Abstract:
Penetration of ¹⁴C-labeled plant growth regulators into the surface of apple
(Malus pumila M.) leaves pre-soaked in 10 ml I⁻¹ solutions of surfactant was
measured by detecting changes in radioactivity of the solution in glass cylinders
sealed to the astomatal, adaxial surface of the leaf discs. Penetration of 2,4-D
[(2,4-dichlorophenoxy)acetic acid] and B-NAA (2-naphthaleneacetic add) was
significantly increased by surfactant Pegosperse 200-ML (polyethylene glycol
monolaurate) but unaffected by four other surfactants with the same
hydrophilic-lipophilic balance (HLB) as Pegosperse 200-ML. Penetration of
2,4-D and B-NAA decreased as the HLBs of Pegosperse (polyethylene glycol or
PEG) surfactants increased. Penetration of maleic hydrazide (l,2-dihydro-3,6-
pyridazinedione) was unaffected by any surfactant tested.
To understand surfactant-cuticle interactions, enzymatically-isolated cuticles
of apple leaves were soaked in PEG 100-O (diethylene glycol oleate) solution
and the weight change of cuticles or the surface tension change of the solutions
was measured to determine the amount of surfactant sorbed or desorbed by the
cuticles. PEG 100-O sorption and desorption of both cuticular membranes
(CMs) and dewaxed cuticular membranes (DCMs) occurred primarily in the first
three hours. The sorption was concentration-dependent and the DCMs always
sorbed more surfactant than the CMs. PEG 100-O could be completely
desorbed from both CMs and DCMs.
The impacts of surfactant on cuticular penetration of 2,4-D were studied by
placing ¹⁴C-2,4-D (0.78 mM) in cylinders affixed to the external surface of cuticles
after the cuticle segments had been treated with 10 ml I⁻¹ of various PEG
surfactant. The same surfactant pretreatments were included in sorption studies
in which cuticle segments were immersed in ¹⁴C-2,4-D solution. Penetration of
2,4-D were inversely related to the HUB of the surfactant. Sorption of 2,4-D by
CMs was unaffected by surfactant pretreatment. PEG 100-O significantly
increased cuticular penetration of dissociated 2,4-D at pH 4.5. The enhanced
penetration occurred only when surfactant was in the cuticles. Results of this
study support the 'hydrophilic channel' hypothesis, viz. that surfactant may induce
'hydrophilic channels' formation in the cuticle, consequently enhancing the passing
of polar molecules of pesticides through the cuticle.
