Author: Khalil, Shad Khan
Date published: December 1, 2011
Water is the main limiting factor affecting crop production throughout the world. Establishing limited water supply as a management tool could be effective in reducing production costs, conserve water and minimize leaching nutrients and pesticides into ground water (Nuruddin et al., 2003). However, before adopting limited water supply as a management tool, its effect on growth, physiology and yield must be evaluated (Kirda, 2002). There is a need to identify the drought adaptation mechanisms on physiological basis of those plants that can be grown on limited water supply. Reduced carbon assimilation (Stoneman et al., 1994), stomatal conductance as well as transpiration (Balok and St. Hilaire, 2002), osmotic adjustment (Chartzoulakis et al., 1993) and leaf water potential (Feser and St. Hilaire, 2005) and higher antioxidant activities (Pourtaghi et al., 2011) are reported in drought conditions. Drought also reduced the plant growth (Fernandez et al., 2002), leaf area (Feser and St. Hilaire, 2005), plant biomass partitioning (Graves and Wilkins, 1991), and ratios of leaf surface area to root dry weight as well as leaf dry weight to root dry weight (Balok and St. Hilaire, 2002; Feser and St. Hilaire, 2005). Considerable differences in response to drought are known to exist among plant genotypes (Bauerle et al., 2003). Drought tolerant species have higher values of carbon assimilation, stomatal conductance (Padgett-Johnson et al., 2003), water potential and relative water content (Lo Gullo et al., 2003) compared to non drought tolerant species. Genotypic variability in cultivar responses to drought could be exploited to improve cultivar management in a nursery and landscape setting. Yarrow is an herbaceous perennial, and is best suited to cottage rather than a formal garden (Halevy, 1999). It has medicinal and cosmetic uses (Rohloff et al., 2000), and extensively grown in drought-prone environments due to its numerous leaf and several stems developed from the horizontal rootstock (Bartram, 1995). However, drought adaptation mechanisms of yarrow species to limited irrigation are unknown. Information on the effects of moisture deficits on yarrow species Achillea millefolium cv Cerise Queen and Achillea filipendulina cv Parker Gold is non available. The objectives of this study were to quantify short term effect of drought on ψpd , Ts, Sc, ψs RWC and biomass production and partitioning of in yarrow species Achilla "Cerise Queen" and "Parker Gold".
Plant water relations and gas exchange
Drought treatment (D), species (S) and drought cycles (C) significantly affected predawn water potential (ψPd) in 2004 , while the effects of D and C was only significant in 2005 (Fig. 1A-D). Interactions between D × S and D × C showed non significant effects in both years. Maximum ψPd (-0.5 MPa) was recorded for A. mellifolium in 2005 irrigated daily (Fig 1B). ψPd decreased with increase in irrigation interval and minimum ψPd (-3.8MPa) was recorded for A. filipendulina watered every 9 days (Fig. 1 D). Drought treatments, species and drought cycles significantly affected stomatal conductance (Sc) in 2004, while only D and C showed significant effects on Sc in 2005 (Fig. 2A-D). None of the interactive effect was significant in both years. Sc decreased with increase in drought level and minimum Sc (9 m.m mol.m-2.s-1) was recorded for A. filipendulina in 2004 irrigated every 9 days (Fig. 2C). Maximum Sc (600 m.mol.m- 2.s-1) was recorded for A. filipendulina in 2005 irrigated daily (Fig. 2 D). Number of drought cycles significantly impacted Sc and the highest Sc values (365 mmol.m-2.s-1) were recorded at 2nd drought cycle in 2005 (Fig. 2 D). Transpiration rate (Ts ) was significantly affected by D, S, C and S × C interaction in both years while no statistical difference in Ts was observed for S in 2005 (Fig. 3A-D). D × S interaction was non significant in both years (Fig. 3A, B). Maximum Ts (10 m. mol. m-2 s1) was recorded for A. millefolium in 2004 at drought cycle 7 (Fig 3A). Ts decreased with increase in drought level and minimum Ts (0.2 mmol. m-2.s1) was recorded in 2005 for A. millefolium and also for A. filipendulina in drought cycle 2 watered every 9 days (Fig. 3 A, C).
