Quality and Physiology of Selected Mentha Genotypes Under Coloured Shading Nets

Abstract

Improving the quality of compounds in medicinal and aromatic plants is crucial due to their uses in the pharmaceutical, cosmetics, and food sectors. One way of influencing plant composition is through exposure to different light conditions. Therefore, a two-year field study (2023–2024) was conducted to investigate the impact of coloured shading nets on the physiology, essential oil (EO) content, and composition of three Mentha genotypes: Mentha × piperita ‘Multimentha’, Mentha × piperita ‘Fränkische Blaue’, and Mentha rotundifolia ‘Apfelminze’. In addition to an unshaded control, the Mentha plants were grown under red and blue shading nets. Plant height and vegetation indices were collected weekly. Biomass accumulation, EO content, and composition were determined for each harvest. Both red and blue shading were found to influence the physiological responses and EO compositions of the plants, with red shading promoting slightly higher p-menthone levels in ‘Fränkische Blaue’ and ‘Multimentha’, while blue shading slightly increased carvone levels in ‘Apfelminze’. While EO content varied across harvest seasons (spring, summer, and autumn), ‘Fränkische Blaue’ responded to red shading, demonstrating an increased EO content. The findings suggest that targeted use of coloured shading nets can modulate EO quality. However, genotype-specific responses highlight the necessity of further research to define shading applications for different species and genotypes.

1. Introduction

Modern agriculture faces challenges from land degradation and changing climatic conditions [1] while seeking to maintain stable yields and ensure high produce quality [2]. Securing high-quality ingredients for medicinal and aromatic plants (MAPs) is important, as they are components in many pharmaceuticals and are widely used in the cosmetic and food industries [3], where their bioactive compound compositions determine their effectiveness and economic value [4]. A prominent example of MAPs is the genus Mentha, which is used for the production of tea and essential oils (EOs) [5]. Due to the diverse applications of Mentha EO, optimization is crucial; this can be achieved, for example, through genetic, environmental, and management influences, typically abbreviated as G × E × M [6].

Genotype selection in case of Mentha is important because of differing EO contents and compositions among the diverse genotypes. Previous work has shown the differences within Mentha species: the genotype Mentha rotundifolia ‘Apfelminze’ differs significantly from the genotypes Mentha × piperita ‘Multimentha’ and Mentha × piperita ‘Fränkische Blaue’ in terms of EO content and composition [7]. Mentha genotypes can be categorized into three different metabolic pathways, depending on which monoterpene predominates in their EO: the menthol pathway, with menthol, menthone, and menthofuran; the carvone pathway, with carvone, dihydrocarvone, and carveol; the linalool pathway, with linalool and linalyl acetate [8]. Examples of environmental influences on EO content and composition include irradiation [9] and nutrient availability [10]. Herbicide treatments [11], harvest time [12], and post-harvest processing [13] are part of these management decisions. The time of harvest, in particular, can have a significant influence on the EO content and composition. For example, Grulova et al. (2015) observed an increase in menthol, the major monoterpene of peppermint EO [14,15], in the EO of Mentha × piperita L. towards summer (up to 71.35%) and a decline afterwards [16].

Light in particular is one of the most impactful environmental influences on plant growth; it affects plant height, the number of branches and leaves a plant has [17], and the development of secondary metabolites like EOs [18]. By adjusting light conditions, both physiological processes and the quality of plant constituents can be influenced [19]. To better understand these physiological responses, VIs are a non-invasive method for monitoring plants. VIs, which derive from spectral reflectance data, provide an overview into plant health, biomass production, and stress levels [20].

While specific lighting systems (e.g., LEDs) have already been utilized to optimize plant compounds in controlled environments [21], there is still potential for their optimization in open-field cultivation. Shading nets are a promising approach to modify the spectral composition of light and thereby regulate plant growth and the composition of EOs. Hortensias (Hydrangea macrophylla subsp. serrata), for example, developed a slightly higher biomass accumulation under 28% shading (Haygrove tunnel) and under 64% shading (Haygrove tunnel + black shading net) compared to a control without shading [22]. However, for clary sage (Salvia sclarea L.), 25% shading with a green net in the field resulted in higher linalyl acetate and sclareol levels, whereas 50% shading led to a higher germacrene D content, and linalool showed the highest levels under control conditions with no shading [23]. Mentha × piperita cultivated indoors under blue light showed an elongation of the stem, increased growth [24], and reduced EO content [25]. Therefore, the choice of net colour and, consequently, light colour could be crucial in the field. For lemon balm (Melissa officinalis L.), 50% shading under blue netting led to an increase of 116% in plant height, 168% in leaf area, 42% in chlorophyll content, and 30% in EO yield compared to shading with black or red shading net [26]. Patchouli (Pogostemon cablin (Blanco) Benth.) showed a higher EO content and the highest value of patchoulol content in the EO composition under red shading [27].

No consistent results on the effect of light spectrum on the physiology and EO of MAPs have been obtained, and no specific study has yet been conducted with Mentha spp. under field conditions; therefore, this two-year field study investigated the impact of different-coloured shading nets on the physiology and EO of three Mentha genotypes (Mentha × piperita ‘Multimentha’, Mentha × piperita ‘Fränkische Blaue’, and Mentha rotundifolia ‘Apfelminze’). The genotypes were cultivated under three different treatments: a control (no shading), red shading, and blue shading. The following three hypotheses were investigated: first, that red shading leads to a higher EO content (and therefore higher plant quality) and blue shading leads to a higher biomass content; second, that red shading leads to a higher accumulation of undesired EO compounds like pulegone (negatively influencing raw-material quality); and third, that the three genotypes would express different reactions to the shading colours.

2. Materials and Methods

2.1. Plant Material and Cultivation

Three genotypes were used for the field experiment: Mentha × piperita ‘Multimentha’, Mentha × piperita ‘Fränkische Blaue’, and Mentha rotundifolia ‘Apfelminze’, which are abbreviated as ‘Multimentha’, ‘Fränkische Blaue’, and ‘Apfelminze’ in this publication. Plant material was propagated from an established cultivar collection on 5 April 2023 via cuttings. Plantlets were grown under greenhouse conditions and irrigated with tap water every 48 h via an ebb–flow system. Einheitserde ED73 (Einheitserde Werkverband, Sinntal-Altengronau, Germany) was used as substrate. The cuttings were trimmed back to two leaf-pairs on 24 April 2023 and planted out in the field on 27 April 2023 using a mechanical three-row planting machine. This resulted in 27 plots measuring three by five meters. Plots consisted of six rows with 60 plants in total (4 plants/m²; 45 cm row width; 50 cm plant distance). This resulted in three plots (replicates) per genotype and shading combination. The experiment was carried out over two years (2023–2024). Temperature and humidity were recorded over the course of the experiment using a datalogger “OM-EL-USB-2-PLUS” (Omega, Deckenpfronn, Germany). Two dataloggers were used for each treatment. The sum of precipitation was 644.3 mm in 2023 and 679.1 mm in 2024. The long-term average (1956–2010) was 603.4 mm. The average temperature was 11.6 ± 6.6 °C in 2023 and 11.3 ± 5.9 °C in 2024. The long-term average was 9.4 °C (

Table 1. Average temperature (°C) and the sum of precipitation (mm) by month for the years 2023 and 2024.

