Photosynthetic nitrogen-use efficiency of species that differ inherently in specific leaf area

Factors that contribute to interspecific variation in photosynthetic nitrogen-use efficiency (PNUE, the ratio of CO₂ assimilation rate to leaf organic nitrogen content) were investigated, comparing ten dicotyledonous species that differ inherently in specific leaf area (SLA, leaf area:leaf dry mass). Plants were grown hydroponically in controlled environment cabinets at two irradiances (200 and 1000 μmol m^–2 s^–1). CO₂ and irradiance response curves of photosynthesis were measured followed by analysis of the chlorophyll, Rubisco, nitrate and total nitrogen contents of the leaves. At both irradiances, SLA ranged more than twofold across species. High-SLA species had higher in situ rates of photosynthesis per unit leaf mass, but similar rates on an area basis. The organic N content per unit leaf area was lower for the high-SLA species and consequently PNUE at ambient light conditions (PNUE_amb) was higher in those plants. Differences were somewhat smaller, but still present, when PNUE was determined at saturating irradiances (PNUE_max). An assessment was made of the relative importance of the various factors that underlay interspecific variation in PNUE. For plants grown under low irradiance, PNUE_amb of high-SLA species was higher primarily due to their lower N content per unit leaf area. Low-SLA species clearly had an overinvestment in photosynthetic N under these conditions. In addition, high SLA-species allocated a larger fraction of organic nitrogen to thylakoids and Rubisco, which further increased PNUE_amb. High-SLA species grown under high irradiance showed higher PNUE_amb mainly due to a higher Rubisco specific activity. Other factors that contributed were again their lower contents of N_org per unit leaf area and a higher fraction of photosynthetic N in electron transport and Rubisco. For PNUE_max, differences between species in organic leaf nitrogen content per se were no longer important and higher PNUE_max of the high SLA species was due to a higher fraction of N in␣photosynthetic compounds (for low-light plants) and a higher Rubisco specific activity (for high-light grown plants). Received: 11 October 1997 / Accepted: 9 April 1998     

The effects of enhanced ultraviolet-B and nitrogen supply on growth,photosynthesis and nutrient status of <Emphasis Type="Italic">Abies faxoniana</Emphasis> seedlings

Abies faxoniana is a key species in reforestation processes in the southeast of the Qinghai-Tibetan Plateau of China. The changes in growth, photosynthesis and nutrient status of A. faxoniana seedlings exposed to enhanced ultraviolet-B (UV-B), nitrogen supply and their combination were investigated. The experimental design included two levels of UV-B treatments (ambient UV-B, 11.02 KJ m⁻² day⁻¹; enhanced UV-B, 14.33 KJ m⁻² day⁻¹) and two nitrogen levels (0; 20 g N m⁻²). The results indicated that: (1) enhanced UV-B significantly caused a marked decline in growth parameters, net photosynthetic rate (Pn), photosynthetic pigments and F _v/F _m, (2) supplemental nitrogen supply increased the accumulation of total biomass, Pn, photosynthetic pigments and F _v/F _m under ambient UV-B, whereas supplemental nitrogen supply reduced Pn, and not affect biomass under enhanced UV-B, (3) enhanced UV-B or nitrogen supply changed the concentration of nutrient elements of various organs.     

Fluoranthene influences endogenous abscisic acid level and primary photosynthetic processes in pea (<Emphasis Type="Italic">Pisum sativum</Emphasis> L.) plants in vitro

The influence of increasing concentrations (0.1, 1.0 and 5.0 mg l⁻¹) of fluoranthene (FLT) on growth, endogenous abscisic acid (ABA) level and primary photosynthetic processes in 21-day-old pea plants (Pisum sativum L.) in vitro was investigated. Murashige and Skoog’s (MS) medium, with or without FLT, was enriched with indole-3-acetic acid (IAA; 0.1 mg l⁻¹) or a combination of IAA (0.1 mg l⁻¹) plus N⁶-benzyladenine (BA; 0.1 mg l⁻¹). The level of endogenous ABA significantly increased with increasing FLT concentrations in the presence of both IAA and IAA plus BA. An increased level of endogenous ABA was observed in plants treated with IAA alone. The growth of shoot, callus and the content of photosynthetic pigments (chlorophyll a and b, carotenoids), in both IAA- and IAA plus BA-treated plants, were significantly stimulated by FLT at its lowest concentration (0.1 mg l⁻¹) assayed in this study. However, FLT at higher concentrations (1.0 and 5.0 mg l⁻¹) significantly inhibited all these parameters. Chlorophyll fluorescence imaging showed that FLT only at the highest concentration (5.0 mg l⁻¹) in the presence of IAA (0.1 mg l⁻¹) significantly increased F₀, but decreased F_V/F_M and Φ_II.     