Cell osmotic potential and relative water content
Drought treatments, S and C significantly affected cell osmotic potential (ψs) in both years, while D × C interaction was significant only in 2005 (Fig. 4A-D). None of the other interaction showed a significant effect on ψs.. Control had highest ψs (-0.8 MPa) for both species in 2004 (Fig 4A-C) which decreased with increase in drought level and minimum ψs (-4.7 MPa) was recorded for A. millefolium in drought cycle 8 in 2005 that received water every 9 days (Fig. 4 B). Drought levels significantly affected relative water content (RWC) in both years (Fig. 5A-D), while S and D × S interaction was significant only in 2004 (Fig. 5A-C) and C was significant in 2005 (Fig. 5B-D). None of the other interaction showed significant effect on RWC. Highest RWC (88%) was recorded in A. millefolium irrigated daily in drought cycle 8 in 2005 (Fig. 5 B) which decreased with increase in drought level and minimum RWC (40%) was recorded for A. filipendulina in 2005 that received water every 9 days (Fig. 5 D).
Drought significantly impacted leaf area (LA) in both years (Table 1, 2) while S affected LA only in 2004 (Table 1). Leaf area decreased with increase in drought stress. Control plots had two timers higher LA of (3690 cm2) in 2004 and four times (3435 cm2) higher in 2005 than plants irrigated every 9 days. A. millefolium produced more leaf area (3209, 1656) than A. filipendulina (2270, 1648 cm2) in both years (Table 1,2). Leaf weight (LW) and root weight (RW) were significantly affected by drought stress in both years while S affected RW in 2004 only (Table 1,2). Drought decreased LW and RW. Control plots had two times higher LW (38.97 g) and RW (63.25 g) and four times higher LW (8.54g) and RW (10.11g) than plants irrigated every 9 days in 2004 and 2005, respectively (Table 1, 2). Leaf area ratio (LAR) was significantly affected by D in 2004 only (Table 1). LAR of the control (22.84) was lowest and increased with each level of drought stress and was highest (39.96) for plants irrigated every 9 days (Table 1). Drought and S significantly affected root-to-shoot dry weight ratio (RSR) in both years (Table 1, 2). Control had highest RSR (3.19) which decreased with increasing stress level and lowest RSR (1.58) was observed for plants irrigated every 9 days (Table 1, 2). A. filipendulina gave high RSR (2.80, 1.79) than A. millefolium (1.79, 1.12) in both years (Table 1, 2). Species significantly affected NAR in 2004 only, while D showed non significant effect on NAR in both years (Table 1,2). A. filipendulina resulted in maximum NAR (0.120 mg. cm2. d-1) than A. millefolium (0.115 mg. cm-2.d-1). Drought showed significant effect on relative growth rate (RGR) in both years (Table 1, 2). RGR of the control (0.151, 0.059) was twice as high (0.011, 0.033) as plants irrigated every 9 days in both years (Table 1,2).
