Ethylene enhances gibberellin levels and petiole sensitivity in flooding-tolerant Rumex palustris but not in flooding-intolerant R. acetosa

The role of gibberellin (GA) and ethylene in submergence-induced petiole elongation was studied in two species of the genus Rumex. Analysis of endogenous GAs in the flooding-tolerant Rumex palustris Sm. and the intolerant Rumex acetosa L. by gas chromatography-mass spectrometry showed for both species the presence of GA₁, GA₄, GA₉, GA₁₉, GA₂₀ and GA₅₃. Gas chromatography-mass spectrometry analysis of R. palustris petiole tissue of submerged plants showed an increase in levels of 13-OH GAs, especially GA₁, compared with drained plants. This effect could be mimicked by application of 5 μL L⁻¹ ethylene. In R. acetosa, no differences between levels of GAs in drained or submerged plants were found. In R. palustris, both submergence and ethylene treatment sensitized petioles to exogenous gibberellic acid (GA₃). In R. acetosa the effect was opposite, i.e. submergence and ethylene de-sensitized petioles to GA₃. Our results demonstrate the dual effect of ethylene in the submergence response related to flooding tolerance, i.e. in the flooding-tolerant R. palustris ethylene causes an increased concentration of and sensitivity to GA with respect to petiole elongation while in the intolerant R. acetosa ethylene reduces growth independent of GAs. Received: 5 November 1996 / Accepted: 8 February 1997     

Thermomorphogenic and Photomorphogenic Control of Stem Elongation in Fuchsia Is Not Mediated by Changes in Responsiveness to Gibberellins

Stem elongation in Fuchsia × hybrida was influenced by cultivation at different day and night temperatures or in different light qualities. Internode elongation of plants grown at a day (25°C) to night (15°C) temperature difference (DIF+10) in white light was almost twofold that of plants grown at the opposite temperature regime (DIF−10). Orange light resulted in a threefold stimulation of internode elongation compared with white light DIF−10. Surprisingly, internode elongation in orange light was similar for plants grown at DIF−10 and DIF+10. Flower development was accelerated at DIF−10 compared with DIF+10 in both white and orange light. To examine whether the effects of DIF and light quality on shoot elongation were related to changes in gibberellin metabolism or plant sensitivity to gibberellins (GAs), the stem elongation responses of paclobutrazol-treated plants to applied gibberellins were determined. In the absence of applied gibberellins paclobutrazol (>0.32 μmol plant⁻¹) strongly retarded shoot elongation. This inhibition was nullified by the application of about 10–32 nmol of GA₁, GA₄, GA₉, GA₁₅, GA₁₉, GA₂₀, GA₂₄, or GA₄₄. The results are discussed in relation to possible effects of DIF and light quality on endogenous gibberellin levels and gibberellin sensitivity of fuchsia and their effects on stem elongation. Received October 4, 1997; accepted December 17, 1997     

Growth-active gibberellins overcome the very slow shoot growth of Hancornia speciosa, an important fruit tree from the Brazilian “Cerrado”

Shoot elongation of Hancornia speciosa, an endangered tree from the Brazilian savannah “Cerrado”, is very slow, thus limiting nursery production of plants. Gibberellins (GAs) A₁, A₃, and A₅, and two inhibitors of GA biosynthesis, trinexapac-ethyl and ancymidol were applied to shoots of Hancornia seedlings. GA₁ and GA₃ significantly stimulated shoot elongation, while GA₅ had no significant effect. Trinexapac-ethyl and ancymidol, both at 100 μg per seedling, inhibited shoot elongation up to 45 days after treatment, though the effect was statistically significant only for ancymidol. Somewhat surprisingly, exogenous GA₃ more effectively stimulated shoot elongation in SD-grown plants, than in LD-grown plants. The results from exogenous application of GAs and inhibitors of GA biosynthesis imply that Hancornia shoot growth is controlled by GAs, and that level of endogenous growth-active GAs is likely to be the limiting factor for shoot elongation in Hancornia. Application of GAs thus offer a practical method for nursery production of Hancornia seedlings for outplanting into the field.     