	2023		2024
Month	Temperature [°C]	Precipitation [mm]	Temperature [°C]	Precipitation [mm]
Jan.	4.5 ± 4.7	40.8	2.5 ± 5.3	43.1
Feb.	4.8 ± 3.6	21.0	7.9 ± 2.3	45.2
Mar.	6.8 ± 4.2	66.0	8.6 ± 2.2	49.1
Apr.	8.4 ± 2.6	46.0	8.9 ± 5.0	56.9
May	13.4 ± 2.3	73.2	14.9 ± 1.9	170.0
Jun.	19.5 ± 2.5	44.0	16.4 ± 3.4	10.2
Jul.	19.1 ± 2.5	60.3	18.8 ± 2.7	70.6
Aug.	18.4 ± 2.8	77.6	20.4 ± 2.3	71.0
Sep.	18.1 ± 3.1	40.4	15.4 ± 3.7	60.3
Oct.	13.3 ± 4.0	53.6	11.7 ± 2.6	34.0
Nov.	7.0 ± 3.4	72.7	6.2 ± 3.0	36.7
Dec.	5.8 ± 3.8	48.6	3.8 ± 3.1	32.0

).

The shading nets were installed on a wood construction consisting of nine poles (1.2 m height), which were connected by roof battens. The red shading net (Polyethylen; 140 g/m²; Accura NTV KG, Neu-Ulm, Germany) and blue shading net (Polyethylen; 200 g/m²; Accura NTV KG, Neu-Ulm, Germany) were reversibly attached to this construction so that the net could be moved aside during data collection and harvest. The shading performance was 77% for the red shading net and 60% for the blue shading net. UV-A, UV-B, and photosynthetically active radiation (PAR) values were obtained using the Gigahertz-Optik X1₂ Optometer (Gigahertz-Optic; Member of the Gerghof Group, Türkenfeld, Germany). UV radiation (UV-A, UV-B; in W/m²) and PAR (PPFD: photosynthetic photon flux density; in µmol/m²/s) in a wavelength range of 380–710 nm were measured [28] between 11:45 am and 12:15 am (

Table 2. UV-A, UV-B (W/m²), and PAR (PPFD) measurements, obtained with a Gigahertz-Optik X12 Optometer, of control conditions and shading in the years 2023 and 2024. Values are the mean value across the measurement dates.

	2023			2024
	Control	Red	Blue	Control	Red	Blue
UV-A (W/m2)	26.0 ± 10.0	4.6 ± 2.2	4.2 ± 1.6	26.0 ± 8.4	5.4 ± 1.5	4.3 ± 1.4
UV-B (W/m2)	0.6 ± 0.3	0.1 ± 0.1	0.1 ± 0.1	0.6 ± 0.2	0.1 ± 0.0	0.1 ± 0.2
PAR (PPFD)	1187 ± 526	307 ± 178	283 ± 146	1188 ± 445	312 ± 97	270 ± 95

).

To reduce the effects on the microclimate, the shading net construction was open at the sides to allow for air circulation. Therefore, only a core spacing under the shading nets were investigated, as those plants did not receive full and un-shaded sunlight. Harvesting only took place in the inner rows. The spectral composition of the irradiation was determined using an LI-180 spectrometer (LI-COR, Lincoln, United States) (

Table 3. Spectral composition (%), obtained with a LI-180 spectrometer, of the irradiation consisting of the colour bands blue, green, and red (400–700 nm) as well as near-UV (UV; 380–400 nm) and far-red (FR; 700–780 nm).

	UV [%]	Blue [%]	Green [%]	Red [%]	FR [%]
Control	1.77	21.25	27.22	27.52	22.24
Red	0.97	8.89	11.17	40.75	38.22
Blue	1.26	39.98	29.21	12.87	16.67

). Over the winter months (November–March), the nets were removed, and the plot grounds were covered with black ribbon fabric to suppress weeds while the Mentha plants overwintered.

2.2. Harvest and Data Collection

In 2023, nine measurements (

Table 4. Categorization of the measurement dates with the corresponding day of the year (DOY).

2023			2024
Measurement	Date	DOY	Measurement	Date	DOY	Measurement	Date	DOY
1st	3 August	215	1st	23 April	114	10th	18 July	200
2nd	10 August	222	2nd	2 May	123	11th	24 July	206
3rd	18 August	230	3rd	8 May	129	12th	31 July	213
4th	24 August	236	4th	14 May	135	13th	19 September	263
5th	22 September	265	5th	21 May	142	14th	24 September	268
6th	29 September	272	6th	20 June	172	15th	2 October	276
7th	4 October	277	7th	27 June	179	16th	8 October	282
8th	11 October	284	8th	4 July	186
9th	18 October	291	9th	11 July	193

) were conducted on a weekly basis. The shading nets were installed on 27 June 2023 (day of the year (DOY) 179), five weeks prior to the first measurement (3 August 2023; DOY 215). As planting took place in the spring, there was a harvest in summer (29 August 2023; DOY 241) and one in autumn (18 October 2023; DOY 291) in 2023. In 2024, sixteen measurements were carried out on a weekly basis. The shading nets were installed (11 April 2024; DOY 102) two weeks before the first measurement (23 April 2024; DOY 114). Three harvests were carried out in 2024: one in spring (21 May 2024; DOY 142), one in summer (31 July 2024; DOY 213), and one in autumn (16 October 2024; DOY 290). After every harvest, biomass (fresh matter (FM) in g/m², dry matter (DM) in g/m², and dry substance (DS) in %) and EO content (in mL/100 g DM) were determined. For the determination of the biomass, one square meter of plant material was harvested. These samples were dried for five days at 35 °C with a Venticell 707-Eco Line drying oven (MMM Group, Planegg, Germany).

Calculation of VIs (n = 15) was performed using a PolyPen RP400 (UV-VIS) by Photon Systems Instruments (Drásov, Czech Republic) with a spectral response range from 380–790 nm to record the physiological reaction of Mentha. The measurements were taken on the youngest fully developed leaves. The following VIs were obtained: Modified Chlorophyll Absorption Reflectance Index 1 (MCARI1), Photochemical Reflectance Index (PRI), Plant Senescence Reflectance Index (PSRI), and Red Edge Inflection Point 1 (REIP1) as shown in

Table 5. Group and equations of the vegetation indices MCARI1, PRI, PSRI, and REIP1.

Vegetation Index	Group 1	Equation
MCARI1	Biochemical/chlorophyll	1.2 × (2.5 × (R790 − R670) − 1.3 × (R790 − R550))
PRI	Physiology/LUE	(R530 − R570)/(R530 + R570)
PSRI	Biochemical/pigments	(R680 − R500)/R750
REIP1	Physiology/stress	700 + 40 × ((((R670 + R780)/2) − R700)/R740 − R700)

¹ Grouped according Roberts et al., 2011 [33].

