Limits to Crop Yield.

While opportunities exist for increasing yields currently achieved by many growers, there are well-defined limits to the ultimate crop yields that can be achieved. The physiological and physical realities of plants and crop production impose these limits. The ultimate limit is the amount of light energy intercepted by a crop through a growing seasons. Based on light interception and the physiology of CO₂ accumulation, maximum radiation use efficiency is usually about 1.7 g MJ^-1 for maize, 1.4 g MJ^-1 for wheat and rice, and 1.1 g MJ^-1 for soybean. Maximum yields achieved without other restrictions are indicative of the light limit, but in most systems yields well below the light limit are imposed by the availability of water and nitrogen. The water requirement results from transpiration as a consequence of stomatal opening necessary for the diffusion of CO₂ into leaves. The transpiration coefficient for each crop is well defined by physiology and physics with values of 9 Pa for maize, 6 Pa for wheat and rice, and 5 Pa for soybean. Without the large amounts of water available for crop transpiration, yield will be limited to the level allowed by the water supply. Nitrogen through history has been the resource that imposes the most severe constraint on yield. Nitrogen is needed for proteins and nucleic acids to generate active vegetative tissue, and finally to produce grain. The nitrogen content of grain and the final nitrogen harvest index define the grain yield limit imposed by the amount of nitrogen accumulated by a crop. The low nitrogen content of rice is an advantage in achieving high yields, in comparison to the higher nitrogen requirement of grain of other crop species.