Effect of Plant Density on Biochemical Characteristics and Essential Oil in wild Thyme (Thymbra spicata L.)

In all major genera and species in the mint family, different types of organic compounds or metabolites such as phenolic compounds, flavonoids, anthocyanins, ascorbates, and carotenoids with antioxidant, antimicrobial, and anti-inflammatory effects are produced through metabolic processes. Research studies in this field have established that planting distance can have a significant effect on essential oil yield as well as quantity and quality of secondary metabolites in medicinal plants. The main purpose of spacing between plants is also to provide a suitable mix of environmental factors (i.e. water, climate, light, and soil) to achieve optimum quality performance.

The present study was conducted using a factorial experiment based on randomized complete block design (RCBD) with three replications at Ilam University Research Farm, Iran, during 2016-2017. Planting density treatments at three levels (that is, 20, 14, and 10 plants per m2) were determined based on changes in plant spacing on rows (i.e. three distances of 20, 30, and 45 cm). Two ecotypes were accordingly included: Malekshahi and Sumar. The characteristics measured were photosynthetic pigments (chlorophyll a/b, total chlorophyll, and carotenoid), total phenol and flavonoid content, anthocyanin, as well as essential oil content and yield.

The results of this experiment revealed that the amount of antioxidant compound, photosynthetic pigments (i.e., chlorophyll a/b, total chlorophyll, and carotenoid), anthocyanin, and essential oil yield had been significantly affected by ecotype and plant density. Therefore, the highest amounts of total phenol (91.64 mg GAE/g DW) and flavonoid (5.20 mg QUE/ g DW) were observed at 20 and 30 cm row spacing. Total phenol and flavonoid also decreased by 7.2% and 12% as plant density was reduced. Moreover, row spacing of 30 cm increased the amount of chlorophyll a/b by 14 to 16% and carotenoid by 10%. The highest amount of anthocyanin (2.5 μg/g FW) was noticed at 20 cm row spacing. As well, the highest yield of the essential oil was spotted at 20 cm row spacing (91.38 kg/ha) and for Malekshahi ecotype (87.5 kg/ha). Compared with Sumar ecotype, the Malekshahi had higher content of photosynthetic pigments, total phenol and flavonoid, carotenoid, anthocyanin, as well as essential oil yield content and yield.

As plant density is enhanced to an optimum level, competition increases for absorption of water, nutrients, and quantity and quality of light received by leaves due to branching density and moderated penetration into canopy. As a result, the rate of cell division and leaf area development is reduced and the content of photosynthetic pigments is diminished following the destruction of chloroplast structure and inhibition of biosynthesis of new chlorophylls. secondary metabolites such as phenolic compounds, flavonoids, and anthocyanin are produced in the plant in response to environmental conditions in order to protect it against environmental stresses. It was observed that the content of photosynthetic pigments had diminished after increasing the density, but the content of carotenoid, anthocyanin, total phenol and flavonoid, as well as essential oil yield had improved in the wild thyme.