Changes in size and composition of pigweed (Amaranthus hybridus L.) calcium oxalate crystals under CO2 starvation conditions

The functional role(s) of plant calcium oxalate (CaOx) crystals are still poorly understood. Recently, it was shown that crystals function as dynamic carbon pools whose decomposition could provide CO₂ to photosynthesis when stomata are closed (e.g. under drought conditions) and CO₂ starvation conditions may be created within the mesophyll. This biochemical process, named as 'alarm photosynthesis', can become crucial for plant survival under adverse conditions. Here, we study crystal decomposition under controlled CO₂ starvation conditions (either in the shoot or in the root) to obtain a better insight into the process of crystal formation and function. Hydroponically grown pigweed plants were kept in CO₂ -free air and/or CO₂ -free nutrient medium for 9 days. Crystal volume was monitored daily, and carbon stable isotope composition (δ¹³ C) and Fourier transformation Raman spectra were obtained at the end of the experiment. A considerable reduction in the leaf crystal volume was observed in shoot-CO₂ -starved plants at the end of the experiment. The smallest crystals were isolated from the plants in which carbon was excluded from both the shoot and the root and contained potassium nitrate. Crystal δ¹³ C of CO₂ -starved plants was altered in a predicted way. Specifically, it depended on the average calculated isotope fractionation of all carbon fixation processes considered to be contributing in each experimental treatment. The results of the present study confirmed the correlation between CO₂ starvation conditions and the CaOx crystal decomposition. Inorganic carbon fixed in the root may represent a major carbon source for CaOx formation.

Effectiveness of different pre-treatments in recovering pre-burial isotopic ratios of charred plants

RATIONALE

Isotopic analysis of archaeological charred plant remains offers useful archaeological information. However, adequate sample pre-treatment protocols may be necessary to provide a contamination-free isotopic signal while limiting sample loss and achieving a high throughput. Under these constraints, research was undertaken to compare the performance of different pre-treatment protocols.

METHODS

Charred archaeological plant material was selected for isotopic analysis (δ¹³ C and δ¹⁵ N values) by isotope ratio mass spectrometry from a variety of plant species, time periods and soil conditions. Preservation conditions and the effectiveness of cleaning protocols were assessed through Fourier transform infrared spectroscopy and X-ray fluorescence (XRF) spectrometry. An acid-base-acid protocol, successfully employed in radiocarbon dating, was used to define a contamination-free isotopic reference. Acid-base-acid isotopic measurements were compared with those obtained from untreated material and an acid-only protocol.

RESULTS

The isotopic signals of untreated material and the acid-only protocol typically did not differ more than 1‰ from those of the acid-base-acid reference. There were no significant isotopic offsets between acid-base-acid and acid-only or untreated samples. Sample losses in the acid-base-acid protocol were on average 50 ± 17% (maximum = 98.4%). Elemental XRF measurements showed promising results in the detection of more contaminated samples albeit with a high rate of false positives.

CONCLUSIONS

For the large range of preservation conditions described in the study, untreated charred plant samples, water cleaned of sediments, provide reliable stable isotope ratios of carbon and nitrogen. The use of pre-treatments may be necessary under different preservation conditions or more conservative measurement uncertainties should be reported.

Diel variations in carbon isotopic composition and concentration of organic acids and their impact on plant dark respiration in different species

Leaf respiration in the dark and its C isotopic composition (δ(13) CR ) contain information about internal metabolic processes and respiratory substrates. δ(13) CR is known to be less negative compared to potential respiratory substrates, in particular shortly after darkening during light enhanced dark respiration (LEDR). This phenomenon might be driven by respiration of accumulated (13) C-enriched organic acids, however, studies simultaneously measuring δ(13) CR during LEDR and potential respiratory substrates are rare. We determined δ(13) CR and respiration rates (R) during LEDR, as well as δ(13) C and concentrations of potential respiratory substrates using compound-specific isotope analyses. The measurements were conducted throughout the diel cycle in several plant species under different environmental conditions. δ(13) CR and R patterns during LEDR were strongly species-specific and showed an initial peak, which was followed by a progressive decrease in both values. The species-specific differences in δ(13) CR and R during LEDR may be partially explained by the isotopic composition of organic acids (e.g., oxalate, isocitrate, quinate, shikimate, malate), which were (13) C-enriched compared to other respiratory substrates (e.g., sugars and amino acids). However, the diel variations in both δ(13) C and concentrations of the organic acids were generally low. Thus, additional factors such as the heterogeneous isotope distribution in organic acids and the relative contribution of the organic acids to respiration are required to explain the strong (13) C enrichment in leaf dark-respired CO2 .

