Determination of leaf carbon isotope discrimination in C4 plants under variable N and water supply

Hao Yang,Jing Tian,Sheng-Gong Li,Qiang Yu,Wen-Ping Sheng

doi:10.1038/s41598-017-00498-w

Abstract

Understanding the mechanisms underlying variations in carbon isotope discrimination (Δ) in C4 plants is critical for predicting the C3/C4 ratio in C3/C4 mixed grassland. The value of Δ is determined by bundle sheath leakiness (Ф) and the ratio of intercellular to ambient CO2 concentration (Ci/Ca). Leaf nitrogen concentration (Nleaf) is considered a driver of Δ in C4 plants. However, little is known about how Nleaf affects Ф and Ci/Ca, and subsequently Δ. Here leaf carbon isotope composition, Nleaf, Ф, and leaf gas exchange were measured in Cleistogenes squarrosa, a dominant C4 species in the Inner Mongolia grassland. Δ remained relatively stable under variable N and water supply. Higher N supply and lower water supply increased Nleaf, stimulated photosynthesis and further decreased Ci/Ca. High N supply increased Ф, which responded weakly to water supply. Nleaf exerted similar effects on Ci/Ca and on Ф in the field and pot experiments. Pooling all the data, Nleaf explained 73% of the variation in Ci/Ca. Overall, both Ф and Ci/Ca determined Δ; however, the contribution of Ф was stronger. Nleaf influenced Δ primarily though Ci/Ca, rather than Ф. Ф should be considered in estimating Δ of C4 endmember.

Highlights

The carbon isotope discrimination (Δ) of C4 plants, C3 plants, and bulk samples (e. g., bulk vegetation, soil organic matter, wool, and horn) are widely used to calculate the C3/C4 ratio based on a two-member mixed model[1,2,3]
The Δ in C4 plants is influenced by many factors, such as isotope effects during diffusion of CO2 through stomatal pore and cell walls, fixation of bicarbonate by phosphenolpyruvate carboxylase (PEPC) in mesophyll cells, fixation of CO2 by Rubisco in bundle sheath cells, and leakage of CO2 from bundle sheath cells to mesophyll cells[10, 11]
The Δ in C4 plants depends on the ratio of intercellular to ambient CO2 concentration (Ci/Ca) and bundle sheath leakiness (Ф, the proportion of C fixed by PEP carboxylation, which subsequently leaks out of the bundle sheath):

Summary

Introduction

The carbon isotope discrimination (Δ) of C4 plants, C3 plants, and bulk samples (e. g., bulk vegetation, soil organic matter, wool, and horn) are widely used to calculate the C3/C4 ratio based on a two-member mixed model[1,2,3]. The Δ in C4 plants is influenced by many factors, such as isotope effects during diffusion of CO2 through stomatal pore and cell walls, fixation of bicarbonate by phosphenolpyruvate carboxylase (PEPC) in mesophyll cells, fixation of CO2 by Rubisco in bundle sheath cells, and leakage of CO2 from bundle sheath cells to mesophyll cells[10, 11]. In equation (1), Δ varies with Ф and Ci/Ca. Ф is an important C4 photosynthesis parameter and is mostly affected by the CO2 concentration gradient between bundle sheath and mesophyll cells, and by factors influencing the activity ratio of Rubisco to PEPC11. Under nitrogen-rich conditions, Nleaf is high and PEPC activity can increase to a greater extent than Rubisco, resulting in a high CO2 concentration gradient between bundle sheath and mesophyll cells and a high Ф value[15, 16]. The mechanism of how Nleaf influence both Ф and Ci/Ca, and in turn Δ is still poorly understood

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Conclusion