Effects of high temperatures on leaf anatomy and phytochemistry in Lippia javanica (Verbenaceae)

Singini, Edith J
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Lippia javanica (Burm.F.) Spreng is a medicinal plant used in various rural communities as a cheaper, safer and more desirable alternative for treating various ailments. In Southern African countries such as South Africa, Botswana and Zimbabwe, L. javanica is sold in local markets and informal sectors. The shrub contains phytochemicals that are responsible for a wide range of medicinal properties, which are profoundly affected by environmental stresses. High temperatures significantly affect plant growth and production, and it was predicted that South Africa would experience increased heat due to climate change. Therefore, the aim of this study was to assess the potential impact of high temperatures (47/37ºC-day/night simulation) on the leaf morphology, histology, essential oil yield and composition, and the total phenolic and flavonoid content in L. javanica. Mature plants of L. javanica were exposed to 47/37ºC episodically for 48, 96 and 144 hours in a climate test incubator, while control samples were kept at 25/20ºC in the greenhouse. The histological assessment was conducted using the glutaraldehyde-osminium method, while essential oils were analysed using the Gas Chromatography–Mass Spectrometry (GC-MS) and the total phenolic and flavonoid content were estimated using the UV-visible spectrophotometer. Control samples showed 144.75 mg GAE/g (phenolics) and 81.17 mg QE/g (flavonoids). The exposure to high temperatures resulted in an increase in total phenolics from 191.33 mg GAE/g (48 hours) up to 441.94 mg GAE/g (144 hours) (F (7,40) = 1670; P < 0.001). Similarly, the total flavonoid content increased from 115.91 mg QE/g (48 hours) up to 268.66 mg QE/g (144 hours). The essential oil percentage yield increased significantly from 48 hours (3.2%) to 144 hours (6.3%) compared with the control (1.2%) (F (3, 20) = 16.31; P < 0.0001), and the grouped essential oil components also showed an increasing trend. The histological assessment showed that the leaf midrib and blade had thickened. Trichome height (F (7, 162) = 32.09, P < 0.001) and diameter (F (7, 161) = 3.56, P < 0.01) also increased significantly from 48 hours to 144 hours, which possibly correlates with the increased phytochemicals. The present data suggest that L. javanica has possible mechanisms to adapt under high temperatures, which was further indicated by the growth of new shoots when treated samples were returned to the greenhouse (25/20ºC). L. javanica’s response to high temperatures may positively impact its medicinal properties and, consequently, the range of ailments that it treats
This research project is submitted in fulfilment of the requirements for the degree of Master of Science, School of Animal, Plant and Environmental Sciences, Faculty of Science, University of the Witwatersrand, Johannesburg, 2020