FEATURE ESSAY 16.2  Sodium in C4 photosynthesis

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Peter Brownell 

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Figure 1  Associate Professor Peter F. Brownell, OAM

My interest in the role of sodium in plant metabolism began in 1954 when my PhD supervisor, Professor J.G. Wood, suggested a project to determine if sodium was required by higher plants. In 1955, Allen and Arnon, at the University of California, Berkeley, demonstrated sodium to be specifically required by the Cyanobacterium, Anabaena cylindrica. Two years later, Brownell and Wood (1957) demonstrated the essentiality for sodium as a micronutrient for Atriplex vesicaria (bladder saltbush). Subsequently, Brownell and Crossland (1972, 1974) showed that sodium was essential for C4 and possibly crassulacean acid metabolism (CAM) plants. Research into the function of sodium in plants started in the Botany Department, University of Adelaide, and was continued at Waite Agricultural Research Institute prior to my appointment to James Cook University of North Queensland.

Sodium is one of the most abundant elements on earth, so that plant culture under conditions totally free of sodium is extremely difficult. Confirmation that sodium was an essential element for higher plants required rigorous experimental procedures to exclude sodium salts. Water was twice distilled so that it contained less than 0.0002 mg Na mL–1. Salts were purified by up to six recrystallisations. As a result, total sodium in full-strength culture solution was less than 0.0016 mg Na mL–1. This is about one-hundredth of the amount that would have been present if the solution had been made up from unpurified analytical reagents. Even the atmosphere in which plants were grown was filtered to prevent contamination from dust particles. When these techniques were implemented for Atriplex vesicaria, sodium-deficiency symptoms of chlorosis, necrosis and greatly decreased growth were observed. Dry mass of sodium-deficient plants was only one-twentieth that of plants receiving 0.5 mg Na mL–1.

Drawing on studies with other essential nutrient elements, we presumed that sodium would be required by all higher plants. Surprisingly, of the 30 species examined, which included halophytes, other Chenopods and non-endemic species of Atriplex, the only plants that seemed to require sodium were 10 species of Atriplex endemic to Australia. This outcome was perplexing because differences in sodium requirement could not be correlated with any obvious differences between the species studied. We imagined that most higher plants might require extremely small amounts of sodium but perhaps Australian species of Atriplex were exceptional in requiring larger amounts.

Discovery of the C4 photosynthetic pathway by Hal Hatch and Roger Slack in 1966 provided a clue: perhaps only plants having the C4 pathway required sodium! C4 plants from dif-ferent families were subsequently examined and all responded to small amounts of sodium, as did the Australian species of Atriplex. A CAM plant, Bryophyllum tubiflorum, also responded to sodium when growing in CAM mode but this species did not respond to sodium when growing in C3 mode (Section 2.1). By implication, sodium requirement was somehow linked to the C4-fixing system.

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Figure 2   Elevated CO2 alleviates symptoms of sodium deficiency in Atriplex spongiosa. The left pair of plants was held at 330 µL L-1 CO2, whereas both plants on the right side were held at 1500 µL L-1 CO2. Within each pair, +Na plants are on the right side and -Na plants are on the left side. (Photograph courtesy P.F. Brownell)

Our discovery that C4 plants required sodium suggested a possible function of this element in C4 plants: was sodium needed for operation of the C4 appendage in transporting CO2 to bundle sheath cells for reduction to carbohydrates? This view was substantiated by our observations that sodium-deficiency symptoms are alleviated in plants grown in atmos-pheres with elevated CO2 concentrations (Figure 2). Plants supplied with sodium showed little or no response to high CO2. This result suggested that under conditions of sodium deficiency, transport of CO2 to bundle sheath cells decreased, thus limiting overall rates of CO2 assimilation. When atmospheric CO2 was increased from 330 to 1500 µL L–1, CO2 must have then reached bundle sheath cells in sufficient amount by physical diffusion, thus bypassing the C4-fixing system.

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Figure 3 Possible sites of sodum involvement in C4 photosynthesis: stomatal conductance (A), carbonic anhydrase (B), activity of PEP carboxylase (C) were all unaffected by sodium nutrition. By contrast, leaves of sodium-deficient plants had high levels of alanine and pyruvate and low levels of PEP in the mesophyll chloroplasts (Original unpublished diagram courtesy P.F. Brownell)

 

Several possible sites were identified where sodium deficiency might limit the rate of supply of CO2 to bundle sheath cells in normal air (Figure 3). Processes examined, including stomatal conductance (A), carbonic anhydrase (which hydrates CO2 to form bicarbonate, the substrate of phospho-enolpyruvate (PEP) carboxylase, B), and the activity of PEP carboxylase (C) were all unaffected by sodium nutrition. By contrast, leaves of sodium-deficient plants had high levels of alanine and pyruvate and low levels of PEP, the acceptor of CO2 in C4 plants, which suggests a block (D) in the conversion of pyruvate to PEP in the mesophyll chloroplasts.