. With the MCARI1, the chlorophyll content and green coloration can be determined [29]. The PRI can be used to estimate photosynthetic activity and light-use efficiency (LUE) [30]. The PSRI responds to changes in the ratio of carotenoids to chlorophyll and can thus be used to determine leaf senescence [31]. The REIP1 determines the chlorophyll content and allows conclusions to be drawn about the nitrogen content [32].

2.3. Essential Oil Extraction and Analysis

Extraction and analyses of EOs were performed, as described in detail previously [13] via steam distillation according to the methodology of the European Pharmacopoeia [15]. The analysis of the composition of EO was carried out using gas chromatography. Detection was performed on a time-of-flight mass spectrometer

The materials used for EO extraction and analyses were as follows:

• Apparatus “KOL” (behr Labor Technik GmbH, Düsseldorf, Germany).
• Agilent 7890B gas chromatograph (Agilent Technologes, Palo Alto, CA, USA) with a ZEBRON ZB 1 MS (30 m, 0.25 mm i.d. × 0.1 µm df).
• Helium as carrier gas (flow of 1.0 mL/min).
• Column temperature: 50 °C for 2 min, then increased to 200 °C with 3 °C/min steps, then 225 °C for 10 min.

TOF MS (Markes International Ltd., Llantrisant, RCT, UK) in the electron-ionization (EI) mode and an ionizing voltage set at −70 eV.

2.4. Statistical Analysis

Data are given as means with standard deviations. Statistical analysis was performed using JMP Pro 17 (SAS Institute GmbH, Heidelberg, Germany). First, a general linear model with a two-way ANOVA including interaction of genotype and treatment was carried out to identify possible interactions. As the model did not reveal any significant (p ≤ 0.05) interactions for biomass yield, plant height, or EO content, further statistical calculations were simplified to investigate genotype and treatment separately. Calculations were therefore made via analysis of variance (ANOVA) with Tukey HSD as post hoc procedure to determine homogenous subgroups at a p-value of p ≤ 0.05, which is indicated by different letters.

3. Results and Discussion

3.1. Temperature and Relative Humidity

The microclimate within the plots (air temperature and relative humidity) under control conditions and shading is depicted in Figure 1. Statistical analysis revealed a significant difference between the control and the shaded plots. This is mainly due to the large sample size of measurements (n = 23,298). Average temperature of shaded plots (17.9 °C) was 1.2 °C lower than the control plots (19.1 °C) and average relative humidity of the shaded plots (76.5%) was 3.4% higher than the control plots (73.1%). This shows that temperature extremes were mitigated. In other studies, different-coloured shading nets also lead to higher air humidity and lower air temperature compared to the control [34,35]. However, if shading nets were mounted above the crops but open on the sides, like in this trial, these effects were only minor [36].

3.2. Plant Height

In 2023, the effects of seasonal changes as well as genotypic differences were observed. In summer, plant height was the highest under control conditions (‘Apfelminze’ with 52.0 ± 3.6 cm), while in autumn, the highest values occurred under red shading (‘Apfelminze’ with 35.4 ± 3.5 cm) (Figure 2). One possible explanation for the differences between harvests is that the plants first had to adapt to the changed light conditions under the nets [37]. The genotype ‘Apfelminze’ had significantly higher plant height in autumn (31.3 ± 2.1 cm, control) compared to ‘Fränkische Blaue’ (23.2 ± 2.3 cm, blue).

In 2024, it is noticeable that ‘Apfelminze’ showed a significantly higher plant height for all three treatments in the spring harvest than the other two genotypes, e.g., up to 87.7 ± 12 cm under red shading (Figure 3). A reason might be the high precipitation of 170 mm in May, when the spring harvest took place (21.05.; DOY 142). ‘Apfelminze’ responded most clearly to increased rain in autumn, which was also reflected in the DS values, e.g., ‘Apfelminze’ under control conditions with 13.6 ± 0.9% and ‘Multimentha’ with 15.2 ± 0.8% (Chapter 3.3 Biomass). Independently of this, red shading resulted in the highest plant height in all three harvests and genotypes, e.g., ‘Multimentha’ with 67.9 ± 10.3 cm (spring), 49.7 ± 8.0 cm (summer), and 23.7 ± 3.0 cm (autumn). Lippia gracilis Schauer also had the highest plant height under red shading (108.71 cm) in contrast to blue shading (103.50 cm) or the control (60.13 cm) [38]. Other studies found that the use of blue light led to plant elongation. However, the authors also underlined that this area requires further investigation and that light can have species-dependent effects [24]. In this case, the effect of the shading was probably stronger than that of the colour and the higher plant height can be explained by shade-induced elongation responses [39]. In addition, an investigation of the leaf/stem ratio would be interesting, to determine both genotypic differences and treatment-induced influences.

3.3. Biomass

In 2023, only two harvest dates (summer and autumn) were collected as planting took place in spring. The summer harvest showed higher biomass accumulation than the autumn harvest. In the summer harvest, all three genotypes reached the highest dry matter values under control condition (

Table 6. Biomass accumulation (g/m²) and dry substance (%) of the three Mentha genotypes (‘Apfelminze’, ‘Fränkische Blaue’, and ‘Multimentha’) and three treatments (control, red shading, and blue shading) for the year 2023 and two harvests. Significant differences between treatments and genotypes were calculated by ANOVA and Tukey HSD (n = 54, α = 0.05) for each harvest separately and are indicated by letters (a–d) for fresh matter (FM), dry matter (DM), and dry substance (DS).

Genotype	Treatment	Harvest	DOY	FM [g/m2]	DM [g/m2]	DS [%]
‘Multimentha’	Control	Summer	241	1892.3 ± 29.0 a	293.0 ± 6.4 a	15.5 ± 0.2 a
‘Multimentha’	Red	Summer	241	1182.1 ± 131.8 bc	141.4 ± 15.8 cd	12.0 ± 0.3 b
‘Multimentha’	Blue	Summer	241	1158.3 ± 342.1 bc	138.9 ± 34.4 cd	12.1 ± 0.7 b
‘Fr. Blaue’	Control	Summer	241	1547.5 ± 110.1 ab	233.0 ± 24.9 b	15.0 ± 0.7 a
‘Fr. Blaue’	Red	Summer	241	1033.9 ± 77.5 c	119.9 ± 4.7 cd	11.6 ± 0.7 b
‘Fr. Blaue’	Blue	Summer	241	798.3 ± 60.8 c	91.5 ± 5.9 d	11.5 ± 0.5 b
‘Apfelminze’	Control	Summer	241	1144.5 ± 277.3 bc	173.1 ± 35.9 c	15.3 ± 1.4 a
‘Apfelminze’	Red	Summer	241	825.2 ± 27.7 c	98.7 ± 5.2 d	12.0 ± 0.5 b
‘Apfelminze’	Blue	Summer	241	919.5 ± 69.2 c	102.8 ± 7.8 d	11.2 ± 0.3 b
‘Multimentha’	Control	Autumn	291	913.4 ± 146.9 a	161.2 ± 18.9 a	17.7 ± 0.7 a
‘Multimentha’	Red	Autumn	291	775.3 ± 128.7 ab	106.3 ± 18.4 ab	13.7 ± 0.1 cd
‘Multimentha’	Blue	Autumn	291	750.6 ± 155.5 ab	97.2 ± 27.4 ab	12.8 ± 0.9 d
‘Fr. Blaue’	Control	Autumn	291	689.8 ± 106.0 ab	117.0 ± 21.9 ab	16.9 ± 0.9 ab
‘Fr. Blaue’	Red	Autumn	291	461.5 ± 147.1 b	70.2 ± 23.8 b	15.1 ± 1.2 bc
‘Fr. Blaue’	Blue	Autumn	291	499.0 ± 67.5 ab	75.5 ± 12.8 b	15.1 ± 0.7 bc
‘Apfelminze’	Control	Autumn	291	664.7 ± 257.2 ab	109.0 ± 38.5 ab	16.5 ± 0.6 ab
‘Apfelminze’	Red	Autumn	291	570.3 ± 158.2 ab	76.1 ± 22.5 b	13.3 ± 0.3 cd
‘Apfelminze’	Blue	Autumn	291	459.4 ± 50.3 b	58.3 ± 4.4 b	12.7 ± 0.5 d

