Major Element Composition of Epidermal and Mesophyll Tissues2Commelina Communis L. and Vicia Faba L.: Some Further Considerations of the Role of Ions in Stomatal Functioning

Epidermal and mesophyll tissues of Commelina communis L. andVicia faba L. were analysed by atomic absorption spectrometryfor the major plant inorganic cations and anions (K, Na, Ca,Mg, P, NO₃-N, Cl) when stomata of the leaf were open and closed.Water-soluble and residual levels of the elements were estimatedand a charge balance of the soluble fraction made. The major portion of K, Na, Cl, and P was extracted in the water-solublefraction of the epidermal and mesophyll tissues of both species.In both species the bulk of Ca remained in the insoluble residueof the epidermis whereas in mesophyll tissue it was equallydistributed be-between the two fractions in C. communis butmainly in the insoluble residue in V. faba. Magnesium was predominantlyfound in the water-soluble fraction of V. faba mesophyll tissueand distributed approximately equally between the two fractionsin the epidermal tissue. In C. communis Mg was slightly moreabundant in the water-soluble fraction of both mesophyll andepidermis. In both species no statistically significant differences inthe levels of the elements could be detected between epidermaland mesophyll tissues from leaves with open stomata and thesame tissues from leaves with closed stomata, suggesting thatthere was no major flux of ions between mesophyll and epidermisduring stomatal movements. Regardless of whether the stomata were open or closed, therewere considerably more water-soluble inorganic cations thananions present in all tissues of both species with K being themajor cation and Cl being the major anion. In V.faba and C-communis epidermis there was 49–53 per cent and 56%68per cent excess cation respectively. In the mesophyll tissuethe excess cation was 63–75 per cent and 75%78 per centin V.faba and C. communis respectively. When the partitioning of the levels of the elements betweenepidermis and mesophyll of a leaf is considered, except forNO₃-N in both species and Na in V. faba, 20 per cent or lessof each element was present in the epidermis.     

Effect of awns and drought on the supply of photosynthate and its distribution within wheat ears

The presence of awns doubled the net photosynthetic rate of wheat ears and also increased the proportion of ¹⁴CO₂ assimilated by the ear that moved to the grain. The effect of water supply on photosynthesis and movement of assimilates was greater for leaves than ears, so that drought increased the proportion of assimilate contributed by ear photosynthesis to grain filling from 13% to 24% in the awnless ears, and from 34% to 43% in the awned ears. ¹⁴C assimilated by the ears was most important to the economy of the upper spikelets and to the distal florets in each spikelet, whereas flag leaf assimilate went mainly to the spikelets in the lower half of the ear, and to the proximal florets. Awns increased grain yield in the dry but not in the irrigated treatment, despite the large contribution of awned ears to grain filling. Either the supply of assimilate did not limit grain yield when water supply was not limiting, or there were compensating disadvantages to awns. However, they did not seem to have any adverse effect on the development of the upper florets, nor did they reduce grain number per ear.     

Injury to Rice Plants Caused by Complete Submergence; A Contribution by Ethylerie (Ethene)

Complete submergence of rice plants (Oryza sativa L. cv. ‘IR42’)in dilute nutrient solution for 3–6 d almost stopped theaccumulation of dry matter, depressed soluble carbohydrate concentrationby over 75% and promoted chlorosis in fully expanded leaves.Increase in fresh weight by the shoots was not impaired. Extensionby the youngest visible leaf was stimulated. Extension by thenext leaf to appear was retarded by submergence. These growthresponses to submergence were associated with a 1-5-fold increasein the partial pressure of endogenous ethylene (ethene). Applying ethylene (0.3–0.35 Pa) in the gas-phase to non-submergedplants reproduced some, but not all, of these effects of submergence.Thus, greater leaf extension and chlorosis of submerged plantscould be attributable to accumulated ethylene but neither theslow relative growth rate nor the decreased extension of leavesemerging after the start of submergence could be so attributed. Two cultivars (‘FR13A’ and ‘Kurkaruppan’)already known to tolerate submergence, differed little fromsubmergence-intolerant ‘IR42’ in their relativegrowth rate and soluble carbohydrate concentration during submergence.However, their underwater leaf extension was less than in ‘IR42’and chlorosis was much less prevalent, especially in ‘FR13A’.Similarly, ethylene supplied to non-submerged plants was a lesseffective promotor of leaf extension and chlorosis in the twosubmergence tolerant cultivars. Application of 1.0 kPa carbondioxide in the gas-phase prevented the chlorosis response toethylene. The results indicate that accumulated ethylene is a likely causeof fast leaf extension and chlorosis in submergence intolerantforms of rice, particularly when amounts of dissolved carbondioxide are minimal. Key words: Oryza sativa L., aeration, ethylene (ethene), stress-tolerance     

