BOTANICAL BRIEFING: The Function and Metabolism of Ascorbic Acid in Plants

Ascorbate is a major metabolite in plants. It is an antioxidantand, in association with other components of the antioxidantsystem, protects plants against oxidative damage resulting fromaerobic metabolism, photosynthesis and a range of pollutants.Recent approaches, using mutants and transgenic plants, areproviding evidence for a key role for the ascorbate–glutathionecycle in protecting plants against oxidative stress. Ascorbateis also a cofactor for some hydroxylase enzymes (e.g. prolylhydroxylase) and violaxanthin de-epoxidase. The latter enzymelinks ascorbate to the photoprotective xanthophyll cycle. Arole in regulating photosynthetic electron transport has beenproposed. The biosynthetic pathway of ascorbate in plants hasnot been identified and evidence for the proposed pathways isreviewed. Ascorbate occurs in the cell wall where it is a firstline of defence against ozone. Cell wall ascorbate and cellwall-localized ascorbate oxidase (AO) have been implicated incontrol of growth. High AO activity is associated with rapidlyexpanding cells and a model which links wall ascorbate and ascorbateoxidase to cell wall extensibility is presented. Ascorbate hasalso been implicated in regulation of cell division by influencingprogression from G1 to S phase of the cell cycle. There is aneed to increase our understanding of this enigmatic moleculesince it could be involved in a wide range of important functionsfrom antioxidant defence and photosynthesis to growth regulation. Ascorbic acid; ascorbate oxidase; cell division; cell wall; growth; oxidative stress; photosynthesis; ozone; vitamin C     

Enzymic Activities Associated with the Ability of Aerial and Submerged Forms of Littorella uniflora (L.) Aschers to perform CAM

ABSRACT: Groenhof, A. C, Smirnoff, N. and Bryant, J. A. 1988. Enzymicactivities associated with the ability of aerial and submergedforms of Littorella uniflora (L.) Aschers to perform CAM.—J.exp. Bot. 39: 353-361. The submerged form of Littorella uniflora shows a full CAM modeof photosynthesis as shown by diel acid fluctuations and elevatedactivities (in comparison to non-submerged leaves) of the enzymesphosphoenolpyruvate carboxylase (PEPC) and NADP-malic enzyme.Non-submerged plants exhibit no diel fluctuations of acidityand no changes in activity of NADP-malic enzyme or PEPC. PEPCactivity is low and NADP-malic enzyme is not detectable. Furthercharacterization of PEPC extracted from submerged plants duringthe light and dark periods of a diel cycle shows that the enzymeextracted in the dark is more active. In addition, the enzymeshows a decrease in K_m (PEP) and an increase in V_max in thepresence of glucose-6-phosphate, whilst in the presence of malateK_m (PEP) is increased and V_max decreased; this response to malateis only observed in the light and at pH 7.2. Molecular weightdeterminations using a Sephacryl S-300 column show that theenzyme extracted from plants during the dark period has an apparentmol. wt. of 375 KDa and the enzyme extracted from plants duringthe light period has an apparent mol. wt. of 307 KDa. Key words: Littorella uniflora (shoreweed), Crassulacean acid metabolism, PEP carboxylase, malic enzyme     

Proline Metabolism and Transport in Maize Seedlings at Low Water Potential

The growing zone of maize seedling primary roots accumulatesproline at low water potential. Endosperm removal and excisionof root tips rapidly decreased the proline pool and greatlyreduced proline accumulation in root tips at low water potential.Proline accumulation was not restored by exogenous amino acids.Labelling root tips with [¹⁴C]glutamate and [¹⁴C]proline showedthat the rate of proline utilization (oxidation and proteinsynthesis) exceeded the rate of biosynthesis by five-fold athigh and low water potentials. This explains the reduction inthe proline pool following root and endosperm excision and theinability to accumulate proline at low water potential. Theendosperm is therefore the source of the proline that accumulatesin the root tips of intact seedlings. Proline constituted 10% of the amino acids released from the endosperm. [¹⁴C]Prolinewas transported from the scutellum to other parts of the seedlingand reached the highest concentration in the root tip. Less[¹⁴C]proline was transported at low water potential but becauseof the lower rate of protein synthesis and oxidation, more accumulatedas proline in the root tip. Despite the low biosynthesis capacityof the roots, the extent of proline accumulation in relationto water potential is precisely controlled by transport andutilization rate.     

Nitrate Reductase Acivity in Leaves of Barley (Hordeum vulgare) and Durum Wheat (Triticum durum) during Field and Rapidly Applied Water Deficits

Smirnoff, N., Winslow, M. D. and Stewart, G. R. 1985. Nitratereductase activity in leaves of barley (Hordeum vulgare) anddurum wheat (Triticum durum) during field and rapidly appliedwater deficits.-J. exp. Bot 36: 1200-1208. The effect of field and rapidly applied water deficits on nitratereductase activity in the leaves of two barley varieties andone durum wheat variety was investigated. In field experimentsplants were subjected to irrigation at different rates in threeMediterranean environments by means of a line source sprinklerirrigation system. The environments differed in rainfall andnitrogen fertility. Plant water potentials decreased from –1.5MPa to between –2.5 and –3.0 MPa as the irrigationrate decreased. Nitrate reductase activity in the leaves ofthese plants during heading was either unaffected or sometimesincreased where the least water was supplied. Nitrate reductaseactivity was highest in the plants growing with an ample nitrogensupply irrespective of water regime. In contrast, seedlingssubject to rapidly applied water stress over 6 d lost 30-85%of their nitrate reductase activity when leaf water potentialfell from between –0.33 and –0.75 MPa to between–O.93 and –2.04 MPa. The decrease was less in theyoung leaves than in the old leaves. Polyethylene glycol inducedosmotic stress resulted in a drop in leaf water potential from–0.20 MPa to between –1.05 and –1.20 MPa alongwith a loss of 40-85% of leaf nitrate reductase activity after48 h. It is suggested that maintenance of nitrate reductase activityin field grown barley and durum wheat plants reflects an acclimationto water deficit Maintenance of nitrate assimilation duringwater stress may allow continued synthesis of nitrogenous compatiblesolutes using the excess photochemical energy available duringstomatal closure. Key words: Nitrate reductase, water stress, barley, durum wheat     

