This study was conducted to determine the effects of adjuvant, ammonium sulfate, and temperature at the time of treatment on the efficacy, absorption and translocation of thidiazuron (Dropp^®) defoliant on cotton DES 119 (Gossypium hirsutum L.). Treatments were made using a mechanical spray table to greenhouse grown cotton with 15 to 20 leaves. Treated plants were placed immediately into environmentally controlled growth chambers for 4s hours with photoperiods of 14 hours light and 10 hours dark. Dropp was applied at 0.05 lb ai in 20 gallons of water per acre with either no additive, 1.25% v/v crop oil concentrate (83 parts paraffinic oil and 17 parts nonionic surfactant), 1 lb/A ammonium sulfate, or a combination of crop oil concentrate and ammonium sulfate at these rates. Three replications of each treatment were placed into separate growth chambers with one chamber at 85 F day and 70 F night and the other at 70 F day and 55 F night temperature. The experiment was conducted four times.

The number of leaves that fell from the plants were counted daily for 14 days after treatment, and the number of leaves that dropped each day was calculated as the percent of the total number of leaves that dropped in 14 days. Five days after treatment at the higher temperatures, leaf drop was 17% with no additive, 37% with the addition of crop oil concentrate, 40% with ammonium sulfate, and 75% with the two additives combined. At the lower temperatures, there was less than 10% leaf drop with all treatments. At 10 days after treatment, leaf drop was not different between treatments at the higher and lower temperatures. At this time, leaf drop was 75 to 85% with no additive and 82 to 100% with each additive applied separately or combined.

Leaf regrowth was determined at 21 days after treatment as grams fresh weight of leaves on the treated plants as compared to that on the untreated plants. Regrowth from treatments at each the high and the low temperatures was reduced 55 to 60% with addition of the combined additives and 44 to 50% with the separate additives or with no additive.

The effect of additives and temperature was determined on absorption and translocation. (14)c-radiolabeled Dropp with specific activity of 15 mci/mg was applied in the amount of 0.1 pCi in 10 pl of water both with and without additives as described above. Application was to the upper surface of the second true leaf of cotton plants with 4 to 5 leaves. Treatments were applied inside environmentally controlled growth chambers and the plants were maintained under the same photoperiods and temperatures as with the nonradiolabeled Dropp.

After 24 hours, the unabsorbed radiolabeled Dropp was washed from the leaf surface with two consecutive wash procedures, and the amount of radioactivity was determined by liquid scintillation spectrometry. Absorption was calculated as the percent of the applied radioactivity which was not recovered from the leaf surface.

The absorption of C-Dropp was not affected by variations in temperature during the time of treatment but was affected by the additives to the treatment mixture. with no additives, absorption was 7 to 10%. With 1.25% crop oil concentrate, absorption was 33 to 46%. The addition of ammonium sulfate resulted in 18 to 19% absorption and the combination of ammonium sulfate and crop oil concentrate increased absorption to 65 to 68%.

Translocation was determined by autoradiographs of the remainder of the shoots of the treated plants. There was no movement of the radiolabel away from the treated leaf as determined by autoradiographs of the treated plants.