4.1. The Yield of Leek Above-Ground Mass and the Yield of Shallot Bulbs
The influence of the weather conditions on the yielding of two species, Allium ampeloprasum var. porrum and Allium ascalonicum, was varied. The weather conditions had an effect on the leek yield but not on the shallot yield. This was related to the different cultivation periods. The ripe ‘Toto’ shallots were harvested in July, while the ‘Jolant’ leeks were harvested in August. In Poland, the period of intensive plant growth covers July and August (the period of acute sensitivity to water shortage). In 2018, the amount of rainfall in July was higher than in 2019. On the contrary, in August, the amount of rainfall in 2019 was higher than in 2018. In the cultivation of shallots in the short cultivation period, the amount of rainfall was of less importance compared to the season.
In this study, the average yield of the leek cultivar ‘Jolant’ was 31.4 Mg ha
−1, and the yield of the shallot bulbs of the ‘Toto’ cultivar was 23.2 Mg ha
−1. In the study by Golubkina et al. [
43], the yields of the fresh leek mass ranged from 23.8 to 40.2 Mg ha
−1. The total yield of the shallot bulbs of the cultivar ‘Toto’ was similar to that described for this cultivar by Tendaj et al. [
2], at 22.7 Mg ha
−1.
These studies show that the co-cultivation of leeks and shallots with Alexandrian clover does not reduce the marketable yield. The average yield of leeks with clover was 38.32 Mg ha
−1 ha, which was higher by 36% compared to the cultivation without LM. The yield of shallots was 26.24 Mg ha
−1, which was higher by 23%. The unit weight of the leek pseudo-stems in the cultivation with clover was 249 g and was higher by 24%, while the weight of 100 shallot bulbs was 8.04 kg and was 20% higher compared to the cultivation without mulch. Thus, the use of living mulch led to many positive responses in the plants, as shown above. The leek and shallot species of the genus
Allium are cultivated in wide inter-row spaces and are perfect for growing with LM. Thus, it was confirmed that one of the factors determining the success of the cultivation is the choice of the sown species [
8]. According to Campanelli and Canali [
34], plants intended for use as living mulch should have a low demand for nitrogen, so that they do not compete with the crop. Several previous studies have shown that
Trifolium repens,
Ornithopus sativus, and
Tagetes patula plants can work well in co-cultivation with leeks [
31,
32,
44]. Some results indicate that living mulch from
Isatis tinctora plants placed in a belt system can increase the leek yield by 112% compared to cultivation without a living mulch [
27]. Trinchera et al. [
45] explained that clover plants in co-cultivation with other species positively affect the activity, while mycorrhiza colonization guarantees the plant an adequate supply of nutrients.
In our experiment, the most competitive approach for leeks and shallots was the use of Trifolium aleksandrinum sowed three weeks before planting, which resulted in yield reductions for both the leeks and shallots. At this clover sowing time, a smaller diameter and unit weight of the leek pseudo-stems and a lower average weight for 100 shallots were recorded. The interaction between the sowing date and the cultivation season for the yield of the above-ground part of the season is noteworthy. With low rainfall in June and July of 2019, a higher yield was harvested in the clover crop, but only when sown in the third term (3 weeks after planting).
4.2. The Biological Value of Leek Pseudostems and Shallot Bulbs
Sowing clover in the 2nd and 3rd terms (at the time of vegetable crop planting and three weeks after planting, respectively) ensured more dry weight in the leeks and shallots than in the 1st term. Similarly, in the cultivation of eggplants, tomatoes, and peppers, changing the LM sowing date from perennial ryegrass caused an increase in dry weight [
31,
32,
33]. It can be suggested that the appropriate selection of the LM species and sowing date is crucial in influencing the dry weight of the edible parts of plants.
Several studies have showed that the cultivation of vegetables with LM may cause an increase in TP content values in the edible parts of onion [
28], cauliflower [
46], pumpkin [
47], cabbage [
48] plants. In this study, the TP content in the edible parts of the leeks in LM cultivation was 655 mg GAE 100 g
−1 DW, and without mulch it was 616 mg GAE 100 g
−1 DW. On the other hand, in comparison to leeks, the TP content in shallot onions was lower in cultivation with live mulch, amounting to 137 mg GAE 100 g
−1 DW, while it amounted to 129 mg GAE 100 g
−1 DW in the monoculture (no significant differences).
A. ampeloprasum is considered a potential source of TP and was compared favorably with the amount reported by Golubkina et al. [
43] (284–555 mg GAE 100 g
−1 DW). A two-fold higher level of TP (on average, 14 mg GAE g
−1 DW) was found in 30 varieties of
A. ampeloprasum [
49]. In the fresh leek plants, the TP content reported by García-Herrera et al. [
50] was 5.70 mg GAE g
−1 FW, while Proteggente et al. [
51] reported 22 mg GAE 100 g
−1 FW. The content of polyphenols in shallots in the fleshy scales of the bulbs is 40.8–43.2 mg 100 g
−1 FW, and in dry scales it is higher, amounting to 1670–1840 mg 100 g
−1 FW [
2]. The higher TP content is characteristic of the leaves more than the bulb extracts [
52].
