**11.3 Seed oil concentration**

Increasing the seed oil concentration has been a breeding goal for centuries. The ancestor of the domesticated soybean used to have small, hard, black seeds with low oil content, high protein content, and low yield. It is known that an increase in oil content is positively correlated with yield and negatively correlated with protein content. Selection for yield, agronomic characteristics and seed quality, large yellow seeds with typical averages of 20% oil and 40% protein were obtained. However, soybean is appreciated for its high protein meal and versatile vegetable oils; therefore, breeders mostly prefer to obtain modest gains in oil and yield without substantial loss in protein concentration [42]. Breeding for oil quality such as with reduced saturated fatty acids are prime focus as it is responsible for elevating cholesterol. The saturated fatty acids present in soybean oil are palmitic acid, 16:0, and stearic acid, 18:0. Especially, palmitic acid is a health concern as it is correlated to cardiovascular disease. It has been suggested that saturated fatty acids should be kept below 7–10% on a daily basis [42]. Soybean oil contains the monounsaturated fatty acid, oleic acid, 18:1. The oxidative stability of the soybean oil is enhanced by increasing three times higher the concentration of monounsaturated fatty acid such as oleic (18:1) than the normal content which is about 22%. Therefore, breeders target a concentration of 18:1 of about 65–75% of total lipid in soybean. By the means

of genetic engineering, 18:1 levels of about 80% total lipid have been achieved [42]. In general, soybean varieties with unique fatty acid composition such as high oleic acid content, high stearic acid content, low linolenic acid content, or low palmitic acid content are preferred [13].

Assessment of agronomic traits has been used to evaluate phenotypic diversity in 20,570 Chinese soybean accessions and it was reported that seed coat color had the highest diversity index among the qualitative traits [126]. Plant's height had the most variation among quantitative traits, and followed by seed size, protein content, growth period, and oil content. The seed size of those accessions ranged from smaller than 2 to as large as 46 g/100-seeds. The protein content ranged from 30 to 53%; and oil content ranged from 10 to 25%. The variances of seed size, protein content, and oil content of the U.S. cultivars were lower than the Chinese cultivars [127]. The Southern U.S. soybeans were more variable in oil and protein contents and less variable in seed size than the Northern U.S. soybeans. The food-grade soybean breeding aims to increase the nutritional content and quality of protein and oil [128]. Greater genetic diversity of protein content, seed hardness, calcium content, and seed size uniformity than other quality traits in both small and large-seeded genotypes were evaluated [128]. The U.S. soybean genotypes with small seed were more diverse and exhibited higher swell ratio and oil content but lower stone seed ratio and protein content than the Asian accessions [128]. Among the large-seeded accessions, the U.S. genotypes had higher stone seed ratio and oil content but lower swell ratio and protein content, and were less diverse than the Asian genotypes [128]. The characterization of diverse food grade soybeans will facilitate parent selection in specialty soybean breeding [1].
