Professor Howard Griffiths
- Does photoprotection affect plant and canopy biomass production?
- Isotopic landscapes and the modelling the seasonal progression of photosynthesis and respiration in Birch (Betula) and Beech (Fagus)
- Monitoring climate change in tropical environments: analysis of stable isotope signals in epiphytes
- Isotopic molecular physiology of the chloroplast pyrenoid
- Photosynthetic plasticity in crassulacean acid metabolism plants
Project Title: Does photoprotection affect plant and canopy biomass production?
Supervisors : Prof H Griffiths (Plant Sciences, Cambridge ) and Dr Erik Murchie (School of Biosciences, University of Nottingham)*
Photoprotection against excess absorbed light energy is an essential and universal attribute of oxygenic photosynthetic organisms. It has been an important factor in their evolution, and a diverse range of solutions have arisen. It can determine survival, productivity and habitat preference, and in principle it determines the ceiling efficiency of energy conversion in photosynthesis in natural environments and agricultural systems.
Photoprotection consists of a suite of regulatory processes which prevent over-excitation of the photosynthetic apparatus and undesirable photooxidative damage to the plant. However recent work predicts a significant impact of photoprotection on whole plant photosynthetic rate caused by a seemingly typical delayed response to natural fluctuations in the light environment. This may have the effect of reducing overall canopy photosynthesis and biomass production and has a strong interaction with prevailing temperatures. Additionally, canopy architecture plays an important role in self shading and altering the efficiency of C3 and C4 crop carbon gain.
The aim of this project is to use physiological measurements at the leaf, whole plant and canopy level to analyse factors influencing light use efficiency in model crops such as Miscanthus (C4) and rice (C3). Variation in photoprotection at the molecular and the leaf level will be used as a tool to examine and preferably to model and to predict canopy photosynthetic responses.
*Early Stage Researcher funded under Marie Curie Initial Training Networks (ITN) Call: FP7-PEOPLE-ITN-2008 "HARVEST: Control of Light Use Efficiency in Plants and Algae From Light to Harvest"
Project Title: Isotopic landscapes and the modelling the seasonal progression of photosynthesis and respiration in Birch (Betula) and Beech (Fagus)
Supervisor: Professor Howard Griffiths
The potential for stable isotopes to investigate and integrate geochemical and biological systems is now widely appreciated. The aim of this project is to undertake seasonal measurements of photosynthesis, respiration and isotope composition, and determine transfer coefficients for organic material in developing leaves, and below ground carbon sequestration and soil respiratory fluxes. The output will underpin estimates of carbon sequestration and turnover for scaling climate change impacts from leaf to canopy.
Birch and beech present an ideal comparative ecological system, since birch is a high-light demanding species which produces leaves successively throughout the year, whilst beech is a shade-tolerant, late-successional species which produces only one or two flushes of leaves each growing season. A previous studentship (Betson, 2004, Betson and Griffiths, In preparation) showed how changes in isotope composition occur on a seasonal basis in cohorts of mature, fully expanded leaves in both birch and beech (see Figure below). We have identified a number of processes which may lead to the surprisingly large seasonal shift in isotopic composition of leaf organic material: fractionation during carbohydrate export and remobilisation; changes in leaf stomatal physiology and plant water relations; responsiveness to light intensity.
The aim of this project is to develop a new concept in stable isotope analyses (Tcherkez, Ghashghaie and Griffiths, In preparation): this will allow multi-dimensional analyses of isotopic signals to be evaluated and scaled between systems of contrasting complexity. First, we suggest that a fundamental approach is to develop the notion of “isotopic landscapes”, reflecting variations in isotopic behaviour of these systems at each scale of interest. Secondly, we suggest that future isotopic studies will then need to integrate isotopic signals using statistical methods derived from functional genetics (genomics), which we will term “isotopomics”. This is analogous to the highly successful technique for analysing gene expression using microarrays, which indicate by colours the transcription level of many genes. Ultimately, these approaches will lead to the improved parameterisation of canopy, ecosystem and global gas exchange models, which will be able to take into account the complexities and interannual variations in organic material signals, and plant-soil respiratory efflux, as a function of gas exchange fluxes and climate change.
