the slope of the regression on the virulence index based on the mean disease severity. The best estimates of proportions of resistance genes that were specific were obtained by calculating a rating based on the above deviation mean square and slope alone.

Genetic regulation of the catalase developmental program in maize scutellum: Identification of a temporal regulatory gene.

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.

Allozyme Frequency Changes Associated with Selection for Increased Grain Yield in Maize (ZEA MAYS L.).

Frequency changes of alleles at eight enzyme loci were monitored in four long-term maize selection experiments. The results indicate that changes in frequencies of the alleles at these loci are associated with changes due to selection for improved grain yield. The frequencies changed more than is consistent with the hypothesis of selective neutrality. In addition, significant deviations from a random-drift model were nearly always accompanied by significant linear trends as would result if allozyme frequencies respond to directional selection. Evaluations of linkages and linkage disequilibria in the selected populations indicate that the eight enzyme loci responded independently as selection progressed.

Experimental Evaluation of Relationships among Populations Resulting from Intergradation among Cultivars of ZEA MAYS L.

Populations were developed as intergradations among three maize cultivars, two adapted to North Carolina and one to Mexico, with the objective of testing whether diallel information could be used to identify the relationships among populations. The data substantiated theoretical observations that the technique would identify relationships among populations resulting from recent intergradations.