Rates of cuticular penetration of 1-naphthylacetic acid (NAA) as affected by adjuvants, temperature, humidity and water quality

When NAA is used for fruit thinning, results can be unpredictable. Problems with foliar penetration of NAA can contribute to this variability. Using i solated pear leaf cuticles we have investigated effects of humidity, temper ature, hard water and selected adjvants on rates of cuticular penetration o f NAA. If NAA was dissolved in deionised water about 40% of the applied dos e penetrated in 8 h, while water having a pH > 7 practically eliminated cut icular penetration of NAA, even at high humidity (100%) and moderate temper atures (20 degrees C). Rates of penetration were much lower at 10 degrees C and 55% humidity. Adding urea, ammonium sulphate or Tween 20 increased rat es of penetration but not to a great extent. Highest rates of penetration w ere obtained when solutions were buffered at pH 4 with DL-lactic acid and t he accelerator adjuvant Genapol C-100 was added. With this combination, abo ut 70% of the applied NAA dose penetrated within 8 h at 20 degrees C and 40 % at 10 degrees C, respectively, even at low humidity (55 to 60%) and when hard water was used as carrier. It is argued that poor performance of NAA a s chemical thinner must be expected when hard water is used for spraying an d when the pH of the spray liquid is not adjusted to values around 4, such that a significant proportion of the NAA is non-ionised. Only non-ionised N AA can penetrate cuticles. Rates of foliar penetration of NAA can be greatl y increased, especially at low temperatures, by adding 0.2 to 2.0 g l(-1) G enapol C-100. Since NAA is destroyed by UV light, spraying should be done i n the evening. High humidity during the night is favourable for foliar pene tration, but low temperatures are detrimental and addition of an accelerato r adjuvant (i.e. Genapol C-100) is necessary. If NAA must be sprayed during cold weather (5-10 degrees C), higher NAA concentrations may be used becau se rates of penetration during the night are proportional to dose and exces s NAA left on the foliage will be destroyed by UV light after sunrise.