Glyoxylate rather than ascorbate is an efficient precursor for oxalate biosynthesis in rice

Oxalate is widely distributed in the plant kingdom. While excess oxalate in food crops is detrimental to animal and human health, it may play various functional roles in plants, particularly for coping with environmental stresses. Understanding its biosynthetic mechanism in plants, therefore, becomes increasingly important both theoretically and practically. However, it is still a matter of debate as to what precursor and pathway are ultimately used for oxalate biosynthesis in plants. In this study, both physiological and molecular approaches were applied to address these questions. First, it was observed that when glycolate or glyoxylate was fed into detached leaves, both organic acids were equally effective in stimulating oxalate accumulation. In addition, the stimulation could be completely inhibited by cysteine, a glyoxylate scavenger that forms cysteine-glyoxylate adducts. To verify the role of glyoxylate further, various transgenic plants were generated, in which several genes involved in glyoxylate metabolism [i.e. SGAT (serine-glyoxylate aminotransferase), GGAT (glutamate-glyoxylate aminotransferase), HPR (hydroxypyruvate reductase), ICL (isocitrate lyase)], were transcriptionally regulated through RNAi or over-expression. Analyses on these transgenic plants consistently revealed that glyoxylate acted as an efficient precursor for oxalate biosynthesis in rice. Unexpectedly, it was found that oxalate accumulation was not correlated with photorespiration, even though this pathway is known to be a major source of glyoxylate. Further, when GLDH (L-galactono-1,4-lactone dehydrogenase), a key enzyme gene for ascorbate biosynthesis, was down-regulated, the oxalate abundance remained constant, despite ascorbate having been largely reduced as expected in these transgenic plants. Taken together, our results strongly suggest that glyoxylate rather than ascorbate is an efficient precursor for oxalate biosynthesis, and that oxalate accumulation and regulation do not necessarily depend on photorespiration, possibly due to the occurrence of the anaplerotic reaction that may compensate for glyoxylate formation in rice.

Biomineralization by photosynthetic organisms: evidence of coevolution of the organisms and their environment?

Biomineralization is widespread among photosynthetic organisms in the ocean, in inland waters and on land. The most quantitatively important biogeochemical role of land plants today in biomineralization is silica deposition in vascular plants, especially grasses. Terrestrial plants also increase the rate of weathering, providing the soluble substrates for biomineralization on land and in water bodies, a role that has had global biogeochemical impacts since the Devonian. The dominant photosynthetic biomineralizers in today's ocean are diatoms and radiolarians depositing silica and coccolithophores and foraminifera depositing calcium carbonate. Abiotic precipitation of silica from supersaturated seawater in the Precambrian preceded intracellular silicification dominated by sponges, then radiolarians and finally diatoms, with successive declines in the silicic acid concentration in the surface ocean, resulting in some decreases in the extent of silicification and, probably, increases in the silicic acid affinity of the active influx mechanisms. Calcium and bicarbonate concentrations in the surface ocean have generally been supersaturating with respect to the three common calcium carbonate biominerals through geological time, allowing external calcification as well as calcification in compartments within cells or organisms. The forms of calcium carbonate in biominerals, and presumably the evolution of the organisms that produce them, have been influenced by abiotic variations in calcium and magnesium concentrations in seawater, and calcium carbonate deposition has probably also been influenced by carbon dioxide concentration whose variations are in part biologically determined. Overall, there has been less biological feedback on the availability of substrates for calcification than is the case for silicification.

Land plants equilibrate O2 and CO2 concentrations in the atmosphere

The role of land plants in establishing our present day atmosphere is analysed. Before the evolution of land plants, photosynthesis by marine and fresh water organisms was not intensive enough to deplete CO(2) from the atmosphere, the concentration of which was more than the order of magnitude higher than present. With the appearance of land plants, the exudation of organic acids by roots, following respiratory and photorespiratory metabolism, led to phosphate weathering from rocks thus increasing aquatic productivity. Weathering also replaced silicates by carbonates, thus decreasing the atmospheric CO(2) concentration. As a result of both intensive photosynthesis and weathering, CO(2 )was depleted from the atmosphere down to low values approaching the compensation point of land plants. During the same time period, the atmospheric O(2) concentration increased to maximum levels about 300 million years ago (Permo-Carboniferous boundary), establishing an O(2)/CO(2) ratio above 1000. At this point, land plant productivity and weathering strongly decreased, exerting negative feedback on aquatic productivity. Increased CO(2) concentrations were triggered by asteroid impacts and volcanic activity and in the Mesozoic era could be related to the gymnosperm flora with lower metabolic and weathering rates. A high O(2)/CO(2) ratio is metabolically linked to the formation of citrate and oxalate, the main factors causing weathering, and to the production of reactive oxygen species, which triggered mutations and stimulated the evolution of land plants. The development of angiosperms resulted in a decrease in CO(2) concentration during the Cenozoic era, which finally led to the glacial-interglacial oscillations in the Pleistocene epoch. Photorespiration, the rate of which is directly related to the O(2)/CO(2) ratio, due to the dual function of Rubisco, may be an important mechanism in maintaining the limits of O(2) and CO(2) concentrations by restricting land plant productivity and weathering.