Strategic importance of project: the work builds on an existing programme and is also relevant to collaborative project being undertaken with both David Coomes (scaling of xylem physiology from stem to leaf and relationship to hdraulic limitations) and Ed Tanner (soil respiratory CO2 fluxes in tropical soils). It will move me into a systems biology modelling approach, which can be supported by my ongoing collaborations with Guillaume Tcherkez (UPS, Paris) and Graham Farquhar (ANU, Canberra). Given that we have a new continuous-flow mass spectrometers now available in the Godwin Laboratory (funded through a collaborative NERC equipment grant) we have the analytical throughput to match the demands of this new theoretical framework.
Project Title: Monitoring climate change in tropical environments: analysis of stable isotope signals in epiphytes
Supervisor: Professor Howard Griffiths
Tropical montane forests are under the threat from global climate change patterns (mean temperature and large-scale weather systems, such as El Nino) as well as local factors (deforestation, altering local hydrological balance). Epiphytes are sensitive indicators of microclimate, with roots of vascular plants such as bromeliads largely used as holdfasts, whereas bryophytes directly dependent on moisture availability for rehydration and reactivation of photosynthesis. The aim of this project is to undertake a study of epiphyte diversity as a function of altitudinal microhabitats, and evaluate the use of 13C and 18O as sensitive indicators of plant growth and overall water status for epiphytes in situ in rainforest canopies.
Our current work has primarily been focussed on epiphytic bromeliads, and two ongoing projects have evaluated the seasonal shifts in organic material isotope composition as a function of seasonal precipitation inputs (rain, fog, dew) in Chamela, Mexico and across the continental divide in Panama. We now have well-developed models which provide a theoretical framework for CO2 and water exchanges across leaf surfaces (Helliker and Griffiths, In preparation), and additional models have recently been published for bryophytes. In a collaboration with Professor Jan Wolf, (Amsterdam) we will evaluate the interactions between vascular and non-vascular epiphyte diversity along an altitudinal gradient. In particular, we will develop the use of 18O to determine the interactions between water uptake, storage and episodic growth in habitats ranging from seasonal, semi-deciduous forest to “cloudforest”.
In terms of water relations, epiphytes defy conventional water relations models, since the uptake, storage and evaporation of water is highly dependent of regular recharge; in addition, seasonal variations in photosynthetic activity are related to the regularity of precipitation inputs. In between rehydration events, the progressive reduction in relative water content might be expected to lead to progressive enrichment in 18O leaf water and carbohydrate composition, perhaps providing seasonal markers of wet and dry seasons; alternatively, under high humidity, we can use the 18O signal to explore the uptake and exchange with water vapour in air. The work has important outputs for modelling climatic impacts on such sensitive ecosystems, as well as offering training in physiology and taxonomy of bryophytes.
Strategic importance of project:
The work will represent an opportunity for me to restart my personal field work programme in the tropics, with applications for grants to the Royal Society and NERC to undertake an expedition to Trinidad . It represents two new collaborations, one with Brent Helliker: here, we have developed a model explaining the hydrological peculiarities of epiphytes under high humidity, whereby the gross gas exchange flux, which is effectively x10 the net evaporative flux, allows the leaf water isotope signal to be completely dominated by atmospheric water vapour, and possibly offers a way of using CAM 18O signals to monitor global atmospheric water vapour (model currently in preparation for publication, hence details not included in proposal); secondly, with Jan Wolf, who examined Casandra Reyes Garcia’s thesis, who has a remarkable collection of epiphytes along an altitudinal transect in Colombia, and some startling observations on bryopyte diversity in the tropics (extremely low) which has implications for the timing of dispersal and diversificaton of bryophytes once angiosperm canopies began to dominate global vegetation.
