ORIGIN OF ADVENTITIOUS SHOOTS REGENERATED FROM CULTURED TOBACCO LEAF TISSUE

Leaf discs from Nicotiana tabacum L., N. glauca Grahm., and a series of interspecific periclinal chimeras were cultured on Murashige and Skoog (MS) medium containing either 2.5 mg/1 6-benzylaminopurine(BA) or 3.0 mg/16-furfurylaminopurine (kinetin). Most shoots regenerated from chimeral leaf discs were non-chimeral but 51 of 658 shoots were chimeral. The histogenic composition of plants regenerated from leaf discs of periclinal chimeras indicated that any cell layer in a leaf can be the ultimate source of shoots, and that shoots can be multicellular in origin. Certain periclinal arrangements were recovered more frequently and their chimeral nature was more stable during subsequent shoot growth. While N. tabacum leaf discs regenerated shoots on MS medium supplemented with either BA or with kinetin, N. glauca leaf discs did not form shoots on the medium containing kinetin. However, chimeras which possessed cells of both species arose on medium containing BA or kinetin, indicating that the morphogenetically competent (i.e., able to produce shoots in culture), N. tabacum cells either interacted with N. glauca cells leading to their competence or incorporated non-competent N. glauca cells into nascent shoot apical meristems.     

Transient expression of <Emphasis Type="Italic">Human papillomavirus</Emphasis> type 16 L2 epitope fused to N- and C-terminus of coat protein of <Emphasis Type="Italic">Potato virus X</Emphasis> in plants

Transient expression of foreign genes based on plant viral vectors is a suitable system for the production of relevant immunogens that can be used for the development of a new generation of vaccines against a variety of infectious diseases. In the present study the epitope derived from HPV-16 L2 minor capsid protein (amino acids 108–120) was expressed from Potato virus X (PVX)-based vector pGR106 as N- or C-terminal fusion with the PVX coat protein (PVX CP) in transgenic Nicotiana benthamiana plants. The fusion protein L2_108-120-PVX CP was successfully expressed in plants at a level of 170 mg/kg of fresh leaf tissue. The C-terminal fusion protein PVX CP- L2_108-120 was expressed using mutated vector sequence to avoid homologous recombination at a level of 8 mg/kg of fresh leaf tissue. Immunogenicity of L2_108-120-PVX CP virus-like particles was tested after immunization of mice by subcutaneous injection or tattoo administration. In animal sera the antibodies against the PVX CP and the L2_108-120 epitope were found after both methods of vaccine delivery.     

AN EVALUATION OF SELECTED PHYSICAL CHARACTERISTICS AND METABOLISM OF ENZYMATICALLY SEPARATED MESOPHYLL CELLS AND MINOR VEINS OF TOBACCO

An enzymatic digestion procedure is presented which allows for the isolation of large quantities of mesophyll cells and minor veins of Nicotiana tabacum L. cv. ‘Wisconsin 38’. Isolated mesophyll cells exhibited CO₂ fixation rates in excess of 150 μmoles CO₂ mg chl^-1hr^-1, while bundles consistently fixed CO₂ at much lower rates, viz. 10–20 μmoles CO₂ mg chl^-1hr^-1. Various physical and biochemical parameters of the isolates have been compared with intact leaf disks.     

Plant regeneration fromBrassicanigra (L.) Koch stem protoplasts

Summary Protoplasts ofBrassica nigra (L.) Koch were isolated from stem peels of bolting racemes and cultured in 1.5 ml of VN1 liquid medium. The protoplasts in the liquid medium were plated on top of half strength MS medium supplemented with 400 mg/liter glutamine, 15 mg/liter glutathione, 50 mg/literl-serine, 0.25 mg/liter 6-benzylaminopurine, 0.5 mg/liter 2,4-dichlorophenoxyacetic acid, 1.5% sucrose, and 5% mannitol, pH, 5.7, solidified with 0.3% agarose. Ten percent of calli obtained from the protoplasts developed into plantlets within 4 wk after transfer onto 2N regeneration medium which contains MS salts plus 200 mg/liter casein hydrolysate, 0.625 mg/liter 6-benzylaminopurine, 0.625 mg/liter kinetin, 0.625 mg/liter 6-(γ,γ-dimethylallylamino)-purine, 0.625 mg/liter zeatin, 0.5 mg/liter 1-naphthaleneacetic acid, 1.5% sucrose, and 0.4% agarose. THis is the first report of plant regeneration fromB. nigra protoplasts.     

