J. Schonherr et al., Rates of cuticular penetration of 1-naphthylacetic acid (NAA) as affected by adjuvants, temperature, humidity and water quality, PLANT GR R, 31(1-2), 2000, pp. 61-74
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.