Influence of hydration and temperature on the rheological properties of plant cuticles and their impact on plant organ integrity

The rheological properties of enzymatically isolated plant cuticular membranes (CM) of mature leaves of Yucca aloifolia L., Hedera helix L., Nerium oleander L., and Lycopersicon esculentum Mill. fruit were analyzed in a transient load-creep test. Cuticular membrane samples were tested dry and hydrated as submerged in distilled water. Apparent plastic extensibility turned out as delayed elastic extensibility, that is, CM showed visco-elastic behavior, if the system had sufficient time (up to 24 h) for relaxation. Both extensibility and the apparent plastic component increased in the hydrated state. In addition to hydration, different temperature regimes ranging from 7 degrees to 30 degrees C were established during testing to evaluate temperature sensitivity of cuticular rheology. Temperature-dependent changes of the rheological properties as small as 1 degrees C could be detected. Extensibility was correlated with neither the thickness of the cuticles nor the specific structure of the cuticles as determined by scanning electron microscopy (SEM) and digital image analysis. For tomato fruit, no significant differences in extension behavior could be detected between CM and fruit skins, including the cell wall proper as analyzed in hydrated condition. The results demonstrate that the cuticle is a flexible biopolymer with rheological properties that can be dynamically modified by both hydration and temperature. The cuticle appears to have a pronounced impact on the overall mechanical behavior of the tomato fruit, implying a substantial contribution to the mechanical integrity of the whole organ. The described rheological properties of the tomato fruit CM are important features to accommodate growth processes without the loss of physiological integrity, but they may also help to understand fruit cracking as affected by hydration and temperature.