Monoclonal antibodies to plant plasma-membrane antigens

Murine monoclonal antibodies to membrane antigens were generated by immunization with a crude cellular membrane preparation from suspension-cultured cells of Nicotiana glutinosa L. From a panel of thirteen monoclonal antibodies, seven were found to be directed against antigens present on the plasma-membrane by immunofluorescence visualization of antibody binding to the surface of isolated protoplasts. The corresponding set of plasma-membrane antigen(s) were present in root, shoot and leaf tissue and some but not all of these antigens were of wide species distribution, being found in Nicotiana tabacum L., N. plumbaginifolia L., Glycine max L., Phaseolus vulgaris L. and Triticum aestivum L. Topologically specific labeling of intact protoplasts with a monoclonal antibody reactive with an epitope present on the plasma-membrane specifically labeled a membrane fraction which equilibrated at a density of 1.14 kg/l following centrifugation in a sucrose gradient. In addition to use as biochemical markers for fractionation and molecular characterization of plasma-membranes, these monoclonal antibodies provide the basis for new selection tools in plant cell and gene manipulations.     

EFFECTS OF ELEVATED CARBON DIOXIDE ON ESTIMATION OF LEAF AREA AND LEAF DRY WEIGHT OF SOYBEAN

The objectives of this study were to determine the effects of elevated CO₂ on relationships between leaf area (A) and linear leaf dimensions (length [L] and width [W]) and leaf dry weight (M) in soybeans (Glycine max (L.) Merr. cv. Bragg). Based on dimensional measurements made on trifoliolates 1–6 for plants grown under three CO₂ levels (348, 502 and 645 μl l^−-1), the best predictor for both trifoliolate leaf area and for fully expanded central leaflets of the trifoliolates was an equation of the form A = b_o + b₁L·W; these relationships were unaffected by CO₂, although there was a small effect of leaf position. For expanding central leaflets of the fifth trifoliolate, no CO₂, leaf size (age) or CO₂ × leaf size effect was found. Specific leaf weight (i.e., M/A) was significantly affected by CO₂, increasing with increasing CO₂. Hence, trifoliolate dry weight can be nondestructively estimated from trifoliolate area using the equation M = 0.097 + (6.71 × 10^−-3 + 1.04 × 10^−-6[CO₂])A, where [CO₂] is mean daytime CO₂ concentration of the growth environment.     

Effects of diet on longevity and fecundity of the spined stilt bug, Jalysus wickhami

We evaluated the suitability of selected food items for the adult spined stilt bug, Jalysus wickhami Van Duzee (Hemiptera: Berytidae), by providing one of seven diet treatments: (1) prepupae of the parasitoid, Cotesia congregata (Say) (Hymenoptera: Braconidae), on a leaf of tobacco (Nicotiana tabacum L., NC 2326) (Solanaceae); (2) C. congregata prepupae alone (i.e., no tobacco leaf); (3) eggs of the parasitoid's host, Manduca sexta (L.) (Lepidoptera: Sphingidae), on a tobacco leaf; (4) M. sexta eggs alone; (5) tobacco aphids, Myzus nicotianae Blackman (Homoptera: Aphididae), on a tobacco leaf; (6) a tobacco leaf alone; and (7) no prey and no tobacco. A tobacco leaf was necessary for the long-term survival of stilt bugs, but prey source did not affect longevity. Regardless of the prey type, stilt bugs with access to a tobacco leaf lived 21–23 days, while stilt bugs without access to tobacco lived less than 12 days. Animal prey provided a protein source required for egg deposition in J. wickhami, and prey species differed in their relative nutritive values. Female stilt bugs that fed on M. sexta eggs or on C. congregata prepupae were significantly more fecund (102 and 106 nymphs per female, respectively) than females that fed on tobacco aphids (24 nymphs per female). Results suggest that stilt bugs may feed on tobacco aphids and C. congregata prepupae when other prey are unavailable.     

