Photosynthetic Determinants of Growth in Maize Plants: Effects of Nitrogen Nutrition on Growth, Carbon Fixation and Photochemical Features

Maize(Zea mays L.) plants were grown in a greenhouse with differentlevels of nitrate-N (2 to 20 millimolar). Nitrogen nutritionhad dramatic effects on plant growth and photosynthetic characteristicsof mature leaves. Increasing nitrogen resulted in greater biomassproduction, shoot/root ratios, and rates of leaf expansion duringthe day. The elongating zone of high-N plants had higher activities(per gram fresh weight) of sucrose synthase and neutral invertasethan low-N plants, suggesting that increased leaf growth wasrelated to a greater biochemical capacity for sucrose metabolism. Mature leaves of high-N plants had higher rates of photosynthesisand assimilate export (sucrose formation), and partitioned morecarbon into sucrose relative to starch. Increased photosyntheticrates (leaf area basis) were associated with higher levels ofribulose-l,5-bisphosphate carboxylase, phosphoenolpyruvate carboxylaseand pyruvate, phosphate dikinase (determined immunochemically).In addition, N-nutrition affected the functional organizationof chlorophyll in the leaves. Large increases in the numberof PS I reaction centers were observed which fully accountedfor increases in leaf chlorophyll content with increasing nitratesupply. Collectively, the results suggest that increased growth of maizeplants at high light and optimal nitrogen nutrition is relatedto greater capacity for photosynthesis and translocation inmature leaves, and possibly increased capacity for sucrose metabolismin expanding leaves. (Received May 22, 1989; Accepted August 28, 1989)     

A Host-specific, Nitrogen Fixation Mutant of Bradyrhizobium: Physiology on Three Host Plants

NC92 #284 is a transposon mutant of Bradyrhizobium sp. (Arachis)strain NC92 and has a host-specific fixation phenotype. It appearsto be ineffective on the host pigeonpea (90% reduction in shootN compared to that of the wild type), but partially effectiveon two other host plants, groundnut and siratro (50% and 20%reduction in shoot N compared to the wild type, respectively).To understand this phenomenon of host-specific fixation, thephysiological basis of the phenotypes was investigated. Host-dependentdifferences in symbiotic effectiveness were largely explainedby the degree to which nitrogenase activity was impaired inthe various #284 symbioses. Nodulation and the onset of nitrogenfixation were found to be delayed on all three hosts, but tothe greatest degree on pigeonpea. The specific activity of nitrogenaseper gram nodule was also reduced on all three hosts, again tothe greatest extent on pigeonpea. By contrast, the carbon costsand relative efficiencies of each symbiosis were similar tothe wild type. The results indicate that the host-specific fixationphenotype of #284 is reflected in a quantitative reduction inthe amount of N₂ fixed. Thus the phenotypes reflect the differentability of the three host plants to tolerate or support the#284 mutation, rather than a defect in a specific interactionbetween #284 and a particular host plant. Key words: Bradyrhizobium, nodules, nitrogenase activity, relative efficiency, Arachis hypogaea, Cajanus cajan, Macroptilium atropurpureum     

Nitrogen Fixation in the Phyllosphere of the Douglas Fir, Pseudotsuga douglasii

Nitrogen-fixing bacteria have been isolated from the leaf surfacesof the Douglas Fir. ¹⁵N tracer studies have shown that thesebacteria fix nitrogen in culture and in the field on the trees.A considerable proportion of the annual requirement for nitrogenby Douglas Fir may be provided by nitrogen-fixing bacteria onthe leaves and in the soil.     

Occurrence of Phosphate-dissolving Micro-organisms in the Rhizosphere of Rice Plants and in Submerged Soils

S ummary . The incidence of phosphate-dissolving micro-organisms in two submerged tropical rice soils, and in the rhizosphere of the rice plant growing in one of these soils at different stages of plant development was studied. Large numbers of anaerobic and aerobic phosphate-dissolving bacteria were found in both soils after 11 weeks of submergence. Phosphate-dissolving fungi were found in only one of the soils, a latosolic soil. Both anaerobic and aerobic phosphate-dissolving bacteria were stimulated in the rice rhizosphere. However, the rhizosphere effect was greater in the case of the anaerobic phosphate-dissolving bacteria. The rhizosphere effect was greatest at the transplanting stage of rice plant growth but declined with age of the plants. The greatest decline in R/S ratio occurred between transplanting and initial tillering.     