The influence of N metabolism and organic acid synthesis on the natural abundance of isotopes of carbon in plants

This paper relates the ¹³ C/¹² C ratio of C₃ plant material relative to that of source CO₂ to the N source for growth, the organic N content of the plant, and the extent of organic acid synthesis. The ¹³ C/¹² C ratio is quantified as Δ, defined as (δ¹³ C substrate -δ¹³ C product)/(1+δ¹³ C product), where δ¹³ C values of substrate or product (i.e. the samples) are defined as [¹³ C/¹² C]_sample ]/[(¹³ C/¹² C)_standard ]-1. The computation is performed by relating differences in plant composition as a function of N nutrition and acid synthesis to the fraction of plant C which is acquired via Rubisco and via other carboxylases. The fractional contribution of the different carboxylases to C gain is then related, using the known isotopic fractionations exhibited by these carboxylases, in a model to predict the final Δ of the plant (relative to atmospheric CO₂ ). Application of this approach to a 'typical' C₃ land plant yields predictions of the decrease of Δ relative to a hypothetical case in which all C is fixed via Rubisco. The predicted decreases range from 0-24 %, for NH₄ ⁺ assimilation (which always occurs in the roots) to 2-80%, for NO₃ ^- assimilation in shoots with the organic acid salt which results from acid-base balance, plus any additional organic acid salts plus free acids for a plant with a basal C:N molar ratio in organic material of 15. Intermediate values are predicted for symbiotic growth with N₂ , or where NO₃ ^- assimilation in root or shoot is accompanied by some acid-base regulation via OH- loss to the root medium. Comparison with published data on the difference in Δ of Ricinus communis cultured with NH₄ ⁺ or NO₃ ^- shows that the measured influence of nitrogen source is in the right direction (NO₃ ^- grown plants with a smaller Δ, i.e. a larger deviation from the value predicted for the absence of non-Rubisco carboxylations) to be explained by the observed difference in composition and hence fractional C contribution by the various carboxylases. However, the effect of N source on Δ is greater than that predicted by the model, i.e. a 2.1 % decrease as opposed to a 0.10 % decrease. It is likely that the major cause of the difference in δ¹³ C of the plants grown on the two N sources is a change in the ratio of transport and biochemical conductances of leaf photosynthesis. Such a change is quantitatively consistent with the lower water use efficiency of NH₄ ⁺ -grown plants. The predicted, and observed, changes in Δ as a function of N source are of the same magnitude as those found for C₃ terrestrial species grown at different temperatures or photon flux densities, or in environments yielding different water use efficiencies by changing root water supply relative to shoot evaporation potential. Variations in N source should be added to the factors which might alter δ of plants growing in the field.

TANSLEY REVIEW No. 2: REGULATION OF PH AND GENERATION OF OSMOLARITY IN VASCULAR PLANTS: A COST-BENEFIT ANALYSIS IN RELATION TO EFFICIENCY OF USE OF ENERGY, NITROGEN AND WATER

