Growth responses of Acacia seyal, Acacia negrii and Acacia asak trees to water stress under field conditions

Growth responses of Acacia seyal, Acacia negrii and Acacia asak trees to water stress under field conditions
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  Growth response of   Acacia seyal ,  Acacia negrii and  Acacia asak  trees towater stress under field conditions Ibrahim M. Aref and Loutfy I. El-Juhany Plant Production Department, College of Agriculture, King Saud University,P. O. Box 2460 Riyadh 11451, Saudi Arabia   Abstract. A field experiment was carried out at the Agricultural Research andExperiments Station of King Saud University in October 1997 and lasted for twoyears. The aim of this experiment was to evaluate the effects of drought stress on thegrowth of   Acacia asak, Acacia negrii and Acacia seyal .   Six months old seedlings of these species were planted in the field using a randomized complete block design. Thetrees were irrigated at 160, 255 or 500 mm of a class "A" evaporation-pan records.The irrigation regimes significantly affected most of the traits measured for acaciaspecies. The trees in severe water stress treatment (500 mm) had significantly lowerstem height, diameter, leaf area and dry matter production compared with theseedlings in well-water treatment (160 mm). While the trees in moderate water stresstreatments (255 mm) had growth traits values did not change significantly from thoseof either severe water stress or well-water treatment. The species respondeddifferently to irrigation regimes used. Stem height, stem diameter, diameter of themain root, mean diameter of lateral roots and dry matter production of   Acacia asak   were significantly the least among the three species.  Acacia seyal had stem height,stem diameter, total leaf area and dry matter were significantly greater than those of   A. negrii . Allocation of dry matter between different tree parts did not change due towater stress. Keywords:  Acacia seyal ,  A . negrii ,  A . asak  , water stress, evaporation pan, growth  Introduction Acacia species   had a   great value in arid and semi arid regions as well-known trees dueto their adaptation to different climatic conditions and providing the locals with manyof their needs. Therefore, acacia species are widely distributed through the driertropical and subtropical regions, they have been called the most successful survivors    in arid and semiarid regions, and possess most of the features required to withstandsevere climatic conditions [1].   However, the successful achievement of acacia speciesthat obtained in their native regions may not be easily so when they planted outsidethese regions. Planting an acacia species in any region requires preceding tests for itsadaptation to the prevailing environmental conditions in that region. Some acaciaspecies are widely spread in Saudi Arabia and their communities represent the climaxstage of xerophytic vegetation and generally have high cover and low speciesdiversity [2].   It has been noticed that acacia species introduced and planted in thecentral part of Saudi Arabia have performed a growth vary from that attained in theirnative habitats in other parts of the country [3].   Evaluation of the properties of introduced species therefore, is essential to determine how they will cop with the newconditions.   Plants deal with drought through modifying some of their morphologicaland physiological attributes. Some morphological modifications are visible andrelatively easy to be measured and may reflect physiological adaptations that oftenneed elaborated work and specific apparatus to be determined. Acacia trees asxerophytic plants have the ability to resist drought and cope with arid environmentsthrough conserving water [4]. However, most of the research that were carried outconcerning the effects of water stress on acacia species in Saudi Arabia studiesfocused only on trees at seedling stage [4, 5]. Hence, the present study was carried outto examine the   growth responses of   Acacia seyal ,  Acacia negrii and  Acacia asak  towater stress treatment in the field in Riyadh, Saudi Arabia.  Materials and methods Site description The site where the experiment was carried out is located at 24 º 6 ` N, latitude; 46 º 5 ` E,longitude, 650 m altitude with temperature ranged between 10ºC in winter and 37ºCin summer (as an average of season); and 50 mm rainfall, annually. The soil is sandyloam with average content of 61, 23 and 15% sand, silt and clay, respectively [6].  Plant material  Six months old seedlings of     Acacia seyal Del.,  Acacia negrii Pichi-Sermoli and  Acacia asak  (Forssk.) Willd grown from seeds were planted in the field at theAgricultural Research and Experiments Station of King Saud University near Riyadh.    The seeds of   Acacia seyal were collected from Riyadh region and those of the othertwo species from Southwest region of Saudi Arabia. Statistical design and treatments Nine experimental plots of 4 × 6 m each were assigned to carry out the experiment inthe field using a randomized complete block design in factorial arrangement includedthree acacia species and three irrigation regimes. Six trees from each species wereplanted in each experimental plot in October 1997. Three plots were allocated to eachtreatment of the three irrigation regimes namely at 160, 255 and 500 mm of cumulative readings of Class-A evaporation pan representing well water, moderateand severe water stress, respectively (Table 1). On November 11 all the plots wereirrigated to field capacity then the irrigation was given at specific time according totreatments. Each plot was giving one cubic meter of water at the time of irrigation.Imposing irrigation treatments depending on cumulative readings of evaporation panas illustrated in Table (2). The first period was the longest for all treatments (70, 90and 134 days for well water, moderate and severe water stress treatments,respectively) while the fifth period was the shortest (9, 15 and 27 days for the sametreatments, respectively. This variation was due to the time of the year in which theperiod lies. The first period was almost within winter (November 12-March 27) sothat the evaporation was less and consequently the interval lengthened. While thefifth was during the summer (August 10-September 5) so that the evaporation washigh and consequently the interval shortened (Table 1). On the other hand, this trendwas similar for all treatments but they differed in the length of irrigation intervals(Fig. 1).  