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PubMed1997

Asymmetric distribution of acetylcholinesterase in gravistimulated maize seedlings.

Acetylcholinesterase (AChE) activity has previously been studied by this laboratory and shown to occur at the interface between the stele and cortex of the mesocotyl of maize (Zea mays L.) seedlings. In this work we studied the distribution of AChE activity in 5-d-old maize seedlings following a gravity stimulus. After the stimulus, we found an asymmetric distribution of the enzyme in the coleoptile, the coleoptile node, and the mesocotyl of the stimulated seedlings using both histochemical and colorimetric methods for measuring the hydrolysis of acetylthiocholine. The hydrolytic capability of the esterase was greater on the lower side of the horizontally placed seedlings. Using the histochemical method, we localized the hydrolytic capability in the cortical cells around the vascular stele of the tissues. The hydrolytic activity was inhibited 80 to 90% by neostigmine, an inhibitor of AChE. When neostigmine was applied to the corn kernel, the gravity response of the seedling was inhibited and no enzyme-positive spots appeared in the gravity-stimulated seedlings. We believe these results indicate a role for AChE in the gravity response of maize seedlings.

Acetylcholinesterase↗
PubMed1971

Reversibility of chilling injury to corn seedlings.

Seedlings of corn (Zea mays) were tested for recovery from chilling injury incurred at 0.3 +/- 0.3 C. At 0.3 C visual leaf injury appeared in 36 hours, whereas stem and root injuries appeared later. Appearance of leaf injury was preceded by a rise in O(2) uptake and a lessened effect of 2,4-dinitrophenol on O(2) uptake by leaf segments and was accompanied by increased ion leakage from the leaves. These effects were reversible, in that transfer of seedlings to 21 C after 36 hours at 0.3 C produced a return of O(2) uptake, 2,4-dinitrophenol stimulation, and ion leakage to the levels of unchilled leaves, as well as a disappearance of leaf symptoms, within 72 hours. For most seedlings, transfer to 21 C after 48 to 60 hours at 0.3 C reversed the chilling effects on O(2) uptake, 2,4-dinitrophenol stimulation, and injury symptoms but not on ion leakage within 108 hours. However, some seedlings collapsed during 48 to 60 hours of chilling, and these never recovered. Transfer to 21 C after 72 hours at 0.3 C did not produce recovery from any symptom of chilling injury examined, and these seedlings soon died. No growth occurred at 0.3 C, but growth began soon after transfer to 21 C. Seedlings chilled 24 or 36 hours grew at reduced rates during the first 72 hours at 21 C, but within 96 hours at 21 C were growing at the same rate as nonchilled seedlings. These results demonstrate considerable capacity of growing plants to recover from short chilling treatments even though significant physiological changes occurred at low temperatures.

Journal Article↗
PubMed1976

Anomalous Temperature Dependence of Seedling Development in Some Soybean (Glycine max [L.] Merr.) Cultivars: Role of Ethylene.

In Clark and Shelby soybean (Glycine max [L.] Merr.) seedlings, hypocotyl elongation was inhibited and hypocotyl swelling and root dry weight were increased by a temperature of 25 C. At 20 and 30 C, development was normal, as was development of Hawkeye and Mandarin soybean seedlings at all three temperatures. Dry matter distribution at 9 days indicates that inhibition of hypocotyl elongation is not due to a lack of translocation from cotyledons, but to a diversion of dry matter from hypocotyl to root. Ethylene evolution by Clark seedlings at 25 C exceeds that at 20 and 30 C. At all three temperatures, Mandarin seedlings' ethylene evolution is at the same low rates as those of Clark at 20 and 30 C. Clark's enhanced rate at 25 C precedes the deceleration of hypocotyl elongation occurring at 5 days. The abnormal effects of a temperature of 25 C on Clark seedlings' development is partially reversed by CO(2) and GA(3) and can be duplicated in Mandarin by applying ethylene. In Clark, effects of the temperature can be further accentuated by indoleacetic acid, which stimulates ethylene evolution, and by applying ethylene to the seedlings. It is concluded that the temperature-induced symptoms, similar in most respects to the well known "triple response" of legume seedlings, are caused by abnormally high levels of ethylene in tissues of the anomalous cultivars.