Gas exchange and photosynthetic performance of the tropical tree <Emphasis Type="Italic">Acacia nigrescens</Emphasis> when grown in different CO<Subscript>2</Subscript> concentrations

The photosynthetic responses of the tropical tree species Acacia nigrescens Oliv. grown at different atmospheric CO₂ concentrations—from sub-ambient to super-ambient—have been studied. Light-saturated rates of net photosynthesis (A _sat) in A. nigrescens, measured after 120 days exposure, increased significantly from sub-ambient (196 μL L⁻¹) to current ambient (386 μL L⁻¹) CO₂ growth conditions but did not increase any further as [CO₂] became super-ambient (597 μL L⁻¹). Examination of photosynthetic CO₂ response curves, leaf nitrogen content, and leaf thickness showed that this acclimation was most likely caused by reduction in Rubisco activity and a shift towards ribulose-1,5-bisphosphate regeneration-limited photosynthesis, but not a consequence of changes in mesophyll conductance. Also, measurements of the maximum efficiency of PSII and the carotenoid to chlorophyll ratio of leaves indicated that it was unlikely that the pattern of A _sat seen was a consequence of growth [CO₂] induced stress. Many of the photosynthetic responses examined were not linear with respect to the concentration of CO₂ but could be explained by current models of photosynthesis.     

Somatic embryogenesis and plant regeneration in two wild cotton species belong to G genome

The present work describes the plant regeneration via somatic embryogenesis in two wild cotton species belonging to G genome: Gossypium nelsonii Fryx and Gossypium australe F Muell. The role of plant hormones and carbohydrates was also evaluated for somatic embryogenesis and somatic embryo development. Normal plants were obtained from G. nelsonii Fryx; abnormal plants and somatic embryos were obtained from G. australe F Muell. The best medium for callus induction for these G genome wild cotton species was MSB₅ supplemented with 0.1 mg L⁻¹ KT and 0.1 mg L⁻¹ 2,4-D. For embryogenic callus proliferation, the best medium used was MSB₅ supplemented with 0.2 mg L⁻¹ KT and 0.5 mg L⁻¹ IBA. The medium MSB₅ supplemented with 0.15 mg L⁻¹ KT and 0.5 mg L⁻¹ NAA was used successfully for root initiation and plant growth. In addition, adding CuSO₄ and AgNO₃ in the callus-inducing and proliferation medium resulted in a number of somatic embryos. Glucose and maltose, the carbon sources in somatic culture, were used for callus induction, but maltose worked even better than glucose for proliferation of embryogenic callus and development of somatic embryos.     

<Emphasis Type="Italic">Agrobacterium</Emphasis>-mediated genetic transformation and regeneration of transgenic plants using leaf segments as explants in Valencia sweet orange

In this study, attempts were made to develop a protocol for regeneration of transgenic plants via Agrobacterium tumefaciens-mediated transformation of leaf segments from ‘Valencia’ sweet orange (Citrus sinensis L. Osbeck) using gfp (green fluorescence protein) as a vital marker. Sensitivity of the leaf segments regeneration to kanamycin was evaluated, which showed that 50 mg l⁻¹ was the best among the tested concentrations. In addition, factors affecting the frequency of transient gfp expression were optimized, including leaf age, Agrobacterium concentration, infection time, and co-cultivation period. Adventitious shoots regenerated on medium containing Murashige and Tucker basal medium plus 0.1 mg l⁻¹ α-naphthaleneacetic acid (NAA), 0.5 mg l⁻¹ 6-benzyladenine (BA) and 0.5 mg l⁻¹ kinetin (KT). The leaf segments from 3-month-old in vitro seedlings, Agrobacterium concentration at OD₆₀₀ of 0.6, 10-min immersion, and co-cultivation for 3 days yielded the highest frequency of transient gfp expression, shoots regeneration response and transformation efficiency. By applying these optimized parameters we recovered independent transformed plants at the transformation efficiency of 23.33% on selection medium (MT salts augmented with 0.5 mg l⁻¹ BA, 0.5 mg l⁻¹ KT, 0.1 mg l⁻¹ NAA, 50 mg l⁻¹ kanamycin and 250 mg l⁻¹ cefotaxime). Expression of gfp in the leaf segments and regenerated shoots was confirmed using fluorescence microscope. Polymerase chain reaction (PCR) analysis using gfp and nptII gene-specific primers further confirmed the integration of the transgene in the independent transgenic plants. The transformation methodology described here may pave the way for generating transgenic plants using leaf segments as explants.     