Measurements and harvesting By the end of the experiment, stem height from soil surface to the top of tree and stemdiameter at 30 cm height from soil surface were measured using tree hypsometer andtree caliper, respectively. The trees were severed at soil surface then its above partwas divided into leaves, stem and branches. The main and lateral roots of each treewere extracted from the soil by hand and their length and diameter were measured.Immediately after harvesting the trees, total leaf area of each tree was scaled usingleaf area scanner (Hayshai Denkoh Co., LTD. Tokyo, Japan). Leaves, stem, branches    and roots of each tree were oven-dried at 70 ° C for 48 h for the leaves and at 103 ° C forthe other parts. Statistical analysis The collected data were analyzed using analysis of variance statistical method throughSAS computer program [7]. The means of treatments were distinguished using L. S.D. test [8]. Arcsine transformation was done for data when necessary. Table 1. Diagram of the field experiment including three acacia species and threeirrigation treatments using a randomized complete block design.BlocksTreatmentsB1 B2 B3Well water  A . asak  (6 trees) A . seyal (6 trees) A . negrii (6 trees)   Moderate water stress  A . seyal (6 trees) A . negrii (6 trees) A . asak  (6 trees)   severe water stress  A . negrii (6 trees) A . asak  (6 trees) A . seyal (6 trees)   Table 2. Number and dates of eight irrigation periods based on cumulative Class-Aevaporation pan readings within which the trees were irrigated in well-water (160mm), moderate (255 mm) and severe water stress (500 mm) treatments.Number of days after which trees were irrigatedNo. Period160 mm 255 mm 500 mmDescendingorder1 12/11/1997 - 27/03/1998 70 90 134 A2 28/03/1998 - 19/05/1998 18 25 51 D3 20/05/1998 - 26/06/1998 10 17 35 F4 27/06/1998 - 31/07/1998 11 16 28 G5 10/08/1998 - 05/09/1998 09 15 27 H6 06/09/1998 - 16/10/1998 10 18 38 E7 17/10/1998 - 30/01/1999 28 40 70 B8 31/01/1999 - 03/04/1999 25 37 59 C Results Analysis of variance procedure showed significant reductions in most of the growthcharacteristics measured for acacia trees in water stress treatments comparing withthose for the trees in well-water treatment. Averaged across species, stem height and    diameter of the trees in severe water stress treatment decreased by 28% and 40%compared with those of the trees in well-water treatment ( P <0.01) and ( P =0.0007),respectively (Table 3). Leaf, branches, stem, root and consequently total dry weight of acacia trees averaged across species, decreased by 34, 25, 47.5, 41 and 37% inmoderate water stress and by 65, 76, 72, 65 and 70% in the severe water stresscompared with those of the trees in well-water treatment ( P <0.05), ( P <0.05),( P <0.001), ( P <0.001) and ( P <0.0001), respectively (Fig. 1). There were alsoreductions in total leaf area ( P =0.002), diameter of the main root ( P <0.05) and themean diameter of the lateral roots ( P <0.05) of the trees in the severe water stresstreatment comparing with those of the trees in well-water treatment. Stem height, leaf and branches dry weight and the number of lateral roots of the trees in moderate waterstress treatment had values did not change from those of either severe water stress orwell-water treatments. Stem and root dry weight had values did not differ between thetrees in both moderate and severe water stress treatments. Total dry weight was theonly variable that showed significant decrease from well water to moderate to severewater stress treatment.Statistical analysis of the data showed that  Acacia seyal had growth characteristics(averaged across treatments) were significantly greater compared with those of either  A . negrii or A . asak, apart from total leaf area which was significantly greater in  A .  asak  than that of   A . seyal . For instance,  A . negrii and A . asak  trees had mean heightsand diameters were lower by 22, 21, 59 and 51% than those of   A . seyal trees,respectively ( P <0.0001). Consequently, mean shoot dry weight of   A . seyal trees was2.5 and 8 folds as much as that of   A . negrii and   that of  A . asak  trees, respectively( P <0.0001). Moreover, root and total dry weights of   A . negrii and A . asak  were 55,14, 44 and 12% of that of  A . seyal trees ( P <0.0001). However, mean total leaf area of   A . asak  trees was greater than that of   A . seyal by more than 80% ( P =0.008) (Table 3).There were treatment × species interactions for stem diameter, stems and total dryweights, ( P =0.008), ( P <0.01) and ( P =0.003), respectively, indicating changing themagnitude of treatment effects with species. For instance, total dry weights of   Acaciaasak  , A . negrii and A . seyal trees decreased by 39, 42, 31% and 40, 51, 77% inmoderate and severe water stress treatments,   respectively (Fig. 1).
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