Journal of Integrative Plant Biology 2006, 48 (5): 520−526

Received 10 Aug. 2005 Accepted 30 Dec. 2005

Supported by the State Key Basic Research and Development Plant of

China (2004CB720200), the National Natural Science Foundation of China

(90502004) and the Knowledge Innovation Project of the Chinese Academy

of Sciences.

*Author for correspondence. Tel: +86 (0)991 788 5432; E-mail: <chenyn@ms.

xjb.ac.cn>.

Physiological Responses of Three ContrastingPlant Species to Groundwater Level Changes in an

Arid Environment

Li Zhuang and Ya-Ning Chen*

(Xinjiang Institute of Ecology and Geography, the Chinese Academy of Sciences, Urumqi 830011, China)

Abstract

Plants growing on both sides of the Tarim River in western China serve as a natural barrier containing thedeserts and protecting the oasis, and their growth is greatly affected by water conditions in their localhabitat. We studied the physiological responses of three different types plants (i.e. Populus euphraticaOliver, Tamarix ramosissima L., and Apocynum venetumas Linn) to changing groundwater levels by analyz-ing changes in chlorophyll, soluble sugar, proline (Pro), malondialdehyde (MDA), superoxide dismutase(SOD), peroxidase (POD), indoleacetic acid (IAA), giberellic acid, abscisic acid (ABA) and cytokinin (CK).Relationships between these physiological characteristics and groundwater levels were analyzed in orderto assess the drought tolerance of the three plant species based on the values of average membershipfunction. We found that MDA, SOD and ABA were more susceptible to changes in groundwater level, fol-lowed by POD, IAA and CK. Among the three plant species, Populus euphratica responded physiologicallyless to changing groundwater level than T. ramosissima and A. venetumas.

Key words: Apocynum venetumas; groundwater level; membership function method; Populus euphratica; Tamarix ramosissima;Tarim River.

Zhuang L, Chen YN (2006). Physiological responses of three contrasting plant species to groundwater level changes in an aridenvironment. J Integrat Plant Biol 48(5), 520−526.

www.blackwell-synergy.com; www.jipb.net

Plants experience various environmental constraints, suchas low and high temperatures, drought, and salinity, under natu-ral conditions. Extremes of constraining environmental condi-tions often lead to direct or indirect injury to plants. In arid andsemi-arid regions, water is the most limiting factor to plantrecruitment, growth, physiology, nutrient dynamics, and eco-system productivity (Xiao et al. 2005a, b). Understanding theimpacts of prevailing environmental constraints and plant re-sponses are fundamental to effectively managing ecosystemsunder conditions of a changing climate and increasing anthro-

pogenic activity.Many physiological and biochemical processes can reflect

plant responses to environmental stresses. Chlorophyll is animportant molecule in plant photosynthesis, and its level canindicate the potential photosynthetic capacity of plants as aresult of long-term environmental stress (Prakash andRamachandran 2000; Nicotra et al. 2003). When under waterstress, plant cells actively accumulate certain types of solutessuch as free proline (Pro) and soluble sugar to improve theirdrought-resistance by lowering water potential and maintain-ing turgor pressure (Gao et al. 2004; Maggio et al. 2004; Sofo etal. 2001). Accumulation of the end product of membrane lipidperoxidation, malondialdehyde (MDA), is also a result of theoxygenation of plant membrane lipids under stress conditions(Ye and Zhao 2002; Luna et al. 2004; Yamamoto et al. 2001).Superoxide dismutase (SOD) and peroxidase (POD) are themain anti-oxidizing agent of plants for cleaning out excessiveactive oxygen to protect membrane structure, and thusstrengthen their resistance to adverse circumstances (Ren et

Physiological Responses to Groundwater Level Changes 521

al. 2001; Cavalcanti et al. 2001; Babitha et al. 2002; Zoller et al.2003; Jin et al. 2004; Ederli et al. 2004). Abscisic acid (ABA) isalso known to play an important role in the physiological re-sponse to environmental stress (Cao et al. 2004; Conklin andBarth 2004; Goh et al. 2004). Responses of these processesto environmental constraints may indicate levels of adaptationof plants to adverse habitats.

