The effect of irrigation frequency on leaf physiomorphological processes of rose-scented
geranium (Pelargonium capitatum x 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 (Gs) and
transpiration rate (Rt) 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 (yw) and relative water content (RWC). At the
end of the one-week stress period, Gs, Rt, yw 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.