Cuticular Waxes in Altered Environments and Their Role in Leaf Water Loss Dynamics.

Cuticular waxes undeniably form the primary barrier restricting uncontrolled water loss from leaves, yet the functional consequences of environmentally induced wax modifications for residual (cuticular) transpiration remain a matter of debate. This review summarises current knowledge on how major abiotic stimuli influence foliar cuticular wax amount and composition and critically re-evaluates their effect on transpirational properties of leaves. By focusing on a strictly curated set of studies integrating gas chromatography for chemical wax analysis and permeance-based residual (cuticular) transpiration measurements, we demonstrate that light quantity seems to represent the most dominant driver of wax deposition and that naturally increased wax amounts do not inherently translate into an enhanced water barrier, with the latter aspect challenging the recurrently postulated 'more cuticular wax must equal lower rates of residual (cuticular) transpiration' narrative. Instead, bridging modern research with long-standing transport models indicates that the qualitative composition and spatial arrangement of cuticular wax might constitute the more decisive factors orchestrating cuticular water barrier properties. While the available data are still insufficient to identify clear causal relationships between compound classes, chain length distributions and structural heterogeneity affecting water permeability, they might point to further complexities, such as unintended off-target effects as a consequence of genetic engineering.