Charles T. MacKown

Variation in nitrogen use efficiency among soft red winter wheat genotypes

SummaryNitrogen use efficiency (NUE), defined as grain dry weight or grain nitrogen as a function of N supply, was evaluated in 25 soft red winter wheat genotypes for two years at one location. Significant genotypic variation was observed for NUE, nitrogen harvest index, and grain yield. Genotype x environment interaction for these traits was not significant. Several variables including N uptake efficiency (total plant N as a function of N supply), grain harvest index, and N concentration at maturity were evaluated for their role in determining differences in NUE. Nitrogen uptake efficiency accounted for 54% of the genotypic variation in NUE for yield and 72% of the genotypic variation in NUE for protein. A path coefficient analysis revealed that the direct effect of uptake efficiency on NUE was high relative to indirect effects.

Aluminum accumulation and associated effects on 15NO3− influx in roots of two soybean genotypes differing in Al tolerance

A study was conducted to examine aluminum (Al) exclusion by roots of two differentially tolerant soybean (Glycine max L. Merr.) lines, Pl-416937 (Al-tolerant) and Essex (Al-sensitive). Following exposure to 80μM Al for up to 2 h, roots were rinsed with a 10 mM potassium citrate solution and rapidly dissected to allow estimation of intracellular Al accumulation in morphologically distinct root regions. Using 10 min exposures to 300μM15NO3− and dissection, accompanying effects on NO3− uptake were measured. With Al exposures of 20 min or 2 h, there was greater Al accumulation in all root regions of Essex than in those of Pl-416937. The genotypic difference in Al accumulation was particularly apparent at the root apex, both in the tip and in the adjacent root cap and mucilage. Exposure of roots to Al inhibited the uptake of 15NO3− to a similar extent in all root regions. The results are consistent with Al exclusion from cells in the root apical region being an important mechanism of Al tolerance.

Nitrate Fluxes and Nitrate Reductase Activity of Suspension-Cultured Tobacco Cells (Effects of Internal and External Nitrate Concentrations)

Cell suspensions of tobacco (Nicotiana tabacum L., cv KY14) were used to determine the responses of NO3- uptake and NO3- reductase activity (NRA) to exogenous NO3- levels in the absence of long-distance NO3- transport. Tobacco cells grown with complete Murashige and Skoog medium for 7 d were subcultured for 3 d with NH4+-free media containing 0, 5, 10, 20, 30, and 40 mM NO3-. Cell NO3-, in vitro NRA, NO3- influx, and efflux of cell NO3- were determined. The NRA increased as cell NO3- increased. Cell NO3- efflux values increased as cell NO3- level increased. Cells with low intracellular NO3- had greater NO3- influx than cells with high intracellular NO3-. Woolf-Augustinsson-Hofstee transformations of the NO3- influx kinetic data revealed patterns characteristic of a high- and low-affinity two-component NO3- uptake system. Apparent Vmax values generally decreased and Km values increased as cell NO3- concentration increased. The NRA of cells supplied with 10 and 20 mM NO3- after 3-d growth in N- free medium increased about 5-fold within 2 h and then remained constant for the next 2 h, whereas NRA of cells supplied with 5 mM NO3- increased only 2-fold during the 4-h period. Intracellular NO3- and other N metabolites associated with cell NO3- levels exerted differential effects on the NO3- influx activity and NRA of tobacco cells cultured in suspension. Expression of high NRA was correlated with both high external and intracellular NO3-, whereas maximum NO3- influx activity required a low (depleted) level of cell NO3-.

