Stomatal movement and potassium transport in epidermal strips of Zea mays: The effect of CO2

Summary The correlation between stomatal action and potassium movement in the epidermis of Zea mays was examined in isolated epidermal strips floated on distilled water. Stomatal opening in the isolated epidermis is reversible in response to alternate periods of light or darkness, and is always correlated with a shift in the potassium content of the guard cells. K accumulates in guard cells during stomatal opening, and moves from the guard cells into the subsidiary cells during rapid stomatal closure. When epidermal strips are illuminated in normal air, as against CO₂-free air, the stomata do not open and there is a virtually complete depletion of K from the stomatal apparatus. In darkness CO₂-containing air inhibits stomatal opening and K accumulation in guard cells, but does not lead to a depletion of K from the stomata as observed in the light.     

Seasonal differences in the pattern of assimilate movement in branches of Coffea arabica L

At Ruiru, Kenya, ¹⁴CO₂ was fed to single leaves at different distances from the apex on branches of mature fruiting and non-fruiting trees of Coffea arabica on six occasions from the Short Rains (November) 1966 to the Long Rains (April-May) 1967. The location of ¹⁴C-labelled assimilates in the treated branches was determined 26 h later by autoradiography. The direction of movement of labelled assimilates indicated large seasonal differences in the relative sink strengths of the shoot tip, fruit and trunk-root systems. On vegetative trees the sink strength of trunk-roots was much smaller, as compared with the shoots, at the beginning of the Long Rains than at the end of the previous Short Rains or in the intervening dry season. Assimilate use by growing fruits did not alter the pattern of distribution of assimilates to the other sinks at the end of the Short Rains, but it did restrict assimilate movement to both shoot tips and trunk-roots at the_ beginning of the Long Rains. In the dry season, virtually all assimilates were utilized by growing fruits when these were present. Vegetative secondary shoots provided assimilates to growing fruits and trunk-roots at the end of the Short Rains and in the dry season. Some practical implications are noted.     

Effect of assimilate utilization on photosynthetic rate in wheat

Summary Two weeks after anthesis, when the grain is filling rapidly, the rate of photosynthesis by flag leaves of wheat cv. Gabo was between 20 and 30 mg CO₂ dm^-2 leaf surface hour^-1 under the conditions used. About 45% of flag-leaf assimilates were translocated to the ear, and only about 12% to the roots and young shoots.On removing the ear, net photosynthesis by the flag leaves was reduced by about 50% within 3–15 hours, and there was a marked reduction in the outflow of ¹⁴C-labelled assimilates from the flag leaves.Subsequent darkening of all other leaves on plants without ears led to recovery of flag-leaf photosynthesis, as measured by gas analysis and ¹⁴CO₂ fixation, and to increased translocation of assimilates to the roots and young shoots. Minor changes in the rates of dark respiration accompanied these major, reversible changes in photosynthetic rate.After more than a week in continuous, high-intensity light, the rate of photosynthesis by flag leaves of intact plants had fallen considerably, but could be restored again by a period in darkness, or by inhibiting photosynthesis in the ears by spraying them with DCMU. The inhibition of ear photosynthesis increased translocation of labelled assimilates from the flag leaf to the ears, without affecting leaf sugar levels.The application of TIBA to the culm below the ear inhibited auxin movement throught the culm, but had no influence on flag-leaf photosynthesis.These results suggest that, at least in this system, photosynthesis by the flag leaf is regulated directly by the demand for assimilates from the flag leaf and not indirectly through action in the leaf of auxins produced by the sink organs.     

