Using Agricultural Plant Growth Regulators As A Mitigating Strategy For Hail-Damaged Rose Geranium (Pelargonium graveolens L’Hér.)

Abstract

Hailstorms cause significant yield and financial losses to the agricultural sector. There are reports that hail can cause severe damage to rose geranium (Pelargonium graveolens spp.) plants. There are currently no mechanisms in place to alleviate hail-damaged essential oil plants, such as rose geranium. This study aims to develop a strategy for hail damage, using agricultural plant growth regulators (PGRs) as an alleviating remedies for hail-damaged rose geranium. The first experiment was conducted using combined agricultural PGRs (cytokinin [CK] + gibberellic acid [GA]) during the 2016/17 and 2017/18 growing seasons. Treatments comprised of three simulated hail damage levels: zero% defoliation (intact plants); 50%; and 100% defoliation, as well as two levels of PGRs: CK 0.32 mg/L + GA 150 mg/L (low); CK 0.64 mg/L + GA 300 mg/L (high); and distilled water as a control. The second experiment was run simultaneously, using four experimental treatments of abscisic acid (ABA) and methyl jasmonate (MeJA): 75 μM ABA; 150 μM ABA; 10 mM MeJA; 20 mM MeJA; and distilled water as a control. In this experiment, the treatment was applied daily for either seven or 14 days, and the hail damage simulation was 100% defoliation only. Both experiments had a factorial treatment design, laid out in a randomised complete block design with three replications. In all experiments, the herbage yield attributes, essential oil yield parameters, leaf trichomes, and essential oil quality were measured. As hypothesized, 100% simulated hail damage negatively affected the herbage yield in the first experiment, indicating that severely hail-damaged plants require attention to facilitate their recovery. The effect of simulated hail damage, and the subsequent application of CK + GA, did not provide conclusive results on the recovery of the herbage yield attributes (plant parameters), and essential oil yield parameters. However, citronellol content increased by 4.6% when a low concentration of combined agricultural PGRs was applied to plants subjected to 100% simulated hail damage. A principal component analysis (PCA) was then applied to provide an overview of the effects on the plant and essential oil yield and quality parameters. The PCA showed a strong association between plant height, number of branches, the branch to height ratio (B:H ratio) and foliar fresh mass (FFM). The biplot loading explained that plant height, number of branches, B:H ratio and FFM of the plants which sustained only 50% foliage loss could be characterised by all treatments, including distilled water in both seasons. Brevicollate trichome densities declined when the high concentration of combined agricultural PGRs was applied. In the second experiment, 75 μM ABA and 10 mM MeJA affected the biosynthesis of linalool, isomenthone, citronellol, geraniol, and the citronellol to geraniol ratio (C:G ratio). However, the variation in these essential oil compounds were inconsistent between the treatments, and did not improve the overall oil quality. Leaf area decreased by 62% following the application of 20 mM MeJA for 14 days, during the 2016/17 growing season. This significant variation was not observed in the leaf area during the following season; however, variations between growing seasons showed a similar trend. Overall, the application of ABA and MeJA had an adverse effect on the herbage and essential oil yield. The results from the two repeated experiments did not solve the hail damage as espoused. Therefore, an additional experiment was conducted during the 2019 growing season to determine the effects of agricultural PGRs containing GA; brassinosteroids (BRs); and trace CKs on the recovery of three levels (low, medium and high), and a control of simulated hail-damaged rose geranium plants (as before). As expected, the mean number of branches per metre (B:H ratio) and mean FFM declined with increased simulated hail damage. Plants were less bushy (B:H ratio) with smaller leaves (leaf area) at 100% simulated hail damage compared to 50% simulated hail damage. It was also found that 100% defoliated rose geranium plants accumulate most of the phenolic compounds associated with defoliation and wounding, while those with only 50% simulated hail damage accumulate phenolic compounds to a level similar to intact plants. Overall, the highest mean FFM was recorded when the highest concentration of PGRs were applied. In addition, brevicollate trichome densities were higher when the PGR-mixture was applied to both leaf surfaces. Rose geranium plants that suffered 50% simulated hail damage had higher mean essential oil yield compared to plants that suffered 100% simulated hail damage and treated with medium PGR. High-quality essential oil, with a C:G ratio below three, was recorded on all defoliated plants compared to the intact plants. The PCA (71.34%) corroborated the use of low PGRs on rose geranium plants that lost 100% foliage, since the N, P, K, Mg, and Mn were higher at this level. It is therefore recommended to consider applying a low to medium concentration of GA, BRs and traces of CKs to improve, and recover the growth of the herbage and essential oil quality (C:G ratio <3) of rose geranium plants with 100% foliage loss from hail damage. Rose geranium plants are grown for essential oil production; therefore, the aerial herbage material is a crucial component of the crop. It has been shown from this study that hail can causes significant damage to these plants, reducing this valuable material, and directly affecting the essential oil yield and quality. This study contributed to the development of a mitigation strategy for hail-damaged rose geranium plants using agricultural PGRs. The study only focused on the use of existing agricultural PGRs, foliar-applied agricultural PGRs, the in vivo study approaches, and the application frequencies. It may be beneficial to focus on root-applied approach of the PGRs, in vitro studies, and test other sources of biostimulants (containing PGRs) for future studies.