A Transmission and Cryo-Scanning Electron Microscopy Study of the Formation of Aerenchyma (Cortical Gas-Filled Space) in Adventitious Roots of Rice (Oryza sativa)

The initiation and development ofaerenchyma in adventitiousroots of rice (Oryza sativa) was studied in tissuc up to approximately36-h-old using transmission electron microscopy (TEM) and cryoscanningelectron microscopy (cryo-SEM). Aerenchyma resulting from selective cortical cell collapse isa naturally occurring feature of rice roots. Evidence of somecortical cell disruption was noticeable by TEM in cells thatwere approximately 6-h-old; it became more advanced as the cellsaged. At 12 h, early stages of cell wall breakdown and lossof cell turgidity were seen. Complete collapse of columns ofcells had occurred by 24 h. In tissue that was 36-h-old, thesenescent cytoplasm of many remaining cells began to disperse.The visual evidence suggests that cell collapse was the resultof autolysis. This pattern of cortical degeneration in ricewas dissimilar to that reported elsewhere for Zea roots grownin an oxygen depleted environment. Cryo-SEM revealed the occurrence of small structures withinthe cortex with the external appearance of miniature, intactcells which are not preserved during conventional SEM preparativeprocedures. Key words: Rice (Oryza sativa L.), roots, aerenchyma, ultrastructure     

Ventilation in Plant Tissue Cultures and Effects of Poor Aeration on Ethylene and Carbon Dioxide Accumulation, Oxygen Depletion and Explant Development

A simple technique for comparing and quantifying the ventilationcapacity of vessels used for plant tissue culture is described.Ethylene was injected into culture vessels and its rate of lossmonitored by gas chromatography. From the resulting exponentialdecay curves, the time in hours for half the ethylene to belost (t₅₀) was calculated and used to compare different containersand sealing methods. Cultures of Ficus lyrata Warb. and Gerberajamesonii Bolus grown for up to 28 d in plastic vessels sufficientlywell-sealed to generate t₅₀ values of approx. 16 h, accumulatedethylene and carbon dioxide in association with depleted oxygen.The relationship between carbon dioxide accumulation and oxygendepletion within culture vessels indicated little if any anaerobicrespiration. Gerbera explants did not appear to be affectedby these gaseous environments. However, in Ficus, leaf expansionwas approximately halved, although fresh and dry mass of wholeshoots was not decreased. The smaller leaf size is attributedto the action of accumulated ethylene, because when the gaswas absorbed with 'Ethysorb' granules or its action inhibitedby 2,5–norbornadiene, leaf growth was normal. The removalof carbon dioxide with potassium hydroxide did not enhance theethylene effect, indicating little if any antagonism of ethyleneaction by carbon dioxide. Shoots of potato (Solanum tuberosumL. cv. Red Craig's Royal) were shortened in sealed culture vessels,in association with swelling, diageotropism and miniaturizationof the leaves. When tuber production was induced by decreasingthe photoperiod, increasing the sucrose concentration and includingcytokinin in the medium, partial sealing promoted conspicuoushypertrophy of the lenticels. These responses of potato wereprevented if the ethylene absorbant mercuric perchlorate wasenclosed together with the cultures. Plant tissue culture, poor aeration, ethylene, leaf expansion, Ficus lyrata Warb., Gerbera jamesonii Bolus, Solanum tuberosum L. cv. Red Craig's Royal     

THE CONFORMATION OF CLATHRIN LIGHT-CHAINS ON TRISKELIONS PROBED BY ANTIBODY INHIBITION

The conformation of clathrin light-chains along the proximal arm of the clathrin triskelion was studied by using rabbit anti-(light-chain peptides) to inhibit the binding of a mouse monoclonal antibody against an epitope in the amino-terminal region. Prior incubation of triskelions with rabbit antisera raised against the extreme carboxyl-terminal of the light-chains partially inhibited binding. The inhibition was largely removed when tested on light-chains that had been freed from triskelions. This suggests that when the light-chains bind the heavy-chain, they adopt a conformation in which the amino and carboxyl-terminal domains are not fully extended, but fold such that these two domains face each other.     

