Effect of temporary changes in light intensity on carbon transport, partitioning and respiratory loss in young tomato seedlings raised under different light intensities

Tomato plants were grown under light intensities of 36 or 90 W m⁻² [photosynthetically active radiation (PAR)], and then the light intensity was changed to 36, 90 or 180 W m⁻² for 8 h to investigate the effect of temporary changes in light intensity on the carbon budget of photoassimilates from the third leaf using a ¹⁴CO₂ steady-state feeding method. In the plants that were raised under 90 W m⁻², the photosynthetic rate increased when the light intensity was increased to 180 W m⁻², whereas no increase occurred in the plants that were raised under 36 W m⁻². Although the total amount of carbon fixed during the 8-h light period showed a large difference between plants grown at the two initial light intensities, the proportion of carbon exported during the light period did not differ apparently, irrespective of the change in light intensity. However, the amount of carbon exported during the time course was higher in plants that were raised under 90 W m⁻² than those raised under 36 W m⁻², irrespective of the change in light intensity. The partitioning pattern of ¹⁴C-photoassimilates was not changed by the change in light intensity, irrespective of whether the light intensity was increased or not. However, the amount of ¹⁴C-photoassimilates accumulated in each part differed according to the two initial light intensities. The carbon transport from a source leaf was also investigated through a quantitative analysis of carbon balance.     

The effect of phytochrome and white light of high and low intensity on the uptake and distribution of 14C-labelled kinetin in radish seedlings (Raphanus sativus)

The influence of light intensity and phytochrome on the uptake of ¹⁴C-kinetin (6-furfurylamino-[8- ¹⁴C]-purine) by the plant and the translocation of the phytochrome between the roots, the hypocotyl and the cotyledons were investigated with radish seedlings ( Raphanus sativus L. cv. Saxa Treib) grown in the dark or under white light of high (20,000 lux, 90 W m⁻²) or low intensity (2,000 lux, 14 W m⁻²). The highest uptake of labelled kinetin was found in plants grown in continuous darkness. The total uptake of kinetin was decreased by strong light and to a finally higher extent by weak light. Under white light most of the kinetin accumulated in the root, whereas in the dark an enhanced translocation of the phytohormone into the cotyledons was observed. In etiolated radish seedlings, light acting on phytochrome (daily 5 min red or far red light pulses) decreased the translocation of ¹⁴C-kinetin into the cotyledons. Under far red light a pronounced uptake of the phytohormone into the roots was found. The data are discussed with regard to the interaction of light and phytohormones on plant development.     

Transport and accumulation of indole-3-acetic acid in pea cuttings under two levels of irradiance

The transport and accumulation of 2-[¹⁴C]-IAA applied to the apex of cuttings of Pisum sativum L. cv. Alaska was greater in cuttings from stock plants grown under 38 W m⁻² than 16 W m⁻². Accumulation of ^14C in the base of the cuttings from the highest level of irradiance was correspondingly more significant. The level of irradiance to the stock plants greatly affected the rate of accumulation, while the light conditions during IAA transport had a minor effect. The amount of IAA reaching the base of the cuttings increased with increasing concentration of IAA in the treatment solution, but the percentage of applied IAA reaching the base decreased.
The relative chromatographic partition of ethanol-extractable ¹⁴C showed that, after 12 h of IAA-transport, the amount of 2-[¹⁴C]-IAA was higher in the base of cuttings from 38 W m⁻² than in those from 16 W m⁻². After a further 12 h of transport the relative amounts of 2-[¹⁴C]-IAA in the two types of cuttings were reduced to the same lower level.
A possible role of an irradiance-mediated difference in the topographic distribution of IAA in the base of pea cuttings on the subsequent adventitious root formation is discussed.     