Severe conditions of drought, wind, and salinity contribute tosparse vegetation coverage in the Tarim basin of Xinjiang, west-ern China, where Populus, Tamarix and saline meadowdominate. Populus euphratica, Tamarix ramosissima, and Apo-cynum venetumas are representative construction species oftrees, shrubs and herbs in the Tarim basin. The changinggroundwater levels along the Tarim River reaches due to his-torical damming and artificial water recharge efforts in recentyears (Chen et al. 2004c) have greatly affected the growthand ecosystem processes of the local vegetation (Chen et al.2004a, b). In order to assess the state of health and responsesto changing groundwater levels of vegetation in the lowerreaches of the Tarim River, we studied the relationships ofseveral physiological variables such as chlorophyll, solublesugar, Pro, anti-oxidative enzymes, and phytohormone withgroundwater level in three different types of plants (i.e. Populuseuphratica, Tamarix ramosissima, and Apocynum venetumas).Our objective was to determine how changing groundwaterlevels would affect plant growth and ecosystem succession inthe lower reaches of the Tarim River.

Results

Relationships between physiological characteristics andgroundwater level

The measured physiological characteristics correlated well withgroundwater levels in P. euphratica, T. ramosissima and A.venetumas (Tables 1, 2). In the Yahepu section, the groundwa-ter level was positively correlated with MDA, SOD, ABA, andCK, and negatively correlated with chlorophyll and POD in P.euphratica; in T. ramosissima, the groundwater level waspositively correlated with chlorophyll, soluble sugar, MDA, SOD,ABA, and CK, and negatively correlated with POD and in-doleacetic acid (IAA); whereas in A. venetumas, the ground-water level was positively correlated with Pro and ABA, and nega-tively correlated with POD and IAA (Table 3). In the Alagan section,the groundwater level was positively correlated with MDA, SOD,and ABA, and negatively correlated with POD and IAA in P.euphratica; in T. ramosissima, the groundwater level was posi-tively correlated with MDA, SOD, ABA and CK; whereas in A.venetumas, the groundwater level was positively correlated withMDA, SOD, and ABA, and negatively correlated with IAA (Table 4).

Drought resistance

The average membership values of drought resistance of 10physiological characteristics, chlorophyll, soluble sugar, Pro,

Table 1. Relation between physiological indexes of Populus euphratica, Tamarix ramosissima and Apocynum venetumas and groundwaterlevel at Yahepu

SampleDistance

Chloro- Soluble ProlineEnzyme activity Plant growth regulator content

Groundwater

(m) phyll sugar content MDA SOD POD

IAA GA3 ABA CKlevel

(%) content (µg/g) (µg/g) (U/g) (U/g)(ng/g (ng/g (ng/g (ng/g

(m)FW) FW) FW) FW)

Populus euphratica100 89.35 326.72 16.42 1.33 0.41 0.99 80.33 126.99 2.87 4.32 3.12200 68.40 405.26 28.62 2.62 0.49 0.89 46.61 120.56 5.06 4.63 4.42300 67.45 409.33 28.09 3.63 0.56 0.81 44.93 138.52 8.32 6.86 5.78400 66.20 408.39 27.35 6.08 0.52 0.72 36.50 132.29 11.55 8.20 6.46500 58.50 404.22 23.34 6.16 0.61 0.60 47.14 141.54 23.55 5.77 6.76

Tamarix ramosissima100 50.10 121.65 22.48 3.57 0.259 1.01 137.88 118.28 7.86 4.33 3.12200 51.45 183.31 25.65 4.27 0.36 0.94 119.67 115.90 12.89 5.23 4.42300 66.80 189.73 31.76 10.50 0.44 0.77 60.12 86.48 14.98 6.84 5.78400 70.70 160.83 38.92 10.83 0.48 0.84 37.49 101.51 18.23 9.08 6.46500 76.85 170.18 33.50 14.29 0.56 0.74 35.68 97.54 29.1 8.22 6.76

Apocynum venetumas100 55.20 222.33 6.27 1.09 0.26 1.68 130.20 161.13 2.27 7.25 3.12200 60.65 177.65 6.13 2.43 0.35 1.53 110.55 169.94 2.74 5.48 4.42300 53.15 223.28 9.72 8.72 0.19 1.42 68.74 145.72 4.21 6.80 5.78400 66.15 189.16 10.72 4.63 0.23 1.37 49.86 120.37 6.04 8.30 6.46500 59.65 178.86 12.95 9.07 0.23 1.25 46.66 130.26 14.25 5.67 6.76

FW, fresh weight.

