**4. Discussion**

#### *4.1. Phosphorus Content and Accumulation in Barley*

The current study demonstrated no e ffects of water deficit on the P content in barley above-ground parts throughout the growing period. The results of other studies conducted to date on the e ffects of water stress on this feature are inconclusive. Both reports of no change [24] and of a reduction in the P content [52,69] of the barley biomass with an increase in water stress were noted.

The type of sowing had no e ffect on the P content in barley above-ground biomass in the current study. A convergen<sup>t</sup> result was presented by Jastrz ˛ebska et al. [24], who examined the e ffect of red clover as a catch crop accompanying spring barley. On the other hand, Wanic and Michalska [31] demonstrated that mixed barley and pea sowing increased the P content in the cereal above-ground biomass, at the heading and ripening stages.

The reduction in the P accumulation in barley above-ground biomass due to water deficit and the presence of rye-grass was proven in the current study. To compare, Jastrz ˛ebska et al. [24] found that mixed spring barley and red clover sowing did not di fferentiate the amount of P accumulated in the cereal until the heading stage, and at the heading stage, it significantly reduced the amount in the leaves. The current study demonstrated that P accumulation was determined by the biomass produced, and only during the initial period of plant development by P content in the plant. Lower P accumulation under water deficit conditions indicates a smaller biomass. A reduction in barley biomass under drought conditions was confirmed in studies by other authors [52,70–72].

#### *4.2. Phosphorus Content and Accumulation in Rye-Grass*

In the current study, the P content in the above-ground stems and leaves of rye-grass was not affected by the water supply of the plants. AbdElgawad et al. [73] also found that drought and high temperatures had no significant e ffect on the P and other nutrient concentrations in the above-ground biomass of grasses (*Poa pratensis, Lolium perenne*) and legumes (*Lotus corniculatus, Medicago lupulina*). No changes in the P content in rye-grass growing in the vicinity of spring barley were found in the current study either. On the other hand, Høgh-Jensen and Schjoerring [74] demonstrated that P concentration in the dry matter of *Lolium perenne* shoots in the single-species sowing was lower than noted when rye-grass was sown in a mixture with *Trifolium repens*.

There are few studies on the e ffects of water deficit and competition on P accumulation in the rye-grass biomass. Indirectly, conclusions about it can be drawn based on the biomass, since the P accumulation throughout the growing period had a positive correlation with the above-ground biomass accumulation. Brink et al. [75] also indicates a strong correlation between P uptake and the dry matter of *Lolium multiflorum*, while Burkitt et al. [76] explain such a relationship for *Lolium perenne*. Italian rye-grass is sensitive to water scarcity and primarily responds with poor tillering [77], which can result in lower biomass. The sensitivity of grasses to drought is a feature of the species [78], and even a varietal feature [79], while agronomic and physiological e ffects of water deficit are determined by the duration of drought [10]. On the other hand, the strength of the e ffect of a protective (main) crop on the development of catch crop is determined by the protective crop species and the catch crop species. Barley is regarded as a good protective crop for underplanted catch crops [25]. Kuraszkiewicz and Pałys [35] demonstrated that winter rye is a better protective crop than spring barley and oats, and in years with high precipitation, the yields of both fresh and the air-dry above-ground biomass of the catch crop are greater.

#### *4.3. Relative Yields*

The nutrient accumulation in plant biomass can result from inter-species interactions such as competition, facilitation and complementarity [74]. For example, a study by Rahetlah et al. [59] demonstrated that Italian rye-grass and spring vetch make complementary use of resources, and that such a mixture could be an alternative to a rye-grass single-crop system, particularly in the dry season. Based on the P accumulation in the plant biomass, expressed as relative yields, it was demonstrated that barley and rye-grass competed for P during the stem elongation and heading stages under water deficit conditions, while at other stages, irrespective of the water supply of the plants, they had a complementary e ffect (1 < RYT < 2). According to Sobkowicz and Podgórska-Lesiak [60], complementarity always occurs at the early plant development stages in the additive pattern. Before competition or other e ffects of emerging plants occur, RYT in the additive mixture is always equal to 2, since the yield of each species is the same in the mixture as in the single-species sowing. At subsequent stages of development, barley was more competitive against rye-grass. This is a result of the higher initial barley growth rate following the emergence, which determined its competitive advantage over the slower growing catch crop [30]. A species that grows faster makes use of the necessary growth resources and makes them unavailable to other species [30]. In the current study, the strength of species' competition was changing during vegetation. Both under the conditions of the optimal plants' water supply and of water deficit, barley competed more strongly during the tillering and stem elongation stages and this phenomenon, then started to decrease in intensity. At the same time, at the barley stem elongation and heading stages, the competition of rye-grass against the cereal (RYB) began to intensify, which is probably due to the higher growth rate of rye-grass. Similar observations as regards oats as well as rye-grass and vetch catch crop are presented by Paris et al. [80]