Electrical signalling and cytokinins mediate effects of light and root cutting on ion uptake in intact plants

Nutrient acquisition in the mature root zone is under systemic control by the shoot and the root tip. In maize, exposure of the shoot to light induces short-term (within 1–2 min) effects on net K⁺ and H⁺ transport at the root surface. H⁺ efflux decreased (from −18 to −12 nmol m⁻² s⁻¹) and K⁺ uptake (∼2 nmol m⁻² s⁻¹) reverted to efflux (∼−3 nmol m⁻² s⁻¹). Xylem probing revealed that the trans-root (electrical) potential drop between xylem vessels and an external electrode responded within seconds to a stepwise increase in light intensity; xylem pressure started to decrease after a ∼3 min delay, favouring electrical as opposed to hydraulic signalling. Cutting of maize and barley roots at the base reduced H⁺ efflux and stopped K⁺ influx in low-salt medium; xylem pressure rapidly increased to atmospheric levels. With 100 m m NaCl added to the bath, the pressure jump upon cutting was more dramatic, but fluxes remained unaffected, providing further evidence against hydraulic regulation of ion uptake. Following excision of the apical part of barley roots, influx changed to large efflux (−50 nmol m⁻² s⁻¹). Kinetin (2–4  µ m ), a synthetic cytokinin, reversed this effect. Regulation of ion transport by root-tip-synthesized cytokinins is discussed.     

Effects of UV-C radiation on extension growth, H+-extrusion and transmembrane electric potential in maize coleoptile segments

The effect of 253.7 nm ultraviolet radiation on elongation growth, medium acidification and changes in electric potential difference between vacuole and external medium in cells of maize ( Zea mays L.) coleoptile segments was investigated. It was found that irradiation with 390, 1170, 3900 and 5 850 J m⁻² UV-C (ultraviolet radiation 253.7 nm) inhibited elongation growth, whereas at 195 J m⁻² stimulation of growth was observed. The administration of IAA (10⁻⁵ M ) to the incubation medium of coleoptile segments partially abolished the inhibitory effect of UV-C. The pH of the incubation medium, measured simultaneously with growth, showed that the exposure of the segments to UV-C caused inhibition of H⁺-extrusion (or stimulation of H⁺ uptake). The presence of IAA (10⁻⁵ M ) in the incubation medium promoted (except after 5850 J m⁻² irradiation) H⁺-extrusion to a level comparable with that produced by IAA in non-irradiated segments. In UV-C irradiated segments the potential difference underwent significant alterations. Irradiation of coleoptile segments with 390 J m⁻² caused a transient depolarization, which was fully reversible within 30 min, while at higher doses depolarization was irreversible. The hyperpolarization of the membrane potential (MP) in cells of maize coleoptile induced by IAA was completely nullified by subsequent irradiation with UV-C. It is suggested that UV-C inhibited IAA-induced growth by a mechanism independent of cell wall acidification.     

Combined effect of light and temperature on triacylglycerol accumulation in Dicranum elongatum

In the subarctic moss Dicranum elongatum Schleich & Schwaegr., the level of total lipids and triacylglycerols (TAG) was high in late winter and spring and low in autumn and winter. Four-week exposure of field material to continuous light (135μmol m⁻²s⁻¹) at 1°C resulted in a considerable increase in the amount of TAG in the autumn material acclimated to low temperatures and rhythmic light in the field. In contrast, the same treatment did not cause any increase in TAG in the spring material, acclimated to low temperatures and continuous light in the field. Results from experiments, in which moss cultivated for 4 months at 9°C on 12-h photoperiods (135μmol m⁻²s⁻¹) was kept for 3 weeks at low temperatures (9°C and −3°C) either in continuous light (135 or 70 μmol m⁻²s⁻¹) or with 12-h photoperiods (135 μmol m⁻²s⁻¹), indicated that the TAG level was higher at higher light intensity. At 9°C it was also higher in continuous light of both intensities than in rhythmic light. These results strongly suggest that decreasing irradiance and decreasing daylength limits the accumulation of TAG in D. elongatum during autumn in the subarctic.     

