Do alterations in leaf isoprene emission affect the timing of senescence in velvet bean (Mucuna pruriens)?
The abscission of senesced leaves at the end of a growing season is controlled by the concerted action of plant hormones on cells of the abscission layer formed at the base of the petiole, layer which breaks during abscission. However, in some species, for example oaks, senesced leaves are retained until spring, referred to as marcescence. Though the retained leaves may 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 over marcescence are not yet understood. In a previous field study we found a delay in the onset of senescence in leaves of oak trees typically retaining their senesced leaves over the winter, and demonstrated a strong negative correlation between the rates of isoprene emission and pre-senescent foliar abscisic acid (ABA) concentrations, one of the hormones also coordinating leaf abscission. To verify, that sustained isoprene emission may deplete hormones available to control the onset of senescence and later, regular leaf abscission, we followed variations in photosynthesis, pigment composition, ABA amounts and senescence markers during the life-cycle of velvet bean (Mucuna pruriens), grown at differing light intensities, to modulate isoprene emission, in a greenhouse study.
Plants were grown from seeds in pots containing one part of potting soil, peat moss and vermiculite each, with manufacturer recommended slow release fertilizer, at 14 h photoperiod, 30C day/20C night temperatures, and 850 µmolm-2s-1 and 400 µmolm-2s-1 photosynthetically active radiation (PAR) for high, and low light treatments, respectfully. Photosynthesis was measured weekly in a cuvette enclosure and isoprene emission at 30°C and growth PAR was sampled onto adsorbents and analysed by GC-MS. Mature leaves were sampled for pigment, hormone and senescence marker assays. Preliminary results indicate that leaves of plants growing under high light emitted higher amounts of isoprene (35-40 nmolm-2s-1) and had lower ABA contents (19-22 pmolg-1fw) than under low light (15-23 nmolm-2s-1 isoprene and 40-45 pmolg-1fw ABA). Photosynthesis rates were 10-15 µmolm-2s-1 in both treatments. Membrane degradation markers accumulated earlier in leaves grown under low light, to concentrations of 38-45 µmolg-1fw. 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. As leaf isoprene emissions are expected to increase in a warming climate, the occurrence of delay in leaf shedding in isoprene emitters may also affect bio-geochemical nutrient cycles in the future.