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Abstract
Allelopathy is a mechanism by which weeds affect crop growth and production. Studies on the allelopathic effects of weed species on crop plants can be applied for weed management. The importance of allelopathy in nature has also attracted ecologists’ attention with the main goal of using the phenomenon in interpretation of many ecological subjects related to plant community structure and organization.
The present study determines the allelopathic potential of Brassica nigra on germination, growth and productivity of two cultivated crops: Trifolium alexandrinum (Egyptian clover) and Triticum aestivum (wheat), and two weed species: Phalaris paradoxa and Sisymbrium irio. Moreover, the composition and species diversity of Brassica nigra communities in Beni Suef governorate, Egypt were investigated in relation to edaphic factors, crop type and the role of allelopathy.
Laboratory bioassays were used to investigate the effect of different extract types of B. nigra (water, ethanolic and chloroform) on germination speed, percentage of germination, seedling growth, mitotic stages and mitotic indices of the target species for eight days of exposure. Greenhouse experiment was used to demonstrate the effect of mulching and root exudates of B. nigra on emergence, growth and the root and leaf anatomy of the test species at the end of 35 days. Moreover, the composition and species diversity of Brassica nigra communities were investigated in 46 stands representing the winter crops where black mustard grows.
For both water and chloroform extracts, gradual inhibition was observed for most of the measured parameters (germination speed, percentage of germination, seedling growth and mitotic indices) for the target species through increasing the extract concentration. Complete germination inhibition for all target species was recorded at the highest concentration of water extract (40 g/l) while the lowest concentration of ethanol extract (300 ppm) was stimulatory for plumule length of wheat and Egyptian clover as well as radicle length of the later. Increasing the concentration of water extract resulted in a remarkable increase for the percentage of prophase but the rest mitotic phases and mitotic indices decreased for all test species. Phalaris paradoxa was more sensitive for the extracts compared with the remaining species. Some allelochemicals have been detected by HPLC analysis namely: ferulic, protocatechuic, caffeic, syringic and p-coumaric acids, and vanillin (aldehyde) that were amounting to 123.7, 18.22, 58.12, 93.5, 38.68 and 12.03 μg/g dry weight, respectively. These allelochemicals may enhance and support the allelopathicity in B. nigra.
In the greenhouse pot experiment, mulching effect of black mustard, at the rate 5, 10 and 20 g plant residue / kg soil, induced a stimulatory effect on emergence and growth criteria of all test species including root and shoot lengths, root/shoot ratio dry matter and leaf area. Moreover, mulching resulted in a more elaborated vascular system in roots and wider root diameter indicating more efficient conductivity, but its influence on mesophyll thickness of leaf lamina varied among the study species. On contrast, root exudates of living Brassica had inhibitory effect on the test species. They inhibited the emergence of Sisymbrium irio completely while emergence and growth of the remaining test species were significantly suppressed. These exudates greatly inhibited root vascular system and root diameter of all test species indicating less efficient conductivity, while their effect on mesophyll thickness of leaf lamina was not sharp.
The vegetation of the B. nigra communities was classified by the Two Way Indicator Species Analysis (TWINSPAN) into seven vegetation groups. Two vegetation groups were present in the reclaimed land, while three groups represented the old land. Each of the remaining two groups included stands from both reclaimed and old cultivated land. The weed communities of the reclaimed land exhibited higher species diversity (as measured by the species richness, Shannon-Wiener index and evenness index) than those of the old fields due to the difference in agricultural practices such as the intensive manure application and frequent irrigation that may enhance weed diversity in reclaimed land. Also, wheat recorded the highest species diversity than other crops due to the single harvest made for such crop at the end of the growing season and the ancient cultivation (introduced to Egypt some 5 millennia ago). The non significant correlations between the importance value of B. nigra and those of Phalaris paradoxa and Sisymbrium irio indicates that allelopathic effect of B. nigra on these two species do not seem to affect their abundance and, thereby, do not seem to play a role in community composition. Edaphic factors especially soil texture, CaCO3 and organic carbon play a role in the distribution of some weed species.
Based on the results of the present study we may conclude that: (1) the phytotoxicity of B. nigra not only due to the isothiocyanates, but also due to other phenolic compounds, (2) some weeds are highly sensitive for allelochemicals derived from B. nigra (3) the stimulatory effect of Brassica mulch may be caused by trapping of soil moisture by the surface residue or addition of organic matter, mineral nutrients or growth stimulators, (4) root exudates were suppressive for the test species, indicating the presence of less volatile allelochemicals or more toxic allelochemicals in root exudates when compared to mulch of Brassica nigra, (5) the allelopathic potential of B. nigra did not seem to affect the composition of weed communities, and (6) the soil characters especially soil texture, CaCO3, and organic carbon significantly affected the distribution of some weed species.