Biostimulants improve the leaf micro-morphology and essential oil biosynthesis of simulated hail-damaged Pelargonium graveolens (L’Hér.)

This study aimed to determine the ability of exogenous applications of biostimulants to improve the leaf micromorphology, and the essential oil yield in rose geranium (Pelargonium graveolens L’Hér.) exposed to simulated hail damage. The experiment was carried out in a 3×4 factorial treatment design, laid out in a randomized complete block design. The treatments consisted of three hail damage simulation levels (0 (non damaged plants), 50, and 100% leaf defoliation) and four biostimulant mixtures (level 1 was the control (consisting of distilled water), level 2 had gibberellic acid (GA) 1.26 mg kg^‑1, brassinosteroids (BR) 0.51 mg kg^‑1, and traces of cytokinins (CKs) 0.025 mg kg^‑1; level 3 had GA 2.55 mg kg^‑1, BR-1.02 mg kg^‑1, and traces of CKs 0.05 mg kg^‑1; level 4 had GA 3.83 mg kg^‑1, BR 1.53 mg kg^‑1, and traces of CKs 0.075 mg kg^‑1). Analysis of variance and multivariate data analysis showed that the larger the stomatal area, the higher the densities of the stomata and trichome become on non-damaged plants treated with level 3 concentration of biostimulants. The trichome density significantly (P<0.001) increased in plants that endured 50% simulated hail damage and were treated with level 3 of biostimulant. The findings of this study revealed that the application of biostimulants from level 2 to level 3 concentration might increase the essential oil yield on non-damaged plants, and those endured at least up to 50% simulated hail damage. Moreover, it can be concluded that the essential oil yield is directly affected by the stomatal area and the variations between the stomatal and trichome densities. Rose geranium plants that endure up to 50% of hail damage, and are treated with a level 3 concentration of biostimulant may recover the essential oil yield at harvest.