A/Prof. Mike D. Cramer

In accepting the prestigious "Wallenberg Prize" in 2002, the well known plant physiologist Melvin Tyree aired his concerns about the continued swing to molecular sciences at the expense of traditional fields of taxonomy, anatomy, and physiology (Plant Physiology 131: 3-5). He asserted that progress in biological science would be further compromised unless anatomists, morphologists, taxonomists, physiologists, and other "traditional" biologists worked more closely with molecular biologists.

This appeal resonates strongly with me, having witnessed the attrition of South African whole plant physiological research capacity over three decades. The fact that plant physiology is a core discipline, but now somewhat rare in South Africa, has yielded many exciting collaborations for me over the past decade. My research has involved aspects of carbon-nitrogen interactions (metabolism and physiology), rhizosphere CO₂ influences on root carbon metabolism, nutritional physiology and carbon allocation processes. More recently I have become interested in the role of transpiration in plants.

I particularly enjoy working on controversial issues in which I think fuzzy thinking may have resulted in misconceptions as to how biological systems work. This has, for example, spurred my interest in the role of transpiration, the dubious possibility of N₂ fixation in sugarcane (Hoefsloot et al., 2005) and sucrose accumulation (McCormick et al., 2009) as well as in other diverse issues such as the occurrence of giraffe in KwaZulu-Natal (Cramer and Mazel, 2007), why elephants push over trees and why dinosaurs were so large.

Current research areas

1. The functions of transpiration: The idea that transpiration is mostly an "unnecessary evil" that results from plants opening their stomata to acquire CO₂ seems at odds to me with the fact that water constrains plant distributions so extensively and that plants utilise about 250 litres of water for every kg of biomass produced by NPP (global average). We (with Tony Verboom and Heidi Hawkins) have been examining the role of transpiration driven mass-flow of nutrients to the roots of plants and how this contributes to nutrition. We have good data showing that transpiration responds to nutrients (Fig. 1; Cramer et al., 2008), and this observation is supported by many other studies. Questions that we are interested in include: a) Is the mass-flow of nutrients the main reason that plants originally evolved the transpiration mechanism? b) Could stomata originally have evolved to facilitate/control water loss and only subsequently became co-opted into a role of facilitating CO₂ acquisition when the atmospheric CO₂ concentrations dropped? c) Do plants native to regions where nutrients are limiting have mechanisms that actually promote water loss (Fig. 2)? d) Are root casts found in coastal sands the product of mass flow (Fig 3., see Cramer and Hawkins, 2009)?
   
   

   Fig. 1. Summary of the results of an experiment to determine whether nutrient availability modified transpiration in the grass Ehrharta calycina (Cramer et al., 2008). Plants were supplied with slow-release fertiliser applied either directly to the sand surrounding the roots, dubbed “interception”, or in a compartment not directly accessible by the roots (fertiliser sequestered behind a 40 µm mesh), but from which nutrients could move by diffusion and mass-flow, dubbed “mass-flow”. Widths of arrows represent relative transpiration (H₂O) and photosynthetic (CO₂) rates and plant size represents biomass accumulation and allocation. The effect size (Hedge’ s g) of the differences in elemental concentrations, root weight ratios and δ15N values between the “interception” and the “mass-flow” plants are shown (=, |g | not significant; < or >, |g | < 1; << or >> | g | > 1)

   
   
   

   Fig 2. A) Root casts on a dune near Still Bay, South Africa. These structures are probably formed as a consequence of mass-flow of Ca to the roots. B) Root casts of a cluster root collected by Matthew Gilbert in the Eastern Cape. Similar structures have been found near Still Bay, South Africa and are likely to be derived from Morella cordifolia, a dune plant common in the area that has cluster roots (See Cramer and Hawkins, 2009).

   
   

   Fig. 3 Variation in leaf size in Proteaceae. Could reduced leaf size ensure water loss from leaves in cold wet Mediterranean winters?

2. The role of N₂ fixation in savanna ecosystems: The puzzle has been that there has been little evidence of extensive N₂ fixation by leguminous trees in savannas. Why are there so many legumes (e.g. Acacias, i.e. African acacia, not the Australian relative) in some African savannas? We (with William Bond) proposed that grass competition may induce N₂ fixation in leguminous tree seedlings, but that deep roots of larger trees reduce dependence on N₂ fixation (Fig. 4). We have found that grass does induce massive dependence on N₂ fixation in seedlings and that this enables seedlings to partially overcome competition from grass (Fig. 5; Cramer et al., 2007). We have more recently found evidence indicating that P becomes a limiting resource for leguminous trees seedlings competing with grass.
   