Micropropagation and plant regeneration from embryogenic callus of Miscanthus sinensis

Miscanthus sinensis (Poaceae) is a typical perennial giant grass of East Asia. Due to its high photosynthetic efficiency, low input requirements, and high biomass production, M. sinensis shows outstanding potential as a biofuel feedstock. However, the lack of an efficient tissue culture system may limit its utilization potential. Different explants of M. sinensis were evaluated to develop an efficient tissue culture system. Shoot apices from in vitro-germinated seedling explants were tested for adventitious bud proliferation. The highest level of proliferation (multiplication coefficient 6.69) was obtained when shoot apices were cultured on Murashige and Skoog (MS) medium supplemented with 1.0 mg L⁻¹ 6-benzyladenine (BA), 2.0 mg L⁻¹ kinetin, 0.05 mg L⁻¹ α-naphthalene acetic acid (NAA), 3% sucrose, and 0.8% agar. The highest rooting percentage (95.4%) was obtained when adventitious buds were cultured on half-strength MS medium supplemented with 0.2 mg L⁻¹ NAA, 3% sucrose, and 0.8% agar. Significant differences were found in the formation of embryogenic callus among different explant types. The embryogenic callus derived from epicotyls had the highest regeneration capacity when cultured on a medium supplemented with 2.0 mg L⁻¹ 2,4-dichlorophenoxyacetic acid, 0.5 mg L⁻¹ BA, and 0.1 mg L⁻¹ thiamine. Under these conditions, the callus induction percentage was 82%.     

Response of in vitro strawberry to antibiotics

By identifying antibiotics that had the least phytotoxic effects on explants during genetic transformation, we evaluated the effect of various antibiotics on callus induction and morphogenesis from leaf explants and in vitro growth of Fragaria × ananassa Duch. cv. Toyonaka. Results showed that kanamycin (Kan) significantly inhibited callus induction, bud differentiation and root morphogenesis while carbenicillin (Carb), cefotaxime (Cef) and an equal concentration of Cef and Carb up to 500 mg L⁻¹ had no significant effects on callus induction and shoot growth. Kan, even at 2.5 mg L⁻¹, significantly inhibited callus induction, shoot regeneration and root formation, while no shoots regenerated at concentrations above 15 mg L⁻¹. Rooting was completely inhibited in the presence of 50 mg L⁻¹ Kan. Cef had negative effects on shoot regeneration from leaf explants and in vitro growth of strawberry. Compared to Cef, Carb at ≤300 mg L⁻¹ significantly promoted shoot and root organogenesis. However, an equal concentration of Carb plus Cef could alleviate the negative effect of Cef on strawberry. Results from relative electrolyte leakage, root and antioxidant activities, O₂^·− production rate, H₂O₂, proline and MDA contents showed that Kan, Cef and Carb caused electrolyte leakage and triggered active enzymatic processes and metabolism. This offers a possible mechanism for the inhibition or stimulation of strawberry growth caused by these antibiotics.     

