Rising atmospheric CO₂concentration inhibits nitrate assimilation in shoots but enhances it in roots of C₃plants

Abstract

We have proposed that rising atmospheric CO 2 concentrations inhibit malate production in chloroplasts and thus impede assimilation of nitrate into protein in shoots of C 3 plants, a phenomenon that will strongly influence primary productivity and food security under the environmental conditions anticipated during the next few decades. Although hundreds of studies support this proposal, several publications in 2018 and 2019 purport to present counterevidence. The following study evaluates these publications as well as presents new data that elevated CO 2 enhances root nitrate assimilation in wheat and Arabidopsis while it inhibits shoot nitrate assimilation.

Will rising atmosphericCO2concentration inhibit nitrate assimilation in shoots but enhance it in roots ofC3plants?

“…These studies indicated that a wide range of C 3 species, including wheat, show increased growth under CO 2 enrichment, especially if they receive high applications of N (a selected eight references were given for this point that included meta‐analyses and reviews). Continuing, Bloom et al () stated that the four studies we highlighted actually support their conclusions: this is not the case. These studies indicated that for wheat (Hocking and Meyer ), tobacco ( Nicotiana tabacum ; Geiger et al ; Matt et al ) and cucumber ( Cucumis sativus ; Dong et al ) supplied NO 3 − as the sole N source under controlled environment or glasshouse conditions, greatest growth and reduced N accumulation across treatments occurred under elevated CO 2 with high NO 3 − supply.…”

“…Bloom et al () listed several points in our paper (Andrews et al ) where in their view we made false claims in relation to the literature or misinterpreted it. For example, the first point raised was that our statement ‘the weight of evidence in the literature indicates that elevated atmospheric [CO 2 ] does not inhibit NO 3 ‐ assimilation and growth of C 3 vascular plants’ consists of four studies that exposed plants to a specific nitrogen form.…”

“…In some cases, elevated CO 2 can increase photosynthesis in the short term, but if photosynthate utilisation is inadequate, a source sink imbalance can arise, leading to end‐product (carbohydrate) accumulation and subsequent down‐regulation of photosynthesis linked to lower ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco) concentration and activity (Ainsworth and Rogers , Zheng et al , Beechey‐Gradwell et al ). In their Summary, Bloom et al () stated that ‘hundreds of papers’ support their proposal that rising atmospheric CO 2 concentrations inhibit malate production in chloroplasts and thus impede assimilation of NO 3 − into protein in shoots of C 3 plants. We strongly disagree with this statement, but many papers do report a decrease in tissue N/protein content g −1 DW with elevated CO 2 .…”

“…These studies indicated that for wheat (Hocking and Meyer ), tobacco ( Nicotiana tabacum ; Geiger et al ; Matt et al ) and cucumber ( Cucumis sativus ; Dong et al ) supplied NO 3 − as the sole N source under controlled environment or glasshouse conditions, greatest growth and reduced N accumulation across treatments occurred under elevated CO 2 with high NO 3 − supply. Considering the first study (Hocking and Meyer ), Bloom et al () argued that wheat with NO 3 − as a sole N source accumulated less reduced N per DW in its shoots under elevated than ambient CO 2 affirming that elevated CO 2 inhibited total NO 3 − assimilation. This is a major point of difference in our views.…”

“…Also, new data for common bean ( Phaseolus vulgaris ) and wheat ( Triticum aestivum ) were presented that supported this view and suggested that the effects of elevated atmospheric [CO 2 ] on N assimilation and growth of C 3 vascular plants will be similar regardless of the form of N assimilated. Bloom et al () responded to Andrews et al () and three other papers (Dier et al , Abadie and Tcherkez , Tcherkez and Limami ) that ‘purport to present counterevidence’ to their proposal ‘that rising atmospheric CO 2 concentrations inhibit malate production in chloroplasts and thus impede assimilation of NO 3 ‐ into protein in shoots of C 3 plants’. Here we focus on their response to Andrews et al ().…”