Root to shoot ratio of crops as influenced by CO2

Crops of tomorrow are likely to grow under higher levels of atmospheric CO₂. Fundamental crop growth processes will be affected and chief among these is carbon allocation. The root to shoot ratio (R:S, defined as dry weight of root biomass divided by dry weight of shoot biomass) depends upon the partitioning of photosynthate which may be influenced by environmental stimuli. Exposure of plant canopies to high CO₂ concentration often stimulates the growth of both shoot and root, but the question remains whether elevated atmospheric CO₂ concentration will affect roots and shoots of crop plants proportionally. Since elevated CO₂ can induce changes in plant structure and function, there may be differences in allocation between root and shoot, at least under some conditions. The effect of elevated atmospheric CO₂ on carbon allocation has yet to be fully elucidated, especially in the context of changing resource availability. Herein we review root to shoot allocation as affected by increased concentrations of atmospheric CO₂ and provide recommendations for further research. Review of the available literature shows substantial variation in R:S response for crop plants. In many cases (59.5%) R:S increased, in a very few (3.0%) remained unchanged, and in others (37.5%) decreased. The explanation for these differences probably resides in crop type, resource supply, and other experimental factors. Efforts to understand allocation under CO₂ enrichment will add substantially to the global change response data base.Abbreviations R:S root to shoot ratio, dry weight basis     

Influence of rhizodeposition under elevated CO2 on plant nutrition and soil organic matter

Atmospheric CO₂ concentrations can influence ecosystem carbon storage through net primary production (NPP), soil carbon storage, or both. In assessing the potential for carbon storage in terrestrial ecosystems under elevated CO₂, both NPP and processing of soil organic matter (SOM), as well as the multiple links between them, must be examined. Within this context, both the quantity and quality of carbon flux from roots to soil are important, since roots produce specialized compounds that enhance nutrient acquisition (affecting NPP), and since the flux of organic compounds from roots to soil fuels soil microbial activity (affecting processing of SOM).From the perspective of root physiology, a technique is described which uses genetically engineered bacteria to detect the distribution and amount of flux of particular compounds from single roots to non-sterile soils. Other experiments from several labs are noted which explore effects of elevated CO₂ on root acid phosphatase, phosphomonoesterase, and citrate production, all associated with phosphorus nutrition. From a soil perspective, effects of elevated CO₂ on the processing of SOM developed under a C4 grassland but planted with C3 California grassland species were examined under low (unamended) and high (amended with 20 g m^–2 NPK) nutrients; measurements of soil atmosphere 13C combined with soil respiration rates show that during vegetative growth in February, elevated CO₂ decreased respiration of carbon from C4 SOM in high nutrient soils but not in unamended soils.This emphasis on the impacts of carbon loss from roots on both NPP and SOM processing will be essential to understanding terrestrial ecosystem carbon storage under changing atmospheric CO₂ concentrations.Abbreviations SOM soil organic matter - NPP net primary productivity - NEP net ecosystem productivity - PNPP p-nitrophenyl phosphate     

Rhizodeposition under ambient and elevated CO2 levels

As global CO₂ levels rise, can soils store more carbon and so buffer atmospheric CO₂ levels? Answering this question requires a knowledge of the rates of C inputs to soil and of CO₂ outputs via decomposition. Below-ground inputs from roots are a major component of the C flow into soils but are still poorly understood. In this article, new techniques for measuring rhizodeposition are reviewed and discussed and the need for cross-comparisons between methods is identified. One component of rhizodeposition, root exudation, is examined in more detail and evidence is presented which suggests that current estimates of exudate flow into soils are incorrect. A mechanistic mathematical model is used to explore how exudate flows might change under elevated CO₂.     