Plants watered daily (control) showed maximum ψpd which decreased with increase in irrigation interval and lowest ψpd was recorded for plants watered every 9 days. The low ψpd of drought stressed plants suggests that the leaves were not fully rehydrated at the end of drought cycle (Feser and St. Hilaire 2005; Balok and St. Hilaire, 2002). Droughted plants tended to exhibit the lowest ψpd indicating that the quantity of water in the root zone or the capacity of conducting system to transport water was insufficient to allow rehydration of leaves during night (Close et al. 1996). Stomatal conductance (Sc) decreased with increase in drought level and minimum Sc was noted for highly stressed plants. This indicates that the severity of our drought treatments were sufficient to cause severe reduction in Sc. The high Sc in control treatment may be due to optimum availability of water which offered low resistance to Sc compared to the stressed treatments which offered high resistance to Sc due to shortage of water, small changes in the growing substrate moisture level can trigger stomatal closure (Croker et al., 1998). It is likely possible that the closer of stomata in plants exposed to moisture stress might be one strategy that yarrow plants use to tolerate low moisture environment. Species responded differently to moisture levels in 2004, showing that stomatal sensitivity of plants varies with species (Balok and St.Hilaire, 2002; Croker et al., 1998). Variation in Sc by years suggests that species varies in response to planting season of the year as the experiment was conducted during fall to winter season in 2004; while in 2005 the same experiment was conducted in spring. Transpiration rate (Ts) was higher in control treatments compared with stressed treatments. High Ts in control treatment may be due to more availability of water. Transpiration may decrease in response to decrease in plant conductivity to water (Brodribb and Hill, 2000) and drought (Balok and St.Hilaire, 2002; Fernandez et al., 2002). Ts were lower for the drought stressed plants in the drought cycle 2 compared with later cycles in 2004, due to more foliage production. However, this trend was not observed in 2005, and the results were unknown. Ts of water stress imposed plants were consistently lower at all sampling date, indicating that drought may have reduced root hydraulic conductivity (Ramos and Kaufmann, 1979). Control treatments had high cell osmotic potential (ψs) than drought treatments. ψs decreased with increased drought stress showing that drought have reduced ψs. Since water was not a limiting factor for control treatment, therefore control plants showed maximum ψs compared with stressed treatments (Feser and St. Hilaire, 2005). The increase in solute concentration in drought exposed plants lowered ψs. The increase in cell solutes that is triggered by exposure to water deficits lowers the water potential at which stomatal closure occurs (Turner and Jones, 1980). Additionally cell solutes may play a significant role in maintenance of turgor and survival by protecting the plant from dehydration. The influx of water into the tissue will maintain turgor and enable the plant to continue growth despite the fact that plant is faced with moisture deficits (Chaves, 1991). A. millefolium deposited more solute than Achillea filipendulina; suggesting that it may perform better under drought condition. Relative water content (RWC) was highest for the control treatment which decreased with increase in water stress level and lowest RWC was recorded for plants watered every 9 days. High RWC in control may be due to the availability of more water (Feser and St. Hilaire, 2005). Photosynthetic activity may decreases when RWC drop down to 40-70% due to increased cell solutes which may inhibit enzymatic activity in chloroplast (Chaves, 1991). High RWC (74.50%) was recorded in A. filipendulina compared with A. millefolium (69.8%). RWC seems to have direct relation with carbon assimilation, which drop down to zero when RWC falls down below 70%, consequently the possibility of the growth ceases (Chaves, 1991). High RWC in A. filipendulina suggests that it may perform better under drought condition. Leaf area of control plants was twice high in 2004 and four times higher in 2005 than that of plants irrigated every 9 days. The greater leaf area in the following year would be associated with the higher photosynthetic radiation, which accelerate the plant growth and hence the leaf area. The increase in LA of unstressed plants was due to production of more and larger leaves. We observed that drought stressed plants resulted in drying of the leaves and thus LA decreased. Reduction in LA may be considered among the first morphological traits affected by water stress (Rauf and Sadaqat, 2008) because cell expansion depends on the availability of water to maintain turgor (Hale and Orcutt, 1987). Drought reduced leaf weight (LW) and