EFFECTS OF CO2 ENRICHMENT ON THE GROWTH AND MORPHOLOGY OF A NATIVE AND AN INTRODUCED HONEYSUCKLE VINE

Japanese honeysuckle (Lonicera japonica Thunb.), introduced to the United States, and the native coral honeysuckle (Lonicera sempervirens L.) were compared to determine how intrinsic differences in their growth characteristics would affect their response to atmospheric carbon dioxide enrichment. Plants of both species grown from cuttings were harvested after 54 days of growth in controlled environment growth chambers at 350, 675, or 1,000 μl/liter CO₂. The biomass of Japanese honeysuckle was increased 135% at 675 μ∗∗∗l/liter CO₂ and 76% at 1,000 μl/liter CO₂ after 54 days. Morphologically, the main effect of CO₂ enrichment was to triple the number of branches and to increase total branch length six times. Enhanced and accelerated branchingalso increased total leaf area 50% at elevated CO₂ concentrations. In coral honeysuckle, total biomass was only 40% greater in the elevated CO₂ treatments. Branching was quadrupled but had not proceeded long enough to affect total leaf area. Main stem height was increased 36% at 1,000 μl/liter CO₂. The much less significant height response of other woody erect growth forms suggests that vines may increase in importance during competition if atmospheric CO₂ concentrations increase as predicted. The impact of Japanese honeysuckle in the United States may become more serious.     

Regeneration of Lycium barbarum L. plants from leaf tissue,callus culture and callus protoplasts

The possibility of plant regeneration from leaf tissue, callus and callus protoplasts of Lycium barbarum L. has been studied. Leaf segments were cultured on B5 medium (Gamborg et al. 1968) containing 1.5 mg/1 6-benzylaminopurine and 0.5 mg/1 -naphthaleneacetic acid. Regeneration of shoots was initiated after 30 days of cultivation. Callus was obtained from leaf and internode tissues on MS medium (Murashige and Skoog 1962) containing 0.4 mg/1 of 2,4dichlorophenoxyacetic acid. Subsequently, callus was successfully subcultured on the same medium with 1 mg/l of 2,4-dichlorophenoxyacetic acid and 0.2 mg/l -naphthaleneacetic acid. Organogenesis in callus culture was obtained in the course of 40 days after transferring to TM-4 (Shahin 1984). Protoplasts were isolated from callus tissue grown in vitro using an enzymatic method. Cell colonies, minicallus formation and organogenesis were obtained. Shoots were rooted on Murashige and Skoog medium containing 0..1 mg/l -naphthaleneacetic acid. Regenerated plants were transferred to soil and were grown to maturity. Regenerated plants carried normal morphological traits.Abbreviations BA 6-benzylaminopurine - 2,4-D 2,4-dichlorophenoxyacetic acid - NAA -naphthaleneacetic acid - Zea zeatin - GA₃ gibberellic acid - MS Murashige and Skoog medium - B5 Gamborg medium     

Pathogenesis-related protein b1' in plants and in cell suspension cultures of Nicotiana glutinosa Nicotiana debneyi and an amphidiploid cross (N. glutinosa×N. debneyi)

