Abstract:Pot-grown peanut plants (Arachis hypogaea L.) cv. Yue-You No. 551–116 at five-leaf-stage were treated with 0, 266 or 532 ppm growth retardant PP333 aqueous solution as a soildrench and liquid spray to the whole shoot, smeared on the stems or fully developed leaves. Forty days after treatment, growth rate of main stem and transpiratory rate were measured andthe ultrastructures of leaf and chloroplast were also observed under electron microscope. The results obtained are as follows: 1. PP333 resulted in reduction of stem elongation and dryweight of shoot, but there was no influence or even slight enhancement with the dry weight of root, thus, the root: stem ratio was increased. By applicating PP333, transpiratory rate was lessthan that of the control, but the water storage cells in treated leaves were significantly larger. These anatomical and physiological characteristics of treated plant are an advantage in tolerating the drought stress. 2. Compared with the control, PP333 caused the epidermal cellsize smaller and reduced the number of grana lamella and stroma lamella, so development of chloroplast was inhibited. 3. Significant retardation occurred when PP333 was applied to soilas compared with that applied to the shoot. Among shoot applications, the stem is more effective at reducing stem elongation than that to the leaf. Based on the results mentioned above, authors suggest that the most effective method of PP333. application for peanut is soil drench.