Influence of soil water management on plant growth, essential oil yield and oil composition of rose-scented geranium (Pelargonium spp.)

Introducing effective irrigation management in arid and semi-arid regions, like most areas of South Africa, is an indispensable way of maximising crop yield and enhancing productivity of scarce freshwater resources. Holistic improvements in agricultural water management could be realised through integrating the knowledge of crop-specific water requirements. In order to develop effective irrigation schedules for rose-scented geranium (Pelargonium capitatum x P. radens), greenhouse and field experiments were conducted at the Hatfield Experimental Farm of the University of Pretoria, Pretoria, South Africa, from 28 October 2004 to 2006. Results from 20, 40, 60 and 80% maximum allowable depletion (MAD) levels of the plant available soil water (ASW) indicated that plant roots extracted most of the soil water from the top 40 cm soil layer, independent of the treatment. Both essential oil yield and fresh herbage mass responded positively to high soil water content. Increasing the MAD level to 60% and higher resulted in a significant reduction in herbage mass and essential oil yields. An increase in the degree of 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. There was no significant relationship between MAD level and essential oil composition. For water saving without a significant reduction in essential oil yield of rose-scented geranium, a MAD of 40% of ASW is proposed. Response of rose-scented geranium to a one-month irrigation withholding period in the second or third month of regrowth cycles showed that herbage mass and oil yield were positively related. Herbage yield was significantly reduced when the water stress period was imposed during the third or fourth month of regrowth. A remarkable essential oil yield loss was observed only when the plants were stressed during the fourth month of regrowth. Essential oil content (% oil on fresh herbage mass basis) was higher in stressed plants, especially when stressed late, but oil yield dropped due to lower herbage mass. The relationship between essential oil composition and irrigation treatments was not consistent. Water-use efficiency was not significantly affected by withholding irrigation in the second or in the third month of regrowth. With a marginal oil yield loss, about 330 to 460 m3 of water per hectare per regrowth cycle could be saved by withholding irrigation during the third month of regrowth. The overall results highlighted that in water-scarce regions withholding irrigation during either the second or the third month of regrowth in rose-scented geranium could save water that could be used by other sectors of society. In greenhouse pot experiments, rose-scented geranium was grown under different irrigation frequencies, in two growth media. Irrigation was withheld on 50% of the plants (in each plot) for the week prior to harvesting. Herbage and essential oil yields were better in the sandy clay soil than in silica sand. Essential oil content (% oil on fresh herbage mass basis) apparently increased with a decrease in irrigation frequency. Both herbage and total essential oil yields positively responded to frequent irrigation. A one-week stress period prior to harvesting significantly increased essential oil content and total essential oil yield. Hence, the highest essential oil yield was obtained from a combination of high irrigation frequency and a one-week irrigation-withholding period. In the irrigation frequency treatments, citronellol and citronellyl formate contents tended to increase with an increase in the stress level, but the reverse was true for geraniol and geranyl formate. Leaf physiological data were recorded during the terminal one-week water stress in the glasshouse pot trial. Upon rewatering, stomatal conductance (Gs) and transpiration rate (Rt) were significantly lower in the less often irrigated than in the more often irrigated treatments, while leaf water potential (yw) and relative water content (RWC) were the same for all plants, indicating that water stress had an after-effect on Gs and Rt. At the end of the stress period, Gs, Rt, yw and RWC were lower in the plants from the more often irrigated than from the less often irrigated treatments. Irrespective of irrigation treatment, one type of non-glandular and two types (different in shape and size) of glandular trichomes were observed. In water stressed-conditions, stomata and trichome densities increased, while the total number of stomata and trichomes per leaf appeared to remain more or less the same. Water stress conditions resulted in stomatal closure.Thesis (PhD)--University of Pretoria, 2009.Plant Production and Soil Scienceunrestricte

Response of potato (Solanum tuberosum) tuber yield components to gel-polymer soil amendments and irrigation regimes

Two field experiments were conducted to investigate the effects of two gel-polymer formulations (pure and fertiliser-fused) and moisture stress on yield and quality of potato (Solanum tuberosum). The experiments were carried out at the Hatfield Experimental Farm of the University of Pretoria, South Africa. Six gel-polymer rates, consisting of pure gel polymer at 1.5 kg/m3 of soil, with 85% and 70% fertiliser rate of the control, and fertiliser-fused gel polymer at 1.5, 2, and 3 kg/m3 soil, and control (without gel polymer) were assigned to the subplots. Four maximum allowable moisture depletion (MAD) levels, 25%, 40%, 55%, and 70% of the plant available soil moisture, were allocated to the main plots. Rate of phosphorus (P) was the same for all treatments (168 kg/ha). Total nitrogen (N) and potassium (K) budget for the treatments (except one pure gel-polymer treatment received 85%) were balanced to 70% of the control. The fertiliser-fused gel polymer showed no substantial improvement in tuber yield parameters for all rates. The pure gel polymer, especially at higher fertiliser rate, improved total and marketable tuber yield. Marketable tuber number and yield, and total tuber mass showed declining trend with an increase in MAD. Significant reduction in tuber fresh and dry mass was observed at the 55% and 70% MAD irrigation levels. Both high and low soil moisture levels reduced tuber specific gravity. Incidence of common scab was inversely related to the irrigation frequency

Physiomorphological response of rose-scented geranium (Pelargonium spp.) to irrigation frequency

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.The Technology and Human Resource for Industry Programme (THRIP), Biosys Plant Extracts (Pty) Ltd, and Clive Teubes CC.http://www.elsevier.com/locate/sajbnf201

Rose-scented geranium (Pelargonium capitatum x 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 niversity of Pretoria, South Africa, from 2004 to 2006, to investigate the effect of soil water depletion regimes on rose-scented geranium (Pelargonium capitatum x P. radens cv. Rose) essential oil yield, essential oil composition and water-use efficiency in an open field and a rain shelter. ourmaximum 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 epletion 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 ASWfrom 20 to 40%, without a significant reduction in essential oil yield