root weight (RW). LW and RW of the controls were over twice as high in 2004 and over four times as high in 2005 (Table 1, 2). The reduction in leaf and root weight was due to production of fewer and smaller leaves and roots in drought stressed plants compared with control. LW was significantly affected by S in both years (Table 1, 2) showing that LW is species dependent and A. millefolium produced heavy leaves than A. filipendulina even if planted in different growing seasons. Drought stress significantly affected leaf area ratio (LAR) in 2004 only. Control produced lowest LAR. LAR increased with each level of drought stress and highest LAR (39.96) was observed for plants in 2004 irrigated every 9 days (Table 1). High LAR values of drought stressed plants was due to reduction in total number of leaves, smaller LA of individual leaf and thus lighter leaves. Root-to-shoot dry weight ratio (RSR) of the control was highest which decreased with each stress level and lowest RSR was observed for plants irrigated every 9 days. It was observed that drought stress severely reduced shoot growth compared with root growth. Reduction in shoot growth under drought may be considered an avoidance mechanism which minimizes water losses (Ruiz- Sanchez et al., 2000). Species significantly ffected NAR in 2004 only, while D showed non significant effect on NAR (Table 1,2). No statistical differences in RGR and NAR between A. millefolium and A filipendulina were observed except in 2004 where NAR was higher for A filipendulina than A. millefolium. These results showed that A. filipendulina is more sensitive to variation in environment compared with A. millefolium. Although the experiment was conducted in the greenhouse, yearly differences in mean maximum, minimum air temperature and relative humidity were noted that might have affected NAR. Drought showed significant effect on RGR in both years. RGR of the control was twice higher than plants irrigated every 9 days. Our results showed that RGR is sensitive to drought and may drastically reduce if plants are exposed to drought stress. Drought might reduce gas exchange, the growth of expanding tissues by reducing cellular expansion and productivity (Sanchez-Blanco et al., 2002, Nilsen and Orcutt, 1996).
Materials and methods
The experiments was conducted in a greenhouse at New Mexico State University, Las Cruces, N.M USA in 2004 and repeated in 2005. The experimental site is located at 1183 m elevation, 32o 16'4"N latitude, 106o46'18"W, longitude. Experiment 1: Seeds of Achillea millefolium cv. Cerise Queen and Achillea filipendulina cv. Parkers Gold were sown in flat plastic trays on June 2, 2004. Plants were removed from the original flat plastic tray on July 19 and repotted into the plastic pots (3.8 liter) of 18 cm height and diameter. Pots were filled with growing substrate consisted of peat, composted bark, sphagnum peat, perlite and a wetting agent containing 0.01% available phosphorus derived from ammonium phosphate (The Scotts Advantage, Scott Company, Marysville, OH). The seedlings were watered every two days with automatic drip irrigation system and fertilized weekly with 2840 mg L-1 Peters solution containing 20:20:20 N:P2O5:K2O, 0.05% each magnesium and iron (Scott Company, Marysville, OH). During the experimental period, daily maximum and minimum air temperature and humidity were recorded with portable thermo-hygrometer sensor (Spectrum Technologies, Plainfield, IL, USA) and is reported in Fig. 1A, B.
Experimental design and irrigation treatment
The experimental design was a randomized complete block design, with four irrigation levels (1 = control; irrigated daily, 2= irrigated every 3 days, 3 = irrigated every 6 days, and 4 = irrigated every 9 days) and two species (Achillea millefolium cv. Cerise Queen and Achillea filipendulina cv. Parkers Gold with four replications. In each pot, a single plant was left after harvesting four plants during initial harvest. These irrigation intervals were selected because they were associated with a similar degree of wilting assessed visually and plants were unable to tolerate stress longer than 9 days. Control plants were irrigated daily with 1 liter of tap water and plants in the moisture deficit treatments were irrigated in cycles. Control plants were fertilized weekly with Peters at 2840 mg L-1 H2O containing 20:20:20 N:P2O5:K2O, 0.05% each magnesium and iron (Scott Company, Marysville, OH) . Moisture stressed plants were fertilized at the end of each drought cycle using the same fertilizer and quantity. During measurements, leaf temperature averaged 22±4°C. Maximum /minimum temperature in the greenhouse averaged 35± 2°C/12±1. Maximum relative humidity averaged 73± 8 /19±1%. Photosynthetically active radiation at canopy level averaged 964± 167 µmols-1m-2.s-1. No artificial radiation was provided. Leaf temperature was measured with steady state porometer (LI-1600; LI-COR, Lincoln, NE, USA).