Effect of Ophiobolus graminis infection on the assimilation and distribution of 14C in wheat

The uptake of ¹⁴C and movement of ¹⁴C-labelled assimilates in wheat plants inoculated with Ophiobolus graminis was examined following exposure of the second youngest leaf to ¹⁴CO₂. Autoradiographs of plants with infected seminal roots showed that assimilates were not translocated past the sites of root infection but accumulated in the undamaged portions of infected root systems, in particular the developing crown roots. There was no evidence that assimilates accumulated in the vicinity of O. graminis lesions. The net assimilation of ¹⁴CO₂ by wheat plants over a 5 h feeding period was not significantly affected by O. graminis infection. However, infection reduced the amount of ¹⁴C lost through respiration. Infection delayed the transfer of labelled assimilates from the fed leaf to the remainder of the plant but increased the proportion translocated to the roots. The latter effect was not apparent when infected plants were continuously irrigated during, and for 20 h following, the feeding period.     

Promotion of sink activity of developing rose shoots by light

Holding young rose shoots (Rosa hybrida cv. Marimba) in darkness while the rest of the plant was in light reduced the amount of ¹⁴C assimilates recovered from the darkened shoot by half. Relative specific activity of the shoot tip grown in light was 13.5 times greater than that of the darkened one. The flower bud at the shoot tip degenerated in darkness and died. Shoots 2 to 3 centimeters long, after flower initiation, were most sensitive to the dark treatment. The degeneration is a gradual and reversible process in the first 8 days of darkness, followed by irreversible damage and atrophy. Darkening enhanced the ability of the young leaves to compete for the available assimilates over that of the darkened shoot tip. The enhancement of the mobilizing ability of the shoot tip by light is independent of photosynthesis since spraying with 3-(3,4-dichlorophenyl)-1,1-dimethylurea or holding shoots in a CO₂-free atmosphere did not diminish the promoting effect of light on flower bud development or assimilate import. The possibility that light exerts its effect by photoproduction of ATP was also excluded inasmuch as no differences were found in ATP levels of shoot tips held in darkness and those held in light.     

Temperature and the translocation of photosynthate through the leaf of Lolium temulentum

Summary Localized cooling of a 2 cm length of the leaf blade of the grass Lolium temulentum to 0° C, had only a small effect on the mass transfer of assimilates from the distal part of the leaf. There was no evidence of a reduction in the velocity of movement of ¹⁴C-labelled assimilates through the low temperature zone, and the retention of assimilates along the pathway of movement was reduced by low temperature. In contrast to the longitudinal movement, lateral movement of ¹⁴C, induced by steam killing sections of the leaf, was extremely sensitive to temperature and showed no sign of acclimatization, even after 3 days at low temperature. These results suggest that, although loading sugars into the transport system was sensitive to temperature, the movement of assimilates was not directly controlled by metabolic processes along the pathway.     

Source,sink and hormonal control of translocation in wheat

Summary An analysis of the pattern of movement of ¹⁴C-labelled flag leaf assimilates in wheat (Triticum aestivum l. c.v. Gabo) during grain development, indicated that the greater the requirement for assimilates by the ear the more rapid was the speed of movement of these through the peduncle to the ear and also the lower their concentration. Experiments with [¹⁴C] indoleacetic acid ([¹⁴C]IAA) suggested that auxin production by the grains was not responsible for the control of assimilate translocation through the peduncle. Limiting the supply of available assimilates by shading the lower parts of the plant, did not significantly alter the speed of movement of ¹⁴C-photosynthate through the peduncle, while severing half of the vascular tissue in the peduncle altered the pattern of movement of ¹⁴C to the ear and enhanced the speed of movement of ¹⁴C through the remaining functional conducting tissue. These results are discussed in relation to the mechanism of translocation.     

PHYTOCHROME CONTROLLED NYCTINASTY IN ALBIZZIA JULIBRISSIN. II. POTASSIUM FLUX AS A BASIS FOR LEAFLET MOVEMENT

Excised Albizzia leaflet pairs exposed to red (R) light close within 30–90 min after transfer to darkness. Interruption of darkness by far-red (FR) light at any time after R inhibits closure within ca. 10 min. Similarly, irradiation with R at any time after prior FR promotes closure within ca. 10 min, and the increased rate of closure is independent of the time lapse between the FR and R irradiations. Closure in the dark is inhibited by NaN₃ and DNP (5 X 10^–4 m ), by anaerobic conditions and by externally applied salts of monovalent cations, especially K; it is also temperature sensitive. Pulvinule cells are very high in K. Electron microprobe analysis of cryostated, lyophilized pulvinules reveals that during closure, K is lost from ventral cells and enters dorsal cells. FR before darkness inhibits the former but not the latter process. Thus, K flux appears to control the changes in volume of the pulvinule cells that control leaflet movement. While leaflet closure normally requires a dark period, salts of organic acids such as sodium acetate, propionate, and butyrate cause closure in the light.     

