Allelopathic effect of aqueous and alcoholic extracts of Eucalyptus camaldulensis Dehnh. on Amaranthus retroflexus L. growth stages compared to 2,4-Dichlorophenoxyacetic acid herbicide

Pigweed (Amaranthus retroflexus L.) is one of the most widespread weeds in the world, and regarding the amount of damage caused to crops, it is the third dominant dicotyledonous weed in the world. It is recommended to reduce the use of chemical herbicides through the replacement of non-chemical methods in sustainable agricultural management, and the use of plants' allelopathic effects is one of these methods. On the other hand, plants in different growth stages have different allelopathy sensitivity.

To evaluate the allelopathic effect of aqueous and alcoholic extracts of eucalyptus (Eucalyptus camaldulensis Dehnh) on Pigweed (Amaranthus retroflexus) growth stages, an experiment was carried out as a factorial based on a randomized complete block design with three replications in the greenhouse of Kashan Barij Essential Company, Iran. Treatments included aqueous and alcoholic extracts of eucalyptus (50, 75, and 100 %), controlled (0.002 concentration of 2,4-Dichlorophenoxyacetic acid, Ethanol 70%, distilled water), and three growth stages (five-leaf stage, seven-leaf stage, and stem elongation). The studied traits were measured in all three growth stages one week after treatments. To measure traits, ten plants were randomly selected from each pot. The studied traits included leaf length and width, total leaf wet weight, total leaf dry weight, stem wet weight, stem dry weight, stem length, root length, root wet weight, root dry weight, total plant weight, and shoot/root ratio. A ruler was used to determine the length and width of the leaf, the length of the root, and the length of the stem. Analysis of the variance of the data was done by MSTAC software. If the experimental factor was significant, the LSD test was used to compare the means.

Leaf width, total leaf fresh weight, total leaf dry weight, stem fresh and dry weight, stem length, root length, root fresh and dry weight, total plant weight, and shoot/root ratio were affected by the growth stage where the extracts were used. The effect of the concentration of aqueous and ethanol extracts of eucalyptus on leaf length and width, total leaf fresh weight, total leaf dry weight, stem fresh weight, stem dry weight, stem length, root length, root fresh weight, root dry weight, total dry plant weight, and shoot/root ratio was significant. Spraying at the stem and 5-leaf stages had the highest and lowest leaf width, stem length, and root dry weight, respectively. The extract application at the stem elongation and five-leaf stage had the highest and lowest leaf width, stem length, and root dry weight, respectively. The application of distilled water in the stem elongation phase produced the maximum leaf length (3.06 cm), total fresh and dry weight of plant leaf (1.00 and 0.2764 gr), stem fresh and dry weight (1.35 and 0.393 gr), and total fresh and dry plant weight (2.80 and 0.801 gr), respectively. The application of 75% concentration of eucalyptus ethanol extract at the 5-leaf stage produced the lowest total fresh and dry weight of plant leaf (0.188 and 0.0521 gr), stem fresh and dry weight (0.303 and 0.046 gr) and total fresh and dry plant weight (0.633 and 0.1157 gr), respectively.

The aqueous and alcoholic extracts significantly decreased Pigweed growth indices, and its inhibitory effect was not significantly different in most traits with 2-4-D herbicide. The inhibitory effect of alcoholic extract on pigweed growth was greater than aqueous extract. Given that almost all eucalyptus extracts, either alcoholic or aqueous, significantly reduced Pigweed growth, it may be possible to reduce the pressure of this weed on crops by combining and using them as a bio-herbicide.