Evaluation of green fluorescent protein as a reporter gene and phosphinothricin as the selective agent for achieving a higher recovery of transformants in cucumber (<Emphasis Type="Italic">Cucumis sativus</Emphasis> L. cv. Poinsett76) via <Emphasis Type="Italic">Agrobacterium tumefaciens</Emphasis>

Efficient Agrobacterium tumefaciens-mediated transformation and a higher recovery of transformed plants of cucumber cv. Poinsett76 were achieved via direct organogenesis from cotyledon explants. Stable transformants were obtained by inoculating explants with A. tumefaciens strains EHA105 or LBA4404, both harboring the binary vector pME508, which contains the neomycin phosphotransferase II (nptII) and phosphinothricin resistance genes (bar) conferring resistance to kanamycin and PPT, respectively, as selectable markers and the sgfp-tyg gene for the green fluorescent protein (GFP) as a visual marker driven by the constitutive CaMV35S promoter in the presence of acetosyringone (50 μM). Transformed shoots were obtained on MS Murashige and Skoog (Plant Physiol. 15: 473–497, 1962) medium supplemented with 1 mg L⁻¹ benzyladenine (BA), 20 mg L⁻¹ l-glutamine and 2 mg L⁻¹ phosphinothricin (PPT) or 100 mg L⁻¹ kanamycin. The regenerated shoots were examined in vivo using a hand-held long wave UV lamp for GFP expression. The GFP screening helped identify escapes and chimeric shoots at regular intervals to increase the growth of transformed shoots on cotyledon explants. Elongation and rooting of putative transformants were achieved on PPT (2 mg L⁻¹) containing MS media with 0.5 mg L⁻¹ gibberellic acid (GA₃) and 0.6 mg L⁻¹ indole butyric acid (IBA), respectively. PCR and Southern analyses confirmed the integration of the sgfp gene into the genome of T₀ and the progenies. T₁ segregation of transgenic progeny exhibited Mendelian inheritance of the transgene. The use of EHA105 resulted in 21% transformation efficiency compared to 8.5% when LBA4404 was used. This higher rate was greatly facilitated by PPT selection coupled with effective screening of transformants for GFP expression, thus making the protocol highly useful for the recovery of a higher number of transgenic cucumber plants.     

Nitrogen and phosphorus removal through laboratory batch cultures of microalga Chlorella vulgaris and cyanobacterium Planktothrix isothrix grown as monoalgal and as co-cultures

Batch cultures of the green microalga Chlorella vulgaris and cyanobacterium Planktothrix isothrix and their corresponding co-cultures were grown in municipal wastewater in order to study their growth as well as the nitrogen (NH₄–N) and phosphorus (PO₄³⁻–P) removal. The cultures were grown under two irradiances of 20 and 60 μmol photons m⁻² s⁻¹ in shaken and unshaken conditions. The co-culture of unshaken Chlorella and Planktothrix showed the greatest growth under both irradiances. The monoalgal Planktotrix cultures showed better growth when unshaken than when shaken, whereas Chlorella cultures grew better when mixed, but only at the higher irradiance. The highest percentage of nitrogen removal (up to 80%) was attained by the unshaken co-cultures of Chlorella and Planktothrix. The amount of nitrogen recycled in the biomass reached up to 85% of that removed. Shaken monoalgal cultures of Chlorella showed phosphorus removal under both irradiances. They completely removed the initial phosphorus concentration (7.47 ± 0.17 mg L⁻¹) within 96 and 48 h under 20 and 60 μmol photons m⁻² s⁻¹, respectively.     

Biomass and photosynthetic productivity of water hyacinth (Eichhornia crassipes) as affected by nutrient supply and mirid (Eccritotarus catarinensis) biocontrol

The biological control of water hyacinth is affected by water nitrogen and phosphorus content and this was investigated experimentally at five levels of nutrient supply by measuring plant photosynthetic and growth responses, and mirid reproduction and herbivory of nutrient treated plants. Low nitrogen (2–0.2 mg L⁻¹) and phosphorus (0.2–0.01 mg L⁻¹) supply decreased hyacinth photosynthesis, growth and biomass accumulation relative to plants supplied 200 mg L⁻¹ N and 20 mg L⁻¹ P. This effect depended more on nitrogen supply than phosphorus supply. Chlorophyll fluorescence showed that the photosynthetic light reactions of low nutrient plants were affected and leaves had decreased chlorophyll content, density of functional photosystems II and dissipated a greater proportion of absorbed energy as heat. Gas exchange parameters showed reduced carboxylation efficiency, rates of RuBP regeneration and light saturated photosynthetic rates, but not quantum yields. Effects on photosynthesis translated into lower plant dry biomass. Mirid herbivory exacerbated the effects of low nutrients noted for chlorophyll fluorescence, gas exchange parameters and biomass accumulation, however, these effects were not always significant and there was no obvious correlation between the level of nutrients supplied and the effect of mirid herbivory. Low nutrient supply did, however, affect mirid performance reducing the number of adult insects, nymphs and herbivory intensity suggesting that in the long-term mirid populations would be significantly affected by water nutrient status.     