   

   Fig. 4. Adult trees and grass in savanna systems avoid root-level competition by exploiting different soil zones (from Walter, 1971).

   
   
   

   Fig. 5. Acacia karroo seedling with nodules grown in the presence of grass (Photo by Tim Szoke).

3. Sucrose accumulation in sugarcane: Much international research effort has focussed on understanding the processes that control sucrose accumulation in the culms of sugarcane plants. The prevailing notion has been that limiting the amount of carbon being channelled into supposedly "wasteful" processes such as cell wall synthesis or into respiration in the culm would allow "savings" of carbon that would then allow sucrose to accumulate in the vacuoles of the culm cells. However, based on metabolic control theory, we argued that the accumulation of sucrose would result in feedback from the culm tissue to the leaf and consequently suppression of photosynthesis. We (with Derek Watt at SASRI) have found evidence that this is the case (see papers by McCormick et al) and argued that in order to increase sucrose accumulation in sugarcane culms, modification of the feedback process is necessary to allow photosynthesis to continue to supply sucrose to the culm (McCormick et al., 2009).
4. Plant nutrition in nutrient poor ecosystems: In collaboration with Heidi Hawkins (UCT) and Hans Lambers (University Western Australia) I have been addressing issues relating to P-nutrition of plants adapted to soils where P is a scarce resource. Proteaceae (and some other families of plants) have fascinating adaptations that allow them to mine sparingly soluble P from soils where P is in forms that are generally inaccessible to most plants (Fig. 6). We have worked on the physiology of these specialised roots and related structures. Since Proteaceae are used for floriculture in South Africa and elsewhere, we have also become interested in the agricultural management of these plants. We have worked extensively with the Protea Producers of South Africa (PPSA) studying nutrient and water requirements of these plants. Because Proteaceae are able to access scarce P, they are also sensitive to P-toxicity. Our latest research in this area is directed at modifying fertilizer compositions so as to reduce P-toxicity to plants. Tony Verboom and I have also recently embarked on a project addressing the diverse strategies of plants in the Fynbos for surviving in nutrient impoverished ecosystems. This includes the rationale for the prevalence of more or less leafless culms (e.g restios, sedges and some grasses) in the Fynbos (a project in collaboration with Peter Linder and Megan Yates) and the benefits of hemi-parasitism in the genus Thesium (with Tim Moore).
   
   

   Fig. 6. Specialised root structures: Compound cluster roots of (A) Banksia grandis and (F) B. idononia (Proteaceae), grown in nutrient solution with less than 1 mM P. Simple cluster roots of (B) Hakea prostrata (Proteaceae), (C) Lupinus albus (Fabaceae) grown in nutrient solution with 1 mM P, and (D) Protea repens (Proteaeceae) excavated in its natural habitat. Very dense and long root hairs give the appearance of a cotton ball to (E) dauciform roots of Cyperaceae. Scale bars: 30 mm (A), 15 mm (B), 12.5 mm (C), 7.5 mm (E) and 20 mm (F). (From Lambers et al., 2003)

5. Biological control: I have worked with John Hoffmann (UCT, Zoology Dept) on biological control of alien Acacias. We have examined the physiological costs of galls induced on Acacias. We are currently interested in the physiological control of gall formation in these plants.

Qualifications

• BSc, Univ. Witwatersrand, South Africa, 1980
• BSc (Hons), Univ. Witwatersrand, South Africa, 1981
• MSc, Univ. Witwatersrand, South Africa, 1987
• PhD, Univ. Cape Town, South Africa, 1992.
• Post-doc, Ben Gurion University of the Negev, Israel, 1992-1995.

Current teaching

• Bio1000F: Cell biology: Metabolism and photosynthesis.
• Bio2003S: Form and function (Botany course-convenor): Leaf form and function (including photosynthesis), growth regulators, flowering, seeds.
• Bio3007S: Global change ecology (Course-convenor): Plant physiological responses to environmental changes.
• BSc Honours: Ecophysiology module (Convenor).

Current research students

Post-doctoral fellow:

Dr. Heidi Hawkins: Heidi leads the research effort on all aspects of research related commercial Protea production, but also works on aspects of physiology of indigenous naturally occurring species.

Doctoral:

Alistair McCormick: Regulation of carbon allocation in sugarcane (with Derek Watt at SASRI).

Mariette Smart: Regulation of flowering in Proteaceae (with Laura Roden, MCB, UCT).

Masters

Alex Schutz: Tree life history traits in savannas and how resprouting depends on carbon allocation in Acacia karoo (with William Bond, UCT).