Journal Article↗
PubMed1978

Role of potassium and malate in nitrate uptake and translocation by wheat seedlings.

Wheat seedlings (Triticum vulgare) treated with 1 mm KNO(3) or NaNO(3), in the presence of 0.2 mm CaSO(4), were compared during a 48-hour period with respect to nitrate uptake, translocation, accumulation and reduction; cation uptake and accumulation; and malate accumulation. Seedlings treated with KNO(3) absorbed and accumulated more nitrate, had higher nitrate reductase levels in leaves but less in roots, accumulated 17 times more malate in leaves, and accumulated more of the accompanying cation than seedlings treated with NaNO(3). Within seedlings of each treatment, changes in nitrate reductase activity and malate accumulation were parallel in leaves and in roots. Despite the great difference in malate accumulation, leaves of the KNO(3)-treated seedlings had only slightly greater levels of phosphoenolpyruvate carboxylase than leaves of NaNO(3)-treated seedlings. NADP-malic enzyme levels increased only slightly in leaves and roots of both KNO(3)- and NaNO(3)-treated seedlings. The effects of K(+) and Na(+) on all of these parameters can best be explained by their effects on nitrate translocation, which in turn affects the other parameters. In a separate experiment, we confirmed that phosphoenolpyruvate carboxylase activity increased about 2-fold during 36 hours of KNO(3) treatment, and increased only slightly in the KCl control.

Journal Article↗
PubMed1980

Photoactivation of Chlorophyll Synthesis and Cytochrome Oxidase Activity in Anaerobically Germinated Seedlings of Echinochloa crusgalli var. Oryzicola.

When seeds of Echinochloa crusgalli var. oryzicola are germinated in dark anaerobic conditions (99.995% N(2)), the seedlings do not have detectable protochlorophyll(ide). Two hours after exposure to light aerobic conditions, they begin to synthesize chlorophyll. The lag in greening is shorter in seedlings exposed to light for 24 hours before exposure to air. Seedlings maintained in light anaerobic conditions exhibit no lag in greening upon transfer to an aerobic environment. Preillumination of anaerobically grown seedlings does not result in any chlorophyll accumulation. Phytochrome is probably the receptor for photoactivation of chlorophyll synthesis, since activation is achieved by red light alone, but not by far red light or red plus far red light. The cytochrome oxidase activity in anaerobically germinated seedlings is 30% of the normal level found in aerobically grown seedlings. Preillumination was also found to activate the ability of anaerobically germinated seedlings to increase their cytochrome oxidase activity upon exposure to air.

Journal Article↗
PubMed1990

Water Deficit and Abscisic Acid Cause Differential Inhibition of Shoot versus Root Growth in Soybean Seedlings : Analysis of Growth, Sugar Accumulation, and Gene Expression.

Roots often continue to elongate while shoot growth is inhibited in plants subjected to low-water potentials. The cause of this differential response to water deficit was investigated. We examined hypocotyl and root growth, polysome status and mRNA populations, and abscisic acid (ABA) content in etiolated soybean (Glycine max [L.] Merr. cv Williams) seedlings whose growth was inhibited by transfer to low-water potential vermiculite or exogenous ABA. Both treatments affected growth and dry weight in a similar fashion. Maximum inhibition of hypocotyl growth occurred when internal ABA levels (modulated by ABA application) reached the endogenous level found in the elongating zone of seedlings grown in water-deficient vermiculite. Conversely, root growth was affected to only a slight extent in low-water potential seedlings and by most ABA treatments (in some, growth was promoted). In every seedling section examined, transfer of seedlings into low-water potential vermiculite caused ABA levels to increase approximately 5- to 10-fold over that found in well-watered seedlings. Changes in soluble sugar content, polysome status, and polysome mRNA translation products seen in low-water potential seedlings did not occur with ABA treatments sufficient to cause significant inhibition of hypocotyl elongation. These data suggest that both variation in endogenous ABA levels, and differing sensitivity to ABA in hypocotyls and roots can modulate root/shoot growth ratios. However, exogenous ABA did not induce changes in sugar accumulation, polysome status, and mRNA populations seen after transfer into low-water potential vermiculite.

