Adjuvant-induced alterations of cuticular waxes and their consequences for mechanisms of drought stress resistance

Abstract

The most important function of the cuticular waxes is the shielding of the plant from uncontrolled water loss as well as from pathogens, UV radiation and pollutants. Penetrating agrochemicals are also effectively shielded against by the waxes, which often necessitates the addition of adjuvants. While the mechanisms of adjuvant ac-tions on the cuticular waxes for improving spray delivery are well understood, the consequences of these actions for the functionality of the cuticular waxes and the subsequent integrity of the plant organism have hardly been studied. The present work aimed to contribute to this understanding with an emphasis on the consequences for mechanisms of drought stress resistance. <br /> The objective of the first chapter is to thoroughly review the available research on the possible modifications that adjuvants may impose on the cuticular waxes and to summarize the literature on the consequences of these al-terations for the barrier functionality. Wax plasticization was found to be widely accepted as the main modification that adjuvants cause in the waxes and the main reason for the improved penetration of adjuvant-containing sprays. The extent of plasticization is known to depend on the solubility of the adjuvant in the specific wax. Plasti-cizing adjuvants were long assumed to exclusively influence amorphous waxes, but more recent findings call for a reassessment of this assumption. Thus, a more comprehensive explanation of the permeability-increasing mech-anism of adjuvants is proposed in which effects on crystalline waxes and effects of surfactant-admicelles adsorbed into wax crevices are also considered. Adjuvants can also change the appearance of epicuticular waxes. This is usually attributed to the solubilizing effect of surfactants. In this work, loss of integrity due to plasticization of the epicuticular structures and coverage of the wax structures by adsorbed adjuvants are proposed as alternative reasons. There is evidence that adjuvant-induced changes may affect susceptibility to pathogen infection, water repellency, and water loss. However, the available literature is sparse. <br /> The most important function of the waxes embedded in the cuticle is the restriction of water loss. As plasticizing adjuvants enhance the permeability of the waxes for incoming agrochemicals, the hypothesis was made here that as a consequence, the exiting of water through the cuticular barrier of living plants might also be facilitated. Therefore, the second part of the present work investigates the effect of adjuvants on cuticular transpiration in vivo. This question has not been addressed so far in the available literature. Brassica oleracea and Malus domesti-ca were chosen as model organisms due to their very different epicuticular wax characteristics, and were treated with adjuvants with and without surface activity and with differing solubility. In accordance with the hypotheses made in the first chapter of this thesis, the potential to increase minimum transpiration and to alter the epicuticu-lar waxes was not determined by the presence of surface activity, but instead related to the plasticization capabil-ity of the adjuvant, as estimated by its solubility. Minimum transpiration increased up to 10.4-fold immediately after application of rapeseed methyl ester in Brassica oleracea. This increase was associated with a delayed sto-matal closure and a strong increase of leaf wettability. Most effects reverted within the measurement period of 14 days. The minimum conductance of Malus domestica, whose epicuticular waxes were amorphous as opposed to the crystalline epicuticular waxes of B. oleracea, was considerably less affected by the adjuvants. <br /> On a physiological level, drought stress is best avoided by low cuticular water loss, but increased cuticular water loss does not necessarily equal impaired drought resistance. Drought resistance of plants is established through a complex interplay between different mechanisms. Plants may effectively counteract increased water loss on a physiological level by adjusting osmotic potential or elasticity of the cell walls for the goal of maintaining turgidity and thereby functionality. Thus, to fully understand the effect of adjuvants on physiological drought resistance, cuticular conductance must be evaluated in conjunction with turgor-related parameters. The pressure-volume curve is widely accepted to be the most thorough method for understanding the dynamics of turgor, but reliable analysis of pressure volume curves is difficult due to a lack of objective and reproducible analysis techniques. Therefore, in the third chapter of this thesis, an R-package (‘pvcurveanalysis’) was developed and published which meets the need for standardization of the analysis by detecting the turgor loss point via a modeling approach. Reliable detection of the turgor loss point determines the quality of all other pressure volume parameters be-cause the turgor loss point is used to separate between the influence of osmotic and pressure potential on the overall water potential. The package eliminates the need for subjective assessments of curve characteristics and improves the reliability of the analysis when curves consist of few data points or contain random noise. The pack-age was developed using data derived from apple and tested on data from an experiment on B. oleracea showing the plasticity of pressure volume curve parameters in response to drought.