Somatic embryogenesis and plant regeneration from leaf and petiole explants of <Emphasis Type="Italic">Campanula punctata</Emphasis> Lam. var. <Emphasis Type="Italic">rubriflora</Emphasis> Makino

A simple and efficient protocol was developed for somatic embryogenesis from leaf and petiole explants of Campanula punctata Lam. var. rubriflora Makino. Somatic embryos (SE) were obtained with greater frequency from petiole explants than from leaf explants when cultured on Murashige and Skoog (MS) medium supplemented with 2.0 mg L⁻¹ 2,4-dichlorophenoxyacetic acid (2,4-D) and 1.0 mg L⁻¹ 6-benzyladenine (BA). On this medium, a mean number of 19.5 and 31.2 SE were developed per leaf and petiole explants, respectively. Embryos were induced both light and dark conditions but culturing the explants 2 weeks in the dark followed by 3 weeks under light resulted in high frequency of embryo formation. Globular embryos germinated best on MS medium supplemented with 0.3% (w/v) activated charcoal (AC) and 1.0 mg L⁻¹ GA₃. The germinated plantlets grew further on MS medium containing 0.3% AC. Plantlets were successfully acclimatized in the greenhouse with 94% survival rate. This is the first report on induction of somatic embryogenesis in this genus and also has implications for genetic transformation, and mass clonal propagation.     

Internal Nitrogen Dynamics in the Graminoid Molinia caerulea Under Higher N Supply and Elevated CO2 Concentrations

Nutrient resorption from senescing leaves is an important aspect of internal plant nutrient cycling. Global environmental change very likely affects this process. In an 8-month experiment, we investigated the effect of increased nitrogen (N) availability and CO₂ concentration on the contribution of leaf N resorption to the internal nitrogen dynamics of the perennial deciduous graminoid Molinia caerulea (L.) Moench. Plants were grown in a factorial combination of two levels of N (65 and 265 N ha⁻¹ year⁻¹) and CO₂ (380 and 700 μL L⁻¹) in a greenhouse. Both N and CO₂ addition increased the total biomass and the total N pools of mature Molinia plants considerably, without a significant interaction. Nitrogen-resorption efficiency from senescing leaves (% of the mature leaf N pool that is resorbed) was neither affected by the N- nor by the CO₂ treatments. When averaged over the treatments, the N-resorption efficiency was 85% ± 1 (SE). The final N concentration in the litter (N-resorption proficiency) was also not affected by the treatments and was on average 3.6 mg N g⁻¹ ± 0.25 (SE). The contribution of resorbed N from senescing leaves to the late seasonal N requirements (seed and stem production and storage of N for next year’s growth) of M. caerulea plants was (negatively) affected by the N treatment only, and no interaction effects with CO₂ were found. Resorption from stems and/or direct reserve and seed formation during growth became relatively more important. Thus, internal N cycling processes in Molinia caerulea are only affected when N availability is increased, but not under elevated CO₂ concentrations. Under high N conditions, this species shifts from a N recycling strategy to reserve formation during growth.     

Effects of ozone exposure on sub-tropical evergreen Cinnamomum camphora seedlings grown in different nitrogen loads

The present study aims in investigating the individual and combined effects of ozone (O₃) exposure and nitrogen (N) load on the growth and photosynthetic characters of Cinnamomum camphora seedlings, a dominant evergreen broadleaf tree species in sub-tropical regions. The seedlings were supplied with N as NH₄NO₃ solution at 0, 30 and 60 kg ha⁻¹ year⁻¹ (simplified as N0, N30, N60, respectively) and were exposed to ambient O₃ concentration (AA) or elevated [O₃] (E-O₃, AA +60 ppb) for one growth season. E-O₃ induced significant negative effects on foliar photosynthesis, including lower photosynthetic rate, reduced carboxylation efficiency, quantum yield of PSII and photosynthetic pigment contents, despite no effect on growth. In contrast, N load acted as fertilization effects. Medium N (N30) increased photosynthetic pigments and stem-base diameter growth relative to N0, whereas high N load (N60) significantly enhanced the growth, photosynthetic pigments, and dark and light action of photosynthesis of C. camphora seedlings. No significant interactive effects of O₃ and N load on the growth, net photosynthetic rate and pigment contents of the seedlings were found, suggesting that N supply to the soil at ≤60 kg ha⁻¹ year⁻¹ does not significantly change the sensitivity of C. camphora to ozone.     