Project Title: Molecular physiology of the chloroplast pyrenoid
Supervisor: Professor Howard Griffiths
The chloroplast pyrenoid is a unique structure which allows the operation of a photosynthetic CO2 concentrating mechanism in many algae and one line of land plants, the Hornworts (Anthocerotae). The pyrenoid structure remains elusive, whether formed from a starch sheath and/or proteinaceous layer, and there are as yet few molecular markers. It provides an organelle into which Rubisco is packaged, and when surrounded by elevated CO2, such a CO2 concentrating mechanism suppresses oxygenase activity and reduces wasteful photorespiration. We have characterised the system at the physiological level (Griffiths et al 2003). Recent evidence suggests that the occurrence of a pyrenoid may in part be related to the amino acid sequence of Rubisco, in that some plants have distinct amino acid substitutions which might allow the enzyme to aggregate (Nozaki et al 2002 J Mol Evol 55, 414-430).
The aim of this project will be to understand the physiological and molecular controls on pyrenoid differentiation and function. Within the Anthocerotae, there are a range of types of pyrenoid which may also be associated with the absence glycollate oxidase (K. Vaughn, personal communication). In a collaboration with Dr Vaughn, we will identify the molecular basis of the pyrenoid in hornworts, where there is a progression of species which vary on their reliance on a CCM. Having firstly characterised the magnitude and extent of any CCM activity from a range of hornworts, (such as in the genus Megaceros), we will analyse rbcL sequences and Rubisco kinetic characteristics. Ultimately, the aim would be to identify genes that determine pyrenoid structure; informed by developments from the well-characterised Chlamydomonas system, we will identify genes associated with the formation and operation of the pyrenoid and CO2 concentrating mechanism.
- Nozaki et al (2002) J Mol Evol 55, 414- 430.
- Griffiths H Maxwell, K, Richardson D and Robe W (2003) Turning the land green: inferring photosynthetic physiology and diffusive limitations in early bryophytes (In: Evolution of Plant Physiology, eds AR Helmsley and I Poole, Academic Press)
Project Title: Photosynthetic Plasticity in Crassulacean Acid Metabolism Plants
Supervisor: Professor Howard Griffiths
Projects are available which span the ecology, physiology, biochemistry and molecular biology of succulent plants and tropical rainforest epiphytes. Our approaches are integrated via gas exchange, chlorophyll fluorescence and stable isotopes.
Over the diel course the succulent leaves of CAM plants naturally exhibit changes in internal CO2 concentration which may exceed those experienced over the course of palaeohistory. Prolonged periods of CO2 limitation and elevated CO2 may be identified throughout the day. Our data has demonstrated that Rubisco undergoes an extremely protracted activation (4-6 h) during the day, which may be related to changes in intracellular [CO2] and/or metabolite fluxes. We are currently investigating the regulation of Rubisco in obligate and facultative plants, with respect to the roles played by Rubisco activase and diurnal inhibitors of Rubisco. In addition we are interested in examining Rubisco structure and enzyme kinetics (Vmax, Kcat, Srel) in a range of CAM species from terrestrial and aquatic habitats.
Photosynthetic plasticity also encompasses acclimation of the photosynthetic apparatus to the environment. A particular focus for our research group is acclimation of succulent long-lived leaves to the light environment and photoprotective responses to high light exposure. Epiphytic bromeliads adopt a novel strategy whereby high light treatment rapidly (hours timescale) induces a loss of a number of photosynthetic proteins (Rubisco, LHCII, PSII core, LHCI), whilst chlorophyll a/b and photosynthetic capacity remains stable. Preliminary evidence suggests increased activity of a broad spectrum cysteine protease. We are keen to identify both the regulation and the identity of this protease or group of proteases, together with an integrated, more general comparison of sun and shade acclimation.
- Dodd AN, Borland AM, Haslam RP, Griffiths H and Maxwell K (2002) Crassulacean acid metabolism: plastic, fantastic. Journal of experimental Botany, 53, 1-12.
- Pierce S, Winter K and Griffiths H (2002) The role of CAM in high rainfall cloudforests: an in situ comparison of photosynthetic pathways in Bromeliaceae. Plant Cell and Environment, 25, 1181- 1189.