Vacuoles of Mesophyll Cells as a Transient Reservoir for Assimilates

We studied the efflux of radioactive photosynthetic products from the central vacuole into the cytosol of protoplasts isolated from the mesophyll tissue of the sugar beet (Beta vulgaris L.) after their darkening and subsequent cessation of photosynthesis. Among the products accumulated in the vacuole were the ¹⁴C-labelled sugars malate and alanine, small amounts of citric, glutamic, and aspartic acids, and some other amino acids. During the initial 20–30 min of darkness, there was no substantial utilization of photoassimilates accumulated in the vacuole during the preceding light period. An efflux of assimilates occurred later, after 30–40 min of darkness. A decrease in the vacuolar ¹⁴C-sucrose occurred not only due to its exit into the cytosol but also because of its conversion into ¹⁴C-monosaccharides by the vacuolar invertase. In fact, this decrease in the sucrose content correlated well with the accumulation of monosaccharides. Immediately after photosynthesis ceased, the chloroplastic ¹⁴C-starch was utilized for the maintenance of cytoplasmic metabolism. After 30-min darkness, the content of starch in the chloroplasts decreased by several times. We believe that the vacuoles of sugar-beet mesophyll cells are transient reservoirs for assimilates and the products of their conversion (glucose and fructose), which can rapidly leave the vacuole to maintain homeostasis in the cytosol under varying environmental conditions.     

Assimilate Partitioning in the Variegated Coleus Leaf

Mature leaves of a variegated cultivar of Coleus blumei Benth. with a green border and central albino region constitute a source-sink system suitable for studies on assimilate partitioning. Leaves treated with ¹⁴CO₂ on a small part of the intact green border export assimilate via the shortest path into the stem. Leaves with all but a small lobe of the green border removed show different partitioning of labeled assimilates when the leaf is exposed to ¹⁴CO₂ (Fisher and Eschrich, 1985): The whole albino region of the leaf is supplied but no tracer is exported into the stem. When the green border is completely removed, ¹⁴CO₂-treatment of the albino region leads to the fixation of CO₂, obviously by PEP carboxylase, as indicated by the occurrence of labeled malate. Results show that the albino region of the variegated leaf constitutes a potential sink when deprived of its green border. In addition, CO₂-fixation by PEP carboxylase in albino tissue seems to indicate a common capacity of leaves which is normally masked by photosynthesis. The difference of assimilate partitioning between leaves with intact and leaves with partly removed green borders demonstrates that the unlabeled assimilates control the movement of labeled assimilates.     

Utilization of Photosynthates for Growth, Respiration, and Storage in Tops and Roots of Lolium multiflorum