Past research students

MSc

Aleysia Viktor, Physiological factors influencing the nitrogen use efficiency of tomato seedlings.

Andre Oberholzer, The effect of supplementation of rhizosphere dissolved inorganic carbon on fruit yield and quality of tomatoes (cv. 'Daniella') grown with salinity.

Charlene Titus, Sucrose transporters and sucrose uptake mechanisms in sugarcane.

Gabi Turner, Development of in situ hybridisation to examine tissue-specific expression patterns of the invertase genes in sugarcane culm.

Mariette Smart, Physiology of floral induction in Protea spp.

Melissa Lintnaar, Does phosphate deficiency affect the anaplerotic provision of carbon for NH4+ assimilation in mycorrhizal plants?

Rakeshnie Ramoutar, The development of an in situ hybridisation technique to determine the gene expression patterns of UDP-glucose dehydrogenase, pyrophosphate-dependent phosphofructokinase and UDG-glucose pyrophosphorylase in sugarcane internodal tissues.

Ronelle Rowland, Responses of sugarcane to aluminium toxicity.

Tshegohatso Matlhoahela, Mineral nutrition of cultivated South African Proteaceace.

PhD

Theresa Voschenk, The effect of saline irrigation on selected soil properties, plant physiology and vegetative and reproductive growth of Palsteyn Apricots.

Post-doc

Michael Richards: Influences of rhizosphere CO₂ on plants growth.

Michael Shane: Phosphate acquisition by indigenous Fynbos species.

Peer-review publication list

2009

Cramer MD, Hawkins H-J, Verboom GA 2009. The importance of nutritional regulation of plant water flux. Oecologia, in press.

Cramer MD, Midgley JJ 2009. Maintenance costs of serotiny do not explain weak serotiny. Austral Ecology, in press.

McCormick AJ, Watt DA, Cramer MD 2009. Supply and demand: sink regulation of sugar accumulation in sugarcane. Journal of Experimental Botany 60: 357–364, 2009

Moseley CT, Cramer MD, Kleinjan CA, Hoffmann JH 2009. Why does Dasineura dielsi-induced galling of Acacia cyclops not impede vegetative growth? Journal of Applied Ecology 46: 214–222.

Cramer MD, Hawkins H-J. 2009. A physiological mechanism for the formation of root casts. Palaeogeography, Palaeoclimatology, Palaeoecology 74: 125–133

Schutz AEN, Bond WJ, Cramer MD 2009. Juggling carbon: allocation patterns of a dominant tree in a fire-prone savanna. Oecologia DOI 10.1007/s00442-009-1293-1

2008

Hawkins H-J, Hettasch H, Mesjasz-Przybylowicz J, Przybylowicz W, Cramer MD. 2008. Phosphorus toxicity in the Proteaceae: a problem in post-agricultural lands. Scientia Horticulturae, 117: 357-365.

McCormick AJ, Cramer MD, Watt DA. 2008. Changes in photosynthetic rates and gene expression of leaves during a source-sink perturbation in sugarcane. Annals of Botany 101: 89-102.

Cramer MD, Hoffmann V, Verboom GA. 2008. Nutrient availability moderates transpiration in Ehrharta calycina. New Phytologist, 179: 1048-1057.

McCormick AJ, Cramer MD, Watt DA. 2008. Culm sucrose accumulation promotes physiological decline of mature leaves in ripening sugarcane. Field Crops Research 108, 250–258.

McCormick AJ, Cramer MD, Watt DA. 2008. Differential expression of genes in the leaves of sugarcane in response to sugar accumulation. Tropical Plant Biology 1, 142–148.

McCormick AJ, Cramer MD, Watt DA. 2008. Regulation of photosynthesis by sugars in sugarcane leaves. Journal of Plant Physiology 165: 1817-1829.

Wigley B. J., Cramer M. D., Bond W. J. 2008. Sapling survival in a frequently burnt savanna: mobilisation of carbon reserves in Acacia karroo. Plant Ecology 10.1007/s11258-008-9495-x.

McCormick AJ, Watt DA, Cramer MD 2008. Supply and demand: sink regulation of sugar accumulation in sugarcane. Journal Experimental Botany doi:10.1093/jxb/ern310.

Shane MW, Cramer MD, Lambers H 2008. Root of edaphically controlled Proteaceae turnover on the Agulhas Plain, South Africa: phosphate uptake regulation and growth. Plant Cell and Environment 31: 1825-1833.

2007

Cramer MD, Chimphango SBM, van Cauter A, Waldram MS and Bond WJ. 2007. Grass competition induces N₂ fixation in some species of African Acacia. J. Ecology 95: 1123-1133.