Biological Nitrogen Fixation in Two Tropical Forests: Ecosystem-Level Patterns and Effects of Nitrogen Fertilization

Humid tropical forests are often characterized by large nitrogen (N) pools, and are known to have large potential N losses. Although rarely measured, tropical forests likely maintain considerable biological N fixation (BNF) to balance N losses. We estimated inputs of N via BNF by free-living microbes for two tropical forests in Puerto Rico, and assessed the response to increased N availability using an on-going N fertilization experiment. Nitrogenase activity was measured across forest strata, including the soil, forest floor, mosses, canopy epiphylls, and lichens using acetylene (C₂H₂) reduction assays. BNF varied significantly among ecosystem compartments in both forests. Mosses had the highest rates of nitrogenase activity per gram of sample, with 11 ± 6 nmol C₂H₂ reduced/g dry weight/h (mean ± SE) in a lower elevation forest, and 6 ± 1 nmol C₂H₂/g/h in an upper elevation forest. We calculated potential N fluxes via BNF to each forest compartment using surveys of standing stocks. Soils and mosses provided the largest potential inputs of N via BNF to these ecosystems. Summing all components, total background BNF inputs were 120 ± 29 μg N/m²/h in the lower elevation forest, and 95 ± 15 μg N/m²/h in the upper elevation forest, with added N significantly suppressing BNF in soils and forest floor. Moisture content was significantly positively correlated with BNF rates for soils and the forest floor. We conclude that BNF is an active biological process across forest strata for these tropical forests, and is likely to be sensitive to increases in N deposition in tropical regions.     

Improving the Mineral Nutrition in Grafted Watermelon Plants: Nitrogen Metabolism

Watermelon (Citrullus lanatus [Trumb.] Mansfeld cv. Early Star), was used as scion grafted onto three cultivars of pumpkin (Cucurbita pepo L. cvs. Brava, Shintoza and Kamel) used as rootstocks and ungrafted Early Star plants were used as control. The rootstocks showed a high capacity for N uptake and transport to the scion where N reduction and assimilation improved growth of the scion in grafted plants with respect to the control.     

Occurrence of Plant Growth Inhibitors in Tropical and Subtropical Vegetation

Acetone, ethanol and water extracts of mature fruits of yaupon (llex vomitoria Ait.) inhibited germination of mesquite (Prosopis juliflora Swartz DC. var. glandulosa (Torr.) Cockerell and sorghum (Sorghum bicolor (L.) Moench). Extracts of guava fruit (Psidium guajava L.) inhibited cucumber (Cucumis sativus L.) seed germination. Water soluble inhibitors were found in fruits, leaves, roots and bark of several tropical species representing 10 different families. Strong inhibition of cucumber seed germination and growth did not occur in sand when water extracts containing inhibitors were applied. Growth of corn, sorghum, cucumber and bean was reduced in soils collected beneath Malay apple (Eugenia malaccensis L.) trees. Plant growth-inhibitors occurred in all species studied in various plant parts, and some apparently affect the growth and ecology of other plant species.     

Identification of the Sources of Energy for Nitrogen Fixation and Physiological Characterization of Nitrogen-Fixing Members of a Marine Microbial Mat Community

Experimental manipulations of a microbial mat community were performed to determine sources of energy and reductant used for nitrogen fixation and to physiologically characterize the responsible diazotrophs. The dominant photolithotrophic members of this community were nonheterocystous cyanobacteria, but other potential nitrogen-fixing microorganisms were also present. Pronounced diel variability in rates of acetylene reduction was observed, with nighttime rates a factor of three to four higher than daytime rates. Acetylene reduction measured at night was dependent upon the occurrence of oxygenic photosynthesis the preceding day; mats incubated in the dark during the daytime reduced acetylene at rates comparable to those of light-incubated mats but were not able to reduce acetylene at the normally high rates the following night. The addition of various exogenous carbon compounds to these dark-incubated mats did not elicit nighttime acetylene reduction. Nighttime acetylene reduction apparently proceeds under anoxic conditions in these mats; the highest rates of acetylene reduction occur late at night. Additions of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (an inhibitor of oxygenic photosynthesis) to mats resulted in a pronounced stimulation of acetylene reduction during the day, but acetylene reduction the next night proceeded at greatly reduced rates (relative to untreated mats). This daytime stimulation, under the 3-(3,4-dichlorophenyl)-1,1-dimethylurea-induced anoxic conditions in the experimentally treated mats, was light dependent. These results suggest that nitrogen fixation in these mats may be attributed to the activities of nonheterocystous cyanobacteria utilizing storage products of oxygenic photosynthesis under anoxic conditions at night.     

