*2.1. Biomass Yield, Partitioning, and Morphology*

The LED system resulted in distinct yield increases, biomass partitioning, and a differentiated morphological appearance of *Thymus vulgaris* L. in comparison to the HPS and FL systems (Figures 1 and 2). While the LEDs produced a fresh biomass of averagely 28.1 <sup>±</sup> 2.0 g plant−<sup>1</sup> , the HPS systems accounted for a fresh biomass of 15.9 <sup>±</sup> 2.3 g plant−<sup>1</sup> within the same cultivation period. The lowest fresh biomass of 4.9 <sup>±</sup> 0.4 g plant−<sup>1</sup> was produced under FL (Figure 1A). Accordingly, dry matter yields of *Thymus vulgaris* L. were significantly enhanced by the LED system (5.6 <sup>±</sup> 0.8 g plant−<sup>1</sup> ) in comparison to HPS (3.2 <sup>±</sup> 0.5 g plant−<sup>1</sup> ) and FL (0.6 <sup>±</sup> 0.1 g plant−<sup>1</sup> ), representing an increase of 1.75- and eight-fold, respectively (Figure 1B). Thereby, the weight proportion of dry leaves did not differ from the (mostly lignified) weight proportion of stems in thyme plants cultivated under the LED and HPS systems, respectively. Under FL, however, the majority of dry yield consisted of leaves (83.3%) and only 16.7% consisted of (unwooded) shoots (Figure 1C). With a Pearson correlation coefficient of r = 0.97 and R<sup>2</sup> = 0.95 (*p* < 0.001), dry mass yields under the differing supplemental lighting systems were highly related to the individual daily light integrals (DLI). *Plants* **2021**, *10*, x FOR PEER REVIEW 4 of 17

**Figure 1.** Biomass yields and partitioning of *Thymus vulgaris* L. cultivated under different supplemental lighting systems during **Figure 1.** Biomass yields and partitioning of *Thymus vulgaris* L. cultivated under different supplemental lighting systems during fall and winter in Berlin, Germany. LED = light-emitting diode, HPS = high-pressure sodium lamp, FL = fluorescent

fall and winter in Berlin, Germany. LED = light-emitting diode, HPS = high-pressure sodium lamp, FL = fluorescent light. A = Fresh matter yields in gram per plant\*, B = Dry matter yields in gram per plant\*, C = Leaf and shoot dry matter partitioning in gram per plant\*\*. \*Presented are mean plant yields of four independent spatial replications per light treatment (*n* = 4) ± standard deviation (SD) of 32 harvested plants per spatial replication and light treatment (N = 384, *n* = 128 plants per supplemental light treatment, *n* = 32 plants per spatial replication). Significant differences (*p* ≤ 0.01) were determined according to Dunnett's T3 multiple comparisons test after Brown-Forsythe and Welch ANOVA test (*p* ≤ 0.001). Different letters indicate significant differ-

mental light treatment, *n* = 16 plants per spatial replication). Significant differences (*p* ≤ 0.05) were determined according to Dunnett's T3 multiple comparisons test after Brown-Forsythe and Welch ANOVA test (*p* ≤ 0.001). Different letters indicate

**Figure 2.** Visual appearance of *Thymus vulgaris* L. at harvest cultivated under different supplemental lighting systems during fall and winter of Berlin, Germany. LED = light-emitting diode, HPS = high-pressure sodium lamp, FL = fluorescent

> The reason for the outstanding biomass accumulations and the concomitant rapid thyme development under the LED system is clearly found in the heightened DLI between 400 and 700 nm, as shown by the correlation coefficient of r = 0.97 (R2 = 0.95). That increasing DLIs accelerate the development and growth of plants up to a certain point is well established [45,46]. The correlation of DLIs and plant growth is known to be linear between each species-specific light compensation point and light saturation point [7].

significant differences.

light.

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matter

(g

plant

)


0

10

significant differences.

20

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<sup>a</sup> <sup>b</sup> <sup>c</sup> **<sup>A</sup>**

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LED HPS FL

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4.9

light. (**A**) Fresh matter yields in gram per plant \*, (**B**) Dry matter yields in gram per plant \*, (**C**) Leaf and shoot dry matter partitioning in gram per plant \*\*. \* Presented are mean plant yields of four independent spatial replications per light treatment (*n* = 4) ± standard deviation (SD) of 32 harvested plants per spatial replication and light treatment (N = 384, *n* = 128 plants per supplemental light treatment, *n* = 32 plants per spatial replication). Significant differences (*p* ≤ 0.01) were determined according to Dunnett's T3 multiple comparisons test after Brown-Forsythe and Welch ANOVA test (*p* ≤ 0.001). Different letters indicate significant differences. \*\* Presented are mean dry leaf and shoot matter yields of four independent spatial replications per light treatment (*n* = 4) ± SD (standard deviation) of 16 harvested plants per spatial replication and light treatment (N = 192, *n* = 64 plants per supplemental light treatment, *n* = 16 plants per spatial replication). Significant differences (*p* ≤ 0.05) were determined according to Dunnett's T3 multiple comparisons test after Brown-Forsythe and Welch ANOVA test (*p* ≤ 0.001). Different letters indicate significant differences. **Figure 1.** Biomass yields and partitioning of *Thymus vulgaris* L. cultivated under different supplemental lighting systems during fall and winter in Berlin, Germany. LED = light-emitting diode, HPS = high-pressure sodium lamp, FL = fluorescent light. A = Fresh matter yields in gram per plant\*, B = Dry matter yields in gram per plant\*, C = Leaf and shoot dry matter partitioning in gram per plant\*\*. \*Presented are mean plant yields of four independent spatial replications per light treatment (*n* = 4) ± standard deviation (SD) of 32 harvested plants per spatial replication and light treatment (N = 384, *n* = 128 plants per supplemental light treatment, *n* = 32 plants per spatial replication). Significant differences (*p* ≤ 0.01) were determined according to Dunnett's T3 multiple comparisons test after Brown-Forsythe and Welch ANOVA test (*p* ≤ 0.001). Different letters indicate significant differences. \*\*Presented are mean dry leaf and shoot matter yields of four independent spatial replications per light treatment (*n* = 4) ± SD (standard deviation) of 16 harvested plants per spatial replication and light treatment (N = 192, n = 64 plants per supplemental light treatment, *n* = 16 plants per spatial replication). Significant differences (*p* ≤ 0.05) were determined according to Dunnett's T3 multiple comparisons test after Brown-Forsythe and Welch ANOVA test (*p* ≤ 0.001). Different letters indicate