The expression of PR-protein b₁' in plants and cell suspension cultures of Nicotiana glutinosa L., Nicotiana debneyi Domin, and an amphidiploid cross of these two species, a hybrid, has been investigated. An enzyme linked immunosorbent assay has been employed to determine the concentration of PR-protein b₁' in extracts. The PR-Protein b₁' was constitutively produced in intact plants of the hybrid (around 25 μg g⁻¹ leaf tissue), while only trace amounts of the protein (< 50 ng g⁻¹ leaf tissue) were found in plants of the two parents. In suspension culture, the concentrations of PR-protein b₁' were 8, 0.4 and less than 0.1 mg l⁻¹ medium for the hybrid. N. debneyi and N. glutinosa , respectively. Only trace amounts of the protein were found in extracts from cells. Seven days after infection by tobacco mosaic virus (TMV) the concentration of PR-protein b₁' in leaves of N. glutinosa was 22.5 μg g⁻¹ leaf tissue. In N. debneyi and the hybrid a relatively limited induction of PR-protein b₁' by TMV was observed. The influence of various phenoxyacetic acids on the expression of PR-protein b₁' in the 3 cell cultures has been investigated. Cultures of N. glutinosa responded to treatments with 2,4-D and 2,4,5-T while cultures of N. debneyi and the hybrid were essentially unaffected. In the former case a concentration of 5–10 mg l⁻¹ 2,4,5-T was optimal and cells were most responsive to the treatment 4 days after subcultivation. The concentration of PR-protein b₁' in elicited cell cultures of N. glutinosa was 2 to 4 mg l⁻¹ medium.     

QUANTITATIVE ANALYSIS OF LEAF DEVELOPMENT IN XANTHIUM PENSYLVANICUM

Maksymowych , Roman . (Villanova U., Villanova, Pa.) Quantitative analysis of leaf development in Xanthium pensylvanicum. Amer. Jour. Bot. 46(9): 635–644. Illus. 1959.—An attempt was made to find a quantitative way of describing the development of the leaf and to correlate the developmental processes, designating precisely their sequence. The processes were presented in terms of the absolute and relative rates of leaf length, expansion of lamina in surface, increase in thickness, rates of cell division of leaf 9 and 13, and tissue differentiation of 3 portions of the lamina. All rates were estimated over the entire period of development, from initiation of a primordium to its maturity. The leaf plastochron index (L.P.I.) was used as a morphological time-scale. The relative plastochron rates were used for the purpose of correlation of the developmental processes. Leaf 9 elongates exponentially up to 3.0 L.P.I. with an average relative rate (dlnL/dpl) of about 0.78 pl^-1, and it stops growing around 8.0 L.P.I. The lamina stops elongating about 1.5 plastochrons before the petiole. The tip of the lamina expands its surface at a constantly lower relative rate than the middle and the basal portions of the blade. The average relative rate of expansion in area (dlnA/dpl) for the whole lamina is 1.7pl^-1 during the exponential stage. Differentiation of the laminar tissues proceeds basipetally, from the tip toward the base of the leaf. The relative rate of expansion of lamina in thickness (dlnT/dpl) is 0.55 pl^-1 at 1.5 L.P.I. and after 4.0 L.P.I. all cells cease elongating in a plane perpendicular to the leaf surface. The formation of cells proceeds exponentially up to 3.0 L.P.I. and about this time cell divisions stop in all parts of the lamina. The mean relative rate of cell formation (dlnC/dpl) at the exponential phase is 1.41 pl^-1, an increase of about 31% per day. At least 27 generations of cells are involved in the process of leaf formation and the generation time was calculated to be 0.5 plastochron or 2.2 days.     

AN ANALYSIS OF LEAF ELONGATION IN XANTHIUM PENSYLVANICUM PRESENTED IN RELATIVE ELEMENTAL RATES

Maksymowych , Roman . (Villanova U., Villanova, Pa.) An analysis of leaf elongation in Xanthium pensylvanicum presented in relative elemental rates . Amer. Jour. Bot. 49(1): 7–13. Illus. 1962.—Xanthium plants were grown vegetatively, and leaves, whose developmental stages were specified by a previously described leaf plastochron index (L.P.I.), were marked with India ink along the midrib and photographed during 3 successive days. The relative elemental rates of elongation, d(dX/dpl)/dX were estimated during the whole course of development. The pattern of elongation was not constant but was changing with increasing plastochron age of the leaf. The elements of a young leaf of L.P.I. 0.75 elongated with a constant relative rate. In older leaves, the d(dX/dpl)/dX values were progressively declining toward the tip of the lamina. After L.P.I. 6.3 the only increment in length was due to the elongation of the elements of the petiole. The pattern of growth distribution is discussed in terms of relative elemental rates with respect to cell division and cell elongation in various portions of the lamina and is correlated with the basipetal trend of tissue differentiation in the developing Xanthium leaf.     