Observations and measurements
Four plants were randomly uprooted from each pot on after three months of sowing. The fresh weight of seedlings was about 10 g pot-1. The seedlings were separated into roots, leaves and stem. Roots were washed free of growing substrate using water. All components were oven dried for 14 d at 65o C.
Plant water relations and gas exchange
Predawn leaf water potential (ψPd) was measured on young fully expanded leaves with a pressure chamber (Model 3005, Soil Moisture Equipment Corp, Santa Barbara, CA, USA) on the same day in all the treatments at the end of drought cycle, when coincided. Similarly, the transpiration rate (Ts) and stomatal conductance (Sc) were measured between 11:00 to 14:00 hr at same day, when drought cycle coincided with the help of steady-state porometer (LI-1600; LI-COR, USA) on youngest fully expanded leaf. All these measurements were made at the end of cycles 2, 4, 7, 9 and 11, at September 27, October 9, November 10, November 24 and December 15. On those dates the end of a drought cycle coincided for the 1, 3, 6 and 9 days treatments.
Cell osmotic potential (ψ s)
Cell osmotic potential was measured at the end of drought cycles 4, 7, 9 and 11. A young fully expanded leaf was selected, excised and sealed in a zip lock plastic bag, placed on ice, immediately transported to laboratory and stored in freezer at -20o C in dark for 3-5 days. Leaves were removed from the freezer, rolled, placed into a Markhart leaf press (Model LP-27; Wescor, Logan, UT, USA) and pressed to squeeze out cell contents. A 10µL aliquot of the cell contents was transferred onto paper discs (SS-033 sample disc, Wescor, USA). Discs were then placed in a self calibrating vapor pressure osmometer (Vapro model 5520; Wescor, Logan, Utah) to determine cell osmolality. Values for cell osmolality (mmol.kg-1) were converted to cell osmotic potential (-MPa) using van't Hoff's equation.
Relative water content (RWC)
Relative water content was measured at the end of drought cycles 4, 7, 9 and 11. A young fully expanded leaf was selected, excised, sealed in a zip lock plastic bag, placed on ice and immediately transported to the laboratory. Each leaf was weighed to determine fresh weight (FW) and rehydrated in deionized water overnight. Each leaf was blotted with lintless paper to remove excess moisture and re-weighed to obtain turgid weight (TW). Each leaf was then dried in oven for 10 h at 85o C. Dry weight (DW) was recorded, and RWC was determined using the formula; RWC (%) = (FWDW)/( TW-DW) ×100.
Final harvest and growth measurements
The experiment was terminated after 99 d of irrigation treatments. The experiment took 197 days as a whole starting from sowing the seed till final harvest. Leaves were cut 2 cm above the growing substrate surface. Leaf area of all plants was measured with the leaf area meter (LI 3000 A; LI-COR, Lincoln, Nebr.). Roots were water-washed free of growing substrate. Leaves and roots were oven dried at 65o C for 14 days at 65 o C. Leaf surface area (LA) was measured with leaf area meter (LI 3000 A; LI-COR, Lincoln, Nebr, USA). The leaf area ratio (LAR), and root shoot ratio were calculated from the dry weights of roots, shoot and leaf area. Net assimilation rate (NAR) was calculated by using the equation of Harper (1977): Relative growth rate (RGR) was calculated using an equation modified from Gutschick and Kay (1995). Experiment 2: Same treatments and experimental design were used for the 2nd experiment as was used for the 1st experiment. Seeds were planted on November 22, 2004, and plants were repotted into the plastic pots using the same procedures outlined in1st experiment after three months. Irrigation treatments were initiated after two months of the repotting of the plants. In this experiment plants in the moisture stress treatments were irrigated in 8 cycles as against 12 cycles in experiment 1st. After completion of 4 cycles, irrigation interval was reduced to 5 and 7 days in treatments 3 and 4 respectively due to high evaporation losses after mid April. During drought treatments plants were fertilized as in experiment 1. During physiological measurements, leaf temperature averaged 22± 1°C/12±1. Maximum/minimum temperature in the greenhouse averaged 36±2°C/14±4°C, and relative humidity averaged 91± 8 /28±4%. Photosynthetically active radiation at canopy level averaged 1689± 200µmolsm-2.s-1 and received no supplemental radiation. Data on ψPd, Sc , Ts, ψs , RWC, NAR and RGR, and other growth parameters were recorded using procedures outlined in experiment 1. Data for ψPd, Sc , Ts, ψs and RWC were recorded on March 23, April 11, April 25 and May 7, 2005. On May 11, 2005, plants were uprooted 67 days after 8th drought cycles. The experiment took 171 d from sowing to final harvest.