LED HPS FL

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8

*Plants* **2021**, *10*, x FOR PEER REVIEW 4 of 17

**Figure 2.** Visual appearance of *Thymus vulgaris* L. at harvest cultivated under different supplemental lighting systems during fall and winter of Berlin, Germany. LED = light-emitting diode, HPS = high-pressure sodium lamp, FL = fluorescent light. **Figure 2.** Visual appearance of *Thymus vulgaris* L. at harvest cultivated under different supplemental lighting systems during fall and winter of Berlin, Germany. LED = light-emitting diode, HPS = high-pressure sodium lamp, FL = fluorescent light.

The reason for the outstanding biomass accumulations and the concomitant rapid thyme development under the LED system is clearly found in the heightened DLI between 400 and 700 nm, as shown by the correlation coefficient of r = 0.97 (R2 = 0.95). That increasing DLIs accelerate the development and growth of plants up to a certain point is well established [45,46]. The correlation of DLIs and plant growth is known to be linear between each species-specific light compensation point and light saturation point [7]. As indicated in Figure 2, the stem biomass of *Thymus vulgaris* L. was greatly increased under the LED system at the end of the experimental period and led to a profoundly different visual appearance in comparison to thyme plants grown under the other two supplemental lighting fixtures. Despite the lowest corresponding leaf-to-shoot ratio, which was 0.9 for LED, 1.3 for HPS, and 5 for FL, the leaf dry matter (LDM) of thyme was significantly increased and highest under LED (Figure 1C).

The reason for the outstanding biomass accumulations and the concomitant rapid thyme development under the LED system is clearly found in the heightened DLI between 400 and 700 nm, as shown by the correlation coefficient of r = 0.97 (R<sup>2</sup> = 0.95). That increasing DLIs accelerate the development and growth of plants up to a certain point is well established [45,46]. The correlation of DLIs and plant growth is known to be linear between each species-specific light compensation point and light saturation point [7].

Faust stated that optimal DLIs vary from 6 to 50 mol m−<sup>2</sup> d −1 for various crops, and moderately light-dependent thyme requires a DLI of at least 18 mol m−<sup>2</sup> d −1 [46]. The natural average DLI in greenhouses during winter in northern latitudes however is often as low as 1 to 5 mol m−<sup>2</sup> d <sup>−</sup><sup>1</sup> and reached approximately 3.9 mol m−<sup>2</sup> d <sup>−</sup><sup>1</sup> during our greenhouse trial [3–5]. Hence, supplemental lighting is essential for winter greenhouse productions. Since FLs raised the total DLI (natural DLI 3.9 mol m−<sup>2</sup> d <sup>−</sup><sup>1</sup> + supplemental DLI 3 mol m−<sup>2</sup> d −1 ) only to approximately 7 mol m−<sup>2</sup> d <sup>−</sup><sup>1</sup> during winter production, the FLs are neither suitable for the production of thyme nor presumably for the majority

of greenhouse crops under the given cultivation conditions. HPS elevated the total DLI (natural DLI 3.9 mol m−<sup>2</sup> d <sup>−</sup><sup>1</sup> + supplemental DLI 7 mol m−<sup>2</sup> d −1 ) to an estimated level of 11 mol m−<sup>2</sup> d −1 . Therewith, the biomass accumulation of *Thymus vulgaris* L. increased significantly in comparison to FL; however, the DLI remains insufficient for an optimal thyme production during winter. With a total DLI of approximately 16 mol m−<sup>2</sup> d −1 (natural DLI 3.9 mol m−<sup>2</sup> d <sup>−</sup><sup>1</sup> + supplemental DLI 11 mol m−<sup>2</sup> d −1 ) during the low light season, the tested LED system achieved the highest DLI and approached the recommended DLI of <sup>≥</sup> 18 mol m−<sup>2</sup> <sup>d</sup> −1 the most (Table 1, Section 3. Material and Methods). Further, only the LED system would be able to achieve the recommended DLI of the moderately light-dependent thyme by simply extending the photoperiod from 14 to 16 h per day during winter.

**Table 1.** Spectral composition of the supplemental lighting fixtures used in the greenhouse for the cultivation of thyme (*Thymus vulgaris* L.).


\* *PPFD* = photosynthetic photon flux density, *PFD* = photon flux density, *R/FR* ratio = red to far-red ratio, DLI = daily light integral. \*\* LED = light-emitting diode, HPS = high-pressure sodium lamp, FL = fluorescent light. \*\*\* Values represent percentages of total *PFD*. ‡ R/FR ratio is based on the absorption maxima of phytochromes at 660 and 730 nm [47]. ± DLI based on *PPFD/PFD*.