STUDIES ON THE FLORAL HISTOGENESIS AND PHYSIOLOGY OF PERILLA. III. EFFECTS OF INDOLEACETIC ACID ON THE FLOWERING OF APICAL BUDS AND EXPLANTS IN CULTURE

Raghavan , V. (Princeton U., Princeton, N. J.) Studies on the floral histogenesis and physiology of Perilla. III. Effects of indoleacetic acid on the flowering of apical buds and explants in culture. Amer. Jour. Bot. 48(10): 870–876. Illus. 1961.—The responses of apical buds and explants of a short-day plant, Perilla frutescens (L.) Britt. var. 'Tall Late,' when grown in vitro in White's medium supplemented with indoleacetic acid (IAA) and subjected to short-days (SD) or long-days (LD), are described. Additions of varying concentrations of IAA to the medium inhibited the flowering of the photoinduced apical buds in 2 ways. There was a progressive delay in the appearance of the first signs at the apex and a gradual transition from the more flower-like structures in lower concentrations of IAA (0.1 mg/liter) to sterile cones in higher doses. The sterile cones had florets with well-developed calyx and corolla lobes, but lacked the sporogenous tissues. When subjected to LD, visible signs were observed only in buds grown in 0.1 and 1.0 mg/liter IAA, the pronounced effect of the auxin being in the step-wise inhibition in the formation of the non-sporogenous tissues of the differentiating florets. Flowering of the explants with the 1st pair of unfolded leaves was also inhibited by IAA in either SD or LD, but the 1st signs appeared relatively faster than in apical buds. When photoinduced, explants with the 1st and 2nd pairs of unfolded leaves flowered in all concentrations of IAA tried (up to 100 mg/liter) while those kept in LD remained entirely vegetative.     

THE MORPHOLOGICAL EFFECTS OF PROTEIN SYNTHESIS INHIBITION IN MARSILEA

Marsilea vestita and M. drummondii were grown in sterile cultures to which concentrations of the protein synthesis inhibitors, 2-thiouracil (10 mg/liter) and 5-fluorouracil (1 mg/liter) had been added. When young sporelings are grown in a solution of thiouracil at optimum concentration, there is an inhibition of the rate of leaf formation, a retardation of the leaf heteroblastic series, and all leaves develop as land forms. When thiouracil is added to plants which are already producing typical adult, quadrifid leaves, the effects depend on whether the treated plants are water or land forms. Plants which are typically water forms convert to land forms. After treatment successive leaves develop typical sunken stomata on both leaf surfaces. The tissues of the rhizome, root and petiole are more compact and, in general, the cells of the plant have thicker walls. Vascular patterns are not changed, though the size of the rhizome, root and petiole may be reduced. Plants which are typically land forms are less affected than the water forms, but they show a small reduction in apex volume and an apparent reversion of the leaflet number from the typical quadrifid leaf to a trifid, bifid, or single lamina condition. In both land and water forms apical dominance may be broken by treatment with 10 mg/liter thiouracil or 1 mg/liter fluorouracil and numerous lateral branches develop. Higher concentrations (15–25 mg/liter of thiouracil) may result in abnormal development of lateral axillary buds, petiole bases and leaflets. The meristems of the plant are differentially sensitive to thiouracil; leaflet meristems are most sensitive, the root meristems are the least sensitive. It appears that a true reversion to juvenile leaf development need not occur even though protein synthesis and the volume of the apex are reduced. The development of the land or water form in Marsilea appears to depend on rate of growth. Hence inhibition of the growth of typical water forms, through inhibition of protein synthesis, causes a shift in development toward the morphology typical of land forms.     