Effect of ultraviolet radiation on accumulation and leakage of 86Rb+ in guard cells of Vicia faba

The effects of UV-C (254 nm), UV-A (365 nm) and broad-band UV (280–380 nm) on guard cells of Vicia faba L. cv. Long Pod were investigated in the presence of white light (450 μmol m⁻² s⁻¹). UV-C (7 μmol m⁻² s⁻¹) was found to cause leakage of ⁸⁶Rb⁺ from guard cells, while UV-A (0.3 μmol m⁻² s⁻¹) stimulated increased uptake in these cells. A relatively small stimulatory effect was observed by broad-band UV (3 μmol m⁻² s⁻¹) during the first 30 min of irradiation with an apparent equilibration of influx and efflux thereafter. Leakage of ⁸⁶Rb⁺ from guard cells continued despite the removal of UV-C and an increase in the amount of white light from 450 to 1500 μmol m⁻² s⁻¹, suggesting that membranes were irreversibly damaged. Irradiation of guard cells with UV-C for 30, 45 and 90 min indicated that these cells began to be affected already by 30 min UV-C irradiation.     

The acquisition and accumulation of inorganic carbon by the unicellular green alga Chlorella ellipsoidea

Abstract. The uptake and accumulation of inorganic carbon has been investigated in Chlorella ellipsoidea cells grown at acid or alkaline pH. Carbonic anhydrase (CA) was detected in ceil extracts but not in intact cells and CA activity in acid-grown cells was considerably less than that in alkali-grown cells. Both cell types demonstrates low K_1/2 (CO₂) values in the range pH 7.0–8.0 and these were unaffected by O₂ concentration. The CO₂ compensation concentrations of acid- and alkali-grown cells suspended in aqueous media were not significantly different in the range of pH 6.0–8.0, but at pH 5.0, the CO₂ compensation concentrations of acid-grown cells (57.4cm³ m⁻³) were lower than those of alkali-grown cells (79.2cm³ m⁻³). The rate of photo-synthetic O₂ evolution in the range pH 7.5–8.0 exceeded the calculated rate of CO₂ supply two- to three-fold, in both acid- and alkali-grown cells, indicating that HCO₃⁻ was taken up by the cells. Accumulation of inorganic carbon was measured at pH 7.5 by silicone-oil centri-fugation, and the concentration of unfixed inorganic carbon was found to be 5.1 mol m⁻³ in acid-grown and 6.4mol m⁻³ in alkali-grown cells. These concentrations were 4.6- and 5.9-fold greater than in the external medium. These results indicate that photorespiration is suppressed in both acid- and alkali-grown cells by an intracellular accumulation of inorganic carbon due, in part, to an active uptake of bicarbonate.     

Influence of solar ultraviolet radiation on photosynthesis and motility of marine phytoplankton

Abstract: The effects of solar ultraviolet radiation (UV) on carbon uptake, oxygen evolution and motility of marine phytoplankton were investigated in coastal waters at Kristineberg Marine Research Station on the west coast of Sweden (58° 30'N, 11° 30'E). The mean irradiances at noon above the water surface during the investigation period were: photosynthetic active radiation (PAR, 400–700 nm) 1670 μmol m⁻² s⁻¹; ultraviolet-A radiation (UV-A, 320–400 nm) 35.9 W m⁻² and ultraviolet-B radiation (UV-B, 280–320 nm) 1.7 W m⁻². UV-B radiation was much more attenuated with depth in the water column than were PAR and UV-A radiation. UV-B radiation could not be detected at depths greater than 100–150 cm. Inhibition of carbon uptake by UV-A and UV-B in natural phytoplankton populations was greatest at 50 cm depth and the effects of UV-B were greater than those of UV-A. At depths greater than 50 cm there was almost no effect of ultraviolet radiation on carbon uptake. PAR, UV-A and UV-B decreased oxygen evolution by the dinoflagellate Prorocentrum minimum . Inhibition of oxygen evolution was greater after 4 h than 2 h but it was not possible to distinguish the negative effects of the different light regimes. The motility of P. minimum was not affected by PAR, UV-A and UV-B. The importance of exposure of phytoplankton to different light regimes before being exposed to natural solar radiation is discussed.     

