Winter wheat (Triticum aestivum L. cv Norin No. 61) was grown at 25°C until the third leaves reached about 10 cm in length and then at 15°C, 25°C, or 35°C until full development of the third leaves (about 1 week at 25°C, but 2-3 weeks at 15°C or 35°C). In the leaves developed at 15°C, 25°C, and 35°C, the optimum temperature for CO -saturated photosynthesis was 15°C to 20°C, 25°C to 30°C, and 35°C, respectively. The photosystem II (PS II) electron transport, determined either polarographically with isolated thylakoids or by measuring the modulated chlorophyll a fluorescence in leaves, also showed the maximum rate near the temperature at which the leaves had developed. Maximum rates of CO -saturated photosynthesis and PS II electron transport determined at respective optimum temperatures were the highest in the leaves developed at 25°C and lowest in the leaves developed at 35°C. So were the levels of chlorophyll, photosystem I and PS II, whereas the level of Rubisco decreased with increasing temperature at which the leaves had developed. Kinetic analyses of chlorophyll a fluorescence changes and P700 reduction showed that the temperature dependence of electron transport at the plastoquinone and water-oxidation sites was modulated by the temperature at which the leaves had developed. These results indicate that the major factor that contributes to thermal acclimation of photosynthesis in winter wheat is the plastic response of PS II electron transport to environmental temperature.
Increasing grain protein in new higher-yielding cereal grains has recently received added attention due to protein premiums paid to farmers. Hard red winter wheat (Triticum aestivum L.) studies were conducted at two locations in Oklahoma in 1997-1998, 1998-1999, and 1999-2000 to evaluate the effects of late-season foliar N applications on grain yield, total grain N, straw yield, and total straw N. Foliar applications of N were made at two different times (pre- and postflowering) using urea ammonium nitrate (UAN) at rates of 0, 11, 22, 34, and 45 kg N ha(-1). Ammonium sulfate [(NH4)(2)SO4] was also applied at a single rate of 22 kg N ha(-1) both pre- and postflowering. A significant linear increase in total grain N was observed for postflowering applications using UAN in five of six site-years. In four out of the six site-years, a significant linear increase was observed for preflowering applications of UAN. No consistent increases or decreases from foliar N applications were observed for grain yield, straw yield, or straw N. Over years and locations, UAN applied preflowering and postflowering at 4 kg N ha(-1) increased total grain N over that of the check (no foliar N applied) by 2.7 and 2.4 g kg(-1), respectively. Late-season foliar N applications before or immediately following flowering may significantly enhance grain N content and, thus, percent protein in winter wheat.
Many dryland producers in the central Great Plains of the USA express concern regarding the effect that elimination of fallow has on soil water content at winter wheat (Triticum aestivum L.) planting and subsequent yields. Our objectives were to quantify cropping system effects (fallow weed control method and crop sequence), including corn (Zea mays L.) (C) and proso millet (Panicum miliacium L.) (M), on soil water at winter wheat planting and subsequent grain yield, and to determine the frequency of environmental conditions which would cause wheat yield to drop below 2500 kg ha(-1) for various cropping systems. Crop rotations evaluated from 1993 through 2001 at Akron, CO, were W-F, W-C-F, W-M-F, and W-C-M (all no-till), and W-F (conventional till). Yields were correlated with soil water at planting: kg ha(-1) = 373.3 + 141.2 X cm (average and wet years); kg ha(-1) = 897.9 + 39.7 X cm (dry years). Increasing cropping intensity to two crops in 3 yr had little effect on water content at wheat planting and subsequent grain yield, while continuous cropping and elimination of fallow reduced soil water at planting by 11.8 cm and yields by 450 to 1650 kg ha(-1), depending on growing season precipitation. No-till systems, which included a 12- to 15-mo fallow period before wheat planting nearly always produced at least 2500 kg ha(-1) of yield under normal to wet conditions, but no cropping system produced 2500 kg ha(-1) under extremely dry conditions.