Nitrogen demand, availability, and acquisition strategy control plant responses to elevated CO2.

Area of Science:

• Plant physiology
• Ecology
• Biochemistry

Background:

• Plants adapt to rising atmospheric CO2 by altering leaf nitrogen and photosynthesis.
• Eco-evolutionary optimality theory suggests leaf responses to CO2 depend on nitrogen demand for photosynthesis, irrespective of availability.

Purpose of the Study:

• To investigate leaf and whole-plant responses of soybean (Glycine max L.) to elevated CO2 under varying nitrogen fertilization and inoculation conditions.
• To reconcile the roles of nitrogen demand and availability in plant responses to elevated CO2.

Main Methods:

• Soybean plants were subjected to factorial combinations of elevated CO2, inoculation (with nitrogen-fixing bacteria), and nine nitrogen fertilization levels.
• Leaf photosynthetic rates, Rubisco carboxylation, and electron transport rates were measured.
• Whole-plant growth and belowground carbon allocation were assessed.

Main Results:

• Elevated CO2 decreased the maximum rate of Rubisco carboxylation more than electron transport, enhancing net photosynthesis by optimizing rate-limiting steps.
• Leaf photosynthetic responses to elevated CO2 were independent of nitrogen fertilization and inoculation.
• Increased nitrogen fertilization enhanced whole-plant responses to elevated CO2 by promoting nitrogen uptake and belowground carbon allocation.

Conclusions:

• Leaf-level photosynthetic responses to elevated CO2 are primarily regulated by leaf nitrogen demand, aligning with eco-evolutionary optimality theory.
• Whole-plant responses to elevated CO2 are constrained by nitrogen availability, with higher fertilization improving growth.
• Inoculation with nitrogen-fixing bacteria did not significantly influence plant responses to elevated CO2.