Grain yield and grain protein are often negatively associated in wheat. When yield increases and grain protein decreases, there can be an adverse effect on the milling and baking quality if the desired end product is bread flour. It has been suggested that this inverse association is the result of selecting for a higher harvest index (ratio of grain yield to total biomass), to enhance grain yield. Parents, Fl, F2, and F3 generations of three crosses and reciprocal backcrosses of one cross were space-planted to study the association of grain protein content with grain and biological yields, harvest index, and related traits. Selection P5221, a high protein selection, was a common parent in crosses with three different genotypes. Differences were observed among generations within crosses for biological yield, grain yield, harvest index, grain protein content, grain hardness, and protein yield. The coefficients of variation for the measured traits were low for the three crosses. No associations between grain protein content and grain yield were observed in the populations studied. The largest association detected was between harvest index and grain protein. The r values ranged from -0.39 to -0.46, and rho was not different from -0.50 in two of the crosses. Path coefficient analyses revealed that this association was mostly due to the direct effect of harvest index on grain protein content, with little direct or indirect effect via other plant traits. In the cross P5221/ORCR 8313, biological yield exhibited a moderately large (0.64) direct effect on grain protein content; however this was offset by the negative indirect effect of tiller number. The R2 of the path analyses were relatively small for the three crosses, indicating that most of the variation in grain protein content was not explained by the variables included in the analyses. A possible negative association between grain protein content and harvest index, although moderate, suggests that selection for high yield should not be based on further increases of harvest index because grain protein could decrease.

A negative relation between grain protein content and grain yield is frequently observed in wheat (Triticum aestivum L. em Thell) i.e. as grain yield increases, grain protein decreases. It has been suggested that the inverse relation between grain yield and protein is in part the result of developing high yielding semi-dwarf wheat cultivars with an increased harvest index. This investigation was undertaken to determine the nature of the possible association of grain yield and protein content as influenced by harvest index, biological yield, plant height and kernel weight in winter wheat populations grown in Oregon. Progenies derived from three crosses of winter wheat were solid-planted in two environments during two seasons. Phenotypic correlations showed a moderate negative association of grain protein content with both grain yield and harvest index. The magnitude of the genetic correlations suggested the presence of genetic relationships among these traits. Selection for harvest index among these crosses could cause a correlated reduction of grain protein content. To investigate if the relationships between grain protein content and selected plant growth traits were similar when grown under space-planted and solid seeded stands, progenies of two winter wheat crosses were evaluated during two seasons. Performance for grain yield and grain protein content was different under contrasting sowing densities as values were not correlated between sowing densities. This indicates the need to evaluate these traits in solid-seeded stands. Harvest index, as well as plant height and heading date, could be effectively selected under space-planted or solid seeded conditions. Associations among traits were reliably estimated in space-planted stands. To evaluate the effect on grain protein content when grain yield and harvest index are modified, the plant growth regulator Paclobutrazol was applied to selected winter wheat genotypes under field and greenhouse conditions. Paclobutrazol increased grain yield and harvest index values of all genotypes in the greenhouse, while only some genotypes improved these traits under field conditions. Grain protein content, however, remained unchanged. Higher grain yields were obtained in both greenhouse and field experiments.