Journal of Experimental Botany

RAVEN, J., A.

doi: 10.1093/jxb/25.2.221pmid: N/A

Abstract The energy source for active phosphate influx in Hydrodictyon africanum has been investigated using gas mixtures with and without O2 and CO2, light of various wavelengths, and metabolic inhibitors selective for respiratory or photosynthetic electron transport and phosphorylation. It is concluded that, as in the other green algae studied, active phosphate transport requires ATP. In the dark this is supplied by oxidative phosphorylation; in the light the influx is much less sensitive to inhibition of oxidative phosphorylation, and photophosphorylation (including cyclic photophosphorylation) can act as energy source. This situation is more like that for active K influx (coupled to active Na efflux) than to active Cl influx in H. africanum, except that the active dark influx is relatively greater for phosphate influx. The significance of these results for the mechanism of regulation of light-stimulated ATP-requiring processes, and for the role of photosynthetic and oxidative phosphorylation in the energy metabolism of green cells, is discussed. This content is only available as a PDF. © Oxford University Press

SILVERBERG, B., A.;SAWA,, T.

doi: 10.1093/jxb/25.2.230pmid: N/A

Abstract In Nitella flexilis, a species belonging to the section Anarthrodactylae of the genus Nitella (Characeae), some mature cells, with the exception of those of the nodal complex and the rhizoid, show numerous spherical vacuoles occupying the entire central region of the cell. These vacuoles are fairly uniform in size measuring 30–40 μm in diameter. Evidence is presented from investigations with the light and electron microscopes suggesting that some mature cells of a certain group of Nitella species may not have a huge central vacuole. Vacuolation in young cells of the apex apparently begins by progressive dilation of the endoplasmic reticulum. The Golgi complex may also be involved in vacuolation. The necessity for re-examination of certain biological activities in the giant internode cell is explored. This content is only available as a PDF. © Oxford University Press

AYOUB, ALI, T.

doi: 10.1093/jxb/25.2.245pmid: N/A

Abstract The efficiency of calcium in enhancing the tolerance of beans (Phaseolus vulgaris L.) to sodium salinization was studied in pot experiments in both cool and hot seasons. In the cool season increased yields of dry matter, roots, nodules and pods were positively correlated with increased Ca application and negatively correlated with the Na contents of irrigation water and plant tissue. The Na levels both in roots and tops declined significantly as increased amounts of calcium were added. Ca in the range of 2.0 to 8.0 mmol/1 caused competitive inhibition of Na uptake and Na translocation. At Ca levels between 0 to 2.0 mM only Na translocation to tops was markedly inhibited. In warm seasons Ca had no beneficial effects on bean yields at any level of Na. On the contrary, high rates of Ca application resulted in a higher death rate. This content is only available as a PDF. © Oxford University Press

CRAM, W., J.

doi: 10.1093/jxb/25.2.253pmid: N/A

Abstract Cl− and HCO3− ions interact apparently competitively during influx across the plasmalemma of carrot root cells. Cl−, however, reduces HCO3− influx much less than predicted from the effect of HCO3− on Cl− influx. Cl− and HCO3− plasmalemma influxes both increase with time after excision of carrot tissue. Cl− and HCO3− may therefore be transported by a common mechanism. The effect of pH changes on the influx of malate across the plasmalemma in barley roots shows that malate crosses the plasmalemma as the singly charged anion. Stimulations of influx by both K2SO4 and KCl suggest that the malate anion crosses in association with. K+. If malate entry is passive, Pmal- is about 2×10−8 cm s−1, but it is thought that malate entry is partly an active process. A slight, apparently competitive inhibition by Cl− of malate flux into the vacuole of barley root cells suggests that the two anions may be transported by a common process at the tonoplast, but this is not thought to be physiologically significant. The accumulation of 14C from 1 mM HCO3− is drastically reduced by 10 mM Cl−. A quantitative analysis of the kinetics of 14C exchange shows that Cl− directly inhibits the formation of malate from HCO3− ⁠. The decreased influx of endogenously produced malate to the vacuole in the presence of Cl− is probably a secondary consequence of the fall in the cytoplasmic concentration. The nature of the Cl− inhibition of malate formation is discussed. In KCl-loaded tissue the influx of external malate and the accumulation of 14C from external HCO3− are reduced. The location of these effects is not certain, but the effects suggest that regulation of malate synthesis and accumulation may be related to the negative-feedback regulation of Cl− and NO3− transport. This content is only available as a PDF. © Oxford University Press

HARNISCHFEGER,, GÖTZ

doi: 10.1093/jxb/25.2.269pmid: N/A

Abstract The magnesium dependence of NADP-reduction, oxygen evolution and concomitant ATP-formation was studied as a function of the age of the Cucurbita cotyledons from which the chloroplasts were isolated. Whereas NADP-reduction under alkaline assay conditions always exhibits a stimulation upon addition of Mg++, measurements in an acidic medium show an age-dependent reversal to inhibition. The data for oxygen evolution are more complex in nature, due to participation of consecutive reactions. The Mg++ concentration for optimal ATP formation decreases with the age of the cotyledonary material. The results are discussed from the viewpoint of a possible control of essential photosynthetic functions in vivo by magnesium. This content is only available as a PDF. © Oxford University Press

LEE, R., B.;WHITTINGHAM, C., P.

