PS 64-72
The eco-physiology of marcescence in dominant oak species in Missouri
Most leaves abscise from deciduous trees in autumn. A zone of distinct cells with weakened cell walls, called abscission layer forms near the base of the leaf's petiole, which breaks during abscission. Abscission is controlled by the concerted interaction of plant hormones. However, in some deciduous species, like oaks the abscission layer does not form until spring, referred to as marcescence. Though the retained leaves may deter grazing herbivores and protect nascent buds in the spring, delayed leaf shedding negatively impacts litter formation and nutrient cycling. Despite its ecological relevance, the eco-physiology and environmental controls of marcescence are not yet understood. Previous studies suggested a metabolic link between the synthesis and emission of isoprene and leaf level abscisic acid (ABA) contents in emitter species, one of the plant hormones also coordinating abscission. To investigate the molecular eco-physiology and the role of isoprene emission in marcescence we initiated a long term study on the natural conservation areas of the 225 acres Missouri Western State University Campus during the winter of 2012, using geographical information systems for mapping marcescent species. In the identified species we followed seasonal variations in photosynthesis, isoprene emission and hormone synthesis throughout the following year.
Results/Conclusions
Trimble GPS and Arc GIS technology was used to map marcescent species during the winter of 2012 and 2013, identified to be pin oaks (Quercus palustris) and post oaks (Q. stellata). Photosynthesis of south-facing, sun-exposed leaves at 2-4 m canopy height was measured biweekly in a controlled-environment cuvette enclosure. In parallel with photosynthesis measurements, isoprene emission rates (at 30°C and 1000µmolm-2 s-1 PAR) were sampled from the cuvette outflow onto adsorbents and analysed by GC-MS. Photosynthesis and isoprene emission rates were correlated with local climate parameters. Leaves were sampled for biochemical hormone assays. Preliminary results indicate that leaves of trees marcescent during the winter of 2012 emitted higher amounts of isoprene (100-120 µgCg-1h-1) throughout the 2013 summer, and had lower ABA contents (20-23 pmolg-1fw) than control trees (50-80 µgCg-1h-1 isoprene and 40-45 pmolg-1fw ABA), which did not retain their leaves. We hypothesize, that increased isoprene emissions may decrease the amount of ABA available to coordinate hormonal changes necessary for the formation of the abscission layer in the fall in marcescent trees. As leaf isoprene emissions are expected to increase in a warming climate, the occurrence of marcescence in isoprene emitters may also affect bio-geochemical nutrient cycles in the future.