## 3 Results and Discussion  
### Plants Isotopic Values (C-N-S), Comparison Between Regions  
#### 3.1.1 \(\delta^{13}\)C of Plants  
The \(\delta^{34}\)S, \(\delta^{13}\)C, and \(\delta^{15}\)N analyses of various plant products from local farmers and food markets from Greece revealed values ranging from \(-2.1\%\) to \(7.3\%\), \(-29.6\%\) to \(-10.9\%\) and \(-4.0\%\) to \(5.1\%\) (except citrus), respectively. The stable isotope ratio of carbon (\(\delta^{13}\)C) of all plants ranged from \(-29.6\%\) to \(-23.2\%\) (Table 1), indicating that they were C3 plants. Only maize fell into the C4 photosynthetic pathway, varying from \(-10.9\%\) to \(-12.7\%\). The \(86.4\%\) difference observed between the samples is attributed to spatial variability [57] and other factors, such as stomatal conductance, environmental indicators (altitude, temperature, solar radiation, humidity, and rainfall), agricultural regime, and nutrient availability [58; 59]. The \(\delta 1.4\%\) difference between Naousa (North Greece) and Sparta (Peloponnese) can be attributed to the higher temperature conditions that are observed in the region of Peloponnese and especially in the Sparta area during the summer period (Figure 2). The prolonged high temperatures can lead to water stress on the plants. This fact can cause the stomata of the leaves to start closing. This is a typical reflective response of the plants to minimize water loss due to evaporation via the stomata under water stress conditions. As a result, less CO\({}_{2}\) enters the interior of the leaves, while the plant is unable to differentiate between the \({}^{13}\)C and \({}^{12}\)C isotopes. Consequently, more \({}^{13}\)C becomes incorporated into the photosynthesis process, leading to more positive \(\delta^{13}\)C values. When one compares the carbon isotopic data from the northern and three southern regions (e.g., from Naousa to the Sparta and Kiato areas), a difference in carbon isotopes can be observed, possibly because of the limited supply of nutrients and water between the aforementioned regions. This scarcity may have caused the partial closure of stomata, leading to a reduction in the concentration of CO\({}_{2}\) inside the plant compared to the surrounding air (ci/ca). As a result, the plants exhibited a higher \(\delta^{13}\)C value. On the contrary, this remark cannot be observed at Ilia (West Peloponnese), where lower isotopic values are typical for this area. This can be attributed to the higher rainwater accumulation that Ilia receives. When plants are not under water stress, the stomata are opened, and then they photosynthesize at maximum rate, resulting in more negative values of \(\delta^{13}\)C.  
In the case of olive trees, a similar pattern can be detected when \(\delta^{13}\)C values of such plants from the Messinia region (West Peloponese) are compared to those from the Nemea area. More negative isotopic values of carbon are recorded in the former olive trees compared to the \(\delta^{13}\)C values of the latter olive trees, which receive more rainfall. Furthermore, the majority of samples with lower carbon isotopic values are found at higher altitudes [Olympus Mt. area, Parnitha Mt. (Attika region), and Psiloritis Mt. (Rethymno region) in Crete Island]. This follows the above analysis and can be likely attributed to the higher concentration of ci/ca that dilutes the heavier C isotopes (\(\delta^{13}\)C) due to carbon assimilation, resulting in low \(\delta^{13}\)C values. Plants grown in a warmer area generally have higher carbon isotopic values than those grown in a cooler (cold) area. However, in some cases, the plants from the same locality (e.g., Messinia region, Ilia region, and Kiato area) exhibit a difference in carbon isotopes greater than \(1\%\), which is probably due to fertilizers. The carbon isotopic values of cultivated and wild plant species fall within the same range.