Lower photorespiration in elevated CO₂ reduces leaf N concentrations in mature Eucalyptus trees in the field

Abstract

Rising atmospheric CO2 concentrations is expected to stimulate photosynthesis and carbohydrate production, while inhibiting photorespiration. By contrast, nitrogen (N) concentrations in leaves generally tend to decline under elevated CO2 (eCO2), which may reduce the magnitude of photosynthetic enhancement. We tested two hypotheses as to why leaf N is reduced under eCO2: (a) A “dilution effect” caused by increased concentration of leaf carbohydrates; and (b) inhibited nitrate assimilation caused by reduced supply of reductant from photorespiration under eCO2. This second hypothesis is fully tested in the field for the first time here, using tall trees of a mature Eucalyptus forest exposed to Free‐Air CO2 Enrichment (EucFACE) for five years. Fully expanded young and mature leaves were both measured for net photosynthesis, photorespiration, total leaf N, nitrate (NnormalO3-) concentrations, carbohydrates and NnormalO3- reductase activity to test these hypotheses. Foliar N concentrations declined by 8% under eCO2 in new leaves, while the NnormalO3- fraction and total carbohydrate concentrations remained unchanged by CO2 treatment for either new or mature leaves. Photorespiration decreased 31% under eCO2 supplying less reductant, and in situ NnormalO3- reductase activity was concurrently reduced (−34%) in eCO2, especially in new leaves during summer periods. Hence, NnormalO3- assimilation was inhibited in leaves of E. tereticornis and the evidence did not support a significant dilution effect as a contributor to the observed reductions in leaf N concentration. This finding suggests that the reduction of NnormalO3- reductase activity due to lower photorespiration in eCO2 can contribute to understanding how eCO2‐induced photosynthetic enhancement may be lower than previously expected. We suggest that large‐scale vegetation models simulating effects of eCO2 on N biogeochemistry include both mechanisms, especially where NnormalO3- is major N source to the dominant vegetation and where leaf flushing and emergence occur in temperatures that promote high photorespiration rates.

Low sensitivity of gross primary production to elevated CO<sub>2</sub> in a mature eucalypt woodland

“…Previous modelling studies applying MAESPA to eC a experiments both assumed a higher J : V ratio (2) and estimated a higher GPP response to eC a , presumably due to less frequent RuBP regeneration limitation (Wang et al, 1998;Luo et al, 2001). A J : V ratio of 2 was suggested by Wullschleger (1993) and has been used in many modelling studies (e.g. the seven terrestrial biosphere models assessed by Rogers et al, 2017 all assumed a J : V ratio of between 1.9 and 2).…”

“…The transition from RuBP-regeneration-limited to Rubisco-limited photosynthesis of the canopy is determined by the ratio of the maximum capacities for RuBP regeneration and Rubisco activity, J max and V cmax (Friend, 2001;Zaehle et al, 2014;Rogers et al, 2017). Wullschleger (1993) reported a J max : V cmax ratio of 2, which has been widely adopted in models (e.g. Wang et al, 1998;Luo et al, 2001;Rogers et al, 2017).…”

Effects of Elevated CO2 on Wheat Yield: Non-Linear Response and Relation to Site Productivity

“…Another consequence is a reduction in the transpiration-driven mass-flow that has been shown to be closely linked to uptake of some plant nutrients (N, K, S, Ca, Mg, Mn) [33], which are essential for plant growth. At high CO 2 concentration, photosynthesis can be limited by triose-phosphate utilization [34] and eCO 2 may also impair N acquisition in leaves due to the suppressed photorespiration [35,36]. Since photorespiration drives the malate transport, eCO 2 is subject to impact the link between N and carbon metabolism in the plant.…”

High Nitrate or Ammonium Applications Alleviated Photosynthetic Decline of Phoebe bournei Seedlings under Elevated Carbon Dioxide

“…When nitrogen is the limiting factor, the nitrogen in rubisco may be redistributed to other enzymes or tissues and organs, which affects the synthesis of rubisco and, hence, reduces plant photosynthesis [29]. However, previous studies have indicated that eCO 2 may limit plant uptake of nitrogen and thus affect the positive growth response of plants to eCO 2 [30,31]. Furthermore, the form of nitrogen can also affect photosynthetic characteristics and the metabolism of different nitrogen forms is different.…”