This study was conducted to evaluate the effect of 2,4-dichloro phenoxy acidic acid (2,4-D) on seed germination and the early growth of mesquite (Prosopis juliflora) seedlings. The three sizes of mesquite at the flowering stage (small, medium, and large) were treated with 2,4-D at different rates (0, 6×103, 12×103, 18×103 and 24×103 mg a.i. / L) dissolved in two solvents (diesel or water), the application method basal bark treatment was used in the two different seasons. The results showed that: the three factors (2,4-D rates, tree sizes, and solvent types) significantly decreased germination percentage with the increase of 2,4-D rate dissolved in diesel with all sizes of trees, while the emergence percentage, radical length, and shoot height were significantly decreased as 2,4-D rate increase dissolved in diesel or water. In the winter season, the root length of seedlings raised from seeds collected from the three tree sizes treated with 2,4-D dissolved in water was significantly decreased as a 2,4-D rate increased, while in the rainy season, the root length was affected with 2,4-D dissolved in diesel. In addition, the effect of 2,4-D rates and solvent types was evaluated for all parameters, the results found that: 2,4-D dissolved in diesel significantly decreased germination percent than the corresponding rates dissolved in water. Emergence percentage and shoot height and radical length were significantly decreased with 2,4-D dissolved in diesel or water compared to their control. In the winter season, the root length is affected by 2,4-D dissolved in water, while in the rainy season affected by 2,4-D dissolved in diesel.

2,4-dichloro phenoxy acetic acid (2,4-D), known as auxin-like herbicide or synthetic auxin, mimic the effect of the natural auxin indole-3-acetic acid (IAA). Afield and laboratory experiments were arranged to study the effect of (2,4-D) on the proline content of leaves of natural stand mesquite trees (Prosopis juliflora) treated with 2,4-D at different rates (0, 6×10 3 , 12×10 3 , 18×10 3 and 24×10 3 mg a. i. / L). Herbicide rates dissolved in two solvents (diesel or water), which 2,4-D mixtures sprayed around the lower part of the tree stem. The experiment designed in the factorial experiment in Randomized Complete Block Design (RCBD) with three replicates. The results showed that: High rate of 2,4-D (24×10 3 mg) gave a high concentration of proline, which gave 115,59 and 108,10 mg/g.: 142.25 and 145.60 mg/g in the two winter, and two rainy seasons, respectively. In both winter seasons leaves proline increased as the 2,4-D concentration increased dissolved in diesel with the three sizes, while in both rainy seasons, all 2,4-D concentrations dissolved in water or diesel increased proline content compared to the control. The results in this research confirmed that: 2,4-D herbicide led to a significant increase in leaves proline content of mesquite trees to a degree that lead to toxicity of the tree, which is one of the mechanisms of action of auxin overdose.