## 3 Results and Discussion
### Nitrate Content
Young plants are characterised by many of these compounds (in young leaves, their level may exceed even 1% of dry weight). Higher nitrate accumulation is typical for early varieties and plant species with a shorter vegetation period [44; 50]. Additionally, younger leaves are characterised by a lower nitrate content than older leaves because nitrate reductase activity decreases with the age of the plant [60]. Figure 1: Nitrate content (**A**), nitrate reductase (**B**), and glutamine synthetase (**C**) activities in the leaves of selected plant species from organic and conventional crops. The permissible standards for nitrate content in Poland are marked with horizontal lines: red [0.4 mg \(\times\) g\({}^{-1}\)], green [0.75 mg \(\times\) g\({}^{-1}\)], blue [1.5 mg \(\times\) g\({}^{-1}\)]. Means with the same letter are not significantly different (_p_ < 0.05, ANOVA followed by Tukey HSD test). Differences in nitrate accumulation may also result from harvest time, growing season, and storage time [59; 61; 62]. It is believed that excessive accumulation of nitrates is characteristic of plant species with a shorter vegetation period or which are early varieties [63]. During storage, a gradual decrease in nitrates is observed, and delaying harvesting favours a reduction in their content [64]. It has been shown that the ability to accumulate nitrogen compounds may be genetically determined and is a characteristic feature of a given species or variety of vegetables [65]. The cultivation system, the nature of the farm, and the doses and types of fertilisers used are also important. When fertilising plants, the total nitrogen doses recommended in integrated production should be observed, and nitrogen fertilisation should not be applied too late in the growing season [66; 67; 68; 69]. In the case of dill, parsley, and celery, the nitrate content in conventionally grown vegetables was higher than in organic vegetables. Still, these changes were not significant for parsley and celery. The exception is carrots, where a more substantial accumulation of nitrates was recorded in the leaves of plants from organic farming. Similar relationships were observed in beet and onion leaves. In general, the level of nitrates in plants of the _Apiaceae_ family was 3-4 times higher than other families, regardless of the crop kind. For example, for dill from the _Apiaceae_ family, the highest nitrate values were recorded for the local market [2.16 mg \(\times\) g\({}^{-1}\)], while the lowest was for plants from organic farming [0.65 mg \(\times\) g\({}^{-1}\)]. However, lower values were recorded for Chinese cabbage, a representative of the _Brassiaceae_ family: local market [0.13 mg \(\times\) g\({}^{-1}\)] and organic food store [0.29 mg \(\times\) g\({}^{-1}\)]. The content of nitrates depended on species but did not depend on the cultivation system [Table 1]. The nitrate content of the analysed plant species was found to have no significant effect on the remaining analysed parameters, as evidenced by \(p\) values [Table 2]. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**(A) Source of Variation**} & \multicolumn{3}{c}{**Nitrate**} & \multicolumn{3}{c}{**Nitrate Reductase**} \\ \cline{2-7}  & **d.f.** & **F** & \(\mathbf{p}\) & **d.f.** & **F** & \(\mathbf{p}\) \\ \hline Species (S) & 8 & 91.557 & \(<\)0.0001 & 8 & 246.116 & \(<\)0.0001 \\ Cultivation (C) & 2 & 1.451 & NS & 2 & 9.816 & \(<\)0.0001 \\ S \(\times\) C & 16 & 10.013 & \(<\)0.0001 & 16 & 75.976 & \(<\)0.0001 \\ Error & 54 & & & 111 & & \\ \hline \multicolumn{7}{c}{**Glutamine Synthetae**} & \multicolumn{3}{c}{**Total Chlorophyll**} \\ \cline{2-7}  & **d.f.** & **F** & \(\mathbf{p}\) & **d.f.** & **F** & \(\mathbf{p}\) \\ \hline Species (S) & 8 & 38.898 & \(<\)0.0001 & 8 & 70.755 & \(<\)0.0001 \\ Cultivation (C) & 2 & 4.407 & 0.0153 & 2 & 73.39 & \(<\)0.0001 \\ S \(\times\) C & 16 & 53.052 & \(<\)0.0001 & 16 & 25.106 & \(<\)0.0001 \\ Error & 81 & & & 54 & & \\ \hline \multicolumn{7}{c}{NS—non-significant.} \\ \end{tabular}
\end{table}