). ‘Multimentha’ achieved the highest DM with 293.0 ± 6.4 g/m² (control). Also, under red and blue shading ‘Multimentha’ reached the largest DM, with 141.4 ± 15.8 g/m² (red) and 138.9 ± 34.4 g/m² (blue). The same trend was also observed in the autumn harvest. However, biomass values were notably lower due to the later season, lower temperatures, and irradiation in the autumn harvest, as ‘Multimentha’ had a DM of only 161.2 ± 18.9 g/m² under the control conditions. This is in line with the results of other experiments, which have also shown that a late harvest resulted in lower biomass yields [10,40,41].

In 2024, biomass accumulation across all genotypes was highest under control conditions, followed by red shading and blue shading (

Table 7. Biomass accumulation (g/m²) and dry substance (%) of the three Mentha genotypes (‘Apfelminze’, ‘Fränkische Blaue’, and ‘Multimentha’) and three treatments (control, red shading, and blue shading) for the year 2024 and three harvests. Significant differences between treatments and genotypes were calculated by ANOVA and Tukey HSD (n = 81, α = 0.05) for each harvest separately and are indicated by letters (a–c) for fresh matter (FM), dry matter (DM), and dry substance (DS).

Genotype	Treatment	Harvest	DOY	FM [g/m2]	DM [g/m2]	DS [%]
‘Multimentha’	Control	Spring	142	1988.4 ± 443.1 ab	302.0 ± 74.7 ab	15.2 ± 0.8 a
‘Multimentha’	Red	Spring	142	1934.9 ± 235.8 b	220.5 ± 29.3 bc	11.4 ± 1.1 bc
‘Multimentha’	Blue	Spring	142	1838.5 ± 145.3 b	214.6 ± 50.0 bc	11.6 ± 1.9 bc
‘Fr. Blaue’	Control	Spring	142	1970.7 ± 121.0 ab	297.7 ± 11.3 ab	15.1 ± 0.4 a
‘Fr. Blaue’	Red	Spring	142	1846.8 ± 498.3 b	200.5 ± 54.7 bc	10.8 ± 0.0 c
‘Fr. Blaue’	Blue	Spring	142	1538.5 ± 224.9 b	171.3 ± 24.4 c	11.2 ± 1.2 bc
‘Apfelminze’	Control	Spring	142	2867.3 ± 260.6 a	389.4 ± 19.2 a	13.6 ± 0.9 ab
‘Apfelminze’	Red	Spring	142	2455.7 ± 236.3 ab	270.3 ± 32.5 abc	11.0 ± 0.3 bc
‘Apfelminze’	Blue	Spring	142	2168.0 ± 481.9 ab	221.7 ± 41.4 bc	10.3 ± 0.4 c
‘Multimentha’	Control	Summer	213	1241.3 ± 358.7 a	293.7 ± 63.6 a	24.0 ± 2.5 a
‘Multimentha’	Red	Summer	213	1210.3 ± 362.1 a	196.9 ± 71.1 ab	16.1 ± 1.1 b
‘Multimentha’	Blue	Summer	213	1006.9 ± 147.2 a	155.8 ± 40.3 b	15.3 ± 1.7 b
‘Fr. Blaue’	Control	Summer	213	862.1 ± 113.0 a	212.3 ± 52.5 ab	24.4 ± 2.9 a
‘Fr. Blaue’	Red	Summer	213	1121.9 ± 90.2 a	189.9 ± 17.3 ab	16.9 ± 1.3 b
‘Fr. Blaue’	Blue	Summer	213	995.3 ± 82.7 a	182.1 ± 53.8 ab	18.2 ± 4.4 ab
‘Apfelminze’	Control	Summer	213	1326.6 ± 92.0 a	275.0 ± 4.9 ab	20.8 ± 1.6 ab
‘Apfelminze’	Red	Summer	213	1386.0 ± 156.4 a	244.2 ± 42.8 ab	17.5 ± 1.4 b
‘Apfelminze’	Blue	Summer	213	1027.2 ± 159.5 a	150.6 ± 27.2 b	14.6 ± 1.0 b
‘Multimentha’	Control	Autumn	290	390.1 ± 209.7 ab	88.0 ± 38.6 abc	23.4 ± 2.3 a
‘Multimentha’	Red	Autumn	290	618.6 ± 32.7 ab	108.6 ± 2.9 ab	17.6 ± 1.0 bc
‘Multimentha’	Blue	Autumn	290	535.1 ± 78.6 ab	85.1 ± 11.2 abc	15.9 ± 0.5 bc
‘Fr. Blaue’	Control	Autumn	290	298.0 ± 89.0 b	59.2 ± 14.1 bc	20.2 ± 1.6 ab
‘Fr. Blaue’	Red	Autumn	290	544.3 ± 118.2 ab	81.8 ± 22.9 abc	14.9 ± 1.6 c
‘Fr. Blaue’	Blue	Autumn	290	431.7 ± 62.7 ab	63.8 ± 6.9 bc	14.8 ± 0.9 c
‘Apfelminze’	Control	Autumn	290	699.7 ± 146.3 a	134.3 ± 15.1 a	19.5 ± 2.1 ab
‘Apfelminze’	Red	Autumn	290	426.0 ± 54.4 ab	68.1 ± 9.1 bc	16.0 ± 0.3 bc
‘Apfelminze’	Blue	Autumn	290	314.0 ± 177.8 b	44.3 ± 18.2 c	14.8 ± 2.1 c

). However, a high value in FM in the spring harvest was noticeable, whereas the DM of the spring harvest and the summer harvest were relatively similar. The high precipitation of 170 mm in May, as already mentioned in Section 3.2 (plant height) may be a possible explanation for the high FM. Biomass accumulation also decreased again over the course of the year, resulting in lowest values in FM and DM in the autumn harvest [41]. In conclusion, non-shading led to the highest yield in biomass in both years (2023 and 2024); in addition, a significant difference in effects between the genotypes occurred.

