Localization of Thermo-Osmotically Active Partitions in Young Leaves ofNuphar lutea

The submerged roots and rhizomes of the aquatic vascular macrophyte Nuphar lutea (L.) Sm. are aerated, at least in part, by pressurized ventilation. Depending on temperature differences of up to 5 K between the inside of young, just-emerged leaves and the surrounding air, pressure differences of 79 to 100 Pa higher than atmospheric are detectable inside the lacunuous spongy parenchyma of the leaf blades. The pressurization is a consequence of structural features of leaf tissues separating the air filled spaces of the spongy parenchyma from the atmosphere. These tissues are acting as thermo-osmotic partitions. Whereas the dimensions of the stomatal openings (about 5·6 × 2·4 μm) and of the intercellular spaces of the palisade parenchyma (diameters about 15 μm) are too large, those of the monolayers of cells separating the palisade and the spongy parenchyma (diameters: 0·7−1·2 μm) are small enough to impede free gaseous diffusion. This inner non-homogeneous partitioning gives rise to the so-called Knudsen diffusion, a physical phenomenon leading to pressurization of the warmer air inside the spongy parenchyma. The rising pressure difference is strong enough to establish an air flow through the aerenchyma of the whole plant and out of the most porous older leaves in which a temperature induced pressurization is never detectable. These thermo-osmotically active leaves enhance the influx of air to the rhizome and the diffusion path for oxygen to the roots is shortened to the distance between rhizome and root tips. Therefore, pressurized ventilation in Nuphar is seen to be of considerable ecological importance for plant life in anaerobic environments.