Physiological and Genetic Changes of Irrigated Wheat in the Post–Green Revolution Period and Approaches for Meeting Projected Global Demand

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Abstract
Global demand for wheat (Triticum aestivum L.) is growing faster than gains in genetic yield potential are being realized, currently a little under 1% per year in most regions. Improvement in yield of semidwarf wheat has generally been associated with increased harvest index (HI) and grain per square meter. For CIMMYT (International Maize and Wheat Improvement Center) varieties released between 1962 and 1988, yield increase was also associated with higher flag‐leaf photosynthetic rate and related traits, but not higher biomass. Nevertheless, significantly higher biomass has been reported in more recent CIMMYT lines. Improved HI is associated with higher N use efficiency (yield per unit of available N) and improved yield of semidwarf lines is expressed at high and low levels of N input. Where interplant competition for light and soil factors are manipulated, yield improvement is associated with adaptation to high plant density. Studies have confirmed that the juvenile spike growth phase is critical in determining both grain number and kernel weight (sink) potential. Improving assimilate availability during this stage, perhaps by lengthening its relative duration, may be one way to improve yield potential. Traits that could potentially be exploited for improving assimilate (source) capacity include early vigor, stay‐green, leaf‐angle, and remobilization of stem reserves. Use of alien chromatin is a successful approach for introducing yield‐enhancing genes into elite genetic backgrounds. Examples include the 1B/1R chromosome translocation from rye (Secale cereale L.), and more recently the LR19 segment from tall wheatgrass [Agropyron elongatum (Host) P. Beauv.] Improving the efficiency of early‐generation selection may be another strategy for raising yield potential by increasing the probability of identifying physiologically superior lines by techniques such as infrared thermometry and spectral reflectance.