3.4. Vegetation Indices

MCARI1, an indicator for chlorophyll content, did not differ between shading colours and genotypes (Figure 4). Over the year of 2023, the chlorophyll content decreased slightly and the average MCARI1 under control conditions was 0.887 ± 0.046 in ‘Apfelminze’, 0.834 ± 0.062 in ‘Fränkische Blaue’, and 0.834 ± 0.069 in ‘Multimentha’. In contrast, the PRI, which indicates photosynthetic activity and LUE, was the lowest for each genotype under the control conditions, e.g., ‘Multimentha’ had, on average, 1.024 ± 0.010 (control), 1.028 ± 0.011 (red), and 1.029 ± 0.009 (blue). This is probably due to the increase in LUE as plants adapt to conditions with insufficient quantities of photons and utilize the available light more efficiently [42]. The PSRI, which indicates leaf senescence, did not differ for ‘Apfelminze’ under all three treatments. In ‘Fränkische Blaue’ and ‘Multimentha’; however, PSRI was higher towards the end of the growing season under control conditions than under shading, regardless of the colour. Thus, shading resulted in a delayed leaf senescence and can therefore buffer radiation peaks, especially in summer. The REIP1 results were slightly higher under control conditions, e.g., ‘Multimentha’ with values of 715.7 ± 2.4 (control) and 714.2 ± 2.3 (red). This may indicate an improved nitrogen supply because the chlorophyll content is indirectly related to the nitrogen content [43] or a different leaf morphology and, hence, greener leaves in contrast to the shaded plants.

In 2024, MCARI1 showed the same trend as in 2023 without significant differences between treatments (Figure 5). On average, ‘Apfelminze’ had slightly higher values under control conditions (0.851 ± 0.078) than ‘Fränkische Blaue’ with 0.781 ± 0.080 and ‘Multimentha’ with 0.804 ± 0.084. The PRI was, on average, lower for all three genotypes under control conditions than under the shaded conditions, e.g., ‘Apfelminze’ with 1.010 ± 0.011 (control), 1.012 ± 0.008 (red), and 1.022 ± 0.008 (blue). The PSRI for ‘Apfelminze’ also showed no differences between the treatments in 2024. However, as observed in the previous year for ‘Fränkische Blaue’ and ‘Multimentha’, the PSRI was again higher under control conditions than under shaded conditions, particularly towards the end of the growing season. REIP1 was lowest on average in 2024 under red shading for all three genotypes, ‘Apfelminze’ with 709.7 ± 2.4, ‘Fränkische Blaue’ with 712.2 ± 1.7, and ‘Multimentha’ with 713.2 ± 2.0.

The results suggest that coloured shading influences plant physiology, particularly photosynthetic efficiency and leaf senescence. The observed increase in LUE under shading conditions indicates that plants can adapt to lower photon availability by optimizing their light absorption and utilization [42]. This adaptation may be beneficial for maintaining photosynthetic activity under suboptimal light conditions, potentially leading to more stable growth patterns. ‘Apfelminze’ seems to be the most adaptable across different conditions, while ‘Fränkische Blaue’ and ‘Multimentha’ might benefit more from shading to reduce senescence effects. Red shading, in particular, resulted in lower REIP1 values, suggesting a need for further investigation into its long-term effects on plant nutrition and growth.

3.5. Essential Oil Content

The EO content was recorded for the two harvests of 2023 (Figure 6). In line with the biomass (3.3 Biomass), in autumn, EO content was lower than in summer. In the summer harvest, the peppermints ‘Multimentha’ and ‘Fränkische Blaue’ reached significant higher EO content than ‘Apfelminze’; meanwhile, in the autumn harvest, the peppermints still reached higher values than ‘Apfelminze’ but without significant difference. ‘Multimentha’ showed the highest EO content under control conditions in the summer harvest (4.4 ± 0.1 mL/100 g DM), while ‘Fränkische Blaue’ had the peak in EO content under red shading in the autumn harvest (2.4 ± 0.3 mL/100 g DM). For the first year of cultivation, no general recommendation can be given regarding the best growing condition, but ‘Fränkische Blaue’ seems to respond positively to red shading (3.6 ± 0.4 mL/100 g DM in summer harvest; 2.4 ± 0.3 mL/100 g DM in autumn harvest).

In 2024, highest EO contents were observed in summer, whereas the lowest EO content occurred in autumn (Figure 7). ‘Multimentha’ was again the genotype with the highest EO content, e.g., 4.7 ± 0.2 mL/100 g DM in the summer harvest under blue shading. ‘Apfelminze’ showed the lowest EO content, e.g., 1.7 ± 0.1 mL/100 g DM in the autumn harvest under blue shading. A clear effect of shading colour on EO content was not identified, but ‘Fränkische Blaue’ still tends to react positively to red shading. In the spring harvest, ‘Fränkische Blaue’ reached the same EO content under red shading (2.1 ± 0.5 mL/100 g DM) as under control conditions (2.1 ± 0.5 mL/100 g DM). In the summer (red; 4.0 ± 0.4 mL/100 g DM) and autumn (red; 2.6 ± 0.1 mL/100 g DM) ‘Fränkische Blaue’ had a higher EO content under red shading than under control conditions (summer; 3.5 ± 0.3 mL/100 g DM and autumn; 2.2 ± 0.3 mL/100 g DM). A study of Mentha arvensis showed a lower EO content under 50% black, red, and blue shading (black, 0.57 g/plant; red, 0.50 g/plant; blue, 0.45 g/plant) than the control (0.95 g/plant) [44]. In contrast, patchouli (Pogostemon cablin) was found to have a higher EO content and the highest relative percentage of patchoulol (66.84%) under red shading [27]. The effect of the coloured shading therefore appears to be very species- and genotype-specific.

3.6. Essential Oil Composition

Different shading regimes lead to significant enhancements of EO quality for the three Mentha genotypes. In this study, the main compound in the EO extracted from ‘Apfelminze’ was carvone while that of the peppermints was p-menthone followed by menthol isomer B (

Table 8. EO composition (%) of each compound of the summer harvest of the year 2023 of the three genotypes ’Multimentha’ (MM), ’Fränkische Blaue’ (FB), and ’Apfelminze’ (AM) under control conditions (C), red shading (R), and blue shading (B). Data are given as the mean with the standard deviation (n = 3). Compound levels below 1% are not listed (indicated by hyphen).

