In a Different Light: Irradiation-Induced Cuticular Wax Accumulation Fails to Reduce Cuticular Transpiration

The cuticle, an extracellular hydrophobic layer impregnated with waxy lipids, serves as the primary interface between plant leaves and their environment and is thus subject to external cues. A previous study on poplar leaves revealed that environmental conditions outdoors promoted the deposition of about 10-fold more cuticular wax compared to the highly artificial climate of a growth chamber. Given that light was the most significant variable distinguishing the two locations, we hypothesized that the quantity of light might serve as a key driver of foliar wax accumulation. Thus, this study aimed to isolate the factor of light quantity (photosynthetic photon flux density [PPFD]) from other environmental stimuli (such as relative humidity and ambient temperature) and explore its impact on cuticular wax deposition and subsequent rates of residual foliar transpiration in different species. Analytical investigations revealed a significant increase in cuticular wax amount with increasing PPFD (between 50 and 1200 µmol m^-2 s^-1) in both monocotyledonous (maize and barley) and dicotyledonous (tomato and bean) crop species, without altering the relative lipid composition. Despite the increased wax coverages, rates of foliar water loss did not decrease, further confirming that the residual (cuticular) transpiration is independent of the cuticular wax amount.