Agricultural adjuvants may impair leaf transpiration and photosynthetic activity

Adjuvants such as surfactants are commonly incorporated into agrochemical formulations to enhance the biological efficiency of foliar sprays by improving the wetting behavior of the spray and/or the penetration of the active ingredients into the leaf tissues. Penetration accelerating adjuvants are known to increase the cuticular permeability and may alter the cuticular barrier to water loss. However, none or very little emphasis has been given to the impacts of adjuvants on crop water balance or drought tolerance, a very important factor affecting crop performance under water scarcity. Two model crops with strongly varying leaf traits, kohlrabi (Brassica oleracea) and apple (Malus domestica) seedlings were grown in controlled environments. Three adjuvants with varying solubility in the cuticle, i.e. octanol-water partition coefficients (logKow) were selected: rapeseed methyl ester (RME) and the surfactants alkyl polyglycoside (APG) and polyoxyethylated tallow amine (POEA). The higher the logKow of the adjuvant, the stronger was the increase of minimum epidermal conductance (gmin, an essential parameter describing plant drought tolerance). However, such effects depended on the physio-chemical properties of the leaf surface. In comparison to kohlrabi, the adjuvant effects on gmin of apple leaves were relatively weak. The increase of gmin was associated with a decrease in contact angle and with an alteration of the wax microstructure. Furthermore, POEA affected photochemical efficiency of kohlrabi leaves. Some adjuvants could have a temporal influence on transpirational water loss and gmin. At repeated applications, they might alter the effective water use and possibly reduce drought tolerance of some horticultural crops.