Surfactants and plant systems

Surfactants find widespread use as components of herbicidal formulations. In the absence of herbicide many surfactants show an inherent phytotoxicity at high concentrations (around 1 %). Some surfactants at low concentrations stimulate growth. Surfactants have been reported to influence a variety of plant processes, photophosphorylation [234], protoplasmic streaming [235], mitosis [236], elongation of root hairs [237], and permeability of cell walls [238]. 

As with animal and human studies, isolated plant cell data cannot always be extrapolated to the whole plant or organ. The data of St John et al [239] on isolated leaf cell permeability changes following surfactant contact cannot necessarily be interpreted for the whole system as the significance of the effects depend on the ability of the surfactant to penetrate the leaf surface to affect cells inside the leaf Some of the results on isolated cells are shown in Table 10.27. 

Surfactant and concentration (cpm/100 /il of solution -f cells) (cpm/100 //I of solution — cells) (% of total) 

Aqueous solutions with a surface tension of around 70 mN m do not enter the stomatal pore, but penetration of the pore occurs when the surface tension is lowered by addition of surfactant [243]. The mechanism of penetration is by no means simple. The contact angle between the solution and the surface of the pore is important, but there is a suggestion that the morphology of the pore wall is equally important [244]. However, in the pear Pyrus communis L. cr. Bartlett, stomatal penetration of aqueous solution into leaves was promoted by surfactants according to their surface activity [245]. Dioctyl sodium sulphosuccinate was the most effective surfactant used and Tween 20 least effective. However, only 0.5 to 4.5 % of the stomata were penetrated suggesting that stomatal penetration is, indeed, a relatively unimportant pathway of entry to the leaf. 

Wallihan et al. [246] have produced results with spray solutions on citrus trees which suggest that spray solutions were at least partly entering the leaves by way of the stomata solutions with surface tensions below 30 mN m have since been shown to be able to penetrate into the sub-stomatal regions of leaves [244], the cuticle in this cavity being generally thinner than the external cuticle and thus more readily penetrated by chemicals. 

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