**1. Introduction**

The worldwide importance of soybean and the main limitations to crop yields

Because of its potential for large-scale production, soybean (*Glycine max* (L.) Merrill) has ex‐ celled in the world agricultural economy as a major oilseed crop. At present, soybeans are grown primarily for oil extraction and for use as a high protein meal for animal feed (Singh & Shivakumar, 2010). According to Li-Juan & Ru-Zhen (2010), soybean has a protein content of approximately 40% and an oil content of approximately 20%. In 2010, the area planted with soybeans worldwide was 102.4 million hectares, with total production of 261.6 million tons in the same year (Faoestat, 2012). This crop is currently being produced around the world, including in much of North America, South America and Asia. The U.S. and Brazil are the world's largest producers and exporters of soybean (Kumudini, 2010).

According to Mutei (2011), with the increasing world population, which is expected to reach nine billion people by 2050, the worldwide production of food should increase by 70% to ensure food security, which should be achieved through growth in productivity, rather than simply through the expansion of cultivation areas. Câmara & Heiffing (2000) indicate that to obtain increases in soybean yields, it is necessary to understand the interaction between cul‐ tivars and the production environment. Based on these factors, crop management can be ad‐ justed to achieve proper development of plants in each production environment. Soybean is very responsive to environmental conditions, and the main climatic factors affecting its crop yields include the photoperiod, which influences the availability of full light, temperature and water availability (Mundstock & Thomas, 2005).

© 2013 Souza et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Souza et al.; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Although the effects of various environmental factors interfere with the performance of crops, water restriction is the main limiting environmental factor that contributes to the fail‐ ure to obtain maximum soybean yields (Casagrande et al, 2001), influencing the use of other environmental resources. According to Confalone & Navarro Dujmovich (1999), the efficien‐ cy of the use of solar radiation by soybean remains relatively constant at different develop‐ ment stages. When there is moderate water stress, soybean tends to maximize the efficiency of radiation utilization and reduce the efficiency of the interception of photosynthetically ac‐ tive radiation, while under severe water deficits, there is a reduction of the efficiency of radi‐ ation utilization.

Lisar et al. (2012) report that the impacts of water stress in crop plants can reduce pro‐ ductivity by 50% in various parts of the world. Under stress conditions, the plants present a series of changes in their morphology, physiology and biochemistry, negatively affecting their growth and productivity. According to Gerten & Rost (2010), two-thirds of world food production through cultivation occurs under water stress. In this context and because of the prospect of global climate change, most crops will be exposed to negative impacts caused by drought.
