New publications about carbon in plant leaves

Originally posted in Ülo Niinemet’s Lab blog.

Text by Ülo Niinemets

Carbon, C, is the most frequent chemical element in plant leaves, but leaf carbon content is a surprisingly understudied plant trait. Modern elemental analyzers typically give estimates of nitrogen, N, and C contents, but often only N (that is present in proteins) is used and C is neglected. We have just shown that C in combination with Ca provides key insight into leaf structure and function both at local and global scales.

Looking back at this work, publication of these two papers was a saga. Lots of rejections, invitations to resubmit, revisions and rejections again. Many good comments, but also comments in a style “I just do not believe it”. This was the frustrating part of it. Perhaps it reflected the feeling in the research community that C and Ca are that common elements that there is nothing to find. Yet, people had not looked at these two key elements together. Taken alone, C and Ca are non-informative. Taken together, a marvelous set of patterns with major functional implications emerges.

figure from Xing et al. (2021, New Phytologist)

figure from Xing et al. (2021, New Phytologist)

The work builds upon an earlier study of Niinemets & Tamm (2005) that demonstrated a negative scaling between leaf structural carbon and leaf Ca content across woody species.

figure from Niinemets & Tamm (2005)

Citation: Xing, K., Zhao, M., Niinemets, Ü., Niu, S., Tian, J., Jiang, Y., Chen, H.Y.H., White, P.J., Guo, D. & Ma, Z. 2021. Relationships between leaf carbon and macronutrients across woody species and forest ecosystems highlight how carbon is allocated to leaf structural function. Frontiers in Plant Science 12: 674932 (link to paper).

Abstract: Stoichiometry of leaf macronutrients can provide insight into the tradeoffs between leaf structural and metabolic investments. Structural carbon (C) in cell walls is contained in lignin and polysaccharides (cellulose, hemicellulose, and pectins). Much of leaf calcium (Ca) and a fraction of magnesium (Mg) were further bounded with cell wall pectins. The macronutrients phosphorus (P), potassium (K), and nitrogen (N) are primarily involved in cell metabolic functions. There is limited information on the functional interrelations among leaf C and macronutrients, and the functional dimensions characterizing the leaf structural and metabolic tradeoffs are not widely appreciated. We investigated the relationships between leaf C and macronutrient (N, P, K, Ca, Mg) concentrations in two widespread broad-leaved deciduous woody species Quercus wutaishanica (90 individuals) and Betula platyphylla (47 individuals), and further tested the generality of the observed relationships in 222 woody eudicots from 15 forest ecosystems. In a subsample of 20 broad-leaved species, we also analyzed the relationships among C, Ca, lignin, and pectin concentrations in leaf cell walls. We found a significant leaf C–Ca tradeoff operating within and across species and across ecosystems. This basic relationship was explained by variations in the share of cell wall lignin and pectin investments at the cell scale. The C–Ca tradeoffs were mainly driven by soil pH and mean annual temperature and precipitation, suggesting that leaves were more economically built with less C and more Ca as soil pH increased and at lower temperature and lower precipitation. However, we did not detect consistent patterns among C–N, and C–Mg at different levels of biological organization, suggesting substantial plasticity in N and Mg distribution among cell organelles and cell protoplast and cell wall. We observed two major axes of macronutrient differentiation: the cell-wall structural axis consisting of protein-free C and Ca and the protoplasm metabolic axis consisting of P and K, underscoring the decoupling of structural and metabolic elements inherently linked with cell wall from protoplasm investment strategies. We conclude that the tradeoffs between leaf C and Ca highlight how carbon is allocated to leaf structural function and suggest that this might indicate biogeochemical niche differentiation of species.

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Citation: Xing, K., Niinemets, Ü., Rengel, Z., Onoda, Y., Xia, J., Chen, H.Y.H., Zhao, M., Han, W. & Li, H. 2021. Global patterns of leaf construction traits and their covariation along climate and soil environmental gradients. New Phytologist. (link to paper)

Abstract:

  • Leaf functional traits and their covariation underlie plant ecological adaptations along environmental gradients, but there is limited information on the global covariation patterns of key leaf construction traits.
  • To explore how leaf construction traits co-vary across diverse climate and soil environmental conditions, we compiled a global dataset including cell wall mass per unit leaf mass (CWmass), leaf carbon (C) and calcium (Ca) concentrations, and specific leaf area (SLA) for 2348 angiosperm species from 340 sites world-wide.
  • Our results demonstrated negative correlations between leaf C and Ca concentrations and between leaf C and SLA across diverse nongraminoid angiosperms. Leaf C concentration increased with increasing mean annual temperature (MAT) and mean annual precipitation (MAP) and with decreasing soil pH and calcium carbonate (CaCO3) concentration, whereas leaf Ca concentration and SLA exhibited the opposite responses to these environmental variables. The covariations of leaf Ca–C and of leaf SLA–C were stronger in habitats with lower MAT and MAP, and/or higher soil CaCO3 content.
  • This global-scale analysis demonstrates that the leaf C and Ca concentrations and SLA together govern the C and biomass investment strategies in leaves of nongraminoids. We conclude that environmental conditions strongly shape leaf construction traits and their covariation patterns.
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