Rose-scented geranium (Pelargonium capitatum × P. radens) growth and essential oil yield response to different soil water depletion regimes

Effective irrigation management in arid and semi-arid regions, like South Africa, could increase crop yield and thereby improve productivity of scarce fresh water resources. Experiments were conducted at the Hatfield Experimental Farm of the University of Pretoria, South Africa, from 2004 to 2006, to investigate the effect of soil water depletion regimes on rose-scented geranium (Pelargonium capitatum × P. radens cv. Rose) essential oil yield, essential oil composition and water-use efficiency in an open field and a rain shelter. Four maximum allowable soil water depletion levels (MAD), 20, 40, 60 and 80% of the plant available soil water (ASW) in the top 0.8 m root zone, were applied as treatments. Plant roots extracted most soil water from the top 0.4 m soil layer. Increasing the soil water depletion level to 60% and higher resulted in a significant reduction in herbage mass and essential oil yield. Water stress apparently increased the essential oil concentration (percentage oil on fresh herbage mass basis), but its contribution to total essential oil yield (kg/ha oil) was limited. Irrigation treatments did not affect essential oil composition. An increase in maximum allowable depletion level generally resulted in a decrease in leaf area and an increase in leaf to stem fresh mass ratio. Up to 28% of irrigation water could be saved by increasing maximum allowable depletion level of ASW from 20 to 40%, without a significant reduction in essential oil yield.