Essential oil yield and composition of rose-scented geranium (Pelargonium sp.) as influenced by harvesting frequency and plant shoot age

by Motsa, Nozipho M.

Rose-scented geranium is a world known essential oil plant priced for its high grade essential oil mostly utilized in the perfumery industry. However, South Africa has just joined the geranium oil production industry and still battling with the variations in oil yield and oil composition. It is not clear when and at what growth stage the herbage should be harvested or the stage at which it can produce the greatest essential oil. Another controversy that surrounds the production of geranium essential oil is the variation in oil composition which is speculated to be influenced by environmental factors. This study was carried out to investigate the effect of plant shoot age, temperature, seasonal changes and harvesting frequencies on herbage yield, oil yield and essential oil composition of rose-scented geranium (Pelargonium species, cultivar Bourbon). The response of herbage yield, oil yield and essential oil composition to plant shoot age and temperature effects was conducted in the field. Rose-scented geranium was harvested every week for eighteen weeks per experimental cycle. Irrigation, fertilizer application and general crop management was conducted according to the crop’s requirements. Measurements were made on fresh mass yield which was later computed into herbage yield, oil yield and oil composition. Oil content was generally higher (0.088%) in the spring/summer harvested plants than those harvested in autumn/winter (0.064%) cycle. Decreases in oil content in the xi autumn/winter cycle were negatively related to decreases in night temperatures. Greater herbage yields resulted to higher oil yields. Geraniol concentration decreased with plant shoot age in both experimental cycles and with decreases in night (minimum) temperatures in the autumn/winter experiment. The double decrease effect on geraniol concentrations in the autumn/winter experiment resulted into very high C: G ratios. Citronellol concentration was not affected by plant shoot age but was affected by decreasing night temperatures in the autumn/winter harvesting cycle. Similarly, the response of rose-scented geranium’s herbage yield, oil yield and oil composition to harvesting frequency was studied in the field. The treatments were harvesting after every two, three and four months. Standard crop management was also conducted to suit the crop requirement. Herbage yield, oil yield and oil composition were measured at each harvest. Seasonal effects on the measured variables were recorded. Harvesting frequency had a highly significant effect (P > 0.01) on herbage yield and oil yield. Closer harvesting intervals (after two months) produced less herbage yield and oil yield while longer harvesting intervals produced more herbage yield and oil yield. Harvesting at two month intervals improved the oil composition of rose-scented geranium to lower C: G ratios but did not improve oil yield. The season of harvest also showed highly significant effects on herbage yield, oil yield and oil composition. Herbage yield, oil yield and oil composition improved when harvested in summer, compared to autumn and winter. The results suggested that high oil yield of rosescented geranium can be obtained by harvesting after three or four months in summer. However, low C: G ratio oil, is obtainable from two month harvesting frequency, irrespective of the harvesting season. Leaf samples were collected from the harvesting frequency experiment to study the variations of essential oil glands with leaf age. The leaf samples were treated and fixed under laboratory environments and then viewed under a scanning electron microscope for the appearance of oil glands. Two types of oil glands were identified on the leaf surfaces of rose-scented geranium and both appeared on both leaf surfaces. The adaxial leaf surface recorded higher densities of oil glands and trichomes than the abaxial leaf surfaces. Leaf position on the plant showed a xii significant effect in the density of oil glands. The top younger leaves contained more oil glands (all types) than the bottom older leaves.