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Nitrogen metabolism as ecological driver of grassland diversity

Supervisor: Prof. Howard Griffiths

Importance of the area of research

Around 5- 10 million years ago, there was a major shift in sub-tropical habitats when forests were replaced by seasonal grasslands and savannas. At this time, warmer and drier conditions, and lowering ambient CO2 concentrations, promoted the rapid diversification of grasses  (Poaceae) in the open savannah forests which developed in sub-tropical regions. Many of these grasses independently developed C4 photosynthetic traits, which enhanced the operating efficiency of Ribulose-1,5-Biphosphate-carboxylase-oxygenase (Rubisco), as well as altering plant nitrogen and water requirements.  The biochemistry and metabolism of the C4 pathway are well-documented, and the project will investigate the physiological basis of nitrogen metabolism as an ecological driver leading to C4 diversification. The aim will be to correlate N uptake and retention as compared to C3 grasses from highly diverse temperate limestone/chalk grasslands, which seem able to deplete soil N and restore diversity. The project has relevance for ecological restoration and historical evolution of grasslands.

Project summary

The focus of the research will identify the competitive advantages of contrasting strategies for N use. Some grasses optimise N uptake and internal recapture (e.g. leading to the C4 pathway), whilst others maximise N uptake to outcompete neighbouring forbs and grasses, perhaps by drawing down soil N, analogous to Tilman’s R* potential. The project will identify the physiological basis to these contrasting ecological observations. Primary nitrogen metabolism (uptake, assimilation of inorganic or organic N) is only one-tenth of the N turnover found in leaves due to photorespiration (following the interconversion of glycine to serine in mitochondria).  Understanding the evolution of leaf anatomy (bundle sheath development, inter-veinal distances, hydraulic conductance) and extent of photorespiratory N recapture, will provide valuable insights into C3 and C4 grass differentiation, evolution and phylogeny.

What the student will be doing

Grasses will be grown from seed, representing a range of grasses with contrasting phylogenetic origins within the PACMAD clade. Here we have a good phylogenetic understanding of the origins of C4, in relation to vein width and bundle sheath differentiation.  A basic framework of leaf anatomy, photosynthetic capacity and hydraulic conductance will be developed for each species. We will develop a system to analyse the ammonia compensation point, to identify those plants more effective at N recapture and recycling during photorespiration. Alternative methods will use stable isotope labelling (15N) to evaluate both mean residence time for N, as well as extent of N release during photorespiration. Competition experiments will also be undertaken to compare the effectiveness of N uptake, versus N recycling and storage, in mixed communities of C3 and C4 grasses, as well as responsiveness to pulsed N resources.

Training that will be provided

The traditional skillset associated with physiological ecology (photosynthetic gas exchange, plant water relations and stable isotopes) will be applied to a collection requiring phylogenetic and morphometric analysis. A good understanding of experimental design to allow competition experiments (response surface design) to be statistically validated. Field sampling on natural N concentrations in grasses from chalk grasslands undergoing scrub clearance and restoration would also feature. Opportunities to attend the BES/SEB Plant Environmental Physiology Field Techniques workshop in Portugal in September 2016.

Desirable skills

An interest in plant physiological ecology, field ecology, conservation and palaeohistorical drivers of plant diversification.


  • Griffiths H, Weller G, Toy LFM, Dennis RJ (2013) You’re so vein: Bundle Sheath Physiology, Phylogeny and Evolution in C3 and C4 plants Plant Cell & Environment, 36, 249- 261. DOI: 10.1111/j.1365-3040.2012.02585.x
  • Berendse, F. & Aerts, R. (1987) Nitrogen-Use-Efficiency: A Biologically Meaningful Definition? Functional Ecology, 1, 293–296.
  • Jankju-Borzelebad, M, Griffiths H (2006) Competition for pulsed resources: an experimental study of establishment and coexistence for an arid-land grass Oecologia 148, 555-563
  • Bellasio C & Griffiths H (2014) The operation of two decarboxylases (NADPME and PEPCK) and partitioning of C4 metabolic processes between mesophylll and bundle sheath cells allows light capture to be balanced by the maize C4 pathway. Plant Physiology Vol. 164, pp. 117, doi/10.1104