**3. Results**

#### *3.1. Phosphorus Content and Accumulation*

#### 3.1.1. Spring Barley

The water supply of the plants and the type of sowing had no significant effect on the P content in the above-ground parts of barley throughout the growing period (Table S1).

Water deficit (LW) and the vicinity of rye-grass (BM) reduced P accumulation in the total barley above-ground biomass and individual organs (Table 1). Only at the ripening stage was the P accumulation in barley leaves not affected by the type of sowing. Throughout the cereal vegetation, most P in the above-ground biomass and the organs was accumulated by the plants BP-OW. Water deficit (LW) reduced the P accumulation more than the presence of rye-grass (BM), and the interaction of these factors (BM-LW) resulted in a further reduction in P accumulation.


**Table 1.** Phosphorus accumulation by barley (mg pot−1).

OW—optimal water supply, LW—water supply reduced by 50%; BP—barley as a single species, BM—barley in a mixture with rye-grass; a, b, c, d—homogeneous groups (values followed by the same letters, for each phase and for each part of the plant, within experimental factors and their interactions are not significantly different at *p* < 0.05).

The amount of accumulated P was strongly correlated with the amount of barley above-ground biomass (Table 2). Moreover, a positive correlation was demonstrated between the P accumulation and P content in the vegetative organs of barley during tillering and stem elongation, and between the P accumulation and P content in the barley stems at the heading and ripening stages.



<sup>\*</sup> r significant at *p* < 0.05.

## 3.1.2. Italian Rye-Grass

The P content in the vegetative organs of rye-grass throughout the growing period was not affected by the water supply of the plants or the type of sowing (Table S2).

Both water deficit (LW) and the vicinity of barley (RM) reduced the amount of accumulated P in the rye-grass above-ground biomass (Table 3). The strength of the effects of the interaction between water deficiency and barley's competition (RM-LW) on this feature varied at different development stages of the cereal. At the barley leaf development stage, a higher P accumulation was noted in rye-grass RP-OW than RM-OW, in the absence of differences between RP-LW and RM-LW plants. During barley tillering, the reducing effect of water deficit was still noted, but under these conditions, the competitive effect of barley against rye-grass was also observed. From the stem elongation stage until the end of vegetation, the vicinity of barley reduced the P accumulation in rye-grass leaves and stems more than water deficit, and the interaction of stress factors (RM-LW) deepened this reduction.


**Table 3.** Phosphorus accumulation by rye-grass (mg pot−1).

OW—optimal water supply, LW—water supply reduced by 50%; RP—rye-grass as a single species, RM—rye-grass in a mixture with barley; a, b, c, d—homogeneous groups (values followed by the same letters, for each phase and for each part of the plant, within experimental factors and their interactions are not significantly different at *p* < 0.05).

The P accumulation in the plants was determined by the above-ground biomass of rye-grass (Table 2). Moreover, a positive correlation between the P accumulation and P content in rye-grass vegetative organs until the stem elongation stage and a strong negative correlation between the P accumulation and P content in rye-grass stems during barley ripening were found.

#### *3.2. Competition for Phosphorus*

Throughout the growing period, irrespective of the water supply of the plants, there was competition for P between barley and rye-grass (RYB < 1 and RYR < 1) (Table 4). Only during the period of leaf development under water deficit (LW) were no effects of barley on rye-grass noted (RYR = 1.04). A water deficit increased the competition intensity, especially of rye-grass against barley from the stem elongation stage to the end of barley vegetation (RYB = 0.75–0.80) and of barley against rye-grass from the tillering stage to the barley heading stage (RYR = 0.26–0.32). Consequently, during the barley stem elongation and heading stages under water stress conditions, there was full competition between the species (RYT did not differ from 1) (Figure 1). At other development stages, the plants made use of the resource, partially in a complementary manner (RYT > 1).


**Table 4.** RY values for barley and rye-grass (based on P accumulation) depending on the water supply of the plants.

OW—optimal water supply, LW—water supply reduced by 50%; RYB—RY values for barley, RYR—RY values for rye-grass; a, b, c—homogeneous groups (values in the column of the table within species values followed by the same letters are not significantly different at *p* < 0.05); \* RYB, RYR, significantly different from 1.0 (*p* = 0.05).

**Figure 1.** Changes in the RYT (**a**) and Cb (**b**) values for the mixture of barley and rye-grass catch crop during the growth. OW—optimal water supply, LW—water supply reduced 50%; growth stage of barley: L—leaf development, T—tillering, S—stem elongation, H—heading, R—ripening; \*—RYT significantly di fferent from 1.0 (*p* = 0.05) and Cb significantly di fferent from 0.0 (*p* = 0.05).

Both under conditions of optimal water supply of the plants and water deficit, the competition of rye-grass against barley was most intense during barley stem elongation and heading. On the other hand, the competition of barley against rye-grass from the tillering to heading stages was the most intense during stem elongation. Barley was a stronger competitor than rye-grass (the Cb index was significantly higher than 0) (Figure 1).