Sulphur protects mustard (<Emphasis Type="Italic">Brassica campestris</Emphasis> L.) from cadmium toxicity by improving leaf ascorbate and glutathione

In a pot-soil culture ameliorative effect of sulphur (S) (0 or 40 mg S kg⁻¹ soil) on cadmium (Cd) (0, 25, 50 and 100 mg Cd kg⁻¹ soil)-induced growth inhibition and oxidative stress in mustard (Brassica campestris L.) cultivar Pusa Gold was studied. Cadmium at 100 mg kg⁻¹ soil caused maximum increase in the contents of Cd and thiobarbituric acid reactive substances (TBARS) in leaves. Maximum reductions in growth (plant dry mass, leaf area), chlorophyll content, net photosynthetic rate (P_N) and the contents of ascorbate (AsA), glutathione (GSH) were observed with 100 mg Cd kg⁻¹ soil compared to control. The application of S helped in reducing Cd toxicity, which was greater for 25 and 50 mg Cd kg⁻¹ soil) compared to 100 mg Cd kg⁻¹ soil. Addition of S to Cd-treated plants showed decrease in Cd and TBARS content in leaves and restoration of growth and photosynthesis through increase in the contents of AsA and GSH. Net photosynthetic rate and plant dry mass were strongly and positively correlated with the contents of AsA and GSH. It is suggested that S may ameliorate Cd toxicity and protects growth and photosynthesis of mustard involving AsA and GSH.     

Effects of elevated CO<Subscript>2</Subscript> and nitrogen on wheat growth and photosynthesis

The effects of nitrogen [75 and 150 kg (N) ha⁻¹] and elevated CO₂ on growth, photosynthetic rate, contents of soluble leaf proteins and activities of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and nitrate reductase (NR) were studied on wheat (Triticum aestivum L. cv. HD-2285) grown in open top chambers under either ambient (AC) or elevated (EC) CO₂ concentration (350 ± 50, 600 ± 50 μmol mol⁻¹) and analyzed at 40, 60 and 90 d after sowing. Plants grown under EC showed greater photosynthetic rate and were taller and attained greater leaf area along with higher total plant dry mass at all growth stages than those grown under AC. Total soluble and Rubisco protein contents decreased under EC but the activation of Rubisco was higher at EC with higher N supply. Nitrogen increased the NR activity whereas EC reduced it. Thus, EC causes increased growth and P_N ability per unit uptake of N in wheat plants, even if N is limiting.     

In vitro growth and single-leaf photosynthetic response of Cymbidium plantlets to super-elevated CO2 under cold cathode fluorescent lamps

To examine the effectiveness of super-elevated (10,000 μmol mol⁻¹) CO₂ enrichment under cold cathode fluorescent lamps (CCFL) for the clonal propagation of Cymbidium, plantlets were cultured on modified Vacin and Went (VW) medium under 0, 3,000 and 10,000 μmol mol⁻¹ CO₂ enrichment and two levels of photosynthetic photon flux density (PPFD, 45 and 75 μmol m⁻² s⁻¹). Under high PPFD, 10,000 μmol mol⁻¹ CO₂ increased root dry weight and promoted shoot growth. In addition, a decrease in photosynthetic capacity and chlorosis at leaf tips were observed. Rubisco activity and stomatal conductance of these plantlets were lower than those of plantlets at 3,000 μmol mol⁻¹ CO₂ under high PPFD, which had a higher photosynthetic capacity. On the other hand, plantlets on Kyoto medium grown in 10,000 μmol mol⁻¹ CO₂ under high PPFD had a higher photosynthetic rate than those on modified VW medium; no chlorosis was observed. Furthermore, growth of plantlets, in particular the roots, was remarkably enhanced. This result indicates that a negative response to super-elevated CO₂ under high PPFD could be improved by altering medium components. Super-elevated CO₂ enrichment of in vitro-cultured Cymbidium could positively affect the efficiency and quality of commercial production of clonal orchid plantlets.     