Lolium multiflorum L. was grown in pots in controlled environments. CO₂-exchange rates were continuously measured on two pots during 46 and 52 days, respectively, separating between tops and roots. After 20 days, the plants were entirely defoliated and the plants were then followed during the regrowth period. During the experiment, alternating 2–3 day periods of high and low irradiance were applied. Analogously treated plants were frequently harvested to obtain the distribution of assimilates between tops and roots. From integration of CO₂-exchange rates, diurnal photosynthesis and respiration were obtained, and utilization of assimilates was analysed. The respiration associated with the synthesis of new structural material (growth respiration) was dependent on assimilates originating from both the current and the preceding 24 h diurnal cycles. The amount of new structural material synthesized during the current 24 h diurnal cycle was estimated from the relative contribution of assimilates accumulated from the preceding and the current 24 h and diurnal cycles to growth respiration of the current 24 h. From this approximation, the respiratory components connected to synthesis of new structural material and to maintenance of already established material were found. Growth and maintenance respirations of the tops were alike during the predefoliation and the regrowth periods. For the roots, however, growth respiration was higher and maintenance respiration lower in the regrowth period. The difference between daily integrated CO₂-exchange and the amount converted into new structural material was assumed to be the daily change in assimilates stored. On the first day of a period of high irradiance, the assimilation per unit leaf weight was higher than on the following day of high irradiance, and an accumulation of storage material took place. On the first day of a period of low irradiance, the assimilation per unit leaf weight was lower than on the following day of low irradiance, and there was a depletion of assimilates stored. These effects were most pronounced during the regrowth period, indicating a change in the metabolic sink demand. This indicates a strong feedback mechanism between sources and sinks, in the sense that accumulation of products will inhibit assimilation.     

Distribution of 14CO2 during ontogeny of chickpea (Cicer arietinum) grown at two moisture levels

The distribution of assimilates of ¹⁴CO₂ in ethanol soluble and insoluble fractions was measured at 20-day intervals from 45–135 days after sowing (DAS) in chickpea (Cicer arietinum) grown at two moisture levels. The contribution of pre-flowering assimilates to pods, although very low, was higher under the stress conditions. At the time of harvest, the recovery of ¹⁴C in pods was 0.4 and 0.9% of the total ¹⁴C fed 45 DAS in soluble and 2.5 and 5.1% in insoluble fractions in control and stressed plants, respectively. The %¹⁴C received by nodules continuously decreased with increasing age of plants. Stressed plants diverted more ¹⁴C to nodules, compared to control, during vegetative and flowering stages. During active seed filling, stressed plants diverted more ¹⁴C to reproductive parts and less to nodules, compared to control. Significant amounts of ¹⁴C were retgined by the stem and leaves during the seed-filling period and it appears that there is scope for the remobilisation of pre-flowering, as well as post-flowering assimilates for seed-filling of chickpea.     

Sorption of phosphorus by the iron oxide-impregnated filter paper (Pi soil test) embedded in soils

Incubation experiments were carried out to evaluate the feasibility of extracting phosphorus from soil by embedding iron oxide-impregnanted filter paper strips (P_i strips) in soils having a wide range in pH, texture, and extractable-P contents. Under flooded conditions, the amount of P extracted by the P_i strips increased with the period of submergence and embedding time of the P_i strips. Under unsaturated conditions, the P_i strips were found to extract P from soils over a wide range in moisture conditions; however, keeping the soil at moisture level between saturation and field capacity was found to result in maximal sorption of P by the strips. An embedding time of 16 h was found to be adequate.Phosphorus extracted by embedding P_i strips in soil columns for 16 h at field capacity moisture level correlated significantly with P extracted by shaking the soil with 0.01 M CaCl₂ solution and a P_i strip for 16 h in the laboratory (r=0.94^**). The P extracted by embedding P_i strips correlated best with Bray 1 P in acid soils (r=0.97^**) and with Olsen P in alkaline and calcareous soils (r=0.96^**). The results of the studies demonstrate the feasibility of developing a nondestructive method of monitoring changes in plant-available P in situ under field conditions.     

Regulation of melatonin production by light,darkness, and temperature in the trout pineal

Summary The pineal gland of the rainbow trout, Salmo gairdneri, when kept under in vitro perifusion culture conditions, displays a consistently elevated level of melatonin production in darkness (Gern and Greenhouse 1988). Upon light exposure melatonin production falls and stabilizes at a new lower level that is dependent upon the irradiance of the stimulus. To achieve the maximal response for each irradiance, the duration of the stimulus must exceed 30 min. The response amplitude is maximally sensitive to photons presented over durations of 30–45 min; is very insensitive to shorter light exposures; and is maintained with no evidence of adaptation over longer exposures. Temperature plays a role in regulation of melatonin production both in darkness and during light exposure; increased temperature increases melatonin production in darkness and also increases the sensitivity of the response to light. The action spectrum for the response is best fit by the Dartnall nomogram for a vitamin A₁ based rhodopsin with peak sensitivity near 500 nm. The possible adaptive significance of control of melatonin synthesis by light and temperature is considered.Abbreviations LD lightdark cycle - RIA radioimmunoassay - I ¹²⁵ Iodine - HIOMT hydroxyindole-O-methyltransferase     