Growth of In vitro banana (Musa SPP.) shoots under photomixotrophic and photoautotrophic conditions

Summary In vitro banana (Musa spp.) shoots were cultured under photomixotrophic (30 gl⁻¹ sucrose and 0.2 h⁻¹ number of air exchanges of culture vessels) and photoautotrophic (0 gl⁻¹ sucrose and 3.9 h⁻¹ number of air exchanges) conditions for 28 d in 370 cm³ Magenta boxes (GA7-type) containing 70 ml of half-strength Murashige and Skoog (MS) medium with 22.2 μM N⁶-benzyladenine (BA). The effects of varying CO₂ concentration (475 or 1340 μmol mol⁻¹) and light intensity (photosynthetic photon flux (PPF) of 100 or 200 μmol m⁻² s⁻¹) were investigated. Fresh and dry weights of banana shoots grown photomixotrophically were significantly greater on day 28 than those grown photoautotrophically. Photoautorophic shoots had a larger number of unfolded leaves and greater leaf area than photomixotrophic plants by days 14 and 28, regardless of CO₂ concentration. The shoot fresh and dry weights on day 14 in photoautotrophic conditions were significantly greater at PPF of 200 μmol m⁻² s⁻¹ than at 100 μmol m⁻² s⁻¹. The increase in net photosynthetic rate of photoautotrophic banana shoots was significant compared with photomixotrophic shoots. The multiplication ratio of in vitro banana shoots grown photoautotrophically in a 28-d culture period was the greatest at 100 μmol m⁻² s⁻¹ PPF and 475 μmol mol⁻¹ CO₂.     

Thidiazuron-induced high-frequency direct shoot organogenesis of <Emphasis Type="Italic">Cannabis sativa</Emphasis> L.

Induction of high-frequency shoot regeneration using nodal segments containing axillary buds from a 1-yr-old mother plants of Cannabis sativa was achieved on Murashige and Skoog (MS) medium containing 0.05–5.0 μM thidiazuron. The quality and quantity of regenerants were better with thidiazuron (0.5 μM thidiazuron) than with benzyladenine or kinetin. Adding 7.0 μM of gibberellic acid into a medium containing 0.5 μM thidiazuron slightly increased shoot growth. Elongated shoots when transferred to half-strength MS medium supplemented with 500 mg l⁻¹ activated charcoal and 2.5 μM indole-3-butyric acid resulted in 95% rooting. The rooted plants were successfully acclimatized in soil. Following acclimatization, growth performance of 4-mo-old in vitro propagated plants was compared with ex vitro vegetatively grown plants of the same age. The photosynthesis and transpiration characteristics were studied under different light levels (0, 500, 1,000, 1,500, or 2,000 μmol m⁻² s⁻¹). An increase in photosynthesis was observed with increase in the light intensity up to 1,500 μmol m⁻² s⁻¹ and then decreased subsequently at higher light levels in both types of plants. However, the increase was more pronounced at lower light intensities below 500 μmol m⁻² s⁻¹. Stomatal conductance and transpiration increased with light intensity up to highest level (2000 μmol m⁻² s⁻¹) tested. Intercellular CO₂ concentration (C _i) and the ratio of intercellular CO₂ concentration to ambient CO₂ (C _i/C _a) decreased with the increase in light intensity in both in vitro as well as ex vitro raised plants. The results show that in vitro propagated and hardened plants were functionally comparable to ex vitro plants of same age in terms of gas and water vapor exchange characteristics, within the limits of this study.     

Decrease of phosphoribulokinase activity by antisense RNA in transgenic tobacco: definition of the light environment under which phosphoribulokinase is not in large excess

