Effects of chemical drying agents on the fine structure of alfalfa cuticles and epicuticular waxes

Chemical drying agents reduce the curing time of alfalfa (Medicago sativa L.), but mechanisms have not been defined to explain this reduction. This study was conducted to define the physical effects of drying agents on alfalfa stem surfaces and to determine the distribution of applied cations on the stems. Field-grown alfalfa was treated with water and nine solutions consisting of all possible combinations of three cations (Na+, K+, Cs+) and three anions (C2H3O2-, CO32-, OH-). Following treatment application, plants were air-dried under ambient conditions, stem samples excised and prepared for scanning electron microscopy (SEM) and energy dispersive x-ray microanalysis (EDXA). Epicuticular waxes were removed with chloroform on one set of samples to allow observation of the cuticle. All nine treatments reduced drying time relative to the control. Based on SEM observations, treatment with C2H3O2- compounds resulted in large cracks in epicuticular waxes and the cuticle, while CO32- and OH- treatments altered wax texture and caused pitting of the cuticle. Cesium was detected on the Cs-treated samples with EDXA and its distribution was uniform over the stem surface. Drying agents affected the physical structure of alfalfa epicuticular waxes and cuticles, but treatments that produced the most subtle physical changes caused the greatest reductions in drying times. These observations do not support extensive wax removal, wax platelet reorientation, or inhibition of stomatal closure as mechanisms of increased drying rates.