Elevated Carbon Dioxide and Chronic Warming Together Decrease Nitrogen Uptake Rate, Net Translocation, and Assimilation in Tomato.

The response of plant N relations to the combination of elevated CO (eCO) and warming are poorly understood. To study this, tomato () plants were grown at 400 or 700 ppm CO and 33/28 or 38/33 °C (day/night), and their soil was labeled with NO or NH. Plant dry mass, root N-uptake rate, root-to-shoot net N translocation, whole-plant N assimilation, and root resource availability (%C, %N, total nonstructural carbohydrates) were measured. Relative to eCO or warming alone, eCO + warming decreased growth, NO and NH-uptake rates, root-to-shoot net N translocation, and whole-plant N assimilation. Decreased N assimilation with eCO + warming was driven mostly by inhibition of NO assimilation, and was not associated with root resource limitations or damage to N-assimilatory proteins. Previously, we showed in tomato that eCO + warming decreases the concentration of N-uptake and -assimilatory proteins in roots, and dramatically increases leaf angle, which decreases whole-plant light capture and, hence, photosynthesis and growth. Thus, decreases in N uptake and assimilation with eCO + warming in tomato are likely due to reduced plant N demand.