Energy Supply for Stomatal Opening in Epidermal Strips of Commelina benghalensis

The influence of light or darkness on stomatal opening in epidermal strips of Commelina benghalensis was evaluated in the presence or absence of O₂ and/or metabolic inhibitors. Opening was restricted in nitrogen and was promoted by NADH and acids of the tricarboxylic acid cycle (succinate and α-ketoglutarate) in CO₂-free air in light as well as in darkness. The enhancement by light of stomatal opening was prevalent under nitrogen or in the presence of the respiratory inhibitors (sodium azide and oligomycin). Respiratory inhibitors decreased the opening in light or darkness under CO₂-free air but exhibited no effect under nitrogen, whereas phosphorylation uncouplers were inhibitory in light or darkness under both CO₂-free air and nitrogen. The results suggest that oxidative phosphorylation is a basic source of energy for stomatal opening, although photophosphorylation could be an energy source.     

Transport 14C-markierter Assimilate im Phloem von Pelargonium zonale und Phaseolus vulgaris

Zusammenfassung 6–10 min nach Beginn der ¹⁴CO₂-Assimilation befindet sich ¹⁴C-Aktivität im Stiel des ¹⁴CO₂-exponierten Blattes belichteter Pflanzen von Pelargonium zonale und Phaseolus vulgaris. Die Verteilungsrichtung der Assimilate im Sproß scheint vor allem durch das Alter des ¹⁴C-assimilierenden Blattes bestimmt, jedoch nicht einseitig festgelegt zu sein; geringe, aber faßbare Mengen an ¹⁴C-Saccharose wereden innerhalb von 20 min (bei Phaseolus) und 180 min (bei Pelargonium) auch aus relativ jüngeren in ältere Blätter transportiert.Neben ¹⁴C-Saccharose wurden im Blattstiel-und Stengelgewebe stets markierte Zuckerphosphate, Hexosen und organische Säuren nachgewiesen. Stärke war nur zu einem verschwindend geringen Anteil an der Gesamtmarkierung der analysierten Transportstrecken beteiligt.

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Translocation of ¹⁴C-labelled assimilates in the phloem of Pelargonium zonale and Phaseolus vulgaris

Summary After the laminae of leaves of intact plants had been exposed to ¹⁴CO₂ the translocation of ¹⁴C-labelled assimilates across the petioles starts very quickly: 6 to 10 min later ¹⁴C-activity could be detected in the basal part of the petioles. The way of distribution within the plant seems to be influenced mainly by the age of the ¹⁴CO₂-assimilating leaf, however, but not in the sense of an unidirectional movement; little but distinct amounts of ¹⁴C were carried also from younger yet full expanded leaves down to older leaves, within 20 min (in Phaseolus) or 180 min (in Pelargonium). Besides sucrose, which was shown by paper chromatography to be the main form of assimilates translocated in these species, we identified sugar phosphates, hexoses, and some organic acids in the petiole and stem tissue as being radioactive also. In our experiments, the petiole segments did not contain any remarkable amount of ¹⁴C-labelled starch.