Compound	AM C	AM R	AM B	FB C	FB R	FB B	MM C	MM R	MM B
Eucalyptol/ Limonene	12.86 ± 1.46	12.44 ± 1.21	8.26 ± 6.08	2.25 ± 0.16	1.72 ± 0.15	1.81 ± 0.07	1.32 ± 0.16	1.03 ± 0.02	1.22 ± 0.43
p-Menthone	-	-	-	55.74 ± 2.68	62.76 ± 3.76	60.88 ± 2.77	65.99 ± 0.73	66.39 ± 1.74	59.92 ± 3.04
Isomenthone	-	-	-	7.19 ± 0.10	7.32 ± 0.42	7.32 ± 0.50	4.15 ± 0.11	4.06 ± 0.15	4.90 ± 1.64
Menthofuran	-	-	-	2.96 ± 0.54	2.98 ± 0.26	3.69 ± 0.20	3.42 ± 0.34	4.42 ± 0.33	4.33 ± 0.58
Menthol isomer A	-	-	-	1.84 ± 0.33	1.59 ± 0.20	1.66 ± 0.24	3.45 ± 0.12	2.35 ± 0.23	2.48 ± 0.53
Menthol isomer B	-	-	-	23.38 ± 1.74	17.28 ± 3.31	17.87 ± 2.70	16.31 ± 0.69	14.82 ± 2.13	19.41 ± 2.64
Dihydrocarvone	10.58 ± 8.33	5.07 ± 1.74	5.79 ± 2.29	-	-	-	-	-	-
1,6-Dihydrocarveol	1.32 ± 1.40	-	-	-	-	-	-	-	-
Pulegone	-	-	-	1.07 ± 0.14	0.98 ± 0.11	1.52 ± 0.16	-	3.47 ± 0.32	3.19 ± 1.73
Carvone	69.13 ± 8.33	75.14 ± 1.67	78.35 ± 3.57	-	-	-	-	-	-
Piperitone	-	-	1.03 ± 0.16	1.27 ± 0.09	1.45 ± 0.04	1.40 ± 0.09	1.47 ± 0.10	1.34 ± 0.10	1.29 ± 0.01
Menthyl acetate	-	-	-	1.07 ± 0.39	-	1.18 ± 0.47	-	-	1.04 ± 0.59
β-Copaene	-	1.00 ± 1.58	-	1.15 ± 0.08	1.07 ± 0.03	-	-	-	-
β-Cubebene	1.25 ± 1.14	1.61 ± 1.40	2.97 ± 1.27	-	-	-	-	-	-

,

Table 9. EO composition (%) of each compound of the autumn harvest of the year 2023 of the three genotypes ’Multimentha´ (MM), ’Fränkische Blaue´ (FB), and ’Apfelminze´ (AM), under control conditions (C), red shading (R), and blue shading (B). Data are given as the mean with the standard deviation (n = 3). Compound levels below 1% are not listed (indicated by hyphen).

Compound	AM C	AM R	AM B	FB C	FB R	FB B	MM C	MM R	MM B
Eucalyptol/ Limonene	8.21 ± 2.79	7.49 ± 2.04	7.42 ± 1.77	2.02 ± 0.67	1.61 ± 0.48	1.28 ± 0.61	1.08 ± 0.70	-	-
p-Menthone	-	-	-	38.41 ± 1.50	39.68 ± 4.69	42.20 ± 0.81	49.28 ± 5.29	56.02 ± 2.39	55.56 ± 2.05
Isomenthone	-	-	-	5.07 ± 1.44	5.73 ± 0.49	5.12 ± 0.70	3.27 ± 0.43	3.66 ± 0.35	3.62 ± 0.99
Menthofuran	-	-	-	4.35 ± 0.66	3.13 ± 2.40	3.20 ± 2.85	2.59 ± 2.29	-	3.50 ± 3.04
Menthol isomer A	-	-	-	3.85 ± 1.17	4.64 ± 1.30	2.99 ± 0.60	6.51 ± 1.78	5.89 ± 0.79	3.48 ± 1.24
Menthol isomer B	-	-		33.33 ± 5.40	31.26 ± 3.92	32.57 ± 4.15	31.41 ± 2.62	27.62 ± 0.92	27.33 ± 3.45
Dihydrocarvone	10.53 ± 6.09	15.37 ± 6.80	12.67 ± 2.94	-	-	-	-	-	-
1,6-Dihydrocarveol	-	1.62 ± 1.03	1.42 ± 0.73	-	-	-	-	-	-
Pulegone	-	-	-	-	-	-	-	-	-
Carvone	76.23 ± 5.44	70.19 ± 6.67	73.79 ± 6.22	-	-	-	-	-	-
Piperitone	1.12 ± 0.21	1.25 ± 0.55	1.37 ± 0.14	1.53 ± 0.70	1.89 ± 0.66	1.13 ± 0.72	1.20 ± 0.65	1.42 ± 0.63	-
Menthyl acetate	-	-	-	9.80 ± 1.96	10.47 ± 3.01	10.05 ± 1.51	3.80 ± 2.07	3.53 ± 1.35	4.30 ± 3.58
β-Copaene	-	-	-	-	-	-	-	-	-
β-Cubebene	-	1.86 ± 1.28	1.28 ± 0.51	-	-	-	-	-	-

,

Table 10. EO composition (%) of each compound of the spring harvest of the year 2024 of the three genotypes ’Multimentha´ (MM), ’Fränkische Blaue´ (FB), and ’Apfelminze´ (AM) under control conditions (C), red shading (R), and blue shading (B). Data are given as the mean with the standard deviation (n = 3). Compound levels below 1% are not listed (indicated by hyphen).

Compound	AM C	AM R	AM B	FB C	FB R	FB B	MM C	MM R	MM B
Eucalyptol/ Limonene	8.56 ± 1.89	9.74 ± 1.01	6.51 ± 3.49	1.97 ± 0.78	1.76 ± 0.23	1.57 ± 0.53	1.47 ± 0.20	1.11 ± 0.31	1.43 ± 0.61
p-Menthone	-	-	-	51.23 ± 11.04	57.65 ± 4.69	57.80 ± 2.44	60.81 ± 2.40	62.16 ± 3.24	61.72 ± 1.69
Isomenthone	-	-	-	4.95 ± 1.16	6.13 ± 0.28	6.08 ± 0.23	3.91 ± 0.12	3.83 ± 0.10	4.57 ± 1.40
Menthofuran	-	-	-	-	-	-	-	1.12 ± 0.06	-
Menthol isomer A	-	-	-	4.08 ± 1.06	1.96 ± 0.25	2.25 ± 0.19	5.34 ± 1.12	5.02 ± 0.49	4.10 ± 1.71
Menthol isomer B	-	-	-	26.27 ± 7.89	20.78 ± 4.30	21.23 ± 3.26	19.73 ± 2.76	19.16 ± 3.64	18.71 ± 1.12
Dihydrocarvone	5.83 ± 3.37	5.25 ± 2.17	5.94 ± 3.07	-	-	-	-	-	-
1,6-Dihydrocarveol	-	-	-	-	-	-	-	-	-
Pulegone	-	-	-	-	-	-	-	-	-
Carvone	75.56 ± 4.24	74.81 ± 2.37	77.74 ± 3.46	-	-	-	-	-	-
Piperitone	1.32 ± 0.14	1.65 ± 0.23	1.90 ± 0.23	2.10 ± 0.59	2.73 ± 0.12	2.47 ± 0.32	2.61 ± 0.12	3.01 ± 0.33	2.50 ± 0.35
Menthyl acetate	-	-	-	2.95 ± 1.56	1.44 ± 0.48	2.15 ± 0.77	1.15 ± 0.35	1.27 ± 0.40	1.34 ± 0.43
β-Caryophyllene	1.45 ± 0.17	1.20 ± 0.18	1.04 ± 0.28	1.29 ± 0.74	1.17 ± 0.34	1.21 ± 0.57	-	-	-
β-Copaene	4.13 ± 0.84	4.61 ± 0.89	3.88 ± 1.38	1.70 ± 0.78	1.88 ± 0.58	1.81 ± 0.93	1.07 ± 0.09	-	1.46 ± 1.23

,

Table 11. EO composition (%) of each compound of the summer harvest of the year 2024 of the three genotypes ’Multimentha’ (MM), ’Fränkische Blaue’ (FB), and ’Apfelminze’ (AM) under control conditions (C), red shading (R), and blue shading (B). Data are given as the mean with the standard deviation (n = 3). Compound levels below 1% are not listed (as indicated by hyphen).

