Under semi-arid conditions of southern Iran variation in moisture level can drastically change yield patterns of winter wheat (Triticum aestivum L.) within the same season or over several seasons. In order to study this yield fluctuation, an experiment considering the effects of spring irrigation and fertility levels on components of yield, plant characteristics, and yield of grain and straw of ‘Roushan’ variety of winter wheat was conducted. Thiis 3-year study showed that yields of grain and straw were positively and significantly correlated with levels of irrigation in each crop season and in combined 3 years. The correlations for yield of grain to levels of fertilizer were negative and significant. Multiple and partial correlation coefficients showed components of yield to be more affected by season and irrigation level than other factors under study.

A nitrogen rate study was undertaken to determine the potential utility of the critical nutrient concept as a guide to N fertilization of winter wheat (Triticum aestivum L.), as part of a more extensive soil fertility investigation in western Oregon. This work on wheat was prompted by the need for a suitable diagnostic technique for determining more precisely the optimum levels of available N. Only limited information exists on the applicability of the critical nutrient concept or on critical levels of plant N in winter wheat, particularly the semidwarf types. Top grain yield was reached after N application of 140 kg/ha, but there were no significant yield differences over the range from 84 to 224 kg/ha. Plant NO₃-N values in leaves and shoots at tillering, jointing, and boot stages of growth were increased significantly by the highest N increment of 224 kg/ha compared to the next lower N rate. In contrast to grain yield, the highest values for total N percentage in plant shoots at all the later growth stages and dry matter yield at the soft dough stage were obtained with the highest rate of applied N, which also produced the highest values for plant NO₃-N. Semidwarf wheat utilized the high N rate more efficiently for dry matter production than for grain yield, and high N fertility also increased grain protein percentage. A graph of grain yield vs NO₃-N concentration in plant leaves at jointing showed a definite peak in grain yield at approximately 500 ppm NO₃-N in the leaves. Additional applied N very readily increased leaf NO₃-N considerably above 500 ppm and caused a slight, insignificant drop in grain yield. These results suggest the applicability of the critical nutrient concept to semidwarf wheat, and provide justification for additional work to establish more precisely the critical level for NO₃-N in wheat.

A winter wheat line of Triticum aestivum L. em. Thell. (aestivum group) segregating for a semidominant gene for dwarf stature from ‘Tom Thumb’ was crossed with ‘Wichita’ or ‘Cheyenne’ aneuploids. The F₁ progenies segregated into two height groups in a ratio of 1:1. F₂ disomic and noncritical monosomic progenies segregated in a ratio of 1 short:2 intermediate:1 tall. Monosomic analyses involving all chromosomes except 2A, 4B, and 6B indicated that the dwarfing gene was located on 4A. The heterozygous and hemizygous states of the gene had the same effect on height.