Effect of Rhazya stricta Foliage Leachate on Seedling Growth and Survival of Some Range Plant Species

Rhazya stricta. an evergreen poisonous shrub is rapidly invading large areas of rangelands in Saudi Arabia. A laboratory study was conducted to investigate the effect of its dry foliage leachate on seedling growth and survival of some range plants including R. stricta its self at different stages of seedling growth. Results indicated that seedlings were more susceptible to allelopathic effects at their early days of growth. Seedling growth was depressed and hence survival was negatively affected. Root growth was more restricted than shoots. Species varied in their response to leachate. Sensitivity ranking of species was as follows: R. stricta = Achillea fragrantissima > Farsetia aegyptia > Atriplex halimus > Lasiurus scindicus. Least sensitive species and species having fast and vigorous seedling growth should be selected for revegetation of R. stricta invaded range sites. Introduction Association and disassociation patterns between certain plants species are widely known. Such phenomena may be governed by direct competition for necessary growth factors or through addition of allelopathic chemicals into the soil environment (Rice, 1984). It has been documented that allelopathy may play an important role in plant-plant interference (Johnston, 1961; Rasmussen and Rice 1971; Newman and Rovira, 1975; Bokhari, 1978; Rice, 1984). The allelopathic effects include germination inhibition (Gressel and Holm, 1964; Al-Saadawi and Rice, 1982a,b; Williamson et al., 1992; Patil, 1994), seedling growth retardation (Al-Saadawi and Rice, 1982a,b; Conard, 1985; Bhatt and Todoria, 1990; Kalburtji and Mosjidis, 1993a,b) and poor seedling survival (Conard, 1985; Smith, 1990). Rhazya stricta Decne. (Apocynaceae), an evergreen poisonous shrub, is extensively invading large areas of deteriorated rangelands in Saudi Arabia. The species occurs over most rangelands in the Kingdom (Allred, 1968; Migahid, 1989). Assaeed (1996) reported that R. stricta was a co-dominant species in a A.M. Assaeed and A.A. Al-Doss 14 low diverse vegetation of a deteriorated range site. He suggested that one mechanism by which this species spreads is through its allelopathic effects on other species. The objectives of this study were to: 1) Investigate the effect of R. stricta dry foliage leachate on growth and survival of seedlings of some range plant species and 2) Assess the sensitivity of tested species to the leachate in terms of seedling growth and survival. Materials and Methods Leachate from air dried leaves of Rhazya stricta Decne was prepared by soaking leaves in tap water at a ratio of 1: 10 weight to volume for 12 hours (Muller and Muller, 1956). Leachate was filtered using Watman No. 1 paper. Seeds of Achillea fragrantissima (Forssk.) Sch.Bip., Atriplex halimus L., Farsetia aegyptia Turra, Lasiurus scindicus Henr. and R. stricta were sown in 10 cm diameter pots filled with sandy soil. Seeds of R. stricta were immersed in running tap water for 2 hours before sowing as it is reported to contain a germination chemical inhibitor (El-Naggar, 1965). Plants were thinned to 10 seedlings of uniform size per pot 10 days after sowing. Pots were randomly assigned to one of four irrigation treatments with dry foliage leachate of R. stricta for a period of 10 days at ages of 10, 20, 30 and 40 days from sowing. Another set of pots was irrigated with tap water as a control treatment. Seedlings were grown in a growth chamber. Temperature was 25 ± 2°C constant, light duration was 12 hours and light intensity was 1350 F.C. The experiment was arranged and analyzed as a split plot design with four replicates (Gomez and Gomez, 1984; SAS, 1988). Main plots were assigned to leachate treatments white species occupied the subplots. At the end of the experiment (60 days from sowing), seedling survival, shoot height, root length and total biomass were recorded. Results and Discussion A summary of analysis of variance (Table 1) indicated high significant differences (P < 0.01) among leachate exposure time treatments in all measured growth parameters. Species, also, responded differently (P < 0.01) to R. stricta leachate in all studied traits except for biomass where the difference between species was significant at P < 0.05 (Table 1). A high significant interaction (P < 0.001) occurred between treatments and species except for seedling biomass (Table 1). Seedling growth and survival were highly depressed by R. stricta foliage leachate. The effect was more pronounced when seedlings were exposed at an earlier age to leachate. No differences were observed between seedlings exposed at 10 and 20 days from sowing in terms of shoot and root length. Effect of R. stricta Foliage Leachate on Seedling Growth... 