Temporal and light-based changes in carbon uptake and storage in the spring ephemeral Podophyllum peltatum (Berberidaceae)

The single leaf of the spring ephemeral Podophyllum peltatum (Berberidaceae), emerges in the spring, when forest over-story leaf area index (LAI) is low (∼0.8) and senesces approximately 12 weeks later when LAI is high (∼4.0). This study characterized (i) seasonal changes in gas exchange characteristics (maximum light-saturated photosynthetic rate, foliar dark respiration rate, apparent quantum efficiency, light saturation point and light compensation point) and carbohydrate storage in field populations and (ii) examined the hypothesis that observed seasonal changes in photosynthetic characteristics represent photosynthetic acclimation to maximize photosynthetic carbon gain as light availability declined. In field populations, as LAI increased, total chlorophyll concentration increased while declines occurred in all gas exchange characteristics except apparent quantum efficiency. Over the same period, soluble sugar content of the rhizome declined and starch content increased rapidly during the first 4 weeks after which it stabilized. Foliar tissue exhibited little change in carbohydrate pool over the season. Laboratory populations grown at high and low light levels differed in all gas exchange parameters except light saturation point, but illustrated similar seasonal changes in gas exchange characteristics to both each other and the field population. The similarity of temporal changes in gas exchange characteristics of field (seasonal decline in light availability) and laboratory (no seasonal decline in light availability) suggests that P. peltatum possesses limited ability to photosynthetically acclimate to declining light availability. It appears that seasonal carbon gain in P. peltatum may be primarily controlled by leaf lifespan and the fraction of lifespan occurring under high illumination conditions early in the season.