The effects of elevated CO₂ and nitrogen nutrition on root dynamics

Ambient CO₂ concentration is currently 400 μmol mol^-1, and projections forecast an increase up to 970 μmol mol^-1 by century's end. Elevated CO₂ can stimulate C3 plant growth, whereas nitrogen is the main nutrient plants acquire from soils and often limits growth. Plants primarily obtain two nitrogen sources from the soil, ammonium (NH₄⁺) and nitrate (NO₃^-). At elevated CO₂ levels, plant growth and nitrogen metabolism is affected by the nitrogen source. Most research has focused on shoot traits, while neglecting the plants' hidden half, the root. We studied the effects of elevated CO₂ and nitrogen source on hydroponically grown tomato plants, a C3 model and crop plant. Our main objective was to determine how the nitrogen source and elevated CO₂ affect root development. Our results indicate they affect development in terms of the size and anatomy of different root orders. Specifically, root xylem development was found sensitive to the nitrogen source, whereas NO₃^--supplied plants displayed greater xylem development compared to their NH₄⁺ counterparts, and also to a lesser extent, to elevated CO₂, which we found inhibits this development. Additionally, elevated CO₂ decreased root respiration in different root orders exclusively in plants supplied with NH₄⁺as the sole nitrogen source.