     

Tolerance for Prolonged Darkness of Three Phytoplankton Species, Microcystis aeruginosa (Cyanophyceae), Scenedesmus quadricauda (Chlorophyceae), and Melosira ambigua (Bacillariophyceae)

Phytoplankton community dynamics are affected not only by atural events such as overwintering but also by artificial events such as artificial circulation and related darkness conditions. In order to clarify the effect of tolerance for prolonged darkness on community succession, laboratory cultures of three phytoplankton taxa, Microcystis aeruginosa(Cyanophyceae), Scenedesmus quadricauda(Chlorophyceae), and Melosira ambigua(Bacillariophyceae) were carried out in darkness. The period of darkness was varied: 5, 10, 15, 20 days, and the control. Thereafter, all samples were reilluminated. After more than 10 days of darkness, M. aeruginosa decreased markedly with the length of the darkness period and was reduced to only 1% of the initial cell number after 20 days darkness. In contrast, S. quadricauda and M. ambigua retained their biomass even after 20 days of darkness. After restarting the light–dark cycle, however, all three species similarly increased exponentially and reached their maximum biomass levels. These results suggest that differences in tolerance for prolonged darkness may cause the succession of the phytoplankton under certain conditions.     

Offspring number in a livebearing fish (<Emphasis Type="Italic">Poecilia mexicana</Emphasis>, Poeciliidae): reduced fecundity and reduced plasticity in a population of cave mollies

Life history traits within species often vary among different habitats. We measured female fecundity in mollies (Poecilia mexicana) from a H₂S-rich cave and from a neighbouring surface habitat, as well as in laboratory-reared individuals of both populations raised in either light or continuous darkness. Compared to conspecifics from surface habitats, cave-dwelling P. mexicana had reduced fecundity (adjusted for size) in the field. In the laboratory, the fecundity of surface mollies was higher in light than in darkness, whereas fecundity in the cave mollies was almost unaffected by the ambient light conditions. Our results suggest a heritable component to the reduction in fecundity in female cave mollies. Moreover, the reduced plasticity in fecundity of cave mollies in response to light conditions might be an example of genetic assimilation or channelling of a life history trait in a population invading a new environment.     

Seed germination behavior of five Resedaceae species from arid Arabian desert: gibberellic acid and light effects

Abstract

Light and growth-promoting compounds, such as gibberellic acids (GA₃), are among the most important factors that can break physiological seed dormancy. Here, we investigate the effects of GA₃ and light on germination of five species of Resedaceae that are known to have different levels of physiological dormancy. Seeds were incubated at 20/30?°C in both 12-hr photoperiod and complete darkness. To study the effect of growth hormone on germination, seeds were soaked for 24?h in different concentrations of GA₃ before sowing. The annuals (Reseda aucheri and Oligomeris linifolia) and the perennial Ochradenus arabicus had deep physiological dormancy, and exogenous application of GA₃ enhanced their germination in the light, but not in darkness; few or no seeds germinated in the dark in these species. Ochradenus aucheri and O. baccatus had intermediate and non-deep dormancy, respectively, and application of GA₃ enhanced their germination in both light and darkness. Germination of the annual species was much slower than that of the shrubby perennials. Overall, these results indicate that conditions under which seed developed, matured and stored on maternal plants as well as incubation conditions should be taken into consideration when assessing germination behavior of the perennial species of Resedaceae.     

Barium effects inPhaseolus aureus,Cephalandra indica,Canna indica,Beta vulgaris,Triticum aestivum andLactuca sativa

Barium chloride at 0.1 mM concentration inhibited elongation of mungbean (Phaseolus aureus) roots more strongly than elongation of hypocotyls. Root growth was completely inhibited at 80 mM and there was no germination at 100 mM. Respiration rates which were directly proportional to seedling vigour declined at varying degrees at the growth-inhibitory concentrations of BaCl₂. Barium retarded senescence in isolated leaf discs ofCephalandra indica by maintaining the chlorophyll level in darkness. The carotenes were similarly protected from degradation by Ba in darkness whereas xanthophylls were retained both in darkness and light. Ba also protected anthocyanin pigments in the staminode discs ofCanna indica flowers and reduced to a variable extent the leakage of betacyanin pigment fromBeta vulgaris root discs caused by various membrane active chemicals. Applied either alone or in combination with IAA, Ba inhibited cell enlargement in wheat (Triticum aestivum) coleoptile sections. Barium-induced inhibition of lettuce (Lactuca sativa) hypocotyl elongation was largely overcome by GA₃.