A sensitive,simultaneous analysis of ribulose 1,5-bisphosphate carboxylase/oxygenase efficiencies: Graphical determination of the CO2/O2 specificity factor

A simple approach to determine CO₂/O₂ specificity factor () of ribulose 1,5-bisphosphate carboxylase/oxygenase is described. The assay measures the amount of CO₂ fixation at varying [CO₂]/[O₂] ratios after complete consumption of ribulose 1,5-bisphosphate (RuBP). Carbon dioxide fixation catalyzed by the carboxylase was monitored by directly measuring the moles of ¹⁴CO₂ incorporated into 3-phosphoglycerate (PGA). This measurement at different [CO₂]/[O₂] ratios is used to determine graphically by several different linear plots the total RuBP consumed by the two activities and the CO₂/O₂ specificity factor. The assay can be used to measure the amounts of products of the carboxylase and oxygenase reactions and to determine the concentration of the substrate RuBP converted to an endpoint amount of PGA and phosphoglycolate. The assay was found to be suitable for all [CO₂]/[O₂] ratios examined, ranging from 14 to 215 micromolar CO₂ (provided as 1–16 mM NaHCO₃) and 614 micromolar O₂ provided as 50% O₂. The procedure described is extremely rapid and sensitive. Specificity factors for enzymes of highly divergent values are in good agreement with previously published data.Abbreviations HEPPS N-(2-hydroxyethyl)piperazine-N-(3-propanesulfonic acid) - L large subunit of rubisco - PGA 3-phosphoglyceric acid - rubisco ribulose 1,5-bisphosphate carboxylase/oxygenase - RuBP d-ribulose 1,5-bisphosphate - S small subunit of rubisco - XuBP d-xylulose 1,5-bisphosphate     

The potential applications of cyanobacterial photosynthesis for clean technologies

Natural photosynthesis may be adapted to advantage in the development of clean energy technologies. Efficient biocatalysts that can be used in solar energy conversion technologies are the cyanobacteria. Photobioreactors incorporating cyanobacteria have been used to demonstrate (a) the production of hydrogen gas, (b) the assimilation of CO₂ with the production of algal biomass, (c) the excretion of ammonium, and (d) the removal of nitrate and phosphate from contaminated waters.Abbreviations Chl chlorophyll - DW dry weight - MSX L-methionine-D-L-sulphoximine - PAR photosynthetically active radiation - PU polyurethane - PV polyvinyl - PVC polyvinylchloride     

Mature ferredoxin-NADP reductase with a glutaminyl residue at N-terminus from spinach chloroplasts

When 35%-acetone extract of spinach chloroplasts was separated by SDS-PAGE, ferredoxin-NADP reductase (FNR) appeared as a single band at a molecular mass of 35 kDa. After the polypeptides on the SDS-PAGE plate were electroblotted onto PVDF membrane, the FNR band was cut out and analyzed for N-terminal structure in a gas-phase protein sequencer. Two different FNR peptides were identified: one with glutamine at its N-terminus (Gln-FNR) and the other with -pyroglutamic acid (tFNR) fraction was extracted from chloroplasts with their loosely bound FNR (lFNR) fraction removed in advance. The tFNR fraction contained Gln-FNR only. The Gln-FNR could be highly purified by affinity chromatography using a ferredoxin column. The purified Gln-FNR was digested with arginyl endopeptidase for peptide mapping and partial sequence analysis. Primary structure of Gln-FNR differed from that of lFNR loosely bound FNR - tFNR tightly bound FNR - -pyroglutamic acid at N-terminus     

Characterization of a cDNA encoding the 55 kDa B regulatory subunit of Arabidopsis protein phosphatase 2A

Type 2A serine/threonine protein phosphatases (PP2A) are key components in the regulation of signal transduction and control of cell metabolism. The activity of these protein phosphatases is modulated by regulatory subunits. While PP2A activity has been characterized in plants, little is known about its regulation. We used the polymerase chain reaction to amplify a segment of a cDNA encoding the B regulatory subunit of PP2A from Arabidopsis. The amplified DNA fragment of 372 nucleotides was used as a probe to screen an Arabidopsis cDNA library and a full-length clone (AtB) of 2.1 kbp was isolated. The predicted protein encoded by AtB is 43 to 46% identical and 53 to 56% similar to its yeast and mammalian counterparts, and contains three unique regions of amino acid insertions not present in the animal B regulatory subunit. Genomic Southern blots indicate the Arabidopsis genome contains at least two genes encoding the B regulatory subunit. In addition, other plant species also contain DNA sequences homologous to the B regulatory subunit, indicating that regulation of PP2A activity by the 55 kDa B regulatory subunit is probably ubiquitous in plants. Northern blots indicate the AtB mRNA accumulates in all Arabidopsis tissues examined, suggesting the protein product of the AtB gene performs a basic housekeeping function in plant cells.     