The Multiple Impacts of Tropical Forest Fragmentation on Arthropod Biodiversity and on their Patterns of Interactions with Host Plants

Tropical rain forest fragmentation affects biotic interactions in distinct ways. Little is known, however, about how fragmentation affects animal trophic guilds and their patterns of interactions with host plants. In this study, we analyzed changes in biotic interactions in forest fragments by using a multitrophic approach. For this, we classified arthropods associated with Heliconia aurantiaca herbs into broad trophic guilds (omnivores, herbivores and predators) and assessed the topological structure of intrapopulation plant-arthropod networks in fragments and continuous forests. Habitat type influenced arthropod species abundance, diversity and composition with greater abundance in fragments but greater diversity in continuous forest. According to trophic guilds, coleopteran herbivores were more abundant in continuous forest and overall omnivores in fragments. Continuous forest showed a greater diversity of interactions than fragments. Only in fragments, however, did the arthropod community associated with H aurantiaca show a nested structure, suggesting novel and/or opportunistic host-arthropod associations. Plants, omnivores and predators contributed more to nestedness than herbivores. Therefore, Heliconia-arthropod network properties do not appear to be maintained in fragments mainly caused by the decrease of herbivores. Our study contributes to the understanding of the impact of fragmentation on the structure and dynamics of multitrophic arthropod communities associated with a particular plant species of the highly biodiverse tropical forests. Nevertheless, further replication of study sites is needed to strengthen the conclusion that forest fragmentation negatively affects arthropod assemblages.     

植物的磷营养：磷酸盐运转系统及其调节

对近年来植物磷营养中有关Pi运转系统及其调节等方面的研究进展进行简要介绍,并对今后有关的研究趋势作了初步展望。     

Tropical Vine Growth and the Effects on Forest Succession: A Review of the Ecology and Management of Tropical Climbing Plants

The climbing habit has evolved independently in many plant taxa, offering vines the ability to compete with non-climbing vegetation for resources such as light, nutrients, and water. This review examines the structural and functional characteristics that allow climbing plants to (1) achieve widespread dispersal, (2) transport large amounts of water throughout vessels, (3) maintain high photosynthesis levels through a large leaf area to biomass ratio, (4) achieve rapid vertical and horizontal expansion by fast growth rates and various climbing mechanisms and (5) survive and recover from disturbances. Due to the competitive effects of vines on trees, management of vine growth is used to preserve tropical timber plantations, combat invasive weeds, and promote rainforest recovery. In order to sustainably manage the vines into the future, it is necessary to understand the mechanisms by which they can alter tropical forest succession and the impacts of various management techniques.     

Identifying Conservation Priority Zones for Effective Management of Tropical Forests in Eastern Ghats of India

There are thousands of protected forest areas existing on earth, yet the deforestation rate continues unabated both inside and outside the protected areas especially in the tropical forests. It identifies the less effectiveness of the current conservation strategies, which is normally oriented around the forest area cover rather than the quality of the protected areas. This calls for realistic and effective management strategies for forests. Based on the drawbacks the present study aims at identifying conservation priority sites within the protected areas (Reserved Forests) of Shervarayan hills, Eastern Ghats of Tamil Nadu, India. The remnant forest patches having less effective management/protection is identified and analysed for its qualitative contribution to the ecosystem. Quadrats of 20 × 20 m were laid in different vegetation based on the percentage of forest cover and assess the species diversity pertaining the richness, Endemism and Red list categories. Thematic layers (maps) such as vegetation type, floristic species richness, floristic endemism, and red list flora are created and categorized according to their weightage classes and overlaid in GIS domain to demarcate the Conservation Priority Zones (CPZ). The CPZ are classified according to the priority status i.e., high, moderate and low based on the contributing species richness, levels of endemism and concentration of Red listed plants.     

Diversity and Activity of PAH-Degrading Bacteria in the Phyllosphere of Ornamental Plants

Phyllosphere bacteria on ornamental plants were characterized based on their diversity and activity towards the removal of polycyclic aromatic hydrocarbons (PAHs), the major air pollutants in urban area. The amounts of PAH-degrading bacteria were about 1–10% of the total heterotrophic phyllosphere populations and consisted of diverse bacterial species such as Acinetobacter, Pseudomonas, Pseudoxanthomonas, Mycobacterium, and uncultured bacteria. Bacterial community structures analyzed by polymerase chain reaction–denaturing gradient gel electrophoresis from each plant species showed distinct band patterns. The uniqueness of these phyllosphere bacterial communities was partly due to the variation in leaf morphology and chemical properties of ornamental plants. The PAH degradation activity of these bacteria was monitored in gas-tight systems containing sterilized or unsterilized leaves. The results indicated that phyllosphere bacteria on unsterilized leaves were able to enhance the activity of leaves for phenanthrene removal. When compared between plant species, phenanthrene removal efficiency corresponded to the size of phenanthrene-degrading bacteria. In addition, phyllosphere bacteria on Wrightia religiosa were able to reduce other PAHs such as acenaphthylene, acenaphthene, and fluorine in 60-ml glass vials and in a 14-l glass chamber. Thus, phyllosphere bacteria on ornamental plants may play an important role in natural attenuation of airborne PAHs in urban areas.     