 To test the hypothesis that the contribution of phosphoribulokinase (PRK) to the control of photosynthesis changes depending on the light environment of the plant, the response of transgenic tobacco (Nicotiana tabacum L.) transformed with antisense PRK constructs to irradiance was determined. In plants grown under low irradiance (330 μmol m⁻² s⁻¹) steady-state photosynthesis was limited in plants with decreased PRK activity upon exposure to higher irradiance, with a control coefficient of PRK for CO₂ assimilation of 0.25 at and above 800 μmol m⁻² s⁻¹. The flux control coefficient of PRK for steady-state CO₂ assimilation was zero, however, at all irradiances in plant material grown at 800 μmol m⁻² s⁻¹ and in plants grown in a glasshouse during mid-summer (alternating shade and sun 300–1600 μmol m⁻² s⁻¹). To explain these differences between plants grown under low and high irradiances, Calvin cycle enzyme activities and metabolite content were determined. Activities of PRK and other non-equilibrium Calvin cycle enzymes fructose-1,6-bisphosphatase, sedoheptulose-1,7-bisphosphatase and ribulose-1,5-bisphosphate carboxylase-oxygenase were twofold higher in plants grown at 800 μmol m⁻² s⁻¹ or in the glasshouse than in plants grown at 330 μmol m⁻² s⁻¹. Activities of equilibrium enzymes transketolase, aldolase, ribulose-5-phosphate epimerase and isomerase were very similar under all growth irradiances. The flux control coefficient of 0.25 in plants grown at 330 μmol m⁻² s⁻¹ can be explained because low ribulose-5-phosphate content in combination with low PRK activity limits the synthesis of ribulose-1,5-bisphosphate. This limitation is overcome in high-light-grown plants because of the large relative increase in activities of sedoheptulose-1,7-bisphosphatase and fructose-1,6-bisphosphatase under these conditions, which facilitates the synthesis of larger amounts of ribulose-5-phosphate. This potential limitation will have maintained evolutionary selection pressure for high concentrations of PRK within the chloroplast. Received: 15 November 1999 / Accepted: 27 January 2000     

CO2 exchange of the endolithic lichen Verrucaria baldensis from karst habitats in northern Italy

CO₂ exchange of the endolithic lichen Verrucaria baldensis was measured in the laboratory under different conditions of water content, temperature, light, and CO₂ concentration. The species had low CO₂ exchange rates (maximum net photosynthesis: c. 0.45 μmol CO₂ m⁻² s⁻¹; maximum dark respiration: c. 0.3 μmol CO₂ m⁻² s⁻¹) and a very low light compensation point (7 μmol photons m⁻² s⁻¹ at 8°C). The net photosynthesis/respiration quotient reached a maximum at 9–15°C. Photosynthetic activity was affected only after very severe desiccation, when high resaturation respiratory rates were measured. Microclimatic data were recorded under different weather conditions in an abyss of the Trieste Karst (northeast Italy), where the species was particularly abundant. Low photosynthetically active radiation (normally below 40 μmol photons m⁻² s⁻¹), very high humidities (over 80%), and low, constant temperatures were measured. Thallus water contents sufficient for CO₂ assimilation were often measured in the absence of condensation phenomena. Received: 22 September 1996 / Accepted: 26 April 1997     

Leaf photosynthesis, plant growth and nitrogen allocation in rice under different irradiances

The photosynthetic rates and various components of photosynthesis including ribulose-1,5-bisphosphate carboxylase (Rubisco; EC 4.1.1.39), chlorophyll (Chl), cytochrome (Cyt) f, and coupling factor 1 (CF₁) contents, and sucrose-phosphate synthase (SPS; EC 2.4.1.14) activity were examined in young, fully expanded leaves of rice (Oryza sativa L.) grown hydroponically under two irradiances, namely, 1000 and 350 μmol quanta · m⁻² · s⁻¹, at three N concentrations. The light-saturated rate of photosynthesis measured at 1800 μmol · m⁻² · s⁻¹ was almost the same for a given leaf N content irrespective of growth irradiance. Similarly, Rubisco content and SPS activity were not different for the same leaf N content between irradiance treatments. In contrast, Chl content was significantly greater in the plants grown at 350 μmol · m⁻² · s⁻¹, whereas Cyt f and CF₁ contents tended to be slightly smaller. However, these changes were not substantial, as shown by the fact that the light-limited rate of photosynthesis measured at 350 μmol · m⁻² · s⁻¹ was the same or only a little higher in the plants grown at 350 μmol · m⁻² · s⁻¹ and that CO₂-saturated photosynthesis did not differ between irradiance treatments. These results indicate that growth-irradiance-dependent changes in N partitioning in a leaf were far from optimal with respect to N-use efficiency of photosynthesis. In spite of the difference in growth irradiance, the relative growth rate of the whole plant did not differ between the treatments because there was an increase in the leaf area ratio in the low-irradiance-grown plants. This increase was associated with the preferential N-investment in leaf blades and the extremely low accumulation of starch and sucrose in leaf blades and sheaths, allowing a more efficient use of the fixed carbon. Thus, morphogenic responses at the whole-plant level may be more important for plants as an adaptation strategy to light environments than a response of N partitioning at the level of a single leaf. Received: 23 February 1997 / Accepted: 8 May 1997     