Endogenous auxin levels in terminal stem cuttings of Chrysanthemum morifolium during adventitious rooting

Free and ester-bound IAA were determined in Chrysanthemum morifolium Ramat cv. 'Yellow Galaxy' by means of a solid phase enzyme immunoassay. In shoots, free auxin decreases basipetally whereas ester IAA reaches a maximum in the middle part. After making the cuttings, a strong increase in both free and ester IAA (or total IAA, respectively) is found up to the time when the first adventitious roots become visible. Only prolonged irradiance of stock plants at high light intensities (40 W m⁻²) delays this increase in the cuttings, concomitantly with a lower number of roots compared to the controls (4.5 W m⁻²), although root growth as determined by measuring root length or fresh weight is not affected. A distinct relation is found between IAA content of stock plants at the time when cuttings are taken and the number of adventitious roots formed by the cuttings 20 days later.     

Thylakoid differentiation in endocyanelles

Cyanophora paradoxa Korshikov synchronized autotrophically in a light-dark regime of 14 h light and 10 h dark divides in the last two hours of the dark period. The division rate of the free-living blue-green alga, Synechococcus leopoliensis Raciborski, at identical culture conditions (24°C; 32 W m⁻²) is only slightly lowered in the light period. The comparison of thylakoid differentiation in the endocyanelles of Cyanophora paradoxa and in Synechococcus leopoliensis during the light-dark regime yields (1) the same ensemble of pigment-protein complexes in both organisms, (2) comparable syntheses of chlorophyll and phycobilins of Cyanophora paradoxa grown under 32 W m⁻² and of Synechococcus leopoliensis grown under light intensities below 9.2 W m⁻², and (3) identical photosynthetic oxygen evolution during the light period of the light-dark regime with minima at the beginning, in the middle (6th–7th h), and at the end of the light period. In both organisms this stage-specific oxygen evolution is inhibited by treatment with chloroamphenicol. Cycloheximide, however, causes no significant alterations. Results are discussed in view of the endosymbiotic theory.     

Photosynthetic acclimation to elevated atmospheric carbon dioxide and UV irradiation in Pinus banksiana

Pinus banksiana seedlings were grown for 9 months in enclosures in greenhouses at CO₂ concentrations of 350 or 750 μmol mol⁻¹ with either low (0.005 to 0. 3 W m⁻²) or high (0.25 to 0. 90 W m⁻²) ultraviolet-B (UV-B) irradiances. Total seedling dry weight decreased with high UV treatment but was unaffected by CO₂ enrichment. High UV treatment also shifted biomass partitioning in favor of leaf production. Both CO₂ and UV treatments decreased the dark respiration rate and light compensation point. High UV light inhibited photosynthesis at 350 but not at 750 μmol mol⁻¹ CO₂ due to a UV induced increase in ribulose-1, 5-bisphosphate carboxylase/oxygenase efficiency and ribulose-1, 5-bisphosphate regeneration. Stomatal density was increased by high UV irradiance but was unchanged by CO₂ enrichment.     

Chloroplast structure in normal and pigment-deficient soybeans grown in continuous red or far-red light

Clark L1, a normal green soybean [ Glycine max (L.) Merrill] and Clark y₉y₉, a backross-developed isoline exhibiting pigment deficiency, were grown under continuous red (11 W m⁻² and far-red (9 W m⁻²) light. Chloroplast thylakoids from the unifoliolate leaf (9–10 days old) were isolated and analyzed for pigments, pigment-protein, membrane polypeptides, electron transport and ultrastructural differences. Chloroplasts of soybean plants grown under far-red light have decreased chlorophyll a to chlorophyll b ratio, increased light-harvesting complexes, and grana structure with few stroma-type thylakoids. Photosystem II/photosystem I ratios (PSII/PSI) are higher in far-red due to decreased synthesis of PSI reaction center and/or less antenna associated with PSI.     