Data were analyzed using SAS/STAT software for windows Version 9.1 (SAS Inst. Cary, NC, USA 2004). Means of leaf area, leaf weight, root weight, LAR, root shoot ratio, NAR and RGR were separated using Fisher's least significant difference (LSD) at P ≤ 0.05 after analysis of variance. The relationship of leaf water potentials, transpiration, stomatal conductance, relative water content and cell osmotic potential with drought cycle was analyzed using repeated measures of the Proc Mixed procedure of SAS (SAS Inst., 2004) to assess species (S) effect, drought treatment (D) effects, drought cycle (C) effect, and all interactions (S × D, S × C, S × D × C).
Drought stress reduced ψpd , Sc , TS , ψs , RWC, LA, leaf and root weights, RSR and RGR compared to control plots where no water stress was imposed. Both species performed better in irrigated condition compared with drought stress. Cerise Queen might be more suited to the arid regions of Southwest as it produced larger and heavier leaves and showed better osmotic adjustment in response to drought which enabled it to deposit more solutes and lowered leaf water potential without dehydration than Parker Gold.
The support of Higher Education Commission (HEC) Islamabad for providing funds for this postdoctoral research is acknowledged.
Balok CA, Hilaire RS (2002) Drought responses among seven southwestern land-scape tree taxa. J Amer Soc Hort Sci 127:211-218.
Bartram T (1995) Encyclopedia of Herbal Medicine, 1st edn, Grace Publishers, Bournemouth B D (1995): Encyclopaedia of Herbs and their Uses. Dorling Kindersley, London. ISBN 0-7513-020-31
Bauerle WL, Dudley JB, and Grimes LW (2003) Genotypic variability in photosynthesis, water use, and light absorption among Red and Freeman Maple cultivars in response to drought stress. J Amer Soc Hor Sci 128:327- 332.
Brodribb TJ, Hill RS (2000) Increases in water potential gradient reduce xylem conductivity in whole plants. Evidence from a low-pressure conductivity method. Plant Physiol 123:1021-1028.
Chartzoulakis K, Noitsakis B, Therios I (1993) Photosynthesis, plant growth and dry matter distribution in kiwifruit as influenced by water deficits. Irr Sci 14:1-5.
Chaves MM (1991) Effect of water deficits on carbon assimilation. J Exp Bot 42:1-16.
Close RE, Kielbaso JJ, Nguyen PV, Schutski RE (1996) Urban vs. natural sugar maple growth, II. Water relations. J Arboriculture 22: 187-192.
Croker JL, Witte WT, Auge RM (1998) Stomatal sensitivity of six temperate, deciduous tree species to non-hydraulic root-to-shoot signalling of partial soil drying. J Exp Bot 49:761-774
Fernandez RT, Schutzki RE, Prevete KJ (2002) Influence of spring and fall drought stresses on growth and gas exchanges during stress and post transplant of containergrown Magnolia × soulangiana 'Jane'. J Amer Soc Hort Sci 127:38-44.
Feser C, St-Hilaire R (2005) Development of in-ground container plants of Mexican elders exposed to drought. Hort Sci 40:446-450.
Graves WR, Wilkins LC (1991) Growth of honey locust seedlings during high root-zone temperature and osmotic stress. Hort Sci 26:1312-1315.
Gutschick VP, Kay LE (1995) Nutrient -limited growth rates: quantitative benefits of stress responses and some aspects of regulation. J Exp Bot 46:995-1009.