Compound	AM C	AM R	AM B	FB C	FB R	FB B	MM C	MM R	MM B
Eucalyptol/ Limonene	11.16 ± 2.27	10.48 ± 1.11	9.85 ± 0.46	2.97 ± 0.07	2.59 ± 0.12	2.52 ± 0.33	2.49 ± 0.47	1.62 ± 0.14	2.26 ± 0.74
p-Menthone	-	-	-	45.45 ± 3.06	58.95 ± 2.01	58.71 ± 0.68	62.34 ± 6.66	68.21 ± 0.81	65.57 ± 9.22
Isomenthone	-	-	-	6.32 ± 0.08	6.87 ± 0.17	6.69 ± 0.16	4.03 ± 0.08	4.10 ± 0.07	5.40 ± 1.73
Menthofuran	-	-	-	1.07 ± 0.14	1.44 ± 0.30	1.77 ± 0.33	2.60 ± 1.51	2.54 ± 0.49	1.72 ± 0.42
Menthol isomer A	-	-	-	2.74 ± 0.35	1.60 ± 0.03	1.71 ± 0.11	2.42 ± 0.75	2.87 ± 0.31	2.14 ± 0.67
Menthol isomer B	-	-	-	31.28 ± 3.55	16.43 ± 3.87	17.83 ± 1.11	18.74 ± 3.89	12.91 ± 1.11	13.07 ± 6.06
Dihydrocarvone	1.96 ± 0.44	1.82 ± 0.14	2.04 ± 0.13	-	-	-	-	-	-
1,6-Dihydrocarveol	-	-	-	-	-	-	-	-	-
Pulegone	-	-	-	-	-	-	1.10 ± 0.49	1.00 ± 0.17	-
Carvone	78.81 ± 4.99	78.96 ± 1.10	79.60 ± 0.27	-	-	-	-	-	-
Piperitone	-	-	-	1.39 ± 0.05	1.97 ± 0.08	1.90 ± 0.05	2.42 ± 0.53	2.29 ± 0.10	2.07 ± 0.28
Menthyl acetate	-	-	-	1.22 ± 0.16	-	-	-	-	-
β-Caryophyllene	1.83 ± 0.12	1.24 ± 0.08	1.20 ± 0.10	1.90 ± 0.43	1.71 ± 0.09	1.74 ± 0.09	-	-	1.37 ± 0.69
β-Copaene	3.63 ± 0.51	4.57 ± 0.27	4.28 ± 0.48	2.13 ± 0.52	2.71 ± 0.08	2.62 ± 0.27	1.44 ± 0.43	1.12 ± 0.07	2.24 ± 0.97

and

Table 12. EO composition (%) of each compound of the autumn harvest of the year 2024 of the three genotypes ’’Multimentha’ (MM), ’Fränkische Blaue’ (FB), and ’Apfelminze’ (AM) under control conditions (C), red shading (R), and blue shading (B). Data are given as the mean with the standard deviation (n = 3). Compound levels below 1% are not listed (indicated by hyphen).

Compound	AM C	AM R	AM B	FB C	FB R	FB B	MM C	MM R	MM B
Eucalyptol/ Limonene	7.74 ± 0.77	8.81 ± 0.49	6.52 ± 2.37	1.90 ± 0.04	1.60 ± 0.11	1.57 ± 0.02	1.25 ± 0.16	1.15 ± 0.10	1.19 ± 0.34
p-Menthone	-	-	-	31.40 ± 1.73	42.50 ± 4.28	40.67 ± 3.92	41.57 ± 4.81	48.99 ± 2.87	44.24 ± 6.26
Isomenthone	-	-	-	4.52 ± 0.15	4.81 ± 0.29	4.50 ± 0.22	2.78 ± 0.16	2.61 ± 0.08	3.12 ± 1.07
Menthofuran	-	-	-	6.96 ± 0.71	9.01 ± 1.85	10.90 ± 0.82	7.83 ± 0.72	11.54 ± 1.04	12.12 ± 1.19
Menthol isomer A	-	-	-	3.83 ± 0.17	2.37 ± 0.30	2.30 ± 0.09	5.42 ± 0.77	3.33 ± 0.59	3.13 ± 0.84
Menthol isomer B	-	-	-	40.55 ± 1.50	30.64 ± 1.41	30.16 ± 2.82	34.49 ± 2.74	27.14 ± 2.76	28.65 ± 4.20
Dihydrocarvone	20.35 ± 1.03	12.86 ± 4.84	11.85 ± 5.26	-	-	-	-	-	-
1,6-Dihydrocarveol	3.11 ± 0.60	1.39 ± 0.65	1.43 ± 1.34	-	-	-	-	-	-
Pulegone	-	-	-	-	-	-	-	-	-
Carvone	64.02 ± 2.06	72.87 ± 5.66	77.50 ± 10.96	-	-	-	-	-	-
Piperitone	-	1.13 ± 0.06	-	1.11 ± 0.11	1.37 ± 0.08	1.18 ± 0.12	1.15 ± 0.18	-	-
Menthyl acetate	-	-	-	7.92 ± 0.82	5.98 ± 1.93	6.69 ± 0.80	3.60 ± 0.93	2.06 ± 0.38	4.59 ± 2.49
β-Caryophyllene	-	-	-	-	-	-	-	-	-
β-Copaene	-	-	-	-	-	-	-	-	-

). Pulegone, which is metabolized to menthofuran in the liver, whose phase 1 metabolite was shown to have hepatotoxic effects [45], was only detected in the summer harvests of both years in both peppermints.

In the summer harvest of 2023 (Table 8), carvone was the main compound of ‘Apfelminze’ with the highest amount under blue shading (78.35 ± 3.57%). For ‘Fränkische Blaue’ p-menthone levels were increased by 8–11% under shading (red: 62.76 ± 3.76%; blue: 60.88 ± 2.77%) in comparison to the control (55.74 ± 2.68%). However, the highest menthol isomer B content was investigated under control conditions (23.38 ± 1.74%). Pulegone was detected in the EO conducted under every treatment in the peppermints, but blue shading induced the highest amount with 1.52 ± 0.16%. In line with ‘Fränkische Blaue’, ‘Multimentha’ also had the highest p-menthone content under red shading with 66.39 ± 1.74%. In contrast to ‘Fränkische Blaue’, no pulegone was detected in ‘Multimentha‘ under control conditions. Under red (3.47 ± 0.32%) and blue shading (3.19 ± 1.73%), however, ‘Multimentha’ had higher contents in pulegone compared to ’Fränkische Blaue’.