Mutations in tumour suppressor genes,l(2)gl andl(2)gd,alter the expression ofwingless inDrosophila

To understand the roles of two well known tumour suppressor genes.l(2)gl andl(2)gd in normal imaginal disc development inDrosophila, we have initiated a study to examine effect of mulations of these genes on the expression of genes involved in the patterning of the imaginal discs. In this study we show that the expression ofwingless, theDrosophila orthologue of the mammalian oncogeneWnt, is affected in the imaginal discs ofl(2)gl ⁴ andl(2)gd ¹ mutant individuals. In the tumourous wing imaginal discs froml(2)gl mutant larvae, the pattern ofwingless expression was progressively disrupted with an increase in the area of expression, Tumourous wing imaginal discs froml(2)gd homozygous individuals exhibited progressive broadening and extension of the wingless expressing domains. We suggest thatl(2)gl andl(2)gd might be involved in regulating post embryonic expression ofWingless.     

ITS2 ribosomal RNA indicates Schistosoma hippopotami is a distinct species

The internal transcribed spacer region of the ribosomal RNA, ITS2, was sequenced from a singlé specimen of S. hippopotami collected from a pulmonary artery of the hippopotamus, Hippopotamus amphibius in South Africa. The nucleotide sequence was aligned with those of S. mansoni, S. rodhaini, S. haematobium, S. intercalatum, S. curassoni. S bovis and S. japonicum. Both maximum parsimony and genetic distance analyses were performed on these data sets. Using S. japonicum as outgroup to the African schistosomes, a single most-parsimonious tree was obtained of length 64 steps with a consistency index of 1. S. hippopotami was the sister-group to the remaining African species. This species has lateral-spined eggs and its basal position in the tree suggests that this condition is primitive and that terminal-spined eggs developed secondarily. Molecular data clearly show that S. hippopotami cannot be considered synonymous with S. mansoni. Assuming the hippopotamus is the normal host of S. hippopotami, phylogenetic analysis is consistent with an ancient association between schistosomes and ungulates.     

ATP,pH and Mg2+ modulate a cation current in Beta vulgaris vacuoles: A possible shunt conductance for the vacuolar H+-ATPase

Macroscopic instantaneous and time-dependent currents have been measured in the vacuolar membrane of Beta vulgaris using a patch clamp configuration analogous to whole cell mode. At low cytosolic Ca²⁺ and in the absence of Mg²⁺, only an instantaneous current was observed. This current is carried predominantly by cations (P_KP_Cl 71, p_nap_cl 41 and arginine is also conducted). The instantaneous current can be activated by ATP^4– (e.g., ATP-activated mean K⁺ current density was –20 mA.m^–2 at a membrane voltage of –20 mV) and by increasing cytosolic pH and Mg²⁺ (raising Mg²⁺ from 0 to 0.4 mm induced a mean current density increase of –7 mA.m^–2 at –20 mV). Such current can be activated by simultaneous addition of putative in vivo concentrations of ATP^4–/MgATP/Mg _free ²⁺ (in the presence of bafilomycin to inhibit the vacuolar ATPase) and further modulated by cytosolic pH. With vacuolar K⁺ concentration greater than that of the cytosol, activation of the instantaneous current would mediate vacuolar K⁺ release over the range of physiological membrane voltage. It is argued that the ATP^4–-activated current, in addition to acting as a K⁺ mobilization pathway, could provide a counter-ion (shunt) conductance, allowing the two electrogenic H⁺ pumps which reside in the vacuolar membrane to acidify the vacuolar lumen.A separate time-dependent current, which was not observed at low Ca²⁺ concentrations (less than 500 nm) could also be elicited by addition of Mg²⁺ at the cytoplasmic membrane face. This current was stimulated by increasing cytoplasmic pH.The authors are grateful to the BBSRC for financial support (Grant PG87/529) and to the Royal Society (University Research Fellowship to J.M.D.). We thank C. Abbott, K. Partridge and J. Robinson for plant cultivation; A. Amtmann, A. Bertl, D. Gradmann and G. Thiel for helpful discussion.