COS 207-3 - A Delicate Balance: How Symbiotic Bacteria Influence Plant Water Use Efficiency

Background/Question/Methods

Interactions among water, carbon (C), and nitrogen (N) are key aspects of plant feedbacks to global climate change. The capacity for N to mediate photosynthesis, transpiration, and the ratio between the two (plant water use efficiency; WUE) is a key part of many models. A substantial body of research shows that plants with high N photosynthesize faster and have a higher WUE, but recent work suggests that one group of plants, symbiotic N-fixing plants, work differently. Specifically, a cross-species synthesis showed that symbiotic N-fixers had higher WUE but not higher photosynthesis when their N content was higher. Why do N-fixers use higher N to conserve water but not increase photosynthesis? We tested two hypotheses: H1: The presence of SNF bacteria alters plant physiology. H2: Increased N content stimulates photosynthesis at low leaf N content, but reduces transpiration at high leaf N content, and N-fixers operate in a different region of parameter space (high N content) than non-fixers. We tested these hypotheses by growing two N-fixers, Robinia pseudoacacia and Gliricidia sepium, under two manipulations: the presence of N-fixing bacteria and a range of soil N supply. We used the ratio of 13C/12C as an index of WUE.

Results/Conclusions

Surprisingly, we found that access to SNF bacteria was a dominant driver of WUE. The inoculated Robinia pseudoacacia seedlings had ~2.5‰ δ13C higher across their N supply ranging from about -28.2‰ to 27.5 ‰ in comparison to the uninoculated plants which ranged from about -30.6‰ to -29‰. The Gliricidia sepium had a similar difference (~2.5‰) at low N supply, with the inoculated plants starting at -25.6 ‰ and the uninoculated at -28 ‰, but at high N supply δ13C was similar in inoculate and uninoculated plants at ~ -28 ‰. Even more intriguingly, the N content of foliage did not drive δ13C aside from its covariation with inoculation: inoculated plants had higher %N and higher δ13C. All the plant species we studied are all capable of forming N-fixing symbioses. It was not the capacity to be an N-fixer that drove WUE; rather, it was the plant’s access to symbiotic bacteria. N-fixers play a critical role in climate (the N they provide helps ecosystems grow and removes atmospheric CO2) and in agriculture (they provide protein to feed our global population), so studying their functionality is essential to climate mitigation.