INDUCTION OF ADVENTITIOUS BRANCHES FROM CULTURED CITRUS JUICE VESICLES–A POTENTIAL MEANS OF PROLIFERATION

The noncallusing morphogenetic properties of Citrus juice vesicles cultured in vitro are unknown and were herein studied. Juice vesicles isolated from 120–180-day-old fruits are capable of proliferation via adventitious vesicle branching in vitro. Gibberellic acid levels of 1–100 mg/L greatly enhanced adventitious vesicle branching while delaying vesicle senescence as evidenced by the vesicles' ability to retain its green color for up to 4 to 6 mo in culture. Vesicles grown on media without gibberellic acid readily matured, turning opaque within 2 mo in culture and rarely produced adventitious branches. Additions of 0.1 mg/L α-naphthaleneacetic acid or 1.0 mg/L benzylaminopurine to media containing 10 mg/L gibberellic acid further enhanced vesicle branching in some species and cultivars. Adventitious juice vesicles have been induced from a variety of citrus species, including Citrus grandis (L.) Osb., C. hystrix DC., C. limon (L.) Burm. f., C. medica L., C. paradisi Macf., C. reticulata Blanco, and C. sinensis (L.) Osb. Adventitious vesicle branches originate from primordia initiated on the surface of the cultured vesicle. These primordia commonly occurred on the terminal meristem region of the vesicle. However, some species (e.g., C. grandis and C. paradisi) produced adventitious vesicle branches from their bodies and stalks as well. Distinct vesicle branches begin to appear on preformed vesicles after 30–60 days in culture. A survey to determine the natural occurrence of vesicle branching in various Citrus species was also conducted. Natural vesicle branching commonly occurred in C. grandis, C. paradisi, and in some cultivars of C. reticulata, but was absent in most citrus species and cultivars (e.g., C. aurantifolia (Christm.) Swing., C. canaliculata Hort. ex Y. Tan., C. hystrix, C. limon, C. medica, C. sinensis). It was found that some species which failed to exhibit the adventitious branching phenomenon in nature did so in vitro (e.g., C. hystrix, C. medica, and C. sinensis).     

龙珠果茎段离体培养和组培苗耐盐性分析

为建立龙珠果(Passiflora foetida)的快繁再生体系,以实生苗茎段为外植体,研究了植物生长调节剂对丛生芽诱导、壮苗生根的影响,同时对组培苗的耐盐性进行研究。结果表明,MS+6-BA 0.5 mg/L+NAA 0.05 mg/L培养基有利于诱导丛生芽并促进芽的生长;MS+6-BA 3.0 mg/L+NAA 0.3 mg/L培养基有利于诱导愈伤组织;1/2 MS+IBA 0.2 mg/L培养基适合小芽壮苗生根。组培苗移栽至泥炭土∶蛭石∶珍珠岩(2∶1∶1)的基质中,成活率可达92.6%,且植株生长良好。0～200 mmol/L NaCl处理的组培苗生长不受影响;超过200 mmol/L NaCl处理,植株出现矮化、叶片萎蔫、变黄等现象。随NaCl浓度升高,叶片的SOD活性逐渐升高,POD、CAT和APX活性则呈先升高后降低的趋势。这为龙珠果的种苗繁育、海滨生态修复提供了技术支持。     

Eating the evidence? Manduca sexta larvae can not disrupt specific jasmonate induction in Nicotiana attenuata by rapid consumption