15 However, seedling exposure at 30 days differed significantly from other treatments in both traits (Table 2). Shoot length was less affected compared to root length at initial growth of seedlings. However, at more advanced growth (30 and 40 days from sowing), shoots and roots were equally affected. This result was in agreement with findings of other workers (Conard, 1985; YoderWilliams and Parker, 1987; Bhatt and Todaria, 1990). Root growth retardation would expectedly be reflected on top growth as indicated by the present results. However, Rivard and Woodard (1990) observed continuous shoot growth in Typha latifalia irrespective of extremely restricted root development in presence of an allelopathic agent. TABLE 1. A summary of analysis of variance of the effect of Rhazya Stricta leachate on seedling shoot length, root length, biomass and survival of five range species. Source d.f. Shoot length Root length Biomass Survival Replication 3 NS NS NS NS Treatments (A) 4 ** ** ** ** Error (a) 12 Species (B) 4 ** ** * ** A × B 16 ** ** NS ** Error (b) 60 * and ** indicate significant differences at P < 0.05 and P < 0.01 respectively. NS not significant at P = 0.05 TABLE 2. Percent reduction in shoot length, root length, biomass and survival of seedlings irrigated with water containing Rhazya stricta leachate at different seedling ages. Seedling age Shoot length Root length Biomass Survival days % reduction relative to control 10 67.86 79.27 87.02 60.59 20 66.87 72.13 81.16 55.66 30 54.74 53.77 70.65 45.27 40 34.85 36.03 58.95 22.02 LSD(0.05) 10.95 12.19 10.95 15.94 Similarly, seedling biomass was highly depressed especially at early days of growth (87 and 81 % reduction at 10 and 20 days growth respectively). However, when leachate was applied to 40 days old seedlings, the impact on biomass was less severe (59% reduction). Several researchers have reported similar results in different species (Smith, 1989; Bhatt and Todaria, 1990; Kalburtji and Mosjdis, 1993a). Reduction in total biomass may have been substantially contributed by root restriction especially at initial seedling growth. A.M. Assaeed and A.A. Al-Doss 16 Further, the present results did not indicate any seedling growth recovery when exposed to leachate at initial growth. Seedling survival was drastical1y reduced by leachate. Seedling loss was highest when seedlings were exposed at their initial growth (61 % reduction). At age of 40 days from sowing, only 22% of seedlings were lost which was significantly less than any other treatment (Table 2). Other workers reported similar results (Conard, 1985; Smith, 1989). The present results indicate that young seedlings were more Susceptible to growth retardation and death in presence of R. stricta than older ones. Species varied in their response to leachate. A. fragrantissima and R. stricta were the most severely affected both in terms of seedling growth and survival (Table 3). Lasiurus scindicus was the least affected in all measured parameters except shoot length where F. aegyptia was affected the least. Seedling growth traits of L. scindicus were equally more affected (40-44% reduction), than survival (12% reduction). Although seedling survival of F. aegyptia was slightly affected (16% reduction), biomass was severely affected (76% reduction) and was mostly due to restriction of root growth (Table 3). Atriplex halimus behaved in a similar manner to F. aegyptia. The variation among species in response to leachate is normal and is in agreement with results reported by several researchers (Bhatt and Todaria, 1989; Smith, 1989; and 1990). TABLE 3. Percent reduction in shoot length, root length, biomass and survival of seedlings in five range plant species irrigated with water containing R. stricta leachate. Significant interactions were observed between species and time of seedling exposure to leachate in seedling shoot, root growth and survival. All species were more affected as leachate was applied at early days of growth with root growth being more restricted (Table 4). Shoot and root length of A. fragrantissima were decreased by 34% and 52% respectively as seedlings were exposed to leachate at age of 40 days. Shoots of A. halimus, F. aegyptia and L. scindicus were moderately decreased as seedlings were exposed to leachate at age of 10 days but the effect diminished as seedlings grew. Seedling shoot and root length of R. stricta were very restricted when exposed to leachate at age of 30 days (81 % and 85% reductions respectively). Although no significant interaction occurred between Species Shoot length Root length Biomass Survival % reduction relative to control A. fragrantissima 83.48 87.98 94.91 85.62 A. halimus 39.85 40.53 72.40 27.56 F. aegyptia 29.82 43.82 76.13 16.00 L. scindicus 43.97 40.19 39.48 12.12 R. stricta 83.27 88.97 89.31 88.12 LSD(0.05) 15.61 10.59 13.25 17.67 Effect of R. stricta Foliage Leachate on Seedling Growth... 17 species and time of seedling exposure to leachate on their effect on biomass, seedling biomass generally decreased as seedlings were exposed earlier to leachate. TABLE 4. Seedling shoot length, root length, biomass and survival (60 days) of five range plant species irrigated with water containing R. stricta leachate at different seedling ages. Treatment A. fragrantissima A. halimus F. aegyptia L. scindicus R. stricta