PGPR enhancement chlorophyll content, trichome density, stomatal density and monoterpene accumulation in peppermint (Mentha x piperita L).

CAPPELLARI, L., SANTORO, M., GIORDANO, W., BANCHIO, E.

La falda

Workshop; II Workshop latinoamericano sobre rizobacterias promotoras del desarrollo vegetal.; 2014

Introduction Peppermint (Mentha x piperita L.; family Lamiaceae) is one of the most important crops cultivated worldwide for fresh or dried leaves, and essential oils (EOs). EOs are used extensively as aromatic, pharmaceuticals, and antimicrobial agents, as well as eco-friendly pesticides. The EOs are produced exclusively in epidermal structures known as glandular hairs. Two types of glandular hairs, termed peltate hairs and capitate hairs, are distinguished primarily by the volume of secretion. Numerous species of bacteria, most of which are associated with the plant rhizosphere, have shown beneficial effects on plant growth, yield and quality of crops. Such bacteria are collectively called ?plant growth-promoting rhizobacteria? (PGPR). Materials and methods M. piperita young shoots from Traslasierra Valley (Córdoba province, Argentina) were transferred to test tubes containing sterile distilled water and rooting hormone (alpha naphthalene acetic acid). M. piperita plantlets were transferred to plastic pots containing sterile vermiculite. After 7 days the plants were inoculated with PGPR strains Bacillus subtilis GB03, Pseudomonas fluorescens WCS417r, P. putida SJ04, and co-inoculated with a combination of P. fluorescens WCS417r and P. putida SJ04. Thirty days after inoculation, we measured chlorophyll content, trichome and stomatal density, and levels of monoterpene. Results The treated plants showed increases in all parameters evaluated. Leaf chlorophyll a, chlorophyll b, and total chlorophyll concentrations were significantly higher than the control for each treatment of inoculation and co-inoculation. GB03 displayed the highest values in each type of chlorophylls analyzed. Stomatal density was ~50% higher in all four treatment groups than in the control group, and the value was slightly higher for the WCS417r + SJ04 group than the other three treatment. The density of peltate trichomes on both surfaces was much higher for the treatment groups than the control group . Differences in peltate trichome density among the four treatment groups were not significant. GC analysis of EOs revealed variations in total yield content and relative percentage (R%) of the major monoterpenes. Total EOs content of co-inoculated plants was 6-fold higher than the control. Singly-inoculated plants showed intermediated values. Amounts of the main EO components [(+) pulegone, (-) menthone, (-) menthol, 1,8-cineole, and linalool] were higher in all treatment than in the control. The R% of linalool was elevated plants inoculated with GB03 and R% of (+)-pulegone was decreased in plants inoculated with WCS417r respect to control. Conclusion In the present study, single inoculation with WCS417r or SJO4 or GB03 affected plant growth and development, trichome density, and chlorophyll content to a similar degree. Co-inoculation with WCS417r and SJO4 produced the greatest increases in stomatal density and levels of secondary metabolites, indicating a synergistic effect on monoterpene biosynthesis.