As nitrogen management practices change to achieve economic and environmental goals, effects on weed-crop competition must be examined. Two greenhouse experiments investigated the influence of N amount and form on growth of maize and redroot pigweed (Amaranthus retroflexus L.).In Experiment 1, maize and pigweed were grown together in a replacement series (maize:pigweed ratios of 0 : 4, 1 : 3, 2 : 2, 3 : 1, 4:0) under three NHaNO3-N supplies (0, 110, and 220 mg N kg -~ soil). Maize was planted into established pigweed and plants were harvested 24 days after maize germination. Pigweed responded more to supplemental N than maize and accumulated 2.5 times as much N in shoots at the high N supply. Competition effects were not significant.Maize and pigweed were grown separately in Experiment 2 and supplied 220 mg N kg-l as either Ca(NO3) 2 or (NH4)2SO 4 plus a nitrification inhibitor (enhanced ammonium supply, EAS). In maize, EAS treatment did not affect shoot growth and reduced root growth 25% relative to the NO3-N treatment. In pigweed, shoot and root growth were restricted 23 and 86% by EAS treatment, respectively. Total plant N accumulation under EAS treatment was higher in maize, less in pigweed. Under EAS treatment, pigweed leaves were crinkled and chlorotic; leaf disks extracted in 70% ethanol, pH 3, contained less malate and oxalate but more NH4 compared to the NO3-N treatment. Maize leaf disk malate levels were generally higher compared to pigweed but were less due to EAS treatment. Ammonium level in maize leaf disks was unaffected by N form and EAS treatment increased oxalate levels. Final bulk soil pH was generally lower in pots where pigweed were grown and tended to be lower due to EAS. Leaf disk malate levels and soil pH were positively associated.Results indicate that pigweed is more likely to compete with maize when high levels of NO3-N are provided. Enhancing the proportion of N supplied as NHj-should restrict the growth of NH4-sensitive pigweed.

The influence of nitrogen stress on net nitrate uptake resulting from concomitant '5NO3 influx and 14NO3 efflux was examined in two 12-day-old inbred lines of maize. Plants grown on '4NO 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 efflux, although the relative effects on the two processes may be dissimilar. However, the nitrate influx system also is subject to substrate induction (3,11,13,23), and the induced activity declines upon exposure to nitrate-free media (21). Hence expression of nitrate influx activity during nitrate deprivation initially may reflect the lifting of feedback suppression (derepression or deinhibition), whereas more prolonged deprivation may indicate a decay of the induced system from its fully derepressed or deinhibited state (3). Roots of maize inbreds have been shown to differ in nitrate uptake and assimilation processes (22, 25) and it is possible that the pattern of relief from suppression (15,17) and/or decay of the induced transport system (21) may exhibit genotypic diversity.Accordingly, the present investigation was initiated to examine whether (a) increases in nitrate influx and decreases in nitrate efflux occur in parallel as autotrophic corn plants suppressed in net uptake undergo nitrate-deprivation, (b) decay of influx from an initial stimulated rate occurs during prolonged deprivation, and (c) intraspecific differences exist in the nature of those responses.Depletion of specific nutrients in higher plants is commonly accompanied by an enhanced capacity for uptake of those ions. This response may be viewed as a release from suppression of uptake which occurs in the presence of high endogenous concentrations of the ion and its assimilation products. Influx of phosphate, sulfate, and chloride into roots is subject to this kind of regulation (e.g., Ref. 14).Some experiments with barley, involving pretreatments with various nitrate concentrations, indicate little effect on subsequent nitrate influx, as measured with 36CIO3 or '3NO (5-7, 9). In these instances, net uptake was largely influenced by changes in nitrate efflux. In other experiments with barley, however, the stimulation in net nitrate uptake resulting from nitrogen deprivation (15)

Root characteristics of wheat (Triticum aestivum L.) genotypes are believed to be important in tolerance to drought and flooding, yet neither the extent of differences in root size among modern soft red wheat cultivars nor the degree of association between root size and drought or flooding tolerance is known. This study was conducted to see whether genotypes differ in root size, and to see if root size is associated with tolerance to flooded soil and to drought during early vegetative growth. We found differences in root fresh weight (RFW), shoot fresh weight (SFW), number of roots longer than 40 cm (NR), longest root length (LRL) and total root length (TRL) of 40 winter wheat genotypes grown in hydroponic culture for 4 wk. Each of these parameters was positively correlated with all others. Twelve genotypes with different root sizes selected from these 40 were grown in a greenhouse soil experiment for 3 wk, after which soil moisture treatments of control, flooding, and drought were imposed for a period of 21 d. Flooding did not affect SFW and number of tillers (NT), but decreased RFW. Drought drastically decreased all three parameters. The genotype × moisture treatment interactions for SFW, RFW, and NT were significant. Root and shoot growth of these genotypes in hydroponic culture were correlated to their root and shoot growth under both control and flooded conditions, but not under drought. Thus, it appears that the expression of genotypic root growth potential may be influenced by the availability of soil moisture, and that selection of wheat seedlings for vigorous growth in hydroponic culture will select for vigorous early growth in soil with adequate or excess moisture, but not under severe drought.

Public concern about NO‐3 levels in potable waters, together with improving techniques for maintaining NH4+ availability in soils, has renewed research interest in the effects of N form on crop growth. The effects of NO‐3 versus NH+4 on growth and morphology of juvenile corn (Zea mays L.) were investigated in two experiments. In Exp. 1, 16‐d‐old, solution‐grown plants produced less shoot fresh weight when grown with NH+4 than with NO‐3 nutrition. Root fresh weights were similar, but elongation of the primary root axis and its longest first order lateral was less with NH+4 nutrition. Under NH+4 nutrition, apparent thickness (grams per meter) of primary roots was 54% greater, and the frequency of first order laterals with second order laterals was 65% greater, than under NO‐3 nutrition. In Exp. 2, plants were grown in the greenhouse until 40 d after emergence in a slightly alkaline 3:1 sand:soil mix. Nitrogen (75 mg N kg−1) was supplied as either Ca(NO3)2 or (NH4)2SO4 plus nitrapyrin [2‐chloro‐6‐(trichloromethyl) pyridine]. Ammonium‐grown plants produced 2.3 times the dry matter and had higher concentrations of N in both stems (18.5%) and leaves (27.5%) compared to NO‐3‐ grown plants. Tillering was increased by NH+4‐ nutrition. Apparent root thickness was again greater under NH+4 nutrition, but the difference between N regimes was only 15%. Results indicate that under conditions of course‐textured soils and slightly alkaline pH an enhanced NH+4‐ N regime may be advantageous for growth of corn. Differences in pH regimes between the hydroponic and soil‐based experiments may account for the contrasting results.