## 3 Results and Discussion
### Yield of Stevia
Typically, a farmer growing _Stevia rebaudiana_ plants wants to acquire a large amount of leaves and, with the similar yield of all the analysed samples, a high percentage of the leaves from 0 W would mean that less total plant material from stevia plants that is not typically used by the industry (such as shoots and dried/damaged leaves) would be acquired. This result can be either an advantage or a disadvantage: the higher amount of redundant biomass could result in the increase in costs of utilisation, but, on the other hand, redundant biomass could be used for composting, cattle feed or other practices and prove valuable. Studies by various other researchers, however, have shown variety of the influence of stevia watering on the yield of the plants. In the study of [PERSON] et al. (2018), where stevia plants were grown using different irrigation regimes (0.8 IW/CPE--irrigation water per cumulative pan evaporation, 1.0 IW/CPE and 1.2 IW/CPE), stevia plants irrigated more frequently (1.0 and 1.2 IW/CPE) were characterised with greater total biomass yield than stevia plants irrigated less frequently, similarly as in our own study performed in Poland. However, difference in the total biomass yield also had a significant influence on the fresh leaf yield, contrary to the results acquired in our own study. On the other hand, the study by [PERSON] et al. (2018) about growing stevia plants in Italy with adequate or deficit irrigation treatment (on the basis of soil water consumption) has shown that deficit irrigation (33 and 66% of soil water consumption) can decrease plant weight by even 20-25%. The study by [PERSON] et al. (2018) has also shown that, similarly to this study  
Figure 1: Percentage of yields of various components of stevia in total yield in stevia cultivated in different irrigation regimes. \(n\) = 30; 0 W—not irrigated, 1 W—irrigated once a week, 2 W—irrigated two times a week, NR—irrigated only during the dry periods (no rainfall). conducted in Poland, stevia plants grown with the lowest amount of water used were characterised with the greatest harvest index (i.e. ratio of leaf weight to plant weight) ([PERSON] et al., 2015). A study about stevia grown in Morocco, conducted by [PERSON] et al. (2015), has shown that stevia plants grown with the highest irrigation practice (100% evapotranspiration) yielded a far greater amount of biomass than stevia plants grown with 80% or 50% evapotranspiration, but all the different watering regimes did not have an influence on the percentage of stevia leaves in the total plant weight. An analysis by [PERSON] et al. (2015) about stevia irrigation on the basis of soil water tension conducted in Ontario has shown, contrary to results acquired in stevia grown in Poland, that less frequent irrigation during stevia growth can reduce leaf yield of stevia by even 25%. Additionally, the different weather conditions in the years of the experiment also could have an influence on the stevia yield. However, the differences of the rainfall level in different years of the stevia cultivation during the experiment were effectively eliminated by the irrigation of the plants. Therefore, the most probable weather factor influencing the yield of stevia was the temperature. Lower stevia yields in 2014 could be caused by lower temperature than was noted in the following years. On the other hand, the average temperature in the years 2015 and 2016 was similar; however, in 2015, more temperature drops and increases could be noted, which could have also negatively affected the yield of stevia in 2015. Results acquired in this study, as well as the results presented by the other authors, suggest that it is possible that quantity of the rainfall in Poland can be acceptable for the cultivation of stevia in cooler conditions present in the Polish region, but the various particular aspects of stevia cultivation (amount of water available for stevia connected with the amount of sunlight and average temperature) need to be analysed in the consequent studies.