# Environment- and Genotype-Dependent Irrigation Effect on Soybean Grain Yield and Grain Quality
## 4 Discussion  
The current study period was characterized by a variability in meteorological conditions as a result of climate change, resulting in very demanding conditions for crop production in terms of available water for the plants. As expected and previously reported by many authors [13; 14; 15; 16; 17; 21; 27; 28], the overall irrigation effect on soybean grain yield was positive but largely depended on the environmental conditions (Table 2). For example, the lack of a significant irrigation effect on the soybean grain yield in the extremely humid A1 growing season was expected because sufficient precipitation makes the irrigation redundant [11]. The positive effects of irrigation on the grain yield in the dry A3 growing season with significant differences between the deficit irrigation (A3B2) and the full irrigation treatments (A3B3) (Table 2) were also expected, and similar grain yield responses to irrigation treatments in dry growing conditions were earlier noted by many authors [11; 15; 16; 19]. However, in this research, the irrigation effect on the soybean grain yield was unexpectedly absent in the extremely dry A2 growing season. This could have been caused by the extremely dry and hot weather in August, during which no irrigation was applied (Figures 1 and 3). In other words, all three treatments endured the same negative weather  
Figure 4: Linear regressions between (**a**) the crude protein and grain yields, (**b**) the crude oil and grain yields and (**c**) the crude oil and crude protein yields. effects in the grain-filling stage (R5-R7), in which water availability is known to significantly influence yield formation [10; 29]. According to [PERSON] [11], drought during seed filling has a significantly larger negative effect on the yield due to pod shedding and less dry matter accumulation than if water shortage had occurred earlier in the growing season. Similarly, the unexpectedly high positive effect of irrigation in the humid A4 growing season could be due to a specific rainfall distribution. The plants in the rain-fed treatment (A4B1) received excess rainfall in the early soybean development period (April and May), followed by a water shortage in the reproductive periods (R3-R8, pod and grain formation and grain filling) occurring during June, July and August (Figure 3). Excessive precipitation in early soybean development can increase the plant's vegetative mass and reduce the final grain yield because of lodging and shadowing [11]. Water shortage in the reproductive periods is known to have a negative effect on the grain yield [11; 30]. In comparison, plants in the deficit irrigation (A4B2) and full irrigation treatments (A4B3) had more available water, as irrigation was scheduled during the reproductive periods. Similarly to the results for grain yield, the effect of irrigation was on average positive for the grain protein content and, consequently, for the crude protein yield, with significant influence from the environmental conditions in each growing season (Tables 3 and 4). Furthermore, the grain yield was a more important factor for estimating the crude protein yield than the grain protein content (Figure 4a). The positive effect of irrigation on the grain protein accumulation determined in the present study (Table 3) was previously reported by many authors [11; 14; 19; 20; 21; 31]. However, in this research, the protein content and crude protein yield increases were not always in a linear relationship with the amount of available water. For example, in the extremely wet A1 growing season, the positive effect of deficit irrigation (A1B2) on the grain protein content was bigger compared to the full irrigation effect (A1B3; Table 3), but both irrigation treatments resulted in statistically equal crude protein yields (Table 4). Such results are in accordance with the reports from [PERSON] et al. [14], [PERSON] et al. [32] and [PERSON] et al. [33], who explained that maintaining a high level of available soil water during soybean growing periods could hinder protein accumulation.