Hawkins HJ, Hettasch H, Cramer MD. 2007. Putting back what we take out, but how much? Phosphorus and nitrogen additions to farmed Leucadendron ‘Safari Sunset’ and Leucospermum ‘Succession’ (Proteaceae). Scientia Horticulturae 111: 378–388

Cramer MD, Kleizen C, Morrow C. 2007. Does the prostrate leaved geophyte Brunsvigia orientalis utilise soil-derived CO₂ for photosynthesis? Annals of Botany, 99:835-844.

Cramer MD, Mazel AD. 2007. The past distribution of giraffe in KwaZulu-Natal. South African Journal of Wildlife Research 37: 197-201.

2006

Lambers, H., M. W. Shane, M. D. Cramer, S. J. Pearce and E. J. Veneklaas. 2006. Root structure and functioning for efficient acquisition of phosphorus: matching morphological and physiological traits. Annals of Botany, 98 (4): 693-713.

Dorchin N, Cramer MD, Hoffmann JH. 2006. Photosynthesis and sink activity of Wasp-induced galls in Acacia pycnantha. Ecology, 87 (7): 1781-1791 .

McCormick AJ, Cramer MD, Watt DA. 2006. Sink-strength regulates photosynthesis in sugarcane. New Phytologist, 171 (4): 759-770.

Shane MW, Cawthray GR, Cramer MD, Kuo J, Lambers H. 2006 Specialised ‘dauciform’ roots of Cyperaceae are structurally distinct, but functionally analogous with ‘cluster’ roots. Plant cell and environment, 29 (10): 1989-1999

2005

Viktor A and Cramer MD. 2005 An investigation into the contribution of dissolved inorganic carbon to the partitioning of C and N in tomato plants. New Phytologist. 165, 157-169.

Watt DA, McCormick AJ, Govender C, Carson DL, Cramer MD, Huckett BI, Botha FC 2005. Increasing the utility of genomics in unravelling sucrose accumulation. Field crops research 92 (2-3): 149-158

Hoefsloot G, Termorshuizen AJ, Watt DA, Cramer MD. 2005 The contribution of diazotrophic bacteria to the nitrogen budget of a commercially grown South African sugarcane cultivar. Plant and Soil 277:85–96.

Cramer MD, Shane MW Lambers H. 2005 Physiological changes in Lupinus albus L. associated with variation in root-zone CO₂ concentration and cluster-root P mobilisation. Plant Cell and Environment, 28,120-3-1217.

Miller AJ, Cramer MD 2005. Root nitrogen acquisition and assimilation. Plant and Soil 274: 1–36.

2004 and older

Shane MW, Cramer MD, Funayama-Noguchi S, Cawthray G, Millar AH, Day DA Lambers H 2004 Developmental physiology of cluster-root carboxylate synthesis and exudation in Hakea prostrata R.Br. (Proteaceae): expression of PEP carboxylase and the alternative oxidase. Plant Physiology 135, 549-560.

Cramer MD, Jacobs G. 2004 Causes of leaf-tip burn in the cultivated Protea hybrid ‘Sylvia’. Scientia Horticulturae 103, 65-77.

Lambers H, Cramer MD, Shane MW, Wouterlood M, Poot P and Veneklaas EJ. 2003. Structure and functioning of cluster roots and plant responses to phosphate deficiency. Plant and Soil 248: ix-xix.

Viktor A, Cramer MD. 2003. Variation in root-zone CO₂ concentration modifies isotopic fractionation of carnon and nitrogen in tomato seedlings. New Phytologist 157:45-54.

Mortimer P, Swart JC, Valentine AG, Jacobs G, Cramer MD. 2003 Does irrigation influence the growth, yield and water use efficiency of the protea hybrid “Sylvia” (Protea susannae x Protea eximia)? South African Journal of Botany 69: 135-143.

Cramer MD. 2002. Inorganic carbon utilization by plant roots, 699-716. Plant roots; The hidden half, 3rd edition. Waisel Y, Eschel A, Kafkafi U, eds. New York, USA: Marcel Dekker, 2002, 0-8247-0631-5 

Cramer MD, Oberholzer JA, Combrink NJJ. 2001. The effect of supplementation of rhizosphere dissolved inorganic carbon on fruit yield and quality of tomatoes (cv. ‘Daniella’) grown with salinity. Scientia Horticulturae 89: 269-289.

Cramer MD, Titus CHA. 2001. Elevated root zone dissolved inorganic carbon can ameliorate Al toxicity in tomato seedlings. New Phytologist, 152: 29-39.