Abscisic acid and gibberellin-like substances in roots and root nodules ofGlycine max

Summary The content of endogenous gibberellin (GA)-like substances of roots and root nodules of SOya, and GA production byRhizobium japonicum cultures, were investigated by a combined thin layer chromatographic (TLC)-dwarf pea epicotyl bioassay technique. GAs were more concentrated in root nodules than in the roots, totalling 1.34 and 0.16 nM GA₃ equivalents g⁻¹ dry wt. respectively. GA production byR. japonicum cultures was demonstrated (1.00 nM GA₃ equivalentsl ⁻¹) and comparison of the GA components of plant and bacterial culture medium extracts, suggested that rhizobial GA production may contribute to the nodule GA content. Cis-trans abscisic acid (ABA) was identified in root and nodule extracts by TLC-gas liquid chromatography (GLC), and amounted to 0.18 and 2.21 nM g⁻¹ dry wt. respectively, whereas 0.30 and 4.63 nM ABA equivalents g⁻¹ dry wt. were detected by a TLC-wheat embryo bioassay technique. ABA was not detected in extracts of bacterial cultures.     

Photosynthetic responses of Microstegium vimineum (Trin.) A. Camus, a shade-tolerant, C4 grass, to variable light environments

Microstegium vimineum (Trin.) A. Camus, a shade-tolerant C₄ grass, has spread throughout the eastern United States since its introduction in 1919. This species invades disturbed understory habitats along streambanks and surrounding mesic forests, and has become a major pest in areas such as Great Smoky Mountains National Park. The focus of this study was to characterize the photosynthetic induction responses of M. vimineum, specifically its ability to utilize low light and sunflecks, two factors that may be critical to invasive abilities and survival in the understory. In addition, we were curious about the ability of a grass with the C₄ photosynthetic pathway to respond to sunflecks. Plants were grown under 25% and 50% ambient sunlight, and photosynthetic responses to both steady-state and variable light were determined. Plants grown in both 25% and 50% ambient sun became 90% light saturated between 750–850 μmol m⁻² s⁻¹; however, plants grown in 50% ambient sun had significantly higher maximum steady-state photosynthetic rates (16.09 ± 1.37 μmol m⁻² s⁻¹ vs. 12.71 ± 1.18 μmol m⁻² s⁻¹). Both groups of plants induced to 50% of the steady-state rate in 3–5 min, while it took 10–13 min to reach 90% of maximum rates, under both flashing and steady-state light. For both groups of plants, stomatal conductance during induction reached maximum rates in 6–7 min, after which rates decreased slightly. Upon return to low light, rates of induction loss and stomatal closure were very rapid in both groups of plants, but were more rapid in those grown in high light. Rapid induction and the ability to induce under flashing light may enable this species to invade and dominate mesic understory habitats, while rapid induction loss due to stomatal closure may prevent excess water loss when low light constrains photosynthesis. The C₄ pathway itself does not appear to present an insurmountable barrier to the ability of this grass species to respond to sunflecks in an understory environment. Received: 21 February 1997 / Accepted: 10 October 1997     

Effect of broken dead culms of Phragmites australis on radial oxygen loss in relation to radiation and temperature

The amount of oxygen released from the roots of Phragmites australis was quantified to examine the effects of airflow through dead culms, radiation, and temperature on radial oxygen loss (ROL). To investigate the effect of dead culms on ROL quantitatively, the ROL of individual plants with open dead culms was compared to that of plants with sealed dead culms as a function of light intensity and temperature. The relationship between ROL and plant morphology (aboveground biomass, shoot diameter, shoot height) was investigated. When exposed to 300, 600, and 900 μmol m⁻² s⁻¹ light, the ROL was 15.6, 22.5, and 30.9 μmol O₂ g⁻¹ dry root day⁻¹, respectively, from plants with open dead culms and 11.0, 16.4, and 23.3 μmol O₂ g⁻¹ dry root day⁻¹, respectively, from plants with sealed dead culms. The ROL from plants with open dead culms was obviously higher than that from plants with sealed dead culms in every condition. The ROL from plants with open culms was 37% and 30% higher than that from plants with sealed culms at 20°C and 30°C, respectively. The effects of plant-specific parameters such as leaf area and shoot diameter on radial oxygen loss were evident. From the point of view of rhizosphere oxidation during the growing season, the existence of open dead culms should be taken into consideration for optimal plant management in constructed wetlands. This study provides a theoretical understanding of the effects of open dead culms, light conditions, and temperature on radial oxygen loss. Handling editor: S. M. Thomaz     