Reproductive Traits and Phenology of Plants in Tropical Dry Evergreen Forest on the Coromandel Coast of India

Qualitative reproductive traits of 84 plant species belonging to 41 families were studied in tropical dry evergreen forest on the Coromandel coast of India. Majority of species had rotate type, white-coloured, scented flowers, rewarding nectar and pollen and pollinated chiefly by bees. An association between floral traits and pollination spectrum is evident. Bee pollination was prevalent in pollination systems. Among the fruit types, drupe and berry were common in black and red colour respectively, and dispersed by zoochorous mode. Seeds of brown- and green-coloured dry fruits, without any reward were disseminated by wind and explosion. The reproductive phenophase of trees and lianas occurred mostly during the dry period from January to June, which receives rainfall of less than 50 mm a month. However, shrubs showed a peak in flowering and fruiting in wet period. Detailed phenological observations of 22 woody species revealed a seasonal and unimodal pattern in flowering. Although some species were in flower round the year, flowering activity was skewed towards the dry season. The fruiting activity showed a bimodal pattern, one peak in dry season and another in wet season. Many species displayed a temporal aggregation in flowering and fruiting. The significant relation was obtained between reproductive traits and phenology of plants in the tropical dry evergreen forest.     

Growth and Nitrogen Fixation Activity of Soybeans in the Presence of Bean Pod Mottle Virus

Pathological and physiological responses associated with the host-microsymbiont interaction of soybeans (Glycine max‘Franklin’, ‘Dyer’, ‘Centennial’, ‘Marshall’, ‘Williams’, ‘Bedford’, and ‘Peking’), which were nodulated with Rhizobium japonicum (USDA strain 3I1b-110) and inoculated with bean pod mottle virus (BPMV) in the greenhouse, varied with the cultivar and the substrate in which the plants were grown. In plants grown in a soil mixture, the virus significantly reduced canopy growth, nodulation, total N, and ureide-N content of most cultivars but symbiotic N₂ fixation was significantly reduced only for ‘Centennial’ and ‘Peking’. In vermiculite, in which the plants depended almost exclusively on symbiotically fixed N_,2, the virus reduced canopy and nodule mass of some cultivars but these reductions were significant only for ‘Franklin’. In plants grown in this substrate, however, the virus had little effect as increases or decreases in measured components of symbiotic N metabolism were nonsignificant except for the increase in leaf ureides in ‘Marshall’. Results suggest that in either soil or vermiculite the symbiotic process in certain soybean cultivars can function at nearly a normal level despite root noduleinfection with this virus.     

Morphophysiological Indices of the Source Leaf in Wheat Plants Acclimated to Conditions of Nitrogen Nutrition

Growth, leaf and cell morphology, and the chemical composition of the second leaf were studied in wheat (Triticum aestivumL., cv. Inna) plants grown on the medium containing nitrate, ammonium, or no nitrogen at all. Independent of the nitrogen nutrition, the second leaf of the 21-day-old plants matures and functions as a source of assimilates. Both ammonium nutrition and nitrogen deficiency decreased the fresh weight, area, and cell size in the leaf; however, the conditions of nitrogen nutrition did not affect the dry weight of the leaf. Nitrogen starvation increased and ammonium nutrition decreased the relative content of the cell walls in the dry weight. In the nitrate-fed plants, the leaf content of sucrose increased, and the contents of reduced nitrogen (N_red) and protein were lower than in the ammonium treatment. Reciprocally, the contents of reduced nitrogen and protein were highest in the ammonium treatment, the content of sucrose was lowest, with starch practically absent from the leaf. The nitrogen-starved leaf accumulated a large amount of starch, the N_redcontent was two times lower than in the ammonium-fed plants, and the protein content was similar to that in the nitrate-fed plants. Thus, leaf and cell morphology and the content of N_red, protein, and carbohydrate changes in different ways during wheat acclimation to the condition of nitrogen nutrition. By assessing the cell wall weight, the authors established that, depending on nitrogen nutrition, this cell compartment accepts a variable flow of carbon.     

Symbiotic Nitrogen Fixation at the Late Stage of Nodule Formation in Lotus japonicus and Other Legume Plants

Lotus japonicus nodules are reviewed, and current research data on Nod-35 (uricase) gene, including that of Lotus japonicus as an example of a late nodulin gene are presented. Received 5 September 2000/ Accepted in revised form 2 October 2000