Journal Article↗
PubMed1994

Evidence for Chilling-Induced Oxidative Stress in Maize Seedlings and a Regulatory Role for Hydrogen Peroxide.

We have taken advantage of an acclimation phenomenon in a chilling-sensitive maize inbred to investigate the molecular, biochemical, and physiological responses to chilling in preemergent maize seedlings. Three-day-old seedlings were exposed to 4[deg]C for 7 days and did not survive chilling stress unless they were preexposed to 14[deg]C for 3 days. cDNAs representing three chilling acclimation-responsive (CAR) genes were isolated by subtraction hybridization and differential screening and found to be differentially expressed during acclimation. Identification of one of these CAR genes as cat3, which encodes the mitochondrial catalase3 isozyme, led us to hypothesize that chilling imposes oxidative stress in the seedlings. Hydrogen peroxide levels were elevated during both acclimation and chilling of nonacclimated seedlings. Further molecular and biochemical analyses indicated that whereas superoxide dismutase activity was not affected, the levels of cat3 transcripts and the activities of catalase3 and guaiacol peroxidase were elevated in mesocotyls during acclimation. Accumulation of H2O2 following a short treatment with aminotriazole, a catalase inhibitor, indicated that catalase3 seems to be an important H2O2-scavenging enzyme in the seedlings. Control 3-day-old seedlings pretreated with H2O2 or menadione, a superoxide-generating compound, at 27[deg]C induced chilling tolerance. Both of these chemical treatments also increased cat3 transcripts and catalase3 and guaiacol peroxidase activities. We suggest that peroxide has dual effects at low temperatures. During acclimation, its early accumulation signals the production of antioxidant enzymes such as catalase3 and guaiacol peroxidase. At 4[deg]C, in nonacclimated seedlings, it accumulates to damaging levels in the tissues due to low levels of these, and perhaps other, antioxidant enzymes.

Journal Article↗
PubMed2000

Gravimorphogenesis of Cucurbitaceae plants: development of peg cells and graviperception mechanism in cucumber seedlings.

We examined the effect of microgravity on the peg formation of cucumber seedlings for clarifying the mechanism of gravimorphogenesis in cucurbitaceous plants. The spaceflight experiments verified that gravity controls the formation of peg, hypocotyl hook and growth orientation of cucumber seedlings. Space-grown cucumber developed a peg on each side of the transition zone of the hypocotyl and root, indicating that on the ground peg formation is regulated negatively by gravity (Takahashi et al. 2000). It was found that the auxin-regulated gene, CS-IAA1, was strongly expressed in the transition zone where peg develops (Fujii et al. 2000). In the seedlings grown horizontally on the ground, CS-IAA1 transcripts were much abundant on the lower side of the transition zone, but no such differential expression of CS-IAA1 was observed in the space-grown cucumber (Kamada et al. 2000). These results imply that gravity plays a role in peg formation through auxin redistribution. By the negative control, peg formation on the upper side of the transition zone in the horizontally growing seedlings might be suppressed due to a reduction in auxin concentration. The threshold theory of auxin concentration accounted for the new concept, negative control of morphogenesis by gravity (Kamada et al. 2000). Anatomical studies have shown that there exists the target cells destined to be a peg and distinguishable at the early stage of the growth. Ultra-structural analysis suggested that endoplasmic reticulum develops well in the cells of the future peg. Furthermore, it was found that reorganization of cortical microtubules is required for the change in cell growth polarity in the process of peg formation. The spaceflight experiment with cucumber seedlings also suggested that in microgravity positive hydrotropic response of roots occurred without interference by gravitropic response (Takahashi et al. 1999b). Thus, this spaceflight experiment together with the ground-based studies has shown that cucumber seedling is an ideal for the study of gravimorphogenesis, hydrotropism and their interaction. Although peg formation is seen specifically in cucurbitaceous seedlings, it involves graviperception, auxin transport and redistribution and cytoskeletal modification for controlling cell growth polarity. This system could be a useful model for studying important current issues in plant biology.