Assimilate export by leaves of Ricinus communis L. growing under normal and elevated carbon dioxide concentrations: the same rate during the day, a different rate at night

Castor bean (Ricinus communis L.) plants were grown for 5–7 weeks in a controlled environment at 350 μl l⁻¹ or 700 μl l⁻¹ CO₂. Carbon assimilation, assimilate deposition, dark respiration and assimilate mobilization were measured in leaves 2, 3 and 4 (counted from the base of the plant), and a balance sheet of carbon input and export was elaborated for both CO₂ concentrations. Carbon dioxide assimilation was nearly constant over the illumination period, with only a slight depression occurring at the end of the day in mature source leaves, not in young source leaves. Assimilation was ca. 40% higher at 700 μl l⁻¹ than at 350 μl l⁻¹ CO₂. The source leaves increased steadily in weight per unit area during the first 3 weeks, more at 700 μl l⁻¹ than at 350 μl l⁻¹ CO₂. On top of an irreversible weight increase, there was a large gain in dry weight during the day, which was reversed during the night. This reversible weight gain was constant over the life time of the leaf and ca. 80% higher at 700 μl l⁻¹ than at 350 μl l⁻¹. Most of it was due to carbohydrates. The carbon content (as a percentage) was not altered by the CO₂ treatment. Respiration was 25% higher in high-CO₂ plants when based on leaf area, but the same when based on dry weight. The rate of carbon export via the phloem was the same during the daytime in plants grown at 350 μl l⁻¹ and 700 μl l⁻¹ CO₂. During the night the low-CO₂ plants had only 50% of the daytime export rate, in contrast to the high-CO₂ plants which maintained the high export rate. It was concluded that the phloem loading system is saturated during the daytime in both CO₂ regimes, whereas during the night the assimilate supply is reduced in plants in the normal CO₂ concentration. Two-thirds of the carbon exported from the leaves was permanently incorporated as plant dry matter in the residual plant parts. This “assimilation efficiency” was the same for both CO₂ regimes. It is speculated that under 350 μl l⁻¹ CO₂ the growing Ricinus plant operates at sink limitation during the day and at source limitation during the night. Received: 2 February 1999 / Accepted: 19 April 1999     

The role of gibberellins A1 and A3 in fruit growth of Pisum sativum L. and the identification of gibberellins A4 and A7 in young seeds

Gibberellins A₁ and A₃ are the major physiologically active gibberellins (GAs) present in young fruit of pea (Pisum sativum L.). The relative importance of these GAs in controlling fruit growth and their biosynthetic origins were investigated in cv. Alaska. In addition, the non-13-hydroxylated active GAs, GA₄ and GA₇, were identified for the first time in young seeds harvested 4 d after anthesis, although they are minor components and are not expected to play major physiological roles. The GA₁ content is maximal in seeds and pods at 6 d after anthesis, the time of highest growth-rate of the pod (Garcia-Martinez et al. 1991, Planta 184: 53–60), whereas gibberellic acid (GA₃), which is present at high levels in seeds 4–8 d after anthesis, has very low abundance in pods. Gibberellins A₁₉, A₂₀ and A₂₉ are most concentrated in seeds at, or shortly after, anthesis and their abundance declines rapidly with development, concomitant with the sharp increase in GA₁ and GA₃ content. Application of GA₁ or GA₃ to the leaf subtending an emasculated flower stimulated parthenocarpic fruit development. Measurement of the GA content of the pods at 4 d after anthesis indicated that only 0.002–0.5% of the applied GA was transported to the fruit, depending on dose. There was a linear relationship between GA₁ content and pod weight up to about 2 ng · (g FW)⁻¹, whereas no such correlation existed for GA₃ content. The concentration of endogenous GA₁ in pods from pollinated ovaries is just sufficient to give the maximum growth response. It is concluded that GA₁, but not GA₃, controls pod growth in pea; GA₃ may be involved in early seed development. The distribution of GAs within the seeds at 4 d post anthesis was also investigated. Most of the GA₁, GA₈, GA₁₉, GA₂₀ and GA₂₉ was present in the testa, whereas GA₃ was distributed equally between testa and endosperm and GA₄ was localised mainly in the endosperm. Of the GAs analysed, only GA₃ and GA₂₀ were detected in the embryo. Metabolism experiments with intact tissues and cell-free fractions indicated compartmentation of GA biosynthesis within the seed. Using ¹⁴C-labelled GA₁₂, GA₉, 2,3-didehydroGA₉ and GA₂₀ as substrates, the testa was shown to contain 13-hydroxylase and 20-oxidase activities, the endosperm, 3β-hydroxylase and 20-oxidase activities. Both tissues also produced 16,17-dihydrodiols. However, GA₁ and GA₃ were not obtained as products and it is unlikely that they are formed via the early 13-hydroxylation pathway. [¹⁴C]gibberellin A₁₂, applied to the inside surface of pods in situ, was metabolised to GA₁₉, GA₂₀, GA₂₉, GA₂₉-catabolite, GA₈₁ and GA₉₇, but GA₁ was not detected. Gibberellin A₂₀ was metabolised by this tissue to GA₂₉ and GA₂₉-catabolite. Received: 23 July 1996 / Accepted: 2 September 1996     

