Allelopathic Effects of Rhazya stricta on Germination

 of Some Range Plant Species


Abdulaziz M. Assaeed and Abdullah A. Al-Doss

Dept. of Plant Production, King Saud Univ. P.O. Box 2460

Riyadh 11451, Saudi Arabia


Rhazya stricta, an evergreen poisonous shrub, has invaded large areas of overgrazed rangelands in Saudi Arabia. To understand the mechanism by which this noxious plant spreads, a laboratory experiment was conducted to assess its allelopathic potential to the germination of some range plants. Leachates were prepared from fresh and dry foliage of Rhazya stricta. Seeds were germinated in petri dishes moistened with either leachates or tap water.

          Leachates inhibited germination with fresh foliage leachate being most severe. Complete germination inhibition was observed in Farsetia aegyptia and Pennisetum divisum. Haloxylon salicornicum and Lasiurus sindicus were the least affected. Rate of germination was also significantly reduced. Haloxylon salicornicum was the least affected, 56 and 25% in presence of dry and fresh leachates respectively, while other species were drastically reduced. Washing and regermination of the remaining non-germinated seeds indicated that seed viability of Farsetia aegyptia and Pennisetum divisum may have been lost in presence of leachates. Osmotic effects of leachates may have contributed to germination inhibition in Atriplex halimus, Achillea fragrantissima, Lasiurus sindicus and Rhazya stricta. While inhibiting early germination of other species, Rhazya stricta needs enough water to leach water soluble inhibitor present in its seeds which in turn would also be sufficient to overcome osmotic effect of foliage leachate.

          It is therefore concluded that Rhazya stricta may spread over deteriorated rangelands through its allelopathic effect on other species and least sensitive species should be selected for when revegetation is considered.