Regeneration of PEP from pyruvate involves (1) transport of pyruvate into the mesophyll chloroplast, (2) enzymatic conversion of pyruvate to PEP within the stroma and (3) provision of energy for the conversion reaction.

Sodium was not found to affect the activity of pyruvate phosphate dikinase (the enzyme catalysing the conversion of pyruvate to PEP), but in 1987 Ohnishi and Kanai discovered a sodium-induced uptake of pyruvate into mesophyll chloro-plasts of Panicum miliaceum. This immediately suggested a role for sodium in a majority of C4 plants. Ohnishi and Kanai demonstrated sodium-induced pyruvate uptake in the mesophyll chloroplasts of many other C4 plants with the exception of some species from Andropogoneae and Arundinelleae. These exceptions have now been shown to have proton instead of sodium ion induced pyruvate transport into the mesophyll chloroplasts.

Mark Johnston and Chris Grof then obtained evidence, in sodium-deficient C4 plants, for damage to the light-harvesting photosystem which is the source of energy for pyruvate transport and/or the regeneration of PEP. In sodium-deficient plants, they found lower chlorophyll a/b and fluorescence ratios and lowered photosystem II activity, with altered ultra-structure in the mesophyll chloroplasts. Notwithstanding these extra dimensions to sodium effects, a difficult question remains as to the primary function of sodium. Is sodium needed to maintain the light-harvesting and energy-transducing systems in the mesophyll chloro-plasts and/or to transport pyruvate into the mesophyll chloroplasts? If transport, then damage observed in mesophyll chloroplasts in sodium-deficient plants could have been caused by excess energy that would normally have been used to convert pyruvate to PEP. If sodium can be demonstrated to be an essential element for members of Andropogoneae and Arundinelleae, we would conclude that activation of pyruvate transport is not the sole function of sodium in C4 plants.

Sodium is also involved in transport of metabolites including bicarbonate ions in Cyanobacteria. In 1967, Professor Don Nicholas and I found the activity of nitrate reductase to be many times greater in sodium-deficient compared to normal cells of Anabaena cylindrica. We were unable to obtain a similar effect of sodium deficiency on nitrate reductase in C4 plants. The effect of sodium nutrition on nitrate reductase in A. cylindrica may have been a consequence of some earlier effects of the sodium treatment, perhaps related to inorganic carbon.

Despite our incomplete understanding of sodium nutrition, we can be confident that a lack of sodium will never limit plant growth in nature. However, as sodium has an important role in C4 photosynthesis, exploring functional roles continues to be an extremely challenging and exciting endeavour.

References

Allen, M.B. and Arnon, D.I. (1955). ‘Studies in the nitrogen-fixing blue-green algae. II. The sodium requirement of Anabaena cylindrica’, Physiologia Plantarum, 8, 653–660.

Brownell, P.F. and Crossland, C.J. (1972). ‘The requirement for sodium as a micronutrient by species having the C4 dicarboxylic photosynthetic pathway’, Plant Physiology,
49, 794–797.

Brownell, P.F. and Crossland, C.J. (1974). ‘Growth responses to sodium by Bryophyllum tubiflorum under conditions inducing crassulacean acid metabolism’, Plant Physiology, 54, 416–417.

Brownell, P.F. and Wood, J.G. (1957). ‘Sodium as an essential micronutrient element for Atriplex vesicaria (Heward)’, Nature, 179, 635–636.

Hatch, M.D. and Slack, C.R. (1966). ‘Photosynthesis by sugar-cane leaves: a new carboxylation reaction and the pathway of sugar formation’, Biochemical Journal, 101, 103–111.

Ohnishi, J. and Kanai, M. (1987). ‘Na+-induced uptake of pyruvate into mesophyll chloroplasts of a C4 plant, Panicum miliaceum L., Federation of European Biochemical Societies Letters, 219, 347–350.

Further reading

Brownell, P.F. (1979). ‘Sodium as an essential micronutrient element for plants and its possible role in metabolism’, Advances in Botanical Research, 7, 117–224.

Brownell, P.F., Bielig, L.M. and Grof, C.P.L. (1991). ‘Increased carbonic anhydrase activity in leaves of sodium-deficient C4 plants’, Australian Journal of Plant Physiology, 18, 589–592. 

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