B. C. Loughman

Avoidance of Hypoxia in a Cnidarian Symbiosis by Algal Photosynthetic Oxygen

The algal symbionts in a variety of invertebrates are widely believed to increase the oxygen tension in the animal tissue by producing photosynthetic oxygen (1). This could be advantageous to the animal by maintaining normoxia in anoxic waters (2-4). Equally, it could be detrimental, through hyperoxia and the generation of toxic oxygen radicals (5), and this may contribute to the recurrent incidence of mass bleaching in tropical cnidarian symbioses over the last decade (6). Here, we use in vivo 31p nuclear magnetic resonance spectroscopy (NMR) to assess the effect of photosynthetic oxygen production by the symbiotic alga Symbiodinium sp. on the energy metabolism of a sea anemone, Anemonia viridis and, by using acidification of the tissues and elevated ADP/ATP ratios as linked indices of anaerobiosis (7), we show unequivocally that photosynthetic oxygen can protect an invertebrate from hypoxia. Illumination prevents the rapid acidification and reduction in ATP that occurs under hypoxic conditions in the dark, and we suggest that this effect could be partly responsible for algal enhancement of coral calcification. 31p NMR spectra of Anemonia viridis tentacles showed signals from a range of phosphorylated metabolites including phosphonates, orthophosphate (Pi), and ATP (Fig. 1). The spectra were assigned on the basis of an earlier 31p NMR study of sea anemones (8) and from the characteristic chemical shifts of the signals in the spectra. The intensity of the ATP, ADP, and Pi resonances, as well as the position of the Pi signal, were of particular interest here, and the analysis of the spectra was confined to these regions of the spectrum. The chemical shift of the Pi signal

Metabolic aspects of the transport of ions by cells and tissues of roots

There is no clear cut viewpoint concerning the mechanism of transport of solutes into and within roots that is applicable to all plants and all solutes. Most attention has been given to ionised species, particularly those that are not incorporated into organic molecules, e.g. K+, Na+, Ca+ + and Cl−. Ions that are rapidly metabolised such as NO 3 − , SO 4 − − and H2PO 4 − give rise to special problems as do uncharged solutes such as sugars that become charged during entry by phosphorylation.

The application of in vivo techniques in the study of metabolic aspects of ion absorption in crop plants

SummaryLittle is known about the biochemical basis of the genotypic differences in the capacity for ion absorption and transport shown by many crop species. If these differences reflect the abundance of a specific membrane component or the activity of an enzyme we need to have some indication of thein vivo operation of these systems in whole plants. Thein vivo assessment of glycolytic enzymes is illustrated by the effects of mannose on the transport of phosphate in maize varieties. The application of high resolution31P-NMR to the study of intermediary metabolismin vivo is also helpful in following transport capacity.The five-fold rise in respiratory rate that occurs when freshly cut potato slices are maintained in aerated water for 24 hours is accompained by the turning on of a wide range of biochemical systems. Major increases in the capacity for absorption of phosphate from low concentrations (0.1 μM–10 μM) and in the phosphorylative ability of the tissue are seen, indicating the synthesis of a carrier involved in phosphate transport. These capacities differ markedly between individual tissues of the tuber,i. e. pith, parenchyma, cortex and buds and large differences have been observed between comparable tissue from different varieties. Varieties grown under similar conditions have been compared and shown to exhibit different kinetics with respect to the development of the low concentration absorption site and in their sensitivity to the effects of uncouplers such as 2,4-dinitrophenol.

Varietal differences in physiological and biochemical responses to changes in the ionic environment

Experimental assessment of differences between cultivars of crop species or ecotypes of wild species from different localities in their capacities for ion absorption and transport is made difficult by the problem of obtaining seed material of comparable ionic content. When young seedlings are used this problem is particularly acute if the seeds of the different cultivars have not been raised under identical soil conditions. Propagation of material from ecotypes under controlled conditions is one approach to the solution of this problem. Six maize cultivars have been selected for similarity of phosphate content and the capacity for phosphate absorption from 5 μM KH2PO4 has been shown to vary by threefold whereas the proportion of the accumulated phosphate that reaches the shoot differs by much less. This level of phosphate supply approached that likely to induce deficiency and when the concentration is reduced to 1 μM differences in transport capacity of up to fourfold were observed when the rate of arrival at the tip of the first leaf was continuously monitored. The rapidity with which the transport is shut off by adding 1 mM D(+) mannose to the root environment also varies significantly indicating that sizeable differences in either the accumulation of mannose or the activity of phosphomannoisomerase exist in these cultivars.

Effects of vanadate on the ATP content, ATPase activity and phosphate absorption capacity of maize roots

Although the sensitivity of the plasma membrane H+-ATPase to vanadate is well known, the metabolic response of plant cells to vanadate is less well characterised in vivo and its use as an inhibitor in whole plant experiments has had mixed success. Experiments with maize (Zea mays, L.) roots and with purified plasma membrane fractions from the same tissues showed that exposure to vanadate caused: (i) a reduction in the capacity for phosphate uptake; (ii) a reduction in the extractable ATPase activity from the tissue; and (iii) a significant increase in the ATP level. The measurements on the extractable ATPase activity and the ATP level showed that the effect of vanadate developed slowly, apparently reflecting the slow accumulation of intracellular vanadate. The marked effect of vanadate on the ATP level-exposure to 500 μM vanadate for 5 h doubled the ATP content of the roots tips-indicates that there is no stringent control over the ATP level in the roots and that the plasma membrane H+-ATPase activity is likely to have a significant role in determining the ATP level under normal conditions.

Transformation of phenoxyacetic acid herbicides by Phaseolus vulgaris L. callus cells.

The auxin herbicide (4-chloro-2-methylphenoxy)-acetic acid (MCPA) was absorbed by liquid cultured callus cells of Phaseolus vulgaris L. and subsequently became hydroxylated at the methyl group. The alcohol remained largely unconjugated and was partly released to the culture medium. Several glycosides and an ether-soluble conjugate of MCPA appeared as minor metabolites. Unsubstituted phenoxyacetic acid (POA) was metabolised primarily by 4-hydroxylation and subsequent phenolic glucoside formation. Metabolites of POA were retained by the cells. In the case of both substrates, metabolism in cells correlated well with that observed in apical buds excised from etiolated seedlings.