<Emphasis Type="BoldItalic">In vitro</Emphasis> propagation of the ornamental bromeliad <Emphasis Type="BoldItalic">Acanthostachys strobilacea</Emphasis> (Schult. f.) Klotzsch via nodal segments

Acanthostachys strobilacea (Schult. f.) Klotzsch is an ornamental species of Bromeliaceae that may show an elongated stem when cultivated in vitro. This work reports a micropropagation protocol for A. strobilacea using nodal segments. Seeds were placed in Murashige and Skoog’s medium with macronutrients diluted to 1/5. Nodal segments isolated from the stems of in vitro elongated plants were subcultured in the same medium and kept in different light intensities (14, 41, and 50 μmol m⁻² s⁻¹) or continuous darkness. Another group of nodes was subcultured according to the position in the mother seedling. The plants that showed the most stem elongation were those that were cultured in 14 μmol m⁻² s⁻¹ or that came from isolated nodal segments in the median and basal regions of the mother plant. After 2 mo, all of the plants originating from the development of lateral buds were transferred to a greenhouse. Only those that were not elongated survived ex vitro and flowered after 1 yr.     

Photosynthetic characteristics of dipterocarp seedlings in three tropical rain forest light environments: a basis for niche partitioning?

In three tropical rain forest light environments in Sabah, Malaysia, we compared photosynthesis in seedlings of ten climax tree species with putatively differing shade tolerances. The objectives of the study were (a) to characterise the range of photosynthetic responses in ten species of the Dipterocarpaceae and (b) to elucidate those photosynthetic characteristics that might provide a basis for niche partitioning. Seedlings were acclimated (c. 7 months) in three light environments; understorey, partial shade and a gap (140 m²). The light environments represented a gradation in median diurnal (0630–1830 hours) photon flux density (PFD) ranging from understorey (4.7 μmol m⁻² s⁻¹), through partial shade (21.2 μmol m⁻² s⁻¹) to gap (113.7 μmol m⁻² s⁻¹). Integrated diurnal PFD were in the sequence gap > partial shade > understorey (15.2, 4.7, 1.3 mol m⁻² day⁻¹, respectively). In gap-acclimated plants, species differed in the photosynthetic light-response variables apparent quantum yield, dark respiration rate, light compensation point, net saturated leaf assimilation rate (A _sat), and in stomatal conductance (g _s sat) when assimilation rate (A) was saturated. A light-demanding pioneer species (Macaranga hypoleuca) and a shade-demanding understorey species (Begonia sp.) had, respectively, higher and lower A _sat and g _s sat than the dipterocarp species. In high-light conditions A _sat and g _s sat were strongly positively correlated in dipterocarp species. Differing photosynthetic characteristics of gap-acclimated plants suggest that, in these dipterocarp species, different rates of carbon fixation may be an important factor contributing towards niche partitioning. Mean integrated diurnal A (A _diurnal) in the gap, partial shade and understory were, respectively, 122.9, 52.7, 20.5 mmol m⁻² day⁻¹. Differences occurred in A _diurnal of dipterocarp species between light environments. When Macaranga was included, differences in A _diurnal were evident in the gap and partial shade, and in both cases were attributed to the pioneer. For the variable A _diurnal, there was of a shift in the rank position of Macaranga among light environments, but a shift did not occur among the dipterocarp species. Results from this study are consistent with the idea that rates of carbon fixation per unit leaf area may contribute towards niche differentiation between the climax and single pioneer species, but not within the group of climax species. Other physiological and/or carbon allocation factors may be involved in any niche partitioning; dipterocarp species often have inherently different growth rates and susceptibility to herbivory. As an alternative to niche partitioning, dipterocarp species may co-exist in natural light environments as a result of habitat disequilibrium or purely stochastic processes. Received: 2 April 1997 / Accepted: 13 July 1997     

