## 4 Discussion
_Brassica rapa_ plants gained greater biomass if the frass was N-rich, while the biomass was reduced if the frass was N-poor [63]. Treating frass may also be good to increase nitrogen availability, through anaerobic digestion or composting [64]. Frass can also have a stimulating effect on the number of soil microorganisms: the number of bacteria, archaea and fungi, carbon mineralization and nitrification, which may ultimately translate into the availability of post-food nutrients for the plant [65]. In addition to the yield, an essential part of the work was also to investigate the physiological condition of the plants. The study of chlorophyll fluorescence has become possible thanks to the popularization of mobile chlorophyll fluorometers. However, interpretation and measurement are not easy. The measurement principle is based on the following phenomenon: Light energy absorbed by chlorophyll molecules can either be used to drive photosynthesis, or the excess can be dissipated as heat or emitted back as light--chlorophyll fluorescence. An increase in the efficiency of one causes a decrease in the intensity of the other [66]. By measuring the fluorescence intensity, it is possible to determine the efficiency of photochemistry. Measuring fluorescence has long been used to monitor the photosynthetic efficiency. They can be used for the non-invasive assessment of the operational quantum efficiency of electron transport, and the efficiency is related to CO\({}_{2}\) assimilation. Measurements can provide the information used [67]. Drought stress caused a significant decrease in CO\({}_{2}\) assimilation, which was an expected reaction from plants as a result of stomatal closure as a defense against water loss. For the same reason, a decrease in the level of transpiration and stomatal conductance was noted. However, the decrease in these parameters (E and Gs) was not significant at all levels of fertilization. It was observed that plants fertilized with HI frass retained CO\({}_{2}\) assimilation and transpiration at a higher level than those fertilized with manure and those not fertilized at all in both water regimes. This proves these plants had a better physiological condition, exhibited more efficient photosynthesis and some adaptation to drought. The results of the chlorophyll fluorescence confirm a better condition of the photosynthetic apparatus in these plants; although the measurement results do not indicate statistically significant differences, a clear tendency is visible. Lower minimum fluorescence values were recorded in the plants fertilized with HI frass, which indicates the greater efficiency of the operation of energy antennas and photosynthetic pigments. Higher maximum fluorescence values, in turn, indicate the higher efficiency of electron acceptors in photosystem II (PSII). The Fv/Fm parameter, according to some sources [58; 59], is not sensitive to drought stress, but it can be seen in optimally watered plants and fertilized plants that the values of the maximum photochemical efficiency of PSII are slightly higher, although their values do not indicate stress at any of the experimental levels. Significant differences in the values of the fluorescence parameters under light--yield and ETR--were demonstrated only in the optimally watered plants. They indicate faster electron transport through photosystems as well as the higher quantum yield of the photosynthetic energy in plants fertilized with Hi frass. However, the differences are small. Therefore, we conclude that HI frans fertilizer slightly improved the health and physiological condition of lettuce plants, which allowed for more effective defense against drought stress. Any improvement in plant health seems to be valuable in the era of climate change and water scarcity. Plants are exposed to biotic and abiotic stress throughout their life. As a result of secondary metabolism, plants produce chemical compounds that can protect the plant against abiotic stress factors such as light, UV radiation, temperature and heavy metals.