*2.3. Productivity*

The broad-spectrum LED system enabled a highly significant increase in leaf and stem production of fresh thyme per square meter, representing increases of 43.3% and 82.4% in comparison to the HPS and FL system, respectively. Additionally, the dry matter productions of HPS and FL were highly reduced by 43.1% and 88.6% in comparison to the LED system. Further, the LED system enabled an increase in production of VOCs per square meter under the given greenhouse conditions in comparison to the conventionally used HPS system (at *p*-value of 0.051). Both systems (LED and HPS) considerably promoted the VOC production in comparison to the FL system. Table 3 summarizes the results. Despite the lower leaf-to-shoot ratio for LED lightning of 0.9 as compared to both HPS (1.3) and FL (5, see Section 2.1), absolute LDM and overall quantity of VOCs were highest for LED. Therefore, LED lightning offers an attractive alternative for thyme cultivation, both for essential oil production and delivery to fresh market.


**Table 3.** Fresh and dry plant production as well as content of volatile fraction of thyme (*Thymus vulgaris* L.) per m<sup>2</sup> under three supplemental lighting systems.

\* LED = light-emitting diode, HPS = high-pressure sodium lamp, FL = fluorescent light. \*\* FM = total fresh matter, DM = total dry matter, VOC = total content of volatile organic compounds of total leaf dry matter. \*\*\* Presented data are means of cumulated fresh and dry matter productions of four independent spatial replications per light treatment (*n* = 4) ± *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 data are means of cumulated volatile productions in thyme leaves of four independent spatial replications per light treatment (*n* = 4) ± *SD* of 16 harvested plants per spatial replication and light treatment (*N* = 192, *n* = 64 dry leaf samples per supplemental light treatment, *n* = 16 dry leaf samples per spatial replication). Significant differences were determined according to Dunnett's T3 multiple comparisons test after Brown-Forsythe and Welch ANOVA test (*p* ≤ 0.002). Different letters within the column indicate significant differences at *p* ≤ 0.02, and bold amounts indicate a difference by trend at *p* < 0.06.
