The Effect of Vermicompost and PGPR on Physiological Traits of Lemon Balm (Melissa officinalis L.) Plant under Drought Stress

In this research, effect of vermicompost and plant growth promoting rhizobacteria (PGPR) on physiological traits of lemon balm under drought stress condition was investigated in year 2012 at Karaj Research Institute of Forest and Rangelands. Experiment was conducted as split-split plot in a randomized complete block design with three replications. irrigation (normal: irrigated to field capacity;  drought stress: watered to 60% of field capacity) was considered as main plot, vermicompost fertilizer (consuming 0, 5 and 10 t ha-1) as split plot and biofertilizers (Pseudomonas fluorescent, Azotobacter chrococum+Azospirillm brasilense, Azotobacter+ Azospirillum + Pseudomonas and no fertilizer) as split-split plot. Results showed that water stress significantly increased essential oil content, while decreased essential oil yield, total chlorophyll, cell membrane stability, relative water content and proline. Only relative water content and total chlorophyll were positively affected by biofertilizer application. Essential oil content, essential oil yield, total chlorophyll, cell membrane stability, relative water content and proline were significantly increased by vermicompost application, especially, relative water content increased with an increase in amount of vermicompost application, but there were strong interactions with irrigation. These interactions are important in practical use of vermicompost and biofertilizers.