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The effect of irrigation frequency on leaf physiomorphological processes of rose-scented geranium ( Pelargonium capitatum × P. radens cv. Rose) was investigated in a glasshouse study at the Hatfield Experimental Farm of the University of Pretoria, Pretoria, South Africa, from November 2005 to October 2006. Daily, and every 2nd, 3rd, 4th , and 5th day irrigation were applied as treatments. Leaf samples for electron-microscopic observations were taken one week prior to harvesting, whereafter all plants were re-watered. For each of the irrigation frequency treatments, 50% of the plants were then exposed to a one-week irrigation withholding period (brief stress treatment) prior to harvesting. During this period, physiological properties were recorded on a daily basis to identify or monitor change. Higher irrigation frequency and a brief water stress period increased essential oil yield. Lower irrigation frequency tended to increase the citronellol to geraniol (C:G) ratio to unacceptably high levels (C:G > 3). Upon re-watering, stomatal conductance (G s ) and transpiration rate (R t ) were significantly lower for the lower irrigation frequency treatments, compared to the higher irrigation frequency treatments, while no noticeable differences were observed in water potential (ψ w ) and relative water content (RWC). At the end of the one-week stress period, G s , R t , ψ w and RWC were lower for the plants that were more frequently irrigated compared to the less frequently irrigated treatments. Water stress reduced leaf size, and apparently increased trichome density, whereas the total number of trichomes per leaf remained more or less the same, indicating that total essential oil yield is mainly affected by leaf number (and not by leaf size or trichome number). Stomatal closure was the main water stress avoiding/adaptation mechanism. These results demonstrate that rose-scented geranium plants can make physiomorphological adaptations to save water. However, such a water saving strategy was counter-productive, since it resulted in lower essential oil yield and lower water-use efficiency.

The Pelargonium cultivar grown on Reunion for the production of rose-scented geranium oil is locally known as the cv. Rose. The origin of the cv. Rose is demonstrated by comparing its morphology, chromosome number and essential oil with those of putative ancestral species. It is concluded that P. capitatum (L.) L’Herit. and P. radens H.E. Moore can be considered as the parent species. A comparison of the characters of the cv. Rose and those of artificial hybrids between P. capitatum and P. radens , confirmed the assumption.

Pelargonium capitatum (L.) L’Herit. is one of the parent species oi the cultivar Rose grown on the Island of Reunion for its essential oil. The ultimate aim of this project is to identify chemotypes which could be included in hybridization programmes to ennoble the cultivar Rose. Forty populations of P. capitatum were selected along the west and east coasts of South Africa to describe the essential oil and macromorphological diversity in relation to geographical distribution. A multivariate statistical analysis was performed on the essential oil data. A survey of the composition of the volatile oils proved the species to display high levels of essential oil variation which allows for the species to be divided into chemotypes correlating to geographical distribulion. No clear-cut macromorphological discontinuities were apparent between populations, but an obvious congruence between the essential oil composition and macro-morphology was accentuated in extreme cases.