522 Journal of Integrative Plant Biology Vol. 48 No. 5 2006

Table 2. Relation between physiological indexes of Populus euphratica, Tamarix ramosissima and Apocynum venetumas and groundwaterlevel at Alagan

SampleDistance Chlorophyll

SolublePro MDA SOD POD IAA GA3 ABA CK

Groundwater

(m) (%) sugars

(µg/g) (µg/g) (U/g) (U/g) (ng/g FW) (ng/g FW) (ng/g FW) (ng/g FW)level

(µg/g) (m)Populus euphratica

100 64.30 484.29 15.75 4.80 0.47 0.88 107.71 137.56 7.70 6.01 5.94200 58.00 439.95 17.95 6.06 0.51 0.76 100.12 100.65 9.04 8.29 8.89300 62.75 400.30 16.74 8.46 0.50 0.78 77.12 177.14 13.75 7.58 9.74400 65.90 459.09 13.25 10.03 0.57 0.72 61.33 110.17 18.68 5.14 10.56500 58.35 418.61 12.53 10.91 0.66 0.69 52.49 95.69 23.55 5.43 10.78

Tamarix ramosissima100 58.95 113.73 4.57 6.83 0.29 0.99 123.86 135.39 8.19 7.25 5.94200 59.05 139.49 10.58 8.46 0.33 0.91 57.49 82.42 16.41 7.05 8.89300 56.25 226.69 10.60 8.96 0.37 0.80 76.16 82.03 22.35 8.61 9.74400 58.80 167.84 5.14 11.41 0.42 0.96 45.00 76.33 23.40 7.96 10.56500 47.35 71.89 7.11 16.81 0.50 0.43 44.09 65.81 32.07 7.19 10.78

Apocynum venetumas100 54.60 236.08 5.62 3.45 0.29 1.26 91.80 105.99 5.19 6.32 5.94200 73.35 190.80 4.04 5.95 0.30 1.21 93.52 82.54 9.29 7.01 8.89300 82.40 314.82 6.95 9.15 0.32 0.84 80.25 37.98 10.02 4.62 9.74400 77.60 180.72 7.32 10.06 0.35 1.17 74.08 67.38 17.32 4.62 10.56500 59.90 302.61 12.56 11.87 0.47 1.09 50.64 31.22 21.84 8.97 10.78

FW, fresh weight.

Table 3. Correlations between physiological indexes and groundwater level at Yahepu

PlantChlorophyll

SolublePro MDA SOD POD IAA GA3 ABA CK

(%)sugars

(µg/g) (µg/g) (U/g) (U/g) (ng/g FW) (ng/g FW) (ng/g FW) (ng/g FW)(µg/g)Populus euphratica

r –0.877 59* 0.696 93 0.389 28 0.973 70** 0.902 92* –0.997 63** –0.715 94 0.758 75 0.931 80* 0.969 88**P 0.049 5 0.190 9 0.517 2 0.005 1 0.035 8 0.000 1 0.173 8 0.137 0 0.021 2 0.006 2

Tamarix ramosissimar 0.967 68** 0.440 29* 0.858 11 0.960 56** 0.988 94** –0.885 69* –0.951 38** –0.667 59 0.953 23** 0.926 28*P 0.006 9 0.045 8 0.062 8 0.009 3 0.001 4 0.045 6 0.007 8 0.218 2 0.008 3 0.023 3

Apocynum venetumasr 0.448 86 –0.521 85 0.964 38** 0.794 41 –0.473 03 –0.987 92** –0.963 12** –0.852 35 0.880 91* –0.046 18P 0.448 3 0.367 1 0.008 0 0.108 4 0.421 0 0.001 6 0.008 5 0.066 6 0.048 4 0.941 2

*P < 0.05; **P < 0.01.