<Emphasis Type="Italic">In vitro</Emphasis> propagation of northern red oak (<Emphasis Type="Italic">Quercus rubra</Emphasis> L.)

In vitro propagation of northern red oak (Quercus rubra) shoots was successful from cotyledonary node explants excised from 8-wk-old in vitro grown seedlings. Initially, four shoots per explant were obtained on Murashige and Skoog (MS) medium supplemented with 4.4 μM 6-benzylaminopurine (BA), 0.45 μM thidiazuron (TDZ), and 500 mg l⁻¹ casein hydrolysate (CH) with a regeneration frequency of 64.7% after 3 wk. Subculturing explants (after harvesting shoots) to fresh treatment medium significantly increased shoot bud regeneration (16.6 buds per explant), but the buds failed to develop into shoots. A higher percentage (73.3%) of the explants regenerated four shoots per explant on woody plant medium (WPM) supplemented with 4.4 μM BA, 0.29 μM gibberellic acid (GA₃), and 500 mg l⁻¹ CH after 3 wk. Explants subcultured to fresh treatment medium after harvesting shoots significantly increased shoot regeneration (16 shoots per explant). Shoot elongation was achieved (4 cm) when shoots were excised and cultured on WPM supplemented with 0.44 μM BA and 0.29 μM GA₃. In vitro regenerated shoots were rooted on WPM supplemented with 4.9 μM indole-3-butyric acid. A higher percentage regeneration response and shoot numbers per explant were recorded on WPM supplemented with BA and GA₃, than on MS medium containing BA and TDZ. Lower concentrations of BA and GA₃ were required for shoot elongation and prevention of shoot tip necrosis. Each cotyledonary node yielded approximately 20 shoots within 12 wk. Rooted plantlets were successfully acclimatized.     

Gibberellin physiology of safflower: endogenous gibberellins and response to gibberellic acid

Endogenous gibberellins (GAs) were extracted from safflower (Carthamus tinctorius L.) stems and detected by capillary gas chromatography-mass spectrometry from which GA₁, GA₃, GA₁₉,, GA₂₀, GA₂₉, and probably, GA₄₄ were detected. The detection of these GAs suggests that the early 13-OH biosynthetic pathway is prevalent in safflower shoots. Deuterated GAs were used as internal standards and GA concentrations were determined in stems harvested at weekly intervals. GA₁ and GA₁₉ levels per stem increased but concentrations per gram dry weight decreased over time. GA₂₀ was only detected in young stem tissue.Gibberellic acid (GA₃) was also applied in field trials and both GA₃ and the GA biosynthetic inhibitor, paclobutrazol, were applied in growth chamber tests. GA₃ increased epidermal cell size, internode length, and increased internode cell number causing stem elongation. Conversely, paclobutrazol reduced stem height, internode and cell size, cell number and overall shoot weight. In field tests, GA₃ increased total stem weight, but decreased leaf weight, flower bud number and seed yield. Thus, GA₃ promoted vegetative growth at the expense of reproductive commitment. These studies collectively indicate a promotory role of GAs in the control of shoot growth in safflower, and are generally consistent with gibberellin studies of related crop plants. Author for correspondence     

Light response characteristics of a morphologically diverse group of southern hemisphere conifers as measured by chlorophyll fluorescence