As feeding by the tobacco specialist Manduca sexta L. is known to result in significantly higher jasmonate (JA) concentrations in Nicotiana plants than do mechanical simulations of the larval damage, we investigated whether M. sexta, which is known to rapidly consume large quantities of leaf material, can impair this “recognition” response by consuming the leaf tissue before it can respond with amplified JA levels. We report that oral secretions (OS) from M. sexta, but not from the cabbage specialist Pieris rapae, amplified the wound-induced JA response of Nicotiana attenuata Torr. Ex Wats., regardless of larval diet, instar and molting stage, and were still active after boiling and when diluted to 1/1000. The largest JA response occurred within 40 min in tissues adjacent to the OS application site. When 3 mm of leaf tissue immediately adjacent to the OS application site was excised within 40 s, the signal that elicits JA amplification was found to travel rapidly into the leaf, beyond the mandibular reach of the larvae. We conclude that M. sexta is not able to consume the evidence of feeding activity. Received: 16 July 1999 / Accepted: 12 August 1999     

Decomposition of leaf and root tissue of three perennial grass species grown at two levels of atmospheric CO2 and N supply

Leaf and root tissue of Lolium perenne L., Agrostis capillaris L. and Festuca ovina L. grown under ambient (350 μl l^-1 CO₂) and elevated (700 μl l^-1) CO₂ in a continuously ¹⁴C-labelled atmosphere and at two soil N levels, were incubated at 14°C for 222 days. Decomposition of leaf and root tissue grown in the low N treatment was not affected by elevated [CO₂], whereas decomposition in the high N treatment was significantly reduced by 7% after 222 days. Despite the increased C/N ratio (g g^-1) of tissue cultivated at elevated [CO₂] when compared with the corresponding ambient tissue, there was no significant correlation between initial C/N ratio and ¹⁴C respired. This finding suggests that the CO₂-induced changes in decomposition rates do not occur via CO₂-induced changes in C/N ratios of plant materials. We combined the decomposition data with data on ¹⁴C uptake and allocation for the same plants, and give evidence that elevated [CO₂] has the potential to increase soil C stores in grassland via increasing C uptake and shifting C allocation towards the roots, with an inherent slower decomposition rate than the leaves. An overall increase of 15% in ¹⁴C remaining after 222 days was estimated for the combined tissues, i.e., the whole plants; the leaves made a much smaller contribution to the C remaining (+6%) than the roots (+26%). This shows the importance of clarifying the contribution of roots and leaves with respect to the question whether grassland soils act as a sink or source for atmospheric CO₂. This revised version was published online in June 2006 with corrections to the Cover Date.     

Nicotiana tabacum callus studies. X. ABA increases resistance to cold damage

The effects of cis. trans abscisic acid on response to chilling was investigated in callused Nicotiana tabacum L. pith explants. Explants pretreated with 10^-4M ABA underwent approximately 50% less cellular leakage when chilled at 2°C under short-day conditions for 10 d than the comparable non-treated tissue. Growth in terms of fresh and dry weights, although poor in comparison to non-chilled (20°C, long days) treatments, was more than twice that of the non-ABA-treated material. On an absolute dry weight basis proline content increased on chilling from 0.7 to 3.4 mg g^-1 in non-ABA-treated explants, but rose to nearly 17 mg g^-1 in the tissue treated with ABA. Only in the case of cold-hardened. ABA-treated tissue could some cells survive subzero temperatures and regenerate callus again. It is suggested that at least part of the ameliorating effects of ABA result from an increase in the level of proline.     

Suppression of Plant Growth by Nitrogen Dioxide

Nicotiana glutinosa and pinto bean seedlings (Phaseolus vulgaris) were exposed for short periods (3 days or less) to high concentrations of NO₂ (4.11-20.53 mg/m³ to compare the resulting leaf lesions with ozone damage produced at concentrations of 0.43 to 0.86 mg/m³. Although the same physiological age leaf tissue was damaged by both toxicants, damage caused by NO₂ was unlike that caused by ozone.

Pinto bean (Phaseolus vulgaris) and Pearson improved tomato (Lycopersicon esculentum) seedlings were continuously exposed for 10 to 22 days, to low concentrations of NO₂ (less than 1.03 mg/m³). These exposures caused significant growth suppression, increase in green color (total chlorophyll content), and distortion of leaves.