Photooxidative inhibition of nitrate reductase during chilling

The effect of a temperature close to the freezing point (chilling) on the nitrate reductase system of leaf discs of Cucumis sativus L. cv. Kleine Groene Scherpe was determined in the absence and presence of light. The capacity of leaf discs in the light (250 μE m⁻²s⁻¹) at 20°C to increase in vivo and in vitro nitrate reductase activity, was unaffected by chilling pretreatment in the dark, but 4 h of chilling pretreatment in the light (250 μE m⁻²s⁻¹) decreased the capacity to less than 50% of the unchilled control. The chilling inhibition of the capacity to increase nitrate reductase activity was of a photooxidative nature since it only occurred in the presence of light and oxygen. Plants grown at a low light intensity (65 μE m⁻²s⁻¹) lost 95% of their capacity to increase nitrate reductase activity, while plants grown at 195 μE m⁻²s⁻¹ retained 80% of their nitrate reducing capacity after 6 h chilling pretreatment in the 250 μE m⁻²s⁻¹ light. Previously induced nitrate reductase activity was also affected by light during chilling. A lag phase of 7 h preceded a fast phase of decrease in activity. Both in vivo and in vitro activity decreased to 15% of the control value after 18 h of chilling in the light. It is concluded that the induction mechanism of nitrate reductase is primarily affected by photooxidation during chilling. The decrease in nitrate reductase activity is attributed to a decrease in the amount of activity enzyme.     

Inhibition of the glucose and amino-acid carriers of Lemna gibba by pretreatment with HgCl2

The electrical resting potential across the plasmalemma of Lemna gibba L. (G 1) cells is −230 to −250 mV and the diffusion potential in the presence of 1 mol m⁻³ KCN + 1 mol m⁻³ salicylhydroxamic acid is about −100 mV. A concentration of 0.01 mol m⁻³ HgCl₂ depolarises the transmembrane electrical potential in a largely reversible way. When the cells after 16 min of HgCl₂-application are returned to Hg-free solution, the transmembrane electrical potential is only depolarised by 24 × 13 mV (SD, n = 13) compared with the potential prior to HgCl₂ treatment. In contrast, a 16 min pretreatment with HgCl₂ followed by a wash with mercury-free solution reduces the transient depolarisations of transmembrane potential observed after addition of 5 mol m⁻³ D-glncose or 1 mol m⁻³ L-alaoine to about 60% of controls. These transient depolarisations are due to the onset of solute uptake. Accordingly, HgCl₂-pretreatment inhibits uptake of ¹⁴C-3-O-methyl- d -glucose by more than 50% and uptake of ¹⁴C- l -alanine by more than 70%. Washing with 1 mol m⁻³ 1,4-dithiothreitol does not reverse this inhibition. It is, therefore, concluded that Hg²⁺ irreversibly binds to essential SH-groups of the H⁺-hexose and the H⁺-amino-acid cotransport carriers of Lemna gibba and inhibits these carriers without appreciably affecting the electrogenic proton-extrusion pump.     

A light intensity threshold for schooling in the Atlantic mackerel, Scomber scombrus

The schooling behaviour of Atlantic mackerel was studied in a large tank at different light intensities in the range 12.6–1.8 × 10⁻¹⁰μEs⁻¹ m⁻². Variable light intensity was produced by accurately controlling the current to a green light-emitting diode (LED) 3 m above the experimental tank. Under high light levels (1.8 × 10⁻⁶μEs⁻¹ m⁻²) mackerel always formed a single school, whereas at lower levels (1.8 × 10⁻⁸μEs⁻¹ m⁻²) they swam as individuals. At light levels down to 1.0 × 10⁻⁶μEs⁻¹ m⁻² the mean nearest neighbour distance in a school remained relatively constant (0.3–0.9 body lengths), and individual mackerel swam along a path which deviated from the position of their nearest neighbours by less than 14°. As light dropped below 1.8 × 10⁻⁷μEs⁻¹ m⁻², both nearest neighbour distance and heading angle between nearest neighbours increased, with mean values of 1–1.8 body lengths and 23–92°, respectively, at 1.8 × 10⁻⁹μEs⁻¹ m⁻². The results are discussed in terms of ambient light conditions in the sea.     

