Diazotrophic Endophytes Improve Photosynthesis and Water Relations Leading to Increase Water Use Efficiency of Rice Plants.

Di-nitrogen fixing bacterial and yeast endophytes have been suggested as a sustainable solution to growing crops under unfavorable environmental conditions. Our previous work with rice plants showed overall increases in biomass, but did not provide full physiological responses of hosts. Here we present the key findings of this plant-microbe symbiotic interaction from our CO₂ chamber studies. We measured gas exchange properties of the rice at panicle initiation stage. A CO₂ chamber study was conducted under ambient (400 ppm) and elevated (800 ppm) atmospheric CO₂ conditions to investigate any CO₂ effects on the endophytic symbiosis. The plants were grown in a nitrogen limited condition to better utilize the benefit of having N₂ fixing endophytes. First, we found the alleviation of the photosynthetic acclimation to high CO₂ by enhancing the enzyme capacities that are associated with the CO₂ assimilation process; Second, we found the improvement of water relations of the rice by endophytes’ decreasing the daytime stomatal conductance. These two responses led to the increases in water use efficiency of the plants. Underlying mechanisms are not known; however, quantifiable microbial respiration which could contribute to the CO₂ assimilation process would be a possible explanation for this.