Revisiting Why Plants Become N Deficient Under Elevated CO2: Importance to Meet N Demand Regardless of the Fed-Form

Abstract

An increase in plant biomass under elevated CO 2 (eCO 2 ) is usually lower than expected. N-deficiency induced by eCO 2 is often considered to be a reason for this. Several hypotheses explain the induced N-deficiency: (1) eCO 2 inhibits nitrate assimilation, (2) eCO 2 lowers nitrate acquisition due to reduced transpiration, or (3) eCO 2 reduces plant N concentration with increased biomass. We tested them using C 3 (wheat, rice, and potato) and C 4 plants (guinea grass, and Amaranthus ) grown in chambers at 400 (ambient CO 2 , aCO 2 ) or 800 (eCO 2 ) μL L −1 CO 2 . In most species, we could not confirm hypothesis (1) with the measurements of plant nitrate accumulation in each organ. The exception was rice showing a slight inhibition of nitrate assimilation at eCO 2 , but the biomass was similar between the nitrate and urea-fed plants. Contrary to hypothesis (2), eCO 2 did not decrease plant nitrate acquisition despite reduced transpiration because of enhanced nitrate acquisition per unit transpiration in all species. Comparing to aCO 2 , eCO 2 remarkably enhanced water-use efficiency, especially in C 3 plants, decreasing water demand for CO 2 acquisition. As our results supported hypothesis (3) without any exception, we then examined if lowered N concentration at eCO 2 indeed limits the growth using C 3 wheat and C 4 guinea grass under various levels of nitrate-N supply. While eCO 2 significantly increased relative growth rate (RGR) in wheat but not in guinea grass, each species increased RGR with higher N supply and then reached a maximum as no longer N was limited. To achieve the maximum RGR, wheat required a 1.3-fold N supply at eCO 2 than aCO 2 with 2.2-fold biomass. However, the N requirement by guinea grass was less affected by the eCO 2 treatment. The results reveal that accelerated RGR by eCO 2 could create a demand for more N, especially in the leaf sheath rather than the leaf blade in wheat, causing N-limitation unless the additional N was supplied. We concluded that eCO 2 amplifies N-limitation due to accelerated growth rate rather than inhibited nitrate assimilation or acquisition. Our results suggest that plant growth under higher CO 2 will become more dependent on N but less dependent on water to acquire both CO 2 and N.

Topics & Concepts