Critical nutrient concentrations (CNC), below which a yield reduction occurs, for potato (Solanum tuberosum L.) petiole nitrate (PN) have been reported for the Pacific Northwest growing region but differences in method of determination or definition of this parameter have resulted in some discrepancies in values. The objectives of this research were to document seasonal PN levels with varying levels of applied N, to determine CNC at three growth dates [tuber initiation, TI; 21 d after TI(TI + 21); and 42 d after TI (TI + 42)], and to establish the relationship between yield responses to midseason N application and CNC. Experiments were conducted at two sites on a Typic Argiboroll and three sites on a Pachic Haploxeroll in western Montana. Treatments included six or eight N rates between 0 and 336 kg N ha−1 applied as NH4NO3 before hilling and a topdressing of 67 kg N ha−1 broadcast on subplots at TI + 21. Early season PN depended on applied N, with maximum values of 25 to 30 g NO3‐N kg−1 at TI. Values declined with seasonal progression and became increasingly dependent on fertilizer N rates. The N topdressed at TI + 21 increased PN level 6 g kg−1 at TI + 42 compared to non‐topdressed areas. Linear plateau regression of relative yield against PN revealed CNC values of 25 g kg−1 at TI 14 g kg−1 at TI + 21, and 10 g kg−1 at TI + 42. Statistical analysis, could not separate the TI + 21 and TI + 42 regressions, indicating a CNC of 13 g NO3‐N kg−1 throughout the period of tuber bulking. Yield responses to topdressed N, however, were documented only when PN values were below 11 g NO3‐N kg−1 at TI + 21. Conservative interpretation of critical nutrient concentration is justified.

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Samples of high and low grain protein content (%) were compared from each of eight spring wheat crosses. The high protein samples were obtained by compositing 14 F3 progeny rows from the high end of the F3 distribution curve; the low protein samples were obtained similarly from the low end of the F3 distribution curve. These composites were seeded at three Montana locations and evaluated for agronomicharacteristics and for grain and plant nitrogen to help determine the nature of the expression of grain nitrogen content in spring wheat.Grain nitrogen content was negatively related to grain yield and to the graln‐straw ratio. High and low protein composites at Bozeman produced the same amount of above ground plant nitrogen, and each translocated the same percent of nitrogen to grain. However, grain nitrogen content of the high protein composite was significantly higher because of the distribution of a similar amount of nitrogen to a smaller amount of grain.Differences in plant growth characteristics among the eight crosses seem to account for the differences in grain nitrogen content.

From an F2 population of about 700 plants from across of two spring wheats (Triticum aestivum L.) CI 13242 and ‘Thatcher,’ 10 plants were selected for maximum expression of grain yield per plant, spike number/plant, kernel number/spike, spikelet number/spike, and kernel weight. In each subsequent generation through the F8, selection lines for each of the five characters were advanced by selecting the 10 plants with the highest value from a population of about 400 plants. A performance trial was conducted at three locations in 1 year with five generations (F4 to F8) of each of the five selection lines and the two parents. Yield and yield components were evaluated and compared with midparent values and as linear regressions on generation number. Selection for kernel weight and kernels/spike at the F8 generation gave 11 and 16% increases in yield over the midparent, whereas direct effects of selection for these two characteristics were 13 and 10%, respectively. Direct effects of selection for grain yield and spikes/m2 at the F8 generation were significantly lower than the midparent (—13 and —7%). Nonsymmetrical correlated responses for increased kernel weight with selection for high kernel number and for decreased kernel number with selection for high kernel weight were large and unexpected. Spike number per plant was not an effective selection criterion for increasing spike number per m2 or grain yield. Only 3 of 20 regression coefficients for response in the F4 through F8 generations were significant, compared with 15 of 20 significant differences from the midparent at F8. Apparently selection was effective in the F2 and F3 generations (13 of 20 midparent differences were significant at F4), with little subsequent response. In this population, kernel weight and kernel number per spike were good characters for indirect selection for yield improvement.

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