The results of these studies show that there was a more significant total level of phenolic compounds in the leeks than in the shallots. The level of phenolic compounds in plants is influenced by many factors [
30], so it is difficult to interpret the results. It should be assumed that the more extended period of leek cultivation than for the shallots (by one month) favored the accumulation of more phenols in the leek plants. Lisiewska and Kmiecik [
52] reported that the phenolic content is determined, to a high degree, by environmental conditions, while the temperature remains the main stress factor. Variations in the phytochemical levels of
Allium can be caused by many factors, such as genotypic differences, growing conditions, and agricultural practices [
30].
In our research, shortening the vegetation period of Alexandrian clover by 3 and 6 weeks, thereby reducing the produced biomass at the beginning of leek and shallot vegetation, increased the TP content. These results, in line with the previous results, indicate that when LM is sown too early, it excessively competes with the main crops for nutrients, water, space, and sunlight, and consequently may reduce the TP level and the biological value of the crop [
31,
32,
53]. Several studies have shown a correlation between the increase in TP levels and exposure to UV-B radiation in barley [
54] and
Arabidopsis [
55]. Plants produce UV-B-absorbing phenolic compounds that accumulate on the leaf surfaces [
56]. Additionally, PAR may increase the content of polyphenols [
57]. In the field cultivation of
A. cepa, it was observed that plants accumulate more TP compounds in years with higher solar radiation [
58]. In our studies, delaying the LM sowing time in leek and shallot plants limited the intensity of the competition for light, favoring the biosynthesis of phenols.
Studies that are more recent indicate that delaying the sowing or planting of living mulch is a reliable method to limit competition with the main crop [
20,
23]. Alternatively, competition between the living mulch and the main crop can be reduced by interrupting the companion plant’s growth. For example, trimming barley to a height of 18 cm in the cultivation of onions reduces the competition from the live barley mulch [
59].
The role of environmental factors is emphasized in shaping the level of secondary metabolites; according to Bibi et al. [
60], the level of TP increases or decreases in response to environmental stimuli. In this context, the results of our research allowed us to establish that the applied LM had a positive effect, with increases in TP, ferulic acid, and chlorogenic acid in leek plants. The living mulch may have limited the heating of the soil in various ways, thereby reducing evaporation and promoting water absorption and infiltration. It was noticed that in the cultivation of leeks and shallots, the level of TP was highly influenced by the weather factor. In 2019, with a lower amount of rainfall, more ferulic acid and caffeic acid was found in the leeks, and more
p-coumaric acid, chlorogenic acid, and quercetin was found in the shallots. In general, the differences in the chemical compositions of the leeks and shallots, as in other plant tissues, are influenced by many factors, such as the temperature, rainfall, sun exposure, soil type, growth phase, and interactions of different plants or animals in the ecosystem [
50].
The effectiveness of polyphenolic compounds depends mainly on their molecular weight, structure, and degree of oxidation. The activity of phenolic acids increases significantly if they contain two ortho hydroxyl groups in the molecule. An example of a compound with high antioxidant activity is caffeic acid. In our study, growing leeks with clover sown three weeks after planting increased the level of caffeic acid and decreased the chlorogenic acid. It is worth noting that during leek cultivation, at each clover sowing time, the content of caffeic acid was higher than in the cultivation without living mulch. Contrary to the cultivation of shallots, the date of clover planting did not affect the level of caffeic acid in the bulbs of this species.
4.3. Antioxidant Value
The direct mechanisms of phenolic compounds’ antioxidant activity mainly consist of capturing or scavenging free oxygen radicals and inhibiting lipid peroxidation. The multidirectional action of phenolic compounds as antioxidants makes it challenging to estimate the antioxidant potential of the plant extracts. In our research, we chose to extinguish DPPH and FRAP free radicals. This approach relies on electron transfer to determine the antioxidant capacity [
61].
In the leek cultivation, more TP compounds were determined with clover cultivation, which was accompanied by higher antioxidant activity (AA) as determined only by the FRAP test. The LM sowing date and growing season did not affect the AA. This difference between the DPPH and FRAP values is in line with previously published data from Bernaert et al. [
49]. It is related to the fact that the antioxidant activity of phenolic compounds also depends on their chemical structure and the different evaluation methods used.
Moreover, our results showed that in the cultivation of shallots, clover plants sown at the time of planting seedlings and three weeks after planting increased the quercetin level in the bulbs and the AA (FRAP) value. It is known that the activity levels of TP and the AA levels in plants vary with growth conditions [
62] and with the methods of extracting compounds [
63]. The higher TP values in the
Allium methanol extracts were correlated with higher in vitro radical scavenging capacity and the more robust inhibition of tumor cell proliferation [
5]. Similarly, in our study, the antioxidant effect was more substantial as the quercetin level in shallot increased. There was no difference between the clover sowing dates in terms of the DPPH scavenging activity of the shallot bulb extracts.