Unlike northern hemisphere conifer families, the southern family, Podocarpaceae, produces a great variety of foliage forms ranging from functionally broad-, to needle-leaved. The production of broad photosynthetic surfaces in podocarps has been linked qualitatively to low-light-environments, and we undertook to assess the validity of this assumption by measuring the light response of a morphologically diverse group of podocarps. The light response, as apparent photochemical electron transport rate (ETR), was measured by modulated fluorescence in ten species of this family and six associated species (including five Cupressaceae and one functionally needle-leaved angiosperm) all grown under identical glasshouse conditions. In all species, ETR was found to increase as light intensity increased, reaching a peak value (ETR_max) at saturating quantum flux (PPFD_sat), and decreasing thereafter. ETR_max ranged from 217 μmol electrons · m⁻² · s⁻¹ at a PPFD_sat of 1725 μmol photons · m⁻² · s⁻¹ in Actinostrobus acuminatus to an ETR of 60 μmol electrons · m⁻² · s⁻¹ at a PPFD_sat of 745 μmol electrons · m⁻² · s⁻¹ in Podocarpus dispermis. Good correlations were observed between ETR_max and both PPFD_sat and maximum assimilation rate measured by gas-exchange analysis. The effective quantum yield at light saturation remained constant in all species with an average value of 0.278 ± 0.0035 determined for all 16 species. Differences in the shapes of light response curves were related to differences in the response of non-photochemical quenching (q _n), with q _n saturating faster in species with low PPFD_sat. Amongst the species of Podocarpaceae, the log of average shoot width was well correlated with PPFD_sat, wider leaves saturating at lower light intensities. This suggests that broadly flattened shoots in the Podocarpaceae are an adaptation to low light intensity. Received: 15 April 1996 / Accepted: 30 September 1996     

Growth and net photosynthetic rates of <Emphasis Type="Italic">Hosta</Emphasis> ‘blue vision’ during acclimatization in bright,natural light with CO<Subscript>2</Subscript> enrichment

Summary Hosta ‘Blue Vision’, a shade-adapted perennial, was successfully acclimatized in high, natural light conditions in the research Acclimatron^TM at Clemson University, Clemson, SC during the summer of 2000. The supplemental CO₂ levels achieved during acclimatization were 710±113, 2396±121, and 5641±119 μmol mol⁻¹, approximately 2×, 6×, and 15× ambient CO₂. Plants were maintained in H₂O-saturated atmospheres and protected from temperature increases associated with high light intensity. In the 5 wk following ex vitro transfer, plantlet roots grew at the 2× CO₂ level, but shoot biomass was unaffected. Results for the 6× and 15× CO₂ levels were comparable and provided the best plantlet growth. The “doubling time’ that is characteristic of exponential growth was 10.8 and 9.8 d for root and shoot dry weights, respectively. There was no indication of light saturation of net photosynthetic rate (NPR) over the photosynthetic photon flux density (PPFD) range of 100–1200 μmolm⁻²s⁻¹ experienced during this study. An interaction between CO₂ and light intensity levels was detected for NPR of Hosta ‘Blue Vision’ with CO₂ saturation occurring at approximately 2800 μmol mol⁻¹. regardless of light level. Furthermore, at the optimal CO₂ level, NPR increased quadratically as light intensity increased, and NPR was greatest at the maximum light intensity (PPFD: 1200 μmol m⁻²s⁻¹).     

Micropropagation of juvenile tissue of Eucalyptus erythronema × eucalyptus stricklandii cv. ‘urrbrae gem’

Summary Micropropagation via enhanced axillary shoot proliferation was investigated in the ornamental Eucalyptus cv. ‘Urrbrae Gem’ using in vitro germinated seedlings and was successfully achieved using woody plant medium (WPM) supplemented with 2.2 μM benzylaminopurine, 1.0 μM α-naphthaleneacetic acid, and 1.5 μM gibberellic acid (GA₃), gelled with 5 g l⁻¹ Phytagel^?. Shoot proliferation was greater on WPM and QL media with GA₃ compared to B5, AP, and TK media with or without GA₃. GA₃ was required for shoot elongation as the internodes were otherwise very short and unsuitable for multiplication or root initiation. Root initiation was improved using (1/2) WPM supplemented with 20 μM indole-3-butyric acid (IBA) over a 7 d pulse, followed by subculture to IBA-free medium, compared to placing shoots on low levels of IBA for 4–6 wk. Plantlets were successfully hardened off to the natural environment via a fogger at 67% relative, humidity at 21°C for 3 d and continued to thrive as potted plants. This is the first report of successful, micropropagation in an ornamental eucalypt (subgenus Symphyomyrtus) from seedling explants.