Assessing the effect of extreme temperature conditions on the morphology, anatomy and phytochemistry of Moringa Oleifera leaves

Abstract

Since its discovery in India many centuries ago, Moringa oleifera has been used largely in the tropics and subtropics where it is native for nutritional, medicinal and industrial benefits. These benefits are attributed to the antioxidant, antifungal and anti-inflammatory properties in the phytochemical compounds of M. oleifera. Agriculture of M. oleifera in South Africa is currently minimal despite its multiple benefits. Naturalisation of M. oleifera in the country is important in the aid to eliminate the shortage of nutritional food reserves, and utilise the plant as a source for realising and developing new medicinal compounds. Global warming and consequent climate change have caused extreme high and low temperature seasons in the past in South Africa. Studying the effects of extreme high and low temperatures on the growth, performance and phytochemistry of M. oleifera at controlled conditions is vital in understanding its responses and adaptability.

M. oleifera grown for three months in a greenhouse of the University of the Witwatersrand was subjected to two extreme temperature conditions of 15/10°C and 47/29°C for seven days (144 hours). The control was kept at ambient temperature (25/22°C). The morphology, histology, total phenolic contents, total flavonoid contents, quercetin and kaempferol concentrations were assessed using dissecting microscope, light electron microscope, UV-Vis spectroscopy and HPLC-UV respectively. The effect of high temperatures showed increased senescence. However, plants that were treated with low temperature appeared to be more tolerant by indicating a delay in leaf senescence. Yellowing, browning, curling and drying of the leaves worsened after 144 hours at high temperature treatment. The histological examination at high temperatures showed intact cellular structures such as upper epidermis, lower epidermis, spongy mesophyll, palisade mesophyll and intercellular spaces in green leaves after 24 hours. However, leaf size decreased over the seven days treatment. The cellular structures in leaves that became yellow had collapsed and showed evidence of necrosis. In the low temperature treatment, 1% of leaves showed signs of leaf curling after VII

48 hours while others remained green and intact. After 144 hours, 5% of leaves showed signs of senescence while others turned dark green and remained intact. Leaves cells such as palisade mesophyll had elongated and thus increased the thickness of the leaves. This was evidence of acclimation to cold temperatures.

The total phenolic content increased by 10.1% after 48 hours under high temperatures. However, a decrease of 3.5% and 3.1% was observed after 96 and 144 hours respectively. Quercetin increased by 8.5% after 48 hours then after 96 and 144 hours a decrease of 41.7% and 61.6% was observed respectively. A significant increase in kaempferol concentration of 203.5% after 48 hours was noted. This was followed by a decline after 96 and 144 hours of 43.0% and 24.8% respectively. Concentration of total flavonoid content increased by 11.6% and 33.8% after 48 and 96 hours, and after 144 hours, a decline of 0.4% was observed.

The combined evidence from the present study showed that the three months old M. oleifera plants are sensitive and cannot withstand temperatures as high as 47/29°C. However, the species has showed to be positively more tolerant to cold temperatures (15/10°C). This evidence may have meaningful contribution towards the industry, Moringa oleifera farming practices as well as advance the understanding of the effects of extreme temperatures on the species.