Comparative corticular photosynthetic characteristics in N-fixing vs. non-N-fixing woody species

Background/Question/Methods

We compared actinorhizal and non N-fixing woody species in order to determine whether species with a symbiotic N-fixing association with the bacterium, Frankia, have higher corticular photosynthesis as compared to non N-fixers.  Actinorhizal species used for this study were Myrica cerifera, Myrica pensylvanica, and Elaeagnus umbellata and non N-fixers were Diospyros virginiana, Prunus serotina, and Quercus virginiana.  All species used for this study inhabit the barrier islands along the US Atlantic coast and are subjected to numerous stressors including high light and temperatures and low nutrients and soil moisture.  Corticular photosynthesis is the refixation of respirational CO₂ within woody stems and results in carbon assimilation without any associated water loss thereby increasing whole plant carbon use efficiency and water use efficiency.  We hypothesized that actinorhizal species would have higher corticular photosynthesis because they are not N limited.  We quantified % refixation, photosynthetic photon flux density (PPFD, μmol m^-2 s^-1), pigment concentrations, and A_net for 1^st and 3^rd order branches of each species during the growing season.  We also determined light transmittance through the periderm for 3^rd order branches and leaf A_net for each species.

Results/Conclusions

While leaf A_net did not differ between N-fixers and non N-fixers (p = 0.698), % refixation within stems was significantly higher in N-fixers (65 ± 3 %) compared to non N-fixers (46 ± 4 %) for 1^st order branches (p = 0.001).  Incident PPFD and chlorophyll a concentrations of 1^st order stems were significantly higher for actinorhizal species (p = 0.011 and 0.028, respectively) compared to non N-fixers which may partially explain the observed refixation values.  Corticular photosynthesis occurs at times when leaves are unable to photosynthesize such as during the winter when temperatures are too low for foliar photosynthesis or during drought conditions.  Carbon dioxide builds up within stems which can result in higher temperature optima for stems allowing corticular photosynthesis to occur when temperatures are too high for foliar photosynthesis.  The dominance of actinorhizal species on many Atlantic coast barrier islands may be a result of the symbiosis with Frankia and the increased CUE and WUE associated with corticular photosynthesis which may be particularly important on young sandy soils that are low in both nutrients and moisture.