doi: 10.1093/jxb/25.2.277pmid: N/A

Abstract The incorporation of radioactive carbon into various photosynthetic products was investigated with tomato plants in atmospheres containing between 40 and 1400 parts/106 carbon dioxide. A significantly greater proportion of 14C entered sucrose and alcohol-insoluble material at high concentrations of carbon dioxide. Incorporation into glycine and serine was significantly greater at lower carbon-dioxide concentrations. The pool size of these intermediates was also determined and it was concluded that in the presence of high partial pressures of carbon dioxide the flow of carbon from the photosynthetic cycle through the C2 pathway is decreased. This content is only available as a PDF. Author notes 1 Present address: Letcombe Laboratory, Letoombe Regis, Wantage, Berkshire. © Oxford University Press

THORPE, TREVOR, A.;MEIER, DONALD, D.

doi: 10.1093/jxb/25.2.288pmid: N/A

Abstract Starch, free sugars and protein contents, and the specific activities of enzymes of starch metabolism were determined in tobacco callus cultured under shoot-forming and non-shoot-forming conditions. Shoot-forming cultures contained higher levels of starch, free sugars and protein. Shoot-forming cultures had higher specific activities for starch-synthesising enzymes throughout culture. On the other hand, higher levels of activity for starch-degrading enzymes in shoot-forming tissues were only observed during organized development. The role of phosphorylase in the cultured tissue was not clear. This content is only available as a PDF. © Oxford University Press

MINCHIN, F., R.;PATE, J., S.

doi: 10.1093/jxb/25.2.295pmid: N/A

Abstract Diurnal changes in plant and nodule performance were studied in 28–9 d plants of Pisum sativum L. in two environments, both with a 12 h (27 000 lx):12 h::light:dark cycle, but one (A) with a fluctuating temperature-humidity regime (photoperiod 18 °C, 60 per cent relative humidity:night 12 °C, 85 per cent), the other (B) with constant temperature (18 °C) and humidity (75 per cent). Fixation rate (C2H2 reduction), respiratory output of the nodulated root, and nodule sugar level increased throughout the photoperiod, whereas nodule soluble nitrogen level declined steadily. Reversal of these trends in the night period led, at its end, to minima in fixation rate, sugar level and respiration, but a maximum in soluble nitrogen. The A environment produced the greater day:night fluctuations in transpiration and nodule soluble nitrogen, but B, with its higher night temperature, induced the more pronounced decrease in fixation at night. Slightly less nitrogen was fixed during the photoperiod than during the night in the A environment, yet since some fixation products were retained in the nodules at night and not released until the next photoperiod, the day: night difference in nitrogen export from nodules was 1.8:1. The photoperiod of A was also a time of higher nodule respiration and replenishment of nodule sugar and starch, so that the nodules' requirement for translocated carbohydrate was more than twice that at night. Humidity decrease in the photoperiod (of A) elicited higher rates of transpiration and a more rapid than normal emptying of soluble nitrogen from the nodules: elevation of humidity had the opposite effects. Shoot removal (A-grown plants) caused nodule sugar levels to fall rapidly below those normally encountered in intact plants. This content is only available as a PDF. Author notes 1 Present address: Plant Environment Laboratory, Department of Agriculture and Horticulture, University of Reading RG2 9AD, Berks., England. 2 Present address: Botany Department, University of Western Australia, Nedlands, W. Australia 6009. © Oxford University Press

CLARK, M., C.

doi: 10.1093/jxb/25.2.309pmid: N/A

Abstract Analysis of the distribution of 14C in extracts prepared from leaf tissue which had been exposed to labelled adenine by petiolar uptake revealed that this purine is extensively metabolized in both healthy and Phytophthora-infected potato leaves. Incorporation of labelled adenine into the major ribonucleic acid species of the leaf was also extensive as determined by radioactive assays performed on individual fractions which were separated on columns of methylated albumin kieselguhr. Examination of individual nucleotides released by alkaline hydrolysis showed that both the adenylic and guanylic acid moieties were labelled. Although the labelling patterns were similar for RNA from healthy and infected leaf tissue, the specific activity of the latter was consistently higher than the former. When partially purified leaf extracts were assayed for phosphoribosyltransferase, they exhibited relatively high levels of activity with adenine as substrate, but were virtually devoid of activity with hypoxanthine and guanine. However, direct petiolar uptake of labelled hypoxanthine resulted in highly labelled RNA. A comparison of adenine phosphoribosyltransferase activity in extracts from healthy and blighted leaves failed to reveal measurable differences. Therefore, it was concluded that the differential incorporation of labelled adenine into the RNA of healthy and infected leaves was due neither to increased activity of this enzyme in response to infection nor to its differential activation. Apart from its role in the recovery of preformed purines for nucleic acid synthesis, adenine phosphoribosyltransferase may function as part of a mechanism for regulating levels of adenine in the potato leaf. This content is only available as a PDF. © Oxford University Press

NZE-EKEKANG,, L.;PATILLON,, M.;SCHÄFER,, A.;KOVOOR,, A.

doi: 10.1093/jxb/25.2.320pmid: N/A

Abstract Jerusalem artichoke DNA, extracted from resting rhizomes, was analysed by the reassociation kinetics of sheared denatured fragments. Various fractions were isolated according to their reassociation rates. The slowest fractions consist of unrepeated or single-copy DNA which makes up 55 per cent of the total DNA. The total sequence length of the haploid genome is estimated as corresponding to 0.23×1012 daltons. Among the fractions of repeated DNA isolated, two were relatively homogeneous with an average complexity of 2×108 and 109 daltons respectively. This content is only available as a PDF. Author notes 1 Present address: Institut für Pflanzenernährung der Justus Liebig-Universität. Giessen, Germany. © Oxford University Press