Leaf water shedding: Moving away from assessments based on static contact angles, and a new device for observing dynamic droplet roll-off behaviour

1. Leaf wettability and drainage characteristics of different taxa are often hypothesised to have emerged as a result of evolutionary selection, perhaps to limit the duration of leaf wetness, or to direct water toward efficiently to the soil and root system, rather than suffering loss to evaporation. Methods for quantifying leaf wetting and drainage are however not well-developed. The present work describes a low-cost, electro-mechanical tilting table intended to facilitate precise and reproducible measurements of droplet shedding from leaves, describe by the roll-off angle alpha(roll). 2. The new tilting table uses widely-available components (microcontroller, stepper motor and driver, liquid-crystal display (LCD) and custom operating code) to achieve controlled tilting through the range 0(degrees )to >90(degrees) at user-controlled rates of tilting. It is suitable for field use, such that leaf specimens can be tested within minutes of collection. 3. Water shedding tests on juvenile leaves from Homolanthus populifolius, native to the wet tropics of northern Queensland, Australia, show that testing of whole leaves (rather than small excised samples) reveals quite complex behaviour in which the open leaf surface is hydrophobic but major adaxial veins are strongly hydrophilic and can trap droplets. These can remain attached to the leaf at inclinations beyond vertical. Moreover, the apparent droplet roll-off angles are dependent on the tilt speed applied. 4. Droplet roll-off tests used to characterise the propensity for leaf wetting or water shedding require controlled and reproducible experimental conditions, and a device suitable for studying the whole intact leaf surface. Preliminary results on H. populifolius show complex adaxial leaf surface characteristics, with mixed hydrophobic and hydrophilic components. This suggests that overall propensity to retain or shed water droplets is likely to depend on the size and intensity of rain or canopy drip from above. This makes the inferring of evolutionary costs or advantages more challenging and more likely to co-vary with regional environmental conditions.