English abstract
Mesquite (Prosopis juliflora) is an invasive plant that is threatening agricultural lands and
native floral diversity because of its ability to adapt and grow rapidly under environmental
conditions of arid and semi-arid areas, urging the Omani government to launch the national
campaign for mesquite eradication in 2016. Its invasion appears to be driven by allelopathy,
a biological phenomenon by which P. juliflora releases allelochemicals influencing the
neighboring plants negatively. However, little is known about the effects of the invasive
species on the vegetation cover especially indigenous trees at realistic concentrations of its
allelochemicals. Furthermore, the plants exposed to the allelochemicals extracted from
different parts of P. juliflora show variable responses with limited understanding on how
these allelochemicals interfere with molecular components of the target plants. Here, we
aim to study and determine the allelopathic effects of P. juliflora on two common Omani
plant species (the native ghaf, Prosopis cineraria, and samr or acacia tree, Vachellia
tortilis) using water or aqueous extracts and powders of P. juliflora leaves and pods. Seeds
of P. cineraria and V. tortilis were exposed to low levels (compared to previous
investigations) of leaves/pods powders (0.5 and 5.0% w/w) or concentrations of their
aqueous extracts (10 and 100 mg/L), planted in 1:1 mixed media (sandy soil: compost), and
seedlings were observed for 28 days after germination. Seed germination percentages as
well as growth parameters (length of root and fresh mass of seedlings of both species) were
not significantly affected by the presence of aqueous extracts or powders of P. juliflora
leaves and pods. While the exposed V. tortilis seedlings had higher content of chlorophylls
a and b particularly with treatment of pod extracts and powders, no change in these
pigments was observed for P. cineraria seedlings. On the contrary, there was an increase
in the content of carotenoids in P. cineraria seedlings exposed to the extracts and reduction
in the content for the seedlings exposed to the powders. Neither P. juliflora extracts nor
powders influenced carotenoids content of V. tortilis seedlings. Unlike the V. tortilis
seedlings, exposure to P. juliflora powders resulted in higher accumulation of proline in
the P. cineraria seedlings, indicating a stressful condition imposed by the invasive species.
The relatively low levels of allelochemicals may not impede processes (e.g. mitotic
division) responsible for root elongation and shoot development of the studied plants.
Despite the inconsistent responses observed for leaves and pods extracts and powders, the
measured effects appeared to be more pronounced in presence of the powders compared to
water extracts of P. juliflora. Our laboratory-based experiments generally revealed limited
allelopathic effects of the invasive species on native plants (P. cineraria and V. tortilis),
nevertheless further studies should be carried out in field soils sampled near or under the
canopy of P. juliflora due to accumulation of leaves that may be somewhat affecting
neighboring plants.