Table 4. Correlations between physiological indexes and groundwater level at Alagan

PlantChlorophyll

SolublePro MDA SOD POD IAA GA3 ABA CK

(%)sugars

(µg/g) (µg/g) (U/g) (U/g) (ng/g FW) (ng/g FW) (ng/g FW) (ng/g FW)(µg/g)Populus euphratica

r –0.178 37 –0.537 52 –0.765 85 0.989 13** 0.923 91* –0.913 91* –0.987 00** –0.348 13 0.985 46** –0.494 24P 0.774 1 0.350 2 0.131 1 0.001 4 0.024 9 0.029 9 0.001 8 0.565 9 0.001 2 0.397 4

Tamarix ramosissimar –0.738 98 –0.150 31 –0.019 98 0.930 18* 0.979 78** –0.741 82 –0.821 11 –0.847 06 0.977 10** 0.189 27*P 0.153 7 0.809 3 0.974 6 0.021 9 0.003 4 0.151 2 0.088 3 0.070 1 0.004 1 0.760 5

Apocynum venetumasr 0.198 27 0.313 96 0.845 00 0.983 21** 0.890 33* –0.363 44 –0.928 22* –0.837 80 0.973 92** 0.252 54P 0.749 2 0.606 9 0.071 5 0.002 6 0.042 9 0.547 7 0.022 5 0.076 5 0.005 0 0.681 9

*P < 0.05; **P < 0.01.

MDA, SOD, POD, IAA, gibberellic acid (GA3), ABA, and CK,were obtained by membership function analyses for both theYahepu and Alagan sections (Table 5). Based on the member-

ship function values, P. euphratica was most drought resistant,whereas T. ramosissima and A. venetumas were both rankedlower with similar values.

Physiological Responses to Groundwater Level Changes 523

Discussion

During the past several decades, groundwater levels in thelower reaches of the Tarim River have declined greatly, withsevere impacts on the survival and development of the localvegetation, and putting many plants with ecological and eco-nomic value under threat. For example, the P. euphratica wood-lands in the Tarim River basin contain the largest gene pool ofthat species. Tamarix ramosissima possesses medicinal value.Xinjiang is known as the main growing base of herbs such asA. venetuma. All these plants are experiencing marked habitatdegradation due to changing groundwater levels. In an effortto restore the local vegetation along the lower reaches of theTarim River, an artificial water-recharge program has beenimplemented since 2000, resulting in rising groundwater levels(Chen et al. 2004c). Understanding the influence of groundwa-ter level on the growth and physiological function of the mainconstructive species will not only promote an understanding ofthe mechanisms of drought resistance, but also provide a sci-entific foundation and theoretical guidance to estimate theamount of ecological water needed to accelerate the recoveryof the “Green Corridor” (Chen et al. 2004c).

Different plant species have evolved different mechanismsof drought resistance. During evolution and adaptation to thevery dry environments of the Tarim River basin, P. euphratica,T. ramosissima, and A. venetumas developed their drought-resistant structure and characteristics and adapted to the spe-cial environments by using specific mechanisms. The physi-ological response of T. ramosissima to changes in groundwa-

ter level in the Yahepu section was greatest followed by thoseof P. euphratica and A. venetumas. There was no differencein the physiological responses among the three species at theAlagan section. Different physiological variables demonstrateddifferent sensitivity to water stress. Among the physiologicalvariables investigated, MDA, SOD, and ABA had the highestdegree of sensitivity to the changes in groundwater level, fol-lowed by POD, IAA, and CK.

In the lower reaches of the Tarim River, groundwater is theonly source of water to maintain the natural vegetation. Veg-etation composition, distribution, and growth vigor are closelyrelated to groundwater level; from the upper segment (e.g.Akdun section) to the lower segment (e.g. Kaogan and TaitemaLake), the plant community changes from a mixture of trees,shrubs, and herbs to single shrubbery with the descendinggroundwater level. Plant communities of the upper segmentcomprise trees, shrubs and herbs; the trees are mainly P.euphratica, the shrubs are T. ramosissima, Nitraria sibricaand Halimodendron halodendron, and the herbs include A.ventumas , Kendyr, Alhagi sparsifolia and Phragmitescommunis; with P. euphratica, T. ramosissima and A.ventumas are the dominant species. Plant communities of themiddle segment comprise trees and shrubs, mainly P.euphratica and T. ramosissima, with scarcely any N. sibrica,and herbs are rarely seen. Most vegetation in this region dis-plays a declining trend. The simple architecture of the lowersegment is Tamarix brush or bare ground.

There is a close relationship between plant degenerationand groundwater level in the lower reaches of the Tarim River.

