Physiological and Phytochemical Responses of Lemon Balm (Melissa officinalis L.) to Pluramin Application and Inoculation with Pseudomonas fluorescens PF-135 under Water-Deficit Stress

Water-deficit stress is one of the major global challenges in plant production. Biostimulants can help to reduce the harmful impacts of water deficit and improve the growth of the medicinal and aromatic plant species. This study is aimed to examine the role of pluramin metabolic activator and Pseudomonas fluorescens PF-135 on the performance, physiological properties and secondary metabolites of lemon balm (Melissa officinalis L.) under water-deficit stress condition. For this purpose, plants were subjected to water-deficit stress, followed by foliar application of pluramin and P. fluorescens PF-135 inoculation as biostimulants. The induced stress resulted in a remarkable reduction in a number of plant characteristics including total dry matter, chlorophyll content, and relative water content (RWC). After foliar application of pluramin and inoculation with P. fluorescens PF-135, a significant improvement in these characteristics was observed. Foliar application of pluramin and P. fluorescens PF-135 inoculation resulted in higher tolerance to the water-deficit stress condition due to the increase in RWC, antioxidant enzyme activity, and reduction in malondialdehyde and hydrogen peroxide contents. Furthermore, water-deficit stress resulted in higher caffeic acid, rosmarinic acid, essential oil content, proline, and total sugar contents, and the use of pluramin and P. fluorescens PF-135 inoculation showed a reduction in these compounds contents. After the continuous application of pluramin and P. fluorescens PF-135, plants initiated defense strategies against water-deficit stress condition in most of lemon balm samples, which indicted the interaction of pluramin and P. fluorescens PF-135 under water-deficit stress.