Inhibition of nitrate uptake and assimilation in wheat seedlings grown under elevated CO2

Abstract

Wheat (Triticum aestivum L.) cv PBW 343 was grown in Hoagland solution devoid of nitrogen (-N) under two CO 2 levels viz. ambient (380 lL L -1 , AC) and elevated (600 ± 50 lL L -1 , EC) for 20 days in growth chambers. The rate of uptake, assimilation and accumulation of nitrate was compared. At lows nitrate concentration up to 0.5 mM, rate of nitrate uptake was higher in EC grown seedlings as compared to AC. Under non-limiting supply of external nitrate, the rate of uptake declined in EC grown seedlings. Nitrate reductase (NR) activity increased in EC grown seedlings at low external concentrations of nitrate. However, AC grown plants showed higher NR activity, but at very high concentrations of nitrate. EC grown plants showed low level of accumulation of nitrate in shoots under limited nitrate availability, indicating lower influx towards storage pool and more availability of nitrate in metabolic pool. Increasing nitrogen (N) fertilization therefore may not compensate for slower NO À 3 assimilation rates under EC, as uptake and assimilation both decline under nitrate sufficient conditions. Effective management practices and changes in the pattern of fertigation may be required in response to rising atmospheric CO 2 levels for wheat production.

Properties of nitrogen fertilization are decisive in determining the effects of elevated atmospheric CO2on the activity of nitrate reductase in plants

“…Conversely, some researchers observed the opposite effect of CO 2 enrichment on NR activity, under high nitrate conditions; Geiger et al 6 reported that elevated CO 2 slightly stimulated NR activity in the leaves of tobacco plants fed with 2 mM nitrate, while causing an approximately 30% decrease in leaf NR activity in plants fed with a 6 mM nitrate supply. This result is similar to that of Lekshmy et al, 22 in which CO 2 enrichment increased NR activity (by 11%-24%) in the leaves of wheat seedlings grown in low nitrate conditions (<2 mM), but the opposite occurred when the plants were grown in high nitrate conditions (>5 mM). These findings imply that the effect of elevated CO 2 on NR activity in plants is dependent on the nitrate levels in the growth medium; this was confirmed by our latest report: in low-level nitrate supply conditions, NR activity is stimulated by the basal endogenous nitric oxide (NO) and the NO induced by CO 2 enrichment, via an NOS-dependent pathway.…”

Effect of CO2enrichment and increased nitrogen supply on the induction of sunflower (Helianthus annuusL.) primary leaf senescence

“…Elevated CO 2 inhibits shoot nitrogen assimilation in C3 plants through two different mechanisms: (1) elevated CO 2 inhibits nitrite transport into chloroplasts; (2) elevated CO 2 inhibits the photorespiration process (Bloom et al 2010). Photorespiration stimulates the export of malate from the chloroplast into the cytoplasm, and this malate generates the NADH used to reduce nitrate to nitrite (Bloom et al 2010(Bloom et al , 2012Aranjuelo et al 2013;de la Mata et al 2013;Lekshmy et al 2013;Bloom 2015). Elevated CO 2 with limited N availability disrupts plant growth and gives rise to senescence progression, indicated by chlorosis, anthocyanin accumulation, and increased GS1 expression (de la Mata et al 2013;Aoyama et al 2014).…”

Elevated CO2 levels enhance the uptake and metabolism of organic nitrogen

“…The uptake amount and N contribution of glycine under eCO 2 conditions were significantly higher than those under aCO 2 conditions, while the absorption and N contribution of ammonium and nitrate were generally lower. Several studies have shown that eCO 2 inhibits the uptake of nitrate in environments with a single N source (nitrate) (Bloom et al ) and in the mixed field community (Shimono and Bunce , Lekshmy et al ). Reduced nitrate uptake under eCO 2 conditions may result from the limited nitrate reductase activity (Lekshmy et al ) and further N metabolism (Bloom et al ).…”

“…Several studies have shown that eCO 2 inhibits the uptake of nitrate in environments with a single N source (nitrate) (Bloom et al ) and in the mixed field community (Shimono and Bunce , Lekshmy et al ). Reduced nitrate uptake under eCO 2 conditions may result from the limited nitrate reductase activity (Lekshmy et al ) and further N metabolism (Bloom et al ). Furthermore, the enhanced absorption of glycine might decrease the absorption of nitrate, and increases of root ammonium or amino acid content could induce feedback regulation of nitrate absorption (Thornton and Robinson ).…”

“…The effects of eCO 2 on the relative uptake of different forms of N remain unclear. Several studies have reported that eCO 2 decreases nitrate uptake in wheat, Arabidopsis, rice and plants in a mixed field community (Jackson and Reynolds 1996, Näsholm et al 2009, Bloom et al 2010, while nitrate uptake is enhanced by CO 2 enrichment under low nitrate conditions (Lekshmy et al 2013). Reduced nitrate uptake may result from decreases in both nitrate reductase activity (Lekshmy et al 2013) and further N assimilation (Bloom et al 2010).…”