Table 5. Average membership function value of physiological indexes of Populus euphratica, Tamarix ramosissima and Apocynum venetumas Populus euphratica Tamarix ramosissima Apocynum venetumas

SectionYahepu Alagan Yahepu Alagan Yahepu Alagan

Accumulated membership function valueChlorophyll content 0.682 0.489 0.489 0.588 0.447 0.538Soluble sugar content 0.827 0.478 0.639 0.395 0.452 0.479Pro content 0.652 0.501 0.486 0.562 0.444 0.382MDA activity 0.545 0.532 0.478 0.331 0.514 0.552SOD activity 0.540 0.379 0.533 0.411 0.388 0.311POD activity 0.482 0.600 0.523 0.499 0.535 0.652IAA content 0.333 0.506 0.584 0.372 0.587 0.361GA3 content 0.333 0.351 0.549 0.281 0.507 0.452ABA content 0.544 0.432 0.412 0.417 0.303 0.453CK content 0.422 0.429 0.507 0.376 0.433 0.388

Table 6. Drought-resistance ranking of Populus euphratica, Tamarix ramosissima and Apocynum venetumasPopulus euphratica Tamarix ramosissima Apocynum venetumas

Accumulated membership function value 10.057 9.432 9.238Average membership function value 0.503 0.472 0.462Sequence I II III

524 Journal of Integrative Plant Biology Vol. 48 No. 5 2006

At present, the major types of vegetation are degraded Tamarixbrush, Tamarix brush and scattered P. euphratica trees. At thelower segment, the vegetation goes through two deterioratingstages of herbs and shrubs, with bare shifting dunes and withTamarix brush scattered away from them.

After using membership function of fuzzy mathematics toevaluate the drought resistance of the main constructive spe-cies of natural vegetation at the Tarim River desert bank, theirrelative drought resistance was found to be in the order of P.euphratica (the main constructive tree species) > T.ramosissima (the main constructive shrub species) > A.ventumas (the main constructive herb species). Populuseuphratica is a mesophyte, and has experienced the evolutionprocess from a mesophyte ecosystem type to a desert eco-system type. Especially in the Tarim River where the course ofthe river has been broken over a long period of time and the soilseriously degraded causing severe water deficit, P. euphraticahas developed some characteristics to adapt to the droughtenvironment. For example, flourishing roots enable the trees tomaintain a strong capacity to absorb more water under droughtconditions, and therefore alleviate the drought stress. In addition,the roots of P. euphratica extend downward into the deepsoils, and the ratio of root to shoot is high, which allows P.euphratica to easily survive the drought environment. Adultplants of T. ramosissima have deep roots and can grow wellwhen the groundwater level is not lower than 5 m (Song et al.2000). Compared with P. euphratica, T. ramosissima has stron-ger salt and alkali resistance. Apocynum venetumas is a pe-rennial herb with some biological characteristics developed toresist drought, salt, alkali, and sandstorm. However, it is ahalophyte, not a xerophyte (Liu et al. 2004).

Materials and Methods

Study area and sampling

Our study was conducted in the lower reaches of the TarimRiver. The area extends 428 km in length, and runs from Qialain Weili Xian to Taitema Lake in Ruoqiang Xian north of theTaklamakan Desert (Figure 1). It is in the continental warm tem-perate zone, with a dry desert climate and scarce precipitation,high evaporative demand, and large daily and seasonal tem-perature fluctuations. The landform is a composite mode ofcrescent-shaped dune ranges, and longitudinal sand ribbonand brushy sand. From south to east, the river meandersthrough the narrow alluvial plain between the Taklamakan andKuluke Deserts.

We chose the Yahepu section and the Alagan section lo-cated at the upper and middle lower reaches of the Tarim River,where all three focal plant species occurred. Five sample plotswere established at 100, 200, 300, 400 and 500 m distancefrom the river course in accordance with the positions of thegroundwater observation wells. Fully expanded leaves for eachspecies in each plot were collected, packed in sealed plasticbags, and then placed immediately into a cooler containing icefor transport to the laboratory.

Sample analyses

Leaf samples were processed and analyzed for chlorophyll,soluble sugar, Pro, SOD, POD, MDA, GA3, ABA, CK, and IAA inthe laboratory as follows.

Chlorophyll content

To determine chlorophyll content, 0.5 g ground fresh leaveswere put in 80% acetone overnight, and made up to 25 mL with80% acetone after filtering. Optical density (OD) was deter-mined at 650 nm by using an UV-265 ultraviolet-visible

Figure 1. Distribution of the study areas in the lower reaches of the Tarim River.

Physiological Responses to Groundwater Level Changes 525

spectrometer.

