The role of peltate scales in desiccation tolerance of Pleopeltis polypodioides

The extreme drought tolerance of the resurrection fern is in part the result of the dorsal scales that assist in water distribution and controlled desiccation. We studied the effect of peltate scales on water uptake and loss of the desiccation-tolerant epiphytic fern Pleopeltis polypodioides using optical and FTIR microscopy and staining with calcofluor, solophenyl flavine7GFE, and Ruthenium Red. We provide information on structure, property, and function of the scales by measuring water uptake and dehydration, contact angles, and metabolic activity. Peltate scales mainly contain cellulose, xylogalactans, and pectin. Water is absorbed from the center of scales, and the overlapping arrangement of scales facilitates surface spreading of water. Intact fronds hydrated fully within 5 h of imbibition of the apical pinna, without scales water uptake stopped after 1 h. Hydration rates via rhizomes followed a longer time course but also improved in the presence of scales. Fronds with and without scales lost half of their water content in 15 or 4 h, respectively. The overall metabolism of rapidly dehydrated fronds was significantly reduced compared with slowly dehydrated fronds. Thus, water management and metabolism of Pleopeltis are dependent on surface properties determined by peltate scales.

Protecting against water loss: analysis of the barrier properties of plant cuticles

The cuticle is the major barrier against uncontrolled water loss from leaves, fruits and other primary parts of higher plants. More than 100 mean values for water permeabilities determined with isolated leaf and fruit cuticles from 61 plant species are compiled and discussed in relation to plant organ, natural habitat and morphology. The maximum barrier properties of plant cuticles exceed that of synthetic polymeric films of equal thickness. Cuticular water permeability is not correlated to the thickness of the cuticle or to wax coverage. Relationships between cuticular permeability, wax composition and physical properties of the cuticle are evaluated. Cuticular permeability to water increases on the average by a factor of 2 when leaf surface temperature is raised from 15 degrees C to 35 degrees C. Organic compounds of anthropogenic and biogenic origin may enhance cuticular permeability. The pathway taken by water across the cuticular transport barrier is reviewed. The conclusion from this discussion is that the bulk of water diffuses as single molecules across a lipophilic barrier while a minor fraction travels along polar pores. Open questions concerning the mechanistic understanding of the plant cuticular transport barrier and the role the plant cuticle plays in ensuring the survival and reproductive success of an individual plant are indicated.