The Occurrence of Nitrate Reduction in the Leaves of Woody Plants

Nitrate reductase activities greater than 02 µmol h^–1g^–1 f. wt, measured by an in vivo assay, occurred in 41per cent of a large sample (555 species) of woody plants. Ifseveral taxonomic groups (Gymnosperms, Ericaceae and Proteaceae)with consistently low activities were discounted activitiesgreater than 02 µmol h^–1 g^–1 f. wt occurredin 73 per cent of the species. This compares with 93 per centin herbaceous species, suggesting that leaf nitrate reductionis of common occurrence in woody plants. In a small sample ofspecies leaf nitrate reductase activity correlated with nitrateconcentration in the xylem sap. Low activities occurred consistentlyin the Gymnosperms, Ericaceae and Proteaceae. Feeding cut shootsof representatives of these groups with nitrate caused inductionof leaf nitrate reductase activity in the Gymnosperms and Proteaceae,but only limited induction in the Ericaceae. The Ericaceae,with the exception of two species, had low activities and lownitrate reductase inducibility. Root assimilation may predominatein the Gymnosperms and Proteaceae. It is suggested that nitratereduction generally occurs in the leaves of trees from a varietyof plant communities and that this may be related to the lowerenergy cost of leaf, as opposed to root, nitrate assimilation. Nitrate reductase, trees and shrubs, leaves, nitrate assimilation, nitrate translocation, nitrate reductase induction, energy cost, plant ecology     

Variation in the Structure and Response to Flooding of Root Aerenchyma in some Wetland Plants

The structure and response to flooding of root cortical aerenchyma(air space tissue) in a variety of wetland (flood-tolerant)species was investigated and compared with some flood-intolerantspecies. In some species aerenchyma consisted of enlarged schizogenousintercellular spaces and in others aerenchyma formation involvedlysigeny. Two types of lysigenous aerenchyma were distinguished.In the first the diaphragms between lacunae were arranged radiallyand consisted of both collapsed and intact cells. In the secondtype, which was confined to the Cyperaceae, the radial diaphragmscontained intact cells, and stretched between them were tangentially-arrangeddiaphragms of collapsed cells. Flooding in sand culture generally increased root porosity (airspace content) although there were exceptions. The flood-intolerantspecies Senecio jacobaea produced aerenchyma but did not survivelong-term flooding. Among the flood-tolerant species, Filipendulaulmaria did not produce extensive aerenchyma even when flooded.Eriophorum angustifolium and E. vaginatum produced extensiveaerenchyma under drained conditions which was not increasedby flooding. In Nardus stricta root porosity was increased bylow nutrient levels as well as by flooding. Aerenchyma, root cortex, wetland plants, waterlogging, flooding-tolerance, Ammophila arenaria, Brachypodium sylvalicum, Caltha palustris, Carex curia, Eriophorum vaginatum, Filipendula ulmaria, Glyceria maxima, Hieracium pilosella, Juncus effusus, Myosotis scorpioides, Nardus stricta, Narthecium ossifragum, Phalaris arundinacea, Senecio jacobaea, Trichophorum cespitosum     

The appearance of a new molecular species of phosphoenolpyruvate carboxylase (PEPC) and the rapid induction of CAM in Sedum telephium L.

Abstract. When detached leaves of Sedum telephium are incubated in the absence of water, a rapid switch from C₃ photosynthesis to CAM (as indicated by the onset of day-to-night fluctuations in titratable acidity. ΔH⁺) occurs within the first dark period. The C₃-CAM switch in intact plants occurs within 3 5d. Extractable activity of phospho enol pyruvate carboxylase (PEPC) increases five-fold in intact plants during CAM induction; however, during rapid CAM induction in detached leaves, there is only a very small increase in PEPC activity. Fractionation by anion exchange chromatography of crude extracts from leaves of intact plants subjected to water deficit shows that CAM induction is associated with the appearance of a molecular species of PEPC termed PEPC I. PEPC I is barely detectable in well-watered plants which are not performing CAM. The major form in these plants is termed PEPC II. In leaves from intact plants, there is a significant positive correlation between PEPC I activity and ΔH⁺ during a period of increasing water deficit. PEPC I exhibits day to night fluctuations in malate sensitivity, being less sensitive during the dark period. In contrast, PEPC II is more sensitive to inhibition by malate and has no day to night fluctuation in sensitivity. In detached leaves deprived of water, a small increase in PEPC I capacity is detected at the end of the first dark period (20 h after the start of treatment). The results suggest that PEPC I is required for attainment of maximum nocturnal malic acid synthesis. There is a significant correlation between leaf water status (relative water content), ΔH⁺, total PEPC and PEPC I activity suggesting that the internal water status of the plant may be a trigger for CAM induction. Abscisic acid applied to detached leaves does not cause nocturnal acidification.