Hale, MG, Orcutt DM, (1987) The Physiology of plants under stress. Blackburg Virginia, p. 206.
Halevy AH (1999) New flower crops. p 407-409. In: Janick J. (ed.), Perspectives on new crops and new uses. ASHS Press, Alexandria, VA.
Harper JL (1977) Population biology of plants. Academic Press, London.
Kirda C (2002) Deficit irrigation scheduling based on plant growth stages showing water stress tolerance, p.3-10. In: Heng LK, Moutonnet P, Smith M (compilers). Deficit irrigation practices FAO Water Report. 22 Rome.
Lo-Gullo MA, Salleo S, Rosso R, Trifilo P (2003) Drought resistance of 2-year old saplings of Mediterranean forest trees in the field: relations between water relations, hydraulics and productivity. Plant and Soil 250:259-272.
Nilsen ET, Orcutt DM (1996) The physiology of plants under stress. John Wiley and Sons, Inc., New York.
Nuruddin MMd, Mandramootoo CA, Dodds GT (2003) Effect of water stress at different growth stages on greenhouse tomato yield and quality. Hort Sci 38:1389- 1393.
Padgett-Johnson M, Williams LE, Walker MA (2003) Vine water relations, gas exchange, and vegetative growth of seventeen vitis species grown under irrigated and non irrigated conditions in California. J Amer Soc Hort Sci 128:269-276.
Pourtaghi A, Darvish F, Habibi D, Nourmohammadi G, Daneshian J (2011) Effect of irrigation water deficit on antioxidant activity and yield of some sunflower hybrids. Aust J Crop Sci 5:197-204.
Ramos C, Kaufmann MR (1979) Hydraulic resistance of rough lemon roots. Physiol Plant 45:311-314.
Rauf S, Sadaqat HA (2008) Identification of physiological traits and genotypes combined to high achene yield in sunflower (Helianthus annuus L.) under contrasting water regimes. Aust J Crop Sci 1:23-30.
Rohloff J, Skagen EB, Steen AH, Iversen TH (2000) Production of yarrow (Achillea millifolium in Norway:essential oil content and quality. J Agri Food Chem 48:6205-6209.
Ruiz-Sanchez MC, Domingo R, Torrecillas A, Perez-Pastor A (2000) Water stress preconditioning to improve drought resistance in young apricot plants. Plant Sci 156:245-251.
Sanchez-Blanco, MJ, Rodriuez P, Morales MA, Ortuno MF, Torrecillas A (2002) Comparative growth and water relations of Cistus albidus and Cistus monspeliensis plants during water deficit conditions and recovery. Plant Sci 162:107-113.
SAS Institute (2004) Base SAS 9.1.3 Procedures Guide. SAS/STAT 9.1 Users Guide. SAS/GRAPH 9.1 Reference, SAS Institute, Cary, NC, USA.
Stoneman GL, Turner NC, Dell B (1994) Leaf growth, photosynthesis and tissue water relations of greenhousegrowth Eucalyptus marginata seedlings in response to water deficits. Tree Physiol 14:633-646.
Turner NC, Jones MM (1980) Turgor maintenance by osmotic adjustment, a review and evaluation. P 84-104. In: Turner NC, Kramer PJ (eds), Adaptation of plants to water and high temperature stress.
Shad Khan Khalil1, 2, Rolston St. Hilaire1, Ahmad Khan2,3*, John G. Mexal1
1Department of Plant and Environmental Sciences, New Mexico State University, Box 3003, Las Cruces, NM
2Departments of Agronomy, Khyber Pakhtunkhwa Agricultural University, Peshawar, Pakistan
3School of Earth and Environment, The University of Western Australia, 35 Starling Highway, Crawley, WA
*Corresponding author: email@example.com
Contribution of the New Mexico Agricultural Experiment Station, New Mexico State University, Las Cruces, USA and the
Higher Education Commission, Islamabad, Pakistan