Nitrate use by tobacco cells in response to N-stress and ammonium nutrition

SummaryCharacterization of NO3−use by suspension cultured tobacco cells during a culture cycle is needed to take advantage of cell cultures for further study of the biochemical regulation of NO3−uptake induction and decay processes. Tobacco (Nicotiana tabacum L., cv. Ky14) cells were cultured with media containing different N sources. Cells cultured with a mixture of NO3−and NH4+(40 mM NO3−plus 20 mM NH4+, in Murashige and Skoog media) initially grew slightly faster but attained the same maximum cell culture density as those cultured with 40 mM NO3−only. Cells subcultured with N-free media grew at a similar rate for the first 3 d as those cells grown with N, then ceased further growth. The cessation of growth of cells subcultured with N-free media coincided with depletion of cell NO3−. The NO3−influx of cells subcultured with N-free media increased eleven-fold and those grown with N increased four- to five-fold before declining. Maximal NO3−influx rates occurred at the onset of the stationary growth phase for N-stressed cells, while cells grown with N reached maximums prior to the stationary phase of cell growth. Cells grown with a mixture of NO3−and NH4+had lower NO3−reductase (NR) activity and higher cell NO3−levels than those of cells grown with NO3−only. The NR activity of cells subcultured with N-free media peaked within 1 d after subculture before declining to a constitutive level when cell NO3−was depleted. The level of cell NO3−plays a critical role in the expression of the NO3−uptake and reduction processes. The transitions in the expression of NO3−uptake and reduction activities of tobacco cell suspension cultures should prove valuable for further study of the biochemical and molecular basis for the regulation of these processes.

Kernel growth of in vitro cultured wheat spikes of cultivars with divergent source-sink limitations

Abstract A 50% reduction at anthesis in the main stem reproductive sink size of soft red winter wheat (Triticum aestivum L. em. Thell) elicits differential kernel size (KS) responses among cultivars. In this study, source levels were manipulated to evaluate cultivar source and sink limitations to kernel growth. Detached spikes of ‘Arthur’ (non-responsive, KS unaffected by 50% spikelet removal), ‘Caldwell’, ‘FL 302’ and ‘Adena’ (responsive, KS increases with 50% spikelet removal) cultivars were cultured in vitro from 14 days after anthesis (during endosperm cell expansion stage) to maturity with sucrose levels of 25, 50, 100 and 200 mM. Relative differences in KS among cultivars for each sucrose level were similar to those of field-grown plants at maturity. The KS of all in vitro cultured cultivars reached maximum at sucrose levels of 50 mM or greater; however, these kernels were 11–21% smaller than those of field-grown plants. On average, kernels on spikes cultured in 100 mM sucrose grew at the same rate as kernels on field-grown plants, but for a significantly shorter duration (22.8 vs. 31.7 days). Sucrose concentrations of peduncle and chaff tissues increased as sucrose levels increased. Compared to field-grown plants, peduncle and chaff tissues of cultured spikes averaged over all sucrose levels and cultivars had 4.6-fold greater water soluble carbohydrate concentration and 59% more tissue dry weight at maturity. Smaller kernels but abundant accumulation of water soluble carbohydrates in peduncle and chaff indicate that factors other than carbohydrate supply limited kernel growth of spikes cultured in vitro. When cultured in vitro, the non-responsive cultivar Arthur did not reach maximum KS at a lower sucrose concentration than the responsive cultivars. Thus, the apparent kernel growth limitation due to sink activity observed after partial spikelet removal of field-grown Arthur was not observed when spikes of this cultivar were cultured in vitro.

Nitrate assimilation and partitioning in detached culms of wheat

Abstract Detached wheat culms ( Triticum aestivum L. cv. Doublecrop) collected from field-grown plants 16 days after anthesis were cultured for 5 days with nutrient solution containing glutamine (7.5 mM), K 15 NO 3 (15 mM), or glutamine (7.5 mM) plus K 15 NO 3 (15 mM). Kernel growth rate of cultured culms equaled or exceeded that of intact field-grown plants (1.46 mg kernel −1 day −1 ). Net mobilization of reduced N from the peduncle occured, but this accounted for less than 23% of the net increase in grain N after 5th day of culture, regardless of N source. Glutamine had no effect on the assimilation of [ 15 N]nitrate or the partitioning of reduced [ 15 N]-labelled products. Peduncles contained less than 4–8% of total reduced[ 15 N] excess recovered in all plant parts. Grain accumulation of reduced [ 15 N] was linear,as was solution depletion, and accounted for an increasing proportion of the total reduced[ 15 N] excess in all culm parts. These results provide direct evidence for nitrate assimilation by spike components.