Wednesday, March 24, 2010

C4 photosynthesis and nitrogen

Comparison of foliar N concentrations among clades.

Since the beginnings of our modern understanding of C4 photosynthesis, it has been set that C4's are more efficient with water and nitrogen. Yet, there have long been unexplained patterns for C4's that didn't match the assertion of greater nitrogen use efficiency. For example, C4 grasses in the field often have lower foliar N concentrations, but also lower root N concentrations. Why would this be? If the leaves need less, shouldn't the roots get more? Also, some C3 grasses like Chionochloa can have foliar N concentrations as low as 6 mg g-1. Most C4's have higher concentrations and only a few have been observed to be below that. Also, foliar N concentrations for any given species are highly plastic and dependent on the balance between C and N supplies and demand. If a given species can have N concentrations that range 30 mg g-1, just how important is the C4 photosynthetic pathway.

Turns out, probably not much. Taylor et al. (2010) used a phylogenetically structured screening experiment to measure a number of morphological and physiological traits of grasses. In doing so, they could compare C3 and C4 species controlling for phylogeny. The research upholds the notion that C4 photosynthesis confers greater water use efficiency to plants. Yet, after controlling for phylogenetic relationships, there were no differences between C3 and C4 species in their foliar nitrogen concentrations. 

By no means the last word on the topic. For example, they only measured ~30 species. Yet, the authors have provided the best experiment to date to address the question and evidence to the contrary will have to be weighed against some strong evidence regarding the ecological consequences of the evolution of C4 photosynthesis.

Taylor, S. H., S. P. Hulme, M. Rees, B. S. Ripley, F. I. Woodward, and C. P. Osborne. Ecophysiological traits in C-3 and C-4 grasses: a phylogenetically controlled screening experiment. New Phytologist 185:780-791.