Does Chloroplast Size Influence Photosynthetic Nitrogen Use Efficiency?

High nitrogen (N) supply frequently results in a decreased photosynthetic N-use efficiency (PNUE), which indicates a less efficient use of accumulated Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). Chloroplasts are the location of Rubisco and the endpoint of CO₂ diffusion, and they play a vital important role in photosynthesis. However, the effects of chloroplast development on photosynthesis are poorly explored. In the present study, rice seedlings (Oryza sativa L., cv. ‘Shanyou 63’, and ‘Yangdao 6’) were grown hydroponically with three different N levels, morphological characteristics, photosynthetic variables and chloroplast size were measured. In Shanyou 63, a negative relationship between chloroplast size and PNUE was observed across three different N levels. Here, plants with larger chloroplasts had a decreased ratio of mesophyll conductance (g_m) to Rubisco content (g_m/Rubisco) and a lower Rubisco specific activity. In Yangdao 6, there was no change in chloroplast size and no decline in PNUE or g_m/Rubisco ratio under high N supply. It is suggested that large chloroplasts under high N supply is correlated with the decreased Rubisco specific activity and PNUE.     

Somatic embryogenesis and plant regeneration in oil palm using the thin cell layer technique

An efficient procedure has been developed for inducing somatic embryogenesis and regeneration of plants from tissue cultures of oil palm (Elaeis guineensis Jacq.). Thin transverse sections (thin cell layer explants) of different position in the shoot apex and leaf sheath of oil palm were cultivated in Murashige and Skoog (MS) (Physiol Plant 15:473–497, 1962) medium supplemented with 0–450 μM picloram and 2,4-D with 3.0% sucrose, 500 mg L⁻¹ glutamine, and 0.3 g L⁻¹ activated charcoal and gelled with 2.5 g L⁻¹ Phytagel. Embryogenic calluses were evaluated 12 wk after inoculation. Picloram (450 μM) was effective in inducing embryogenic calluses in 41.5% of the basal explants. Embryogenic calluses were maintained on a maturation medium composed of basal media, plus 0.6 μM NAA and 12.30 μM 2iP, 0.3 g L⁻¹ activated charcoal, and 500 mg L⁻¹ glutamine, with subcultures at 4-wk intervals. Somatic embryos were converted to plants on MS medium with macro- and micronutrients at half-strength, 2% sucrose, and 1.0 g L⁻¹ activated charcoal and gelled with 2.5 g L⁻¹ Phytagel.     

Nitrogen relations of natural and disturbed plant communities in tropical Australia

Nitrogen relations of natural and disturbed tropical plant communities in northern Australia (Kakadu National Park) were studied. Plant and soil N characteristics suggested that differences in N source utilisation occur at community and species level. Leaf and xylem sap N concentrations of plants in different communities were correlated with the availability of inorganic soil N (NH⁺ ₄ and NO⁻ ₃). In general, rates of leaf NO⁻ ₃ assimilation were low. Even in communities with a higher N status, including deciduous monsoon forest, disturbed wetland, and a revegetated mine waste rock dump, levels of leaf nitrate reductase, xylem and leaf NO⁻ ₃ levels were considerably lower than those that have been reported for eutrophic communities. Although NO⁻ ₃ assimilation in escarpment and eucalypt woodlands, and wetland, was generally low, within these communities there was a suite of species that exhibited a greater capacity for NO⁻ ₃ assimilation. These “high- NO⁻ ₃ species” were mainly annuals, resprouting herbs or deciduous trees that had leaves with high N contents. Ficus, a high-NO⁻ ₃ species, was associated with soil exhibiting higher rates of net mineralisation and net nitrification. “Low-NO⁻ ₃ species” were evergreen perennials with low leaf N concentrations. A third group of plants, which assimilated NO⁻ ₃ (albeit at lower rates than the high-NO⁻ ₃ species), and had high-N leaves, were leguminous species. Acacia species, common in woodlands, had the highest leaf N contents of all woody species. Acacia species appeared to have the greatest potential to utilise the entire spectrum of available N sources. This versatility in N source utilisation may be important in relation to their high tissue N status and comparatively short life cycle. Differences in N utilisation are discussed in the context of species life strategies and mycorrhizal associations. Received: 5 July 1997 / Accepted: 13 July 1998