The benefits which this paper addresses are those of maintaining the intracellular acid-base balance during growth, and of generating osmolarity related to regulation of turgor in environments of low water potential. These benefits may incur costs in terms of the quantity of potentially growth-limiting resources (photons, water, nitrogen) which are needed to produce unit quantity of 'baseline' plant biomass. The direction (excess H⁺ or excess OH^- ) and magnitude of acid-base perturbation during growth depends on the nature of the N-source (NH₄ ⁺ , N₂ or NO₃ ^- ), so that the costing of pH homoiostasis involves consideration of the costs of overall N-assimilation for comparison with the other costs of growth of a terrestrial C₃ plant. Photon costs for the various biochemical and transport processes involved in overall growth, N-assimilation, pH regulation and osmolarity generation are computed using known stoichiometries of coupled reactions. Water costs are deduced from the C-requirements for the various processes (including C lost in associated decarboxylations) by assuming a constant value of water lost in transpiration per unit net C fixed in an illuminated shoot. Nitrogen costs are deduced from the N-content of the plants or compounds under consideration. The computed costs for N-assimilation and the generation of osmolarity are referred to the costs of 'baseline' plant synthesis using the cheapest mechanisms (NH₄ ⁺ as source for N-assimilation; inorganic ions as the basis for osmolarity generation) so that the increment of cost related to assimilation of N₂ or NO₃ ^- , or of osmolarity generation using an organic compatible solute, can be presented. Photon costs of growth with N₂ fixation and the processes associated with regulation of pH are (granted the assumptions made as to stoichiometries and plant composition) 9 % higher than are those of growth with NH₄ ⁺ as N˜ source. The predicted cost of growth with NO₃ ^- as N source depends on the location of NO₃ ^- reduction and the mechanism of OH^- disposal, and ranges from 5 to 12% more than that for growth with NH₄ ⁺ as N source. H₂ O (transpiration) costs follow a similar pattern, with growth on N₂ as N source costing 12% more, and growth on NO₃ ^- costing to 1-2 to 167 % more, than growth with NH₄ ⁺ as N source. The extra costs in photons of using compatible solutes (sorbitol, proline or glycine betaine) to generate an osmolarity of 500 osmol m^-3 in all of the non-apoplastic water of the plant add 21·5 to 26·1 % to the total costs of growth, while use of compatible solutes to generate osmolarity in 'N' phases (i.e. cytosol, plastid stroma, mitochondrial matrix) alone would add 5·2 to 6·2% The costs of growth in terms of transpirational water are increased 7·9 to 98 % by the use of compatible solutes for osmolarity generation in the 'N' phases only. The increments for the N-containing solutes are higher when NO₃ ^- is the N-source rather than NH₄ ⁺ . The N-cost of growth with N-containing compatible solutes generating 500 osmol m^-3 in 'N' phases increases the N cost of growth by 33%. These predicted costs are under-estimates of 'real' costs which take into account the occurrence of alternate oxidase activity under some growth conditions and the production of additional organic acid anions with N₂ as opposed to NH₄ ⁺ as N source. Nevertheless, the predicted minimum costs of attaining the benefits of pH regulation and of turgor generation are of use in suggesting where selectively significant (i.e. low requirement for a scarce resource) alternative mechanisms may occur. Examples include a possible photon saving by using NH₄ ⁺ rather than N₂ or NO₃ ^- where all three are available; a possible water saving by use of photoreduction of NO₃ ^- in leaves in arid environments; and a possible N saving by use of non-N-containing compatible solutes (polyols) in environments of low water potential. Proof of these suggestions involves comparisons of inclusive fitness of genotypes possessing the trait under consideration with that of genotypes lacking the trait. CONTENTS Summary 26 I. Introduction 27 II. pH Regulation and Osmolarity Generation 27 III. Photon Costs of Various Syntheses Related to pH Regulation and Osmolarity Generation 31 IV. Conclusions on Energy Costs of pH Regulation During Nitrogen Assimilation and Growth 56 V. Conclusions on Energy Costs of Osmolarity Generation 60 VI. Water Costs of pH Regulation and Nitrogen Assimilation 61 VII. Water Costs of Osmolarity Generation 67 VIII. Nitrogen Costs of Osmolarity Generation 69 IX. Conclusions 70 Acknowledgements 72 References 73.

Calcium-oxalate crystals and crystal cells in determinate root nodules of legumes

Early reports of the presence of calciumoxalate crystals in the cortices ofPhaseolus vulgaris root nodules have been confirmed. Crystals were found in all six genera examined (Cajanus, Desmodium, Glycine, Lespedeza, Phaseolus, Vigna) that have determinate nodules and export ureides. They were absent from six genera examined that have indeterminate nodules and export amides. The possible physiological significance of these structures is discussed.

Photosynthetic pathways in the Bromeliaceae of Trinidad: relations between life-forms, habitat preference and the occurrence of CAM

An investigation was carried out into the photosynthetic pathways of the complete bromeliad flora of Trinidad (West Indies). Carbon-isotope ratios (δ¹³C values) were used to distinguish obligate C₃ and crassulacean acid metabolism (CAM) species. Measurements were also carried out on some species in the field to test for day-night changes in leaf titratable acidity.A wide range of δ¹³C values was found. The obligate CAM species had values of -10 to -20‰ and the obligate C₃ species of -23 to -35‰ CAM was found (a) in the majority of Tillandsia spp. (Tillandsioideae) and (b) in all species of Bromelioideae. The other genera of the Tillandsioideae appeared to be at least predominantly C₃. One species, Guzmania monostachia var. monostachia, was identified as a C₃-CAM intermediate, and others may well exist in the Trinidad flora. The influence of factors such as source CO₂, photosynthetic photon flux density and ambient humidity in determining the δ¹³C values is discussed.The taxonomic distribution of C₃ and CAM species within the Bromeliaceae is analyzed in terms of the life-forms and ecological types recognized by Pittendrigh (1948). The most xerophytic species (the light-demanding "atmospherics") all show CAM and are restricted to the drier parts of the island. Most of the species with waterstoring "tanks" have a wide geographic distribution: these include light-demanding C₃ plants and less light-demanding CAM plants. The shade-tolerant bromeliads, which show a requirement for high ambient humidity, are all C₃ plants. We discuss the phylogenetic origins of CAM and the epiphytic habit in the Bromeliaceae.