# Environment- and Genotype-Dependent Irrigation Effect on Soybean Grain Yield and Grain Quality
## 4 Discussion
In the extremely dry A2, the protein content increased as the available water increased (Figure 3, Table 3), confirming the higher positive impact of irrigation on protein accumulation in dry conditions compared to well-watered conditions [19]. However, due to the unexpectedly absent irrigation effects on the grain yield in the extremely dry A2 growing season, the crude protein yield was lower in A2B3 than in A2B2 (Table 4). In the dry A3 growing season (Figures 1 and 2), the irrigation had an unexpectedly negative effect on the grain protein content (Table 3), but the crude protein yields increased with the increase in water availability (Table 4). According to [PERSON] and [PERSON] [13] and [PERSON] et al. [22], protein content can decrease with irrigation, indicating that water use efficiency for protein accumulation is better in conditions with less available water. Unlike grain yield and grain protein content, the grain oil content was on average negatively affected by irrigation, but the differences between the treatments per growing season (AB) were not significant (Table 5). Lower oil contents as a result of irrigation were previously reported by [PERSON] [11] and [PERSON] et al. [20; 21], while no irrigation effect was reported by [PERSON] et al. [14]. On the other hand, [PERSON] and [PERSON] [13] concluded that soybean grain oil content increased with increasing the irrigation treatments in sub-humid climates and was highest when the water requirements were fully met by irrigation. The lower mean oil contents in the irrigation treatments in the present study could have been a result of the protein content increase in the deficit irrigation (B2) and full irrigation (B3) treatments and a commonly known negative relationship between protein and oil contents [34], where every 2% of protein content increase usually decreases the oil content by 1% [35]. Although the oil content was negatively affected by irrigation, as a result of the positive effect of irrigation on the grain yield, the crude oil yield increased in all of the growing seasons except for the extremely wet A1 growing season (Table 6). This was expected, as the grain and crude protein yields were the more important factorsfor estimating the crude oil yields according to the correlation and regression analyses (Figure 4b,c). The irrigation effects on the soybean grain yield, grain protein and oil content and crude protein and oil yields varied not only depending on the environmental conditions but on the genotype as well (Tables 2-6). This was expected because the soybean MG choice affects the irrigation water use efficiency, and the profit-maximizing MG selection varies for irrigated compared to non-irrigated conditions [36]. The biggest mean grain yield increase due to irrigation was observed in the short season C1 (MG 00), indicating that it had a better irrigation response than C2 (MG 0), C3 and C4 (MG I) (Table 2). While researching the effect of irrigation on different soybean MGs, [PERSON] et al. [36] noted that earlier-maturing (short-season) varieties had a greater irrigation water use efficiency (IWUE) than later-maturing varieties due to shorter seed-filling periods. As a result of the better irrigation effect in short-season genotypes with lower genetic yield potential, the differences between the genotypes in this study decreased with the increase in water availability (Table 2). Furthermore, the highest overall grain yield was achieved by C2 in the deficit irrigation treatment (B2C2); thus, C2 production could be the most profitable in the tested growing area if irrigation is available, not only because it produces more grain yield with less available water but also because it has a shorter growing season than the other two highest-yielding genotypes (C3 and C4), which reduces the input costs.