الفهرس 
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Abstract
Allelopathy is a phenomenon by which a plant species affect germination and growth of the other associated plants through the potential release of toxic compounds into the environment. Studies on the allelopathic effects of tree species on understory vegetation are common in the forest ecosystems. But in the new urban regions, these studies are scarce. The study of green areas in the new cities is a topic of particular interest to urban ecology. In addition, the potential use of leaf litter from the urban trees in field of allelopathy, in general, and in organic agriculture, in particular, may be a site of interest.
Ficus retusa is one of the most common ornamental trees in Egypt and all over the world. As a result of the continuous annual pruning of F. retusa, its leaf litter accumulated in large quantities in different locations in urban ecosystem. Theses leaf residues can cause an ecological problem due to their harmful effects on the understory vegetation. However, the allelopathic effect of this tree has not been investigated. The present study aimed to: (1) evaluate the allelopathic effects of F. retusa leaf litter on cover, composition and diversity of the understory species, and (2) determine whether the application of F. retusa leaf residues in agroecosystem could improve crop productivity and provide a safe method for weed control.
To achieve the first goal, a field study was conducted to assess the effect of F. retusa leaf litter on the cover, richness and diversity of understory vegetation. A total of 62 plots were selected in different seasons. Thirty-one plots located under tree canopies were designated as treatments and the other thirty-one plots outside those canopies were set as controls. Some properties of litter affected and unaffected soils were measured to determine the potential effect of Ficus tree litter on the soil properties of urban gardens. A greenhouse pot experiment was performed to investigate the allelopathic effect of litter-affected soils collected under tree canopies on germination and growth of three understory species detected in the field study namely: Amaranthus viridis, Melilotus indicus and Trifolium resupinatum. The osmotic potentials corresponding to those of litter-affected soils were prepared using polyethylene glycol (PEG) (6000) solution and assessed on the same tested species to separate the osmotic effect of litter infested soil from its allelopathic effect.
To accomplish the second goal of this study, leaf residues from F. retusa were applied in the agroecosystem at different rates (250, 500 and 1000 g m-2) to soils in green beans fields to determine the effects of these treatments on weed density and biomass as well as chemical characteristics of the amended soils.
Finally, it would be necessary to use an adequate analytical tool such as high performance liquid chromatographic (HPLC) analysis to identify the possible allelochemicals, particularly phenolic and flavonoid compounds, in the leaf litter and in the litter-affected soils under the tree canopies as well.
The field investigation in the urban ecosystem revealed that F. retusa leaf litter had an inhibitory effect on the cover of many understory species, total plant cover and species richness. Besides, the litter-affected soil significantly reduced the germination and some of the growth variables of the selected target species. The litter-infested soils had lower pH and higher EC values. The PEG solution with an osmotic potential equal to that of the litter-affected soil did not affect the germination and growth parameters of the same target species.
The incorporation of F. retusa leaf residues into two cultivated fields cropped with green beans (Phaseolus vulgaris L. cv. bronco) significantly reduced the density and biomass of many weed species, total weed density and total weed biomass. This inhibition varied according to the rate of applied leaf residues and the weed species found. In contrast, application of leaf residues had no effects on the growth and yield parameters of P. vulgaris. The amendment of leaf residues with field soils decreased pH and increased EC, organic carbon and some available nutrients such as N and K. The effects of leaf residues in the cultivated fields depended on the dose applied.
HPLC analysis of F. retusa leaves indicated the presence of several phenolic and flavonoid compounds among which chlorogenic acid was the most abundant phenol (15.3 µg g-1 dry leaf residue), while quercetin and kaempferol were the most prominent flavonoids (8.21 and 6.1 µg g-1 dry leaf residue, respectively). HPLC analysis for litter-affected soils revealed significant concentrations of some phenolics such as caffeic acid, coumaric acid, ellagic acid and resorcinol, and of some flavonoids such as quercetin compared to the unaffected soils (control soil). In addition, the total phenolics and flavonoids were considerable higher (by 65.84% and 47.54%, respectively) in litter-affected soil than the control soil.
Conclusively, the results of the present study revealed that:
1) Ficus retusa leaf litter had inhibitory effects on the understory weed vegetation in the urban gardens as represented in the reduction obtained in cover and richness of understory species. Weed suppression in the urban ecosystem was mostly parallel to that obtained in the cultivated fields cropped with green beans.
2) The significant reduction in germination and growth parameters of target species under greenhouse and in agro- and urban ecosystem was not related to the change in soil properties, but it could be attributed to the phytotoxins released from F. retusa leaf residues into field soils.
3) Weeds were more sensitive to the phytotoxins derived from F. retusa leaf residues in comparison to the crop assayed (P. vulgaris), and the response of the target plants was dose- and weed species-dependent.
4) The leaf litter derived from the urban trees has to be controlled and continuously collected under the canopies of the trees to maintain plant cover and potentially conserve native plants. Besides, maintaining the cleaning process of urban gardens and parks is certainly a civilized principle to be accomplished throughout the governments. On the other hand, use of F. retusa leaf residues has become possible to be applied as a safe tool for weed management in organic farming systems.