A relationship between irradiation, carbohydrates and rooting in cuttings of Pisum sativum

Rooting ability was studied for cuttings derived from pea plants ( Pisum sativum , L. cv. Alaska) grown in controlled environment rooms. When the cuttings were rooted at 70 μmol m⁻² s⁻, ¹ (photosynthetic photon flux density) or more, a stock plant irradiance at 100 μmol m⁻² s⁻¹ decreased rooting ability in cuttings compared to 5 μmol m⁻², s⁻¹, However, cuttings rooted at 160 μmol m⁻² s⁻¹ formed more roots compared to 5 (μmol m⁻² s⁻¹. Although a high irradiance increased the number of roots formed, it could not overcome a decreased potential for root formation in stock plants grown at high irradiance. Light compensation point and dark respiration of cuttings decreased by 70% during the rooting period, and the final levels were strongly influenced by the irradiance to the cuttings. Respiratory O₂ uptake decreased in the apex and the base of the cutting from day 2 onwards, whereas a constant level was found in the leaves. Only the content of extractable fructose, glucose, sucrose and starch varied during the early part of the rooting period. We conclude that the observed changes in the cuttings are initiated by excision of the root system, and are not involved in the initiation of adventitious roots.     

Photoinhibition of photosynthesis in Lemna gibba as induced by the interaction between light and temperature. I. Photosynthesis in vivo

The floating angiosperm Lemna gibba L. was exposed for 2 h to various combinations of photosynthetic photon flux densities and temperature. The extent of photoinhibition of photosynthesis was assayed by measuring the net CO₂ uptake before and after a photoinhibitory treatment, and the time course for photoinhibition was studied. It was found that the maximum quantum yield and the light-saturated rate of CO₂ uptake were affected by the interaction between light and temperature during the photoinhibitory treatment. At a constant photon flux density of 650 μmol m⁻² s⁻¹ the extent of photoinhibition increased with decreasing temperature showing that even a chilling-resistant plant like L. gibba is much more susceptible to photoinhibition at chilling temperatures. About 60% photoinhibition of the quantum yield for CO₂ uptake could be obtained either by a high photon flux density of 1 750 μmol m⁻² s⁻¹ and 25°C or by a moderate photon flux density of 650 μmol m⁻² s⁻¹ and 3°C. The time courses of recovery from 60% photoinhibition produced by either of these two treatments were similar, indicating that the nature of the photoinhibition was intrinsically similar. The extent of photoinhibition was related to the amount of light absorbed in excess to what could be handled by photosynthesis at that temperature. The vital importance of photosynthesis in alleviating photoinhibition is discussed.     

Effect of light intensity on proton extrusion by roots of intact maize plants

Shading of maize plants ( Zea mays L. cv. Blizzard) reduced net H⁺ extrusion by roots and increased K⁺ release, whereas there was no significant effect on anion efflux in deionized water. With lower light intensity the concentrations of carbohydrates in the roots decreased, but ATP levels and energy charge remained unchanged. Also, shading raised the tissue pH of roots and made the cytoplasmic pH of root cells drop. There was a significant influence of light intensity on H⁺ uptake by roots from an acidified test solution and CCCP (carbonylcyanide-3-chlorophenylhydrazone)-in-duced H⁺ uptake was modified by shading.
It is concluded that low light intensity does not limit active H⁺ release by plasmalemma ATPase activity in the root cells, but that a reduced carbohydrate supply brings about a change in biochemical reactions which alter the membrane permeability for protons. An increased passive reflux of H⁺ into the cells rather than a reduced H⁺ ATPase activity explains the decrease of net H⁺ release by roots of intact maize plants under low light intensity.