In the autumn harvest of 2023 (Table 9), carvone remained the main compound in ’Apfelminze´ (control; 76.23 ± 5.44%), with a slight decrease under red shading (70.19 ± 6.67%). For the peppermints p-menthone also remained the main compound with higher values under shading than under control conditions, e.g., ’Multimentha´ with 49.28 ± 5.29% under control conditions and 56.02 ± 2.39% (red) and 55.56 ± 2.05% (blue) under shading. Menthol isomer B showed higher values in the autumn harvest than in the previous summer harvest, e.g., ’Fränkische Blaue´ with 33.33 ± 5.40% (control), 31.26 ± 3.92% (red), and 32.57 ± 4.15% (blue).

In spring of 2024 (Table 10), carvone in ’Apfelminze´ was slightly higher under blue shading (77.74 ± 3.46%) compared to red shading (74.81 ± 2.37%) and the control (75.56 ± 4.24%). However, ’Apfelminze´ under red shading had the highest eucalyptol/limonene value with 9.74 ± 1.01% in comparison to control (8.56 ± 1.89%) and blue shading (6.51 ± 3.49%). For the peppermints, the p-menthone values were higher under both shading colours than in the control, e.g., ’Fränkische Blaue´ with 51.23 ± 1.04% (control), 57.65 ± 4.69% (red), and 57.80 ± 2.44% (blue). The menthol isomer B value was highest for the peppermints under control conditions, e.g., ’Multimentha´ with 19.73 ± 2.76% (control), 19.16 ± 3.64% (red), and 18.71 ± 1.12% (blue).

In the summer harvest of 2024 (Table 11), there was almost no difference in carvone content in ’Apfelminze´ between the treatments. The p-menthone content was higher under shading for ’Fränkische Blaue´ and ’Multimentha´, e.g., ’Fränkische Blaue´ with 45.45 ± 3.06% (control), 58.95 ± 2.01% (red), and 58.71 ± 0.68% (blue). Pulegone was only detected in small proportions in ’Multimentha´ under control conditions (1.10 ± 0.49%) and red shading (1.00 ± 0.17%). A pulegone content of ≤1% is considered to be safe [46].

In autumn of 2024 (Table 12), the carvone content of ’Apfelminze’ was highest under blue shading with 77.50 ± 10.96% compared to red shading (72.87 ± 5.66%) and control (64.02 ± 2.06%). For the peppermints, the p-menthone content was higher under shading and the menthol isomer B content was higher under control conditions, e.g., ’Fränkische Blaue’ with 40.55 ± 1.50% (control), 30.64 ± 1.41% (red), and 30.16 ± 2.82% (blue).

These investigations indicate that red shading may enhance the biosynthesis of menthone-based compounds in ’Fränkische Blaue’ and ’Multimentha’, while blue shading slightly influences carvone contents in ’Apfelminze’. However, the effects remain moderate, suggesting that genotype and seasonal variations may have a stronger influence on EO composition than the colour of shading alone. The presence of pulegone only in the summer harvest could indicate an influence of increased radiation in the summer months or also a temperature influence. This observation corresponds with the results of previous studies, where no significant influence of 50% black, red, and blue shading nets on the EO composition of a Mentha arvensis was found. The effect of light intensity on plant growth, EO content, and EO composition was greater than the effect of spectral light quality transmitted by the shading nets [44]. Under indoor farming conditions, the modification of UV radiation could be more promising to modulate the EO content more effectively, e.g., ’Multimentha’ reacted to increased UV-B radiation with a significantly higher EO content than ’Apfelminze’ [47]. However, increased UV-B radiation may affect EO composition negatively. A study with Mentha × piperita L. found that the EO content was slightly increased by UV-B radiation, but the menthol content was significantly decreased because of increased synthesis of menthone, menthofuran, and menthyl acetate [48].

4. Conclusions

This study demonstrated that coloured shading nets had significant effects on the physiology, biomass accumulation, and EO content and composition of three Mentha genotypes: Mentha × piperita ’Multimentha’, Mentha × piperita ’Fränkische Blaue’, and Mentha rotundifolia ’Apfelminze’. This is highly relevant for farmers, supporting them in making scientifically sound management decisions; no data on this topic using widely cultivated genotypes from Germany were available prior to this study. The data indicated that red shading slightly enhanced p-menthone levels in ’Fränkische Blaue’ and ’Multimentha’, while blue shading slightly increased carvone levels in ’Apfelminze’. Furthermore, ’Fränkische Blaue’ exhibited an increase in EO content under red shading across different harvests (summer 2023 until autumn 2024), suggesting that spectral modifications can be used to optimize EO production in certain genotypes. This is especially important if certain terpenoids are in the focus of production, so farmers can optimize management strategies accordingly. Other plants of the Lamiaceae family that react to certain changes in the light spectrum and support this finding are sage and basil. One study investigated the yield, chemical composition, and antioxidant activity of sage (Salvia officinalis L.) EOs from plants grown under different-coloured shading nets and non-shaded conditions. The highest EO yield and antioxidant activity were observed in plants grown under blue nets. The results indicate that blue light shading promotes the biosynthesis of secondary metabolites in sage, enhancing both oil yield and bioactivity [49]. For the studied mints, this promotion seems to be genotype-dependent. Conclusively, the effects of coloured nets should be monitored in a crop-specific manner. Accordingly, another study investigated how different-coloured shade nets (pearl, red, and blue) affect the aroma profile and antioxidant activity of basil (Ocimum basilicum) compared to unshaded field conditions. The main aroma compounds were linalool, 1,8-cineole, and myrcene, while the highest levels of phenols, flavonoids, and antioxidant activity were found in basil grown under a red net [50].

However, despite these changes in EO content and composition, the overall effects of shading on EO of the studied Mentha genotypes were moderate, indicating that genotype and seasonal variation play a more dominant role in determining EO properties than shading alone. The delayed leaf senescence observed under shading, as indicated by VIs such as PSRI, suggests that plants may experience less stress and prolonged physiological activity. As effects of genotypic differences were more pronounced than management strategies, further research is needed in that area. Additionally, this study presents the first, broad overview on the physiology of Mentha under different shading net colours. More detailed analyses are needed to determine the interactions that occur among the measured parameters (e.g., biomass and EO content).

In conclusion, while coloured shading nets show promise in modulating EO content and composition, their effectiveness is highly dependent on genotype-specific traits and environmental conditions. For practical applications in agriculture, it is crucial to tailor shading strategies to specific Mentha genotypes and growth conditions to achieve optimal EO content and composition. Additionally, studies on the underlying molecular and biochemical mechanisms driving these genotype-specific responses would provide deeper insights into the role of light spectrum manipulation and could be the basis for the breeding of Mentha.