Cramer M.D., Van der Westhuizen M.M. 2000. The influence of elevated rhizosphere dissolved inorganic carbon concentrations on carbon and nitrogen metabolism in tomato roots. In: Loucao MA and Lips SH, eds, Nitrogen in a sustainable ecosystem – from the cell to the plant. Backhuys Publishers, Leiden. pp 139-144.

Van der Merwe C.A., Cramer M.D. 2000. The influence of dissolved inorganic carbon in the root zone on nitrogen uptake and the interaction between carbon and nitrogen metabolism. In: Loucao MA and Lips SH, eds, Nitrogen in a sustainable ecosystem – from the cell to the plant. Backhuys Publishers, Leiden. pp 145-151.

Van der Merwe C.A., Cramer M.D. 2000. The effect of enriched rhizosphere carbon dioxide on nitrate and ammonium uptake in hydroponically grown tomato plants. Plant and Soil 221:5-11.

Cramer M.D., Richards M.B. 1999. The effect of rhizosphere dissolved inorganic carbon on the growth of tomato seedlings. Journal of Experimental Botany, 50: 79-87.

Cramer M.D., Gao Z.F., Lips S.H. 1999. The influence of rhizosphere dissolved inorganic carbon on carbon and nitrogen metabolism in salinity-treated tomato plants. New Phytologist. 142: 441-450

Kraaij T., Cramer M.D. 1999. Do the gas exchange characteristics of alien acacias enable them to successfully invade the fynbos? South African Journal of Botany, 63:232-238.

Hawkins H.J., Cramer M.D. 1999. Root respiratory quotient and nitrate uptake in hydroponically grown non-mycorrhizal and mycorrhizal wheat. Mycorrhiza, 9:57-60.

van der Westhuizen M.M., Cramer M.D. 1998. The influence of elevated rhizosphere dissolved inorganic carbon concentrations on respiratory O 2 and CO₂ flux in tomato roots. Journal of Experimental Botany 49: 1977-1985.

Cramer, M.D., Savidov, N. and Lips, S.H. 1996. The influence of enriched rhizosphere CO₂ concentrations on N uptake and metabolism in NR-deficient and wild type barley. Physiologia Plantarum: 97: 47-54.

Cramer, M.D., Nagel, O.W., Lips, S.H. and Lambers H. 1995. Reduction, assimilation and transport of N in wild type and gibberellin-deficient tomato plants. Physiologia Plantarum: 95:347-354

Cramer, M.D., Schierholt, A, Wang, Y.Z. and Lips, S.H. 1995. The influence of salinity on utilisation of root anaplerotic carbon and nitrogen metabolism in tomato seedlings. J. Experimental Botany 46: 1569-1577

Cramer, M.D. Lips, S.H. 1995. Enriched rhizosphere CO₂ concentrations can ameliorate the influence of salinity on hydroponically grown tomato plants. Physiologia Plantarum 94: 425-432.

Cramer, M.D. Lips, S.H. 1995. The influence of enriched root-zone CO₂ concentrations on growth, nitrogen metabolism and root HCO 3 ­ - incorporation in salinity stressed Lycopersicon esculentum. Acta Phytopathologica et Entomologica Hungarica 30: 105-118.

Cramer, M.D. and Lewis, O.A.M. 1993. The Influence of NO₃^- and NH₄⁺ nutrition on the Gas Exchange Characteristics of the Roots of Wheat (Triticum aestivum) and Maize (Zea mays) Plants. Annals of Botany, 72: 37-46.

Cramer, M.D. and Lewis, O.A.M. 1993 A Comparison of the Influence of Nutrient NO₃^- and NH₄⁺ on the Growth and Gas Exchange Characteristics of C₃ Wheat (Triticum aestivum) and C₄ Maize (Zea mays) Plants. Annals of Botany, 72: 359-364.

Cramer, M.D. and Lewis, O.A.M. 1993. The influence of NO₃^- and NH₄⁺ nutrition on the carbon and nitrogen partitioning characteristics of wheat (Triticum aestivum) and maize (Zea mays) plants. Plant and Soil, 154: 289-300

Cramer, M.D., Lewis, O.A.M., Lips, S.H. 1993. Inorganic carbon fixation and metabolism in maize roots as affected by nitrate and ammonium nutrition. Physiologia Plantarum 89: 632-639.

Lewis O.A.M., Cramer M. and van der Leij T. 1990. Influence of nitrogen source on carbon distribution in plants exhibiting the C₃ and C₄ photosynthetic pathways. In: Ullrich W.R., Rigano C., Fuggi A., Aparicio P.J. (eds), Inorganic Nitrogen Metabolism, Springer, Berlin, Heidelberg, New York, pp 330-335.