Thu, Aug 18, 2022: 8:30 AM-8:45 AM
515B
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.
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.