Endogenous Ammonium Generation in Maize Roots and its Relationship to other Ammonium Fluxes

An investigation to determine the magnitude of the back reactionswhich occur during net ammonium uptake by roots and during netammonium assimilation within roots was undertaken with maize(Zea mays L.). Ten-day-old seedlings, which had been grown on250 mmol m^–3 ammonium at pH 4 or 6, were pretreated for3 h in the absence or presence of 500 mmol m ^–3 MSX (methionine-DL-sulphoximine),an inhibitor of the glutamine synthetase-catalysed pathway ofammonium assimilation. They were then exposed for 2 h to 99A% ¹⁵N-ammonium ± MSX. Substantial ammonium cycling occurredduring net ammonium uptake. Efflux was enhanced by MSX treatment,reflecting a 2- to 3-fold accumulation of ammonium in the roottissue. Influx of ammonium was also increased by treatment withMSX, indicating that influx was enhanced when products of ammoniumassimilation were dissipated. The decline in root ¹⁴N-ammoniumaccounted for only a small fraction of the ¹⁴N-ammonium recoveredin the ambient ¹⁵N-ammonium solution, revealing a substantialgeneration of endogenous ¹⁴N-ammonium during the 2 h exposure.The net quantity of ammonium generated was increased appreciablywhen assimilation of ammonium was restricted by MSX and it wasestimated to occur at least 50% faster than net ammonium uptake.Presence of MSX severely decreased translocation of ¹⁵N to shootsbut had a smaller influence on incorporation of ¹⁵N into macromoleculesof the root tissue. The various ammonium flux rates were notgreatly affected by growth at pH 4.0, implying a considerableresistance of ammonium assimilation processes in these maizeroots to the high ambient acidity commonly induced by exposureto ammonium Key words: Ammonium generation, uptake, assimilation     

Cardiovascular Function in Turtles During Anoxia and Acidosis: In vivo and in vitro Studies

During prolonged experimental submergence, freshwater turtlesbecome anoxic and develop a combined respiratory and non-respiratoryacidosis. Anoxia and acidosis are known to depress cardiac functionin turtles and other vertebrate species. In vitro studies ofventricular and atrial tissue of turtles indicate that increasedextracellular Ca⁺⁺ concentration can reverse these depressantactions. Intact turtlesutilize this compensatory mechanism duringanoxia and other conditions leading to acidosis byincreasingplasma Ca⁺⁺ concentration. In addition, cardiac cells may releaseCa⁺⁺ from cell organelles to compensate for induced respiratoryacidosis. Both mechanisms presumably improve cardiac contractilityby elevating the level of sarcoplasmicCa⁺⁺.     

Inferring past zooplanktivorous fish and macrophyte density in a shallow lake: application of a new regression tree model

1. Eutrophication has a profound effect on the biological structure and function of shallow lakes, altering the composition and abundance of submerged macrophyte and fish assemblages. Relatively little is known, however, about decadal to centennial‐scale change in these important aspects of shallow lake ecology. 2. Established palaeolimnological inference models are limited to reconstructing a single variable. As macrophyte and zooplanktivorous fish abundance exert dual and interacting controls on cladoceran assemblages a single variable inference model may contain significant error. To obviate this problem, we applied a new cladoceran‐based multivariate regression tree (MRT) model to cladoceran subfossil assemblages from dated cores from a small shallow lake (Felbrigg Lake, U.K.) to assess long‐term change in fish and submerged macrophyte abundance. Plant macrofossil, chironomid and mollusc subfossil assemblages were also analysed to track changes in biological structure and function and to evaluate the inferences of the MRT model. 3. Over the 200+ year period covered by the sediment cores, there was good agreement in the timing and nature of ecological change reflected by the plant macrofossil, mollusc, chironomid and cladoceran data. The sediment sequence was divided into three dated zones: c. 1797–1890, c. 1890–1954 and c. 1954–present. Prior to 1890 plant‐associated mollusc, cladoceran and chironomid assemblages indicated a species‐rich macrophyte community; a scenario confirmed by the plant macrofossil data. From c. 1890 to 1954 macrophyte‐associated species of all three invertebrate groups remained abundant but the proportion of pelagic cladocerans rose. Post‐1954 mollusc and chironomid assemblages changed to sediment associated detrital feeders and the proportion of pelagic cladoceran taxa increased further. 4. The cladoceran‐based MRT model indicated a long period of stability, c. 1790–1927, characterised by abundant submerged macrophytes and zooplanktivorous fish. From c. 1927 to 1980, the MRT model inferred a decline in zooplanktivorous fish density (ZF) but relative stability in August macrophyte abundance. From 1980 to 2000, an increase in zooplanktivorous fish was inferred tallying well with available data on the fish population (since the 1970s), which indicated extirpation of perch in the 1970s and a subsequent increase in the rudd population. The model inferred little change in August macrophyte abundance until post‐c. 1980 at which point it indicated a decline. The surface sediment assemblage was placed in MRT group A, where submerged plants are absent or very rare in late summer in good agreement with current conditions at the site. 5. The MRT model, applied here for the first time, appears to have successfully tracked changes in macrophyte abundance and ZF over the last 200 years at Felbrigg Lake. The inferences agreed with historical observations on the fish community and the supporting palaeolimnological data. Given that multiple structuring forces shape most biological communities, the application of a model capable of allowing for this represents a significant advance in palaeolimnology.