Soluble sugar content

To determine soluble sugar content, (0.500 ± 0.005) g freshleaves were oven-dried at 80 °C, leached for 30 min in 80 °Chot water after the addition of 6–7 mL 80% ethanol, and thencentrifuged for 5 min; this procedure was repeated twice. Threeextracts were made up to the required volume with 80% ethanol,then tubes containing 1–2 mL extract were placed in a waterbath to boil off the ethanol, and 10 mL water was added. Themixture was agitated until the sugar was dissolved. Opticaldensity was determined at 620 nm by using a UV-265 ultravio-let-visible spectrometer.

Proline content

Proline content was determined based on the method of Zhu(1983). A quantity of 0.03 g ground fresh leaves were mixedwith 10 mL distilled water without ammonia, placed in a boilingwater bath for 30 min, and then filtrated after cooling. A 5 mLvolume of percolating liquid and 5 mL ninhydrin were coloredfor 60 min in boiling water and then extracted with toluene.Color matching was carried out at 515 nm by using a UV-265ultraviolet spectrophotometer.

Activity of SOD, POD, and MDA

To determine SOD, POD and MDA activity, 0.5 g fresh leaveswere added to 4.5 mL phosphate-buffered saline (PBS; pH 7.8)and ground in a mortar in an ice water bath, then centrifugedfor 15 min at 10 000 r/min. Supernatant liquid (i.e. the enzymeliquid) was used to determine SOD, POD and MDA activity.Activity of MDA was determined based on the method of Yue etal. (1994): 1 mL enzyme liquid and 1 mL 10% TCA were sealedin tubes, placed into boiling water for 15 min, cooled rapidlyand then centrifuged for 15 min at 10 000 r/min; OD was deter-mined by using a UV-265 ultraviolet-visible spectrometer at 532nm and 600 nm. Activity of SOD was determined by using theNBT photoreduction method as described by Liu (1994): a mix-ture of 2.4 mL PBS (pH 7.8), 0.2 mL riboflavin, 0.2 mL methionine,0.1 mL ethylenediamine tetraacetic acid, 0.1 mL enzyme liquid,and 0.2 mL NBT was placed in 4 000 lux sunlight for reduction,then OD was determined by using a UV-265 ultraviolet-visiblespectrometer at 650 nm. Activity of POD was determined basedon the method of Yang et al. (2004): 2 mL mixture of 50 mL PBS(pH 6.0), 28 µL methyl catechol and 19 µL 30% H2O2 was mixedwith 1 mL enzyme liquid, and then OD was determined immedi-ately by using a UV-265 ultraviolet-visible spectrometer at 470nm at 1-min intervals.

Content of GA3, ABA, CK and IAA

To determine GA3, ABA, CK and IAA content, 0.100 ± 0.005 gfresh leaves were ground in liquid nitrogen, extracted with 500µL methanol at 4 °C overnight, and centrifuged. The superna-tant liquid was freeze-dried, dissolved in 30 µL 10% CH3CN,and then analyzed by high performance liquid chromatography.The plant hormone was quantified by using an external stan-dard method. Standard plant hormone was purchased fromSigma. The method of analysis was described by Ruan andWang (2000).

Membership function analysis

Drought resistance in plants involves many pathways and com-plex mechanisms concerning morphological structure, andphysiological and biochemical characteristics interacting withvarious environmental factors. The membership function methodaccounts for the influences of multiple characteristics in as-sessing plant drought resistance.

In the present study, membership function values of fuzzymathematics were used for comprehensive evaluation of thethree plant species with respect to drought resistance by mak-ing use of the 10 measured physiological variables (chlorophyll,soluble sugar, Pro, MDA, SOD, POD, IAA, GA3, ABA and CK).The computational methods used were as follows:

First, equations (1) and (2) were used to calculate the spe-cific membership function values that were correlated withdrought resistance:

Xi = (Xij −Ximin) / (Ximax −Ximin) (1)Xij = 1−(Xij −Ximin) / (Ximax−Ximin) (2)

where Xij is the value of character j for species i, Ximin is theminimum value of character j, Ximax is the maximum value ofcharacter j, and Xij is the membership value of drought resis-tance for species i and character j.

The average membership value of drought resistance of allcharacters is then:

Xi = ΣXij / nwhere X i is the average of membership values of droughtresistance. The values of X i indicate the level of droughtresistance.

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