Efeitos da irradiância sobre a fotossíntese, crescimento e produção de óleo essencial em poliplóides de Lippa alba (mill.) N.e.Brown (Verbenaceae)

Abstract

Lippia alba (Mill.) N.E. Brown (Verbenaceae) is one of the most widely used medicinal species in Brazil. In the last decades, a great number of works have been investigating and proving the pharmacological effects of the components found in the essential oil of this plant. Studies related to genomics demonstrate the occurrence of natural polyploidy in the species, a condition that may interfere in the composition and production of these substances. In contrast, environmental factors, such as irradiance and water availability, are also important modellers of the biosynthesis of these molecules, and may interact with the genetic factor. Considering the above, the effects of contrasting levels of irradiance, at 100% (full sun) and 30% (in the shade), during the dry and rainy seasons of cultivation, on the gas exchanges and their subsequent effects on growth were investigated and the production of essential oil in L. alba accesses that had three distinct levels of ploidy (Di-, Tetra- and Hexaploid). For most gaseous exchange parameters, shade-grown plants presented lower values when compared to plants grown in full sun, regardless of the growing season. In both levels of irradiance, tetraploid access presented the lowest values for liquid photosynthesis, crude, photorespiration, stomatal conductance and electron transport rate for carboxylation and oxygenation. Shade - grown accesses presented responses typically associated to this luminosity condition, with a greater specific and unitary leaf area, preferential allocation of biomass in aerial part and higher content of chlorophylls, as well as lower values in root biomass, total mass and net assimilation rate. Compared to those grown in the full sun, all accesses cultivated in the shade had an essential oil content, on average, 40% lower. When cultivated in full sun the diploid access had the highest essential oil content, which was on average 150% higher than that observed for the tetra- and hexaploid accessions. In the shade, the essential oil content of the diploid access was 120% greater than others accessions. However, in this condition, this access presented the lowest total biomass. The results show that diploid access, irrespective of the irradiance in which it was cultivated, presents the highest yield of essential oil production, suggesting higher activity of the secondary metabolism pathways, reducing the accumulation of biomass under low irradiance.