Comparative Effects of Two Elicitors on the Essential Oil Biosynthesis of Simulated Hail-Damaged Rose Geranium (Pelargonium graveolens L'Her. cv. `BOURBON')

Extreme abiotic stress factors such as hail, defoliation, salinity, and moisture stress can affect the biosynthesis of essential oils. To mitigate this, most commercial farmers globally use agricultural crop insurance; however, this is not the case for most smallholder and emerging farmers in southern Africa, as agricultural insurance tends to be expensive when included in the production system. Therefore, this study aimed to investigate the comparative effects between two elicitors: abscisic acid (ABA) and methyl jasmonate (MeJA) on essential oil biosynthesis of simulated hail-damaged rose geranium (Pelargonium graveolens L’Hér.) as an alternative strategy to improve the essential oil biosynthesis following severe hail damage of rose geranium. The study was conducted in a 72 m2 temperature-controlled greenhouse, using a 4 x 2 factorial treatment design arranged in a randomized complete block design. Treatments comprised four levels of two elicitors at 75 μM (ABA), 150 μM (ABA), 10 mM (MeJA), and 20 mM (MeJA), applied in two different application periods (daily application for either 7 or 14 days). The simulation of hail damage was through 100% defoliation, with decapitation of the terminal buds. A significant interaction was observed between the elicitor treatment, and length of the period of application on essential oil yield (mass), where the highest yield was observed when MeJA (10 mM) was applied for seven consecutive days compared to the 14-day treatment. Geraniol esters were partially improved by the application of MeJA at 10 mM. Application of the two elicitors improved citronellol, geraniol, and linalool, regardless of the length of the period of application. The citronellol to geraniol ratio, a determining factor of essential oil quality, was 2.7:1 when MeJA was applied at 10 mM, significantly better than any application level of ABA. Therefore, it could be concluded that the application of MeJA at 10 mM may improve the essential oil biosynthesis of haildamaged rose geranium plants compared to ABA at any level, and this may be adopted as a possible mitigating strategy by the emerging farmers and growers in southern Africa for improving the biosynthesis of rose geranium post hail damage. On the other hand, where growers are interested in higher contents of linalool and citronellol, ABA may be used at any level.