Influence of Drought Stress on Growth and Essential Oil Yield of Ocimum Species

Comprehensive evaluation of drought stress on medicinal plants: a meta-analysis

Drought stress significantly affects plants by altering their physiological and biochemical processes, which can severely limit their growth and development. Similarly, drought has severe negative effects on medicinal plants, which are essential for healthcare. The effects are particularly significant in areas that rely mostly on traditional medicine, which might potentially jeopardize both global health and local economies. Understanding effects of droughts on medicinal plants is essential for developing strategies to enhance plant adaptability to drought stress, which is vital for sustaining agricultural productivity under changing climatic conditions. In this study, a meta-analysis was conducted on 27 studies examining various parameters such as plant yield, chlorophyll content, relative water content, essential oil content, essential oil yield, non-enzymatic antioxidants, enzymatic antioxidants, phenols, flavonoids, and proline content. The analysis explored the effects of drought across different stress conditions (control, moderate, and severe) to gain deeper insights into the drought's impact. The categorization of these stress conditions was based on field or soil capacity: control (100-80%), moderate (80-50%), and severe (below 50%). This classification was guided by the authors' descriptions in their studies. According to meta-analysis results, enzymatic antioxidants emerge as the most responsive parameters to stress. Other parameters such as relative water content (RWC) and yield also exhibit considerable negative mean effect sizes under all three stress conditions. Therefore, when evaluating the impacts of drought stress on medicinal plants, it is beneficial to include these three parameters (enzymatic antioxidants, RWC, and yield) in an evaluation of drought stress. The chlorophyll content has been determined not to be a reliable indicator for measuring impact of drought stress. Also, measuring antioxidants such as flavonoids and phenols could be a better option than using radical scavenging methods like DPPH (2, 2-difenil-1-pikrilhidrazil), FRAP (ferric reducing antioxidant power), and ABTS (2, 2'-Azino-bis (3-ethylbenzothiazoline-6-sulfonic acid)).

Evaluation of basil (Ocimum basilicum) accessions under different drought conditions based on yield and physio-biochemical traits

BACKGROUND

Basil is one of the most famous herbs, which has broad usage as a fresh vegetable and therapeutic and pharmaceutical services. The main abiotic stress limiting basil production globally is drought. As a result, appropriate drought screening-which effectively separates high-yielding but drought-sensitive genotypes from drought-tolerant genotypes-is necessary for the optimal selection of high-yielding basil cultivars under drought stress conditions. So, a split plot experiment with three replications based on a completely randomized design were carried out in a pot under field conditions for this investigation. Water levels (full irrigation or control, moderate stress, and severe stress) were assigned as main plots, while 22 basil accessions were given as sub-plots. In this study, leaf yield as well as physio-biochemical traits had measured on accessions.

RESULTS

Our results revealed large variation in yield, essential oil (%), protein, proline, chlorophyll, total phenol and flavonoids traits across the 22 accessions. The percentage of leaf yield reduction in moderate drought stress than normal conditions showed that G1 (-6.5%), G17 (-7.05%), G20 (-9.01%), and G12 (-10.9%) accessions had the least changes, respectively. Although in severe drought stress than normal conditions, the G1 (-32.01%), G12 (-33.12%), G4 (-33.24%), G7 (-34.11%), and G17 (-34.93%) accessions had the least amount of change in plant leaf yield, respectively. Furthermore, the highest yield reduction occurred in moderate and severe stress conditions in G18 (-25.36%) and G8 (-42.98%) accessions, respectively. Cluster analysis based on the ward method in both conditions (moderate and severe drought conditions) placed the accessions in three groups, and accessions were identified as tolerant, whose average traits in that group were higher than the total average. The principal component analysis also showed that in moderate drought conditions, the first two components explained about 95.28% of the total variation, while in severe drought conditions, these two components explained about 96.37% of the total variation.

CONCLUSIONS

The different multivariate analyses (cluster analysis, PCA, mean comparison) were used to identify tolerant and sensitive accessions based on all traits. The accessions G3, G4, G6, and G7 were found to be tolerant to stress, while G10, G15, G16, and G20 were found to be sensitive to drought. These accessions are a useful step in producing drought-tolerant, high-yielding accessions and can be utilized in breeding programs for basil.

Morphological and biochemical responses of selected Ocimum species under drought

Introduction: Drought stress is known to change biochemical activities resulting in altered plant growth and development.

Objective: Due to the shortage of research evidence in a certain taxa of basil, it is imperative to examine the relationship between drought stress and morphological changes as well as essential oil accumulation.

Methods: An open field experiment was conducted in 2020 to determine the effects of drought stress on the morphological and biochemical responses of selected Ocimum species. Five basil species, namely O. basilicum ‘Genovese’, O. basilicum ‘Ohre’, O. × africanum, O. americanum, O. selloi, and O. sanctum ‘Krishna’ were grown under irrigated (control) and non-irrigated (drought stress treatment) plots with two block replications.

Results: The result showed that irrigation had a positive effect on production. Consequently, fresh and dry biomass yields were higher on irrigated plots by 39% and 33%, respectively. No significant changes were detected between the treatments in essential oils (EO) yield and EO composition of major compounds. However, a slight increase in camphor (O. × africanum), nerol (O. americanum), and trans-β-caryophyllene (O. sanctum ‘Krishna’) ratios were observed under irrigation treatments. On the other hand, drought stress increased EO content, polyphenol content, and antioxidant capacity. Furthermore, drought stress had a positive effect on the ratios of 1,8-cineole (O. × africanum), and eugenol (O. sanctum ‘Krishna’). There is only a slight increase in EO content under drought stress, which is not comparable to the significant yield reduction recorded. Morphological and biochemical variations were also detected among basil species. Accordingly, higher biomass (616.33 g/plant) and EO yield (3.72 ml/100 g) among the species were obtained from O. basilicum ‘Genovese’ and O. × africanum, respectively.

Conclusion: Irrigation is imperative for maximizing biomass and EO yield of tested basil species.