Cucumis sativus↗
PubMed2001

Heat shock-mediated APX gene expression and protection against chilling injury in rice seedlings.

Rice (Oryza sativa L.) seedlings, when kept at 42 degrees C for 24 h before being kept at 5 degrees C for 7 d, did not develop chilling injury. Chilling resistance was enhanced in parallel with the period of heat-treatment. The level of APX activity was higher in heated seedlings whereas CAT activity was decreased by heat stress. There was no significant difference in SOD activity between heated and unheated seedlings. The elevated activity of APX was sustained after 7 d of chilling. The cytosolic APX gene expression in response to high and low temperature was analysed with an APXa gene probe. APXa mRNA levels increased within 1 h after seedlings were exposed to 42 degrees C. Elevated APXa mRNA levels could also be detected after 24 h of heating. The APXa mRNA level in preheated seedlings was still higher than unheated seedlings under cold stress. The promoter of the APXa gene was cloned from rice genomic DNA by TAIL-PCR, and characterized by DNA sequencing. The promoter had a minimal heat shock factor-binding motif, 5'-nGAAnnTTCn-3', located in the 81 bp upstream of the TATA box. Heat shock induction of the APXa gene could be a possible cause of reduced chilling injury in rice seedlings.

Ascorbate Peroxidases↗
PubMed1971

Cellular changes in wheat seedlings during orbital flight.

Wheat seedlings in weightlessness aboard NASA Biosatellite 2 differ from ground control seedlings in mitotic count, cell length and nuclear volume as well as in orientation, starch grain distribution, organ length and malformations previously reported to COSPAR. Dividing cells are fewer in roots of orbited seedlings than in erect or clinostat ground controls. The greatest difference is among cells in early prophase. Root cells, proximal to the zone of cell division are longer in flight seedlings than erect or clinostat ground controls. As the roots are the same length, greater elongation compensates for the reduced rate of cell division. Volume of interphase nuclei in all seedling organs is increased by orbital flight. In clinostat controls, only nuclei of primary roots increase in size. Between 58 and 65 hours of age, nuclear volume in erect 1 g coleoptiles decreases; in flight seedlings it increases. Simulated launch vibration, alone or followed by growth on the clinostat, increases nuclear volume in some roots, but coleoptile nuclei do not respond as in flight seedlings. Thus, at the cellular level, orbital flight cannot be exactly duplicated by the clinostat whether preceded by vibration or not.

Cell Nucleus↗
PubMed1996

Cucumber chloroplast trnL(CAA) gene: nucleotide sequence and in vivo expression analysis in etiolated cucumber seedlings treated with benzyladenine and light.

The nucleotide sequence of a 714 bp BamHI-EcoRI fragment of cucumber chloroplast DNA was determined. The fragment contained a gene for tRNA(Leu) together with its flanking regions. The trnL(CAA) gene sequence is about 99% in similarity to broad bean, cauliflower, maize, spinach and tobacco corresponding genes. The relative expression level of the gene was determined by Northern (tRNA) gel blot and Northern (total cellular RNA) slot-blot analyses using the trnL gene probe in 6-day old etiolated cucumber seedlings and the seedlings that had been kept in the dark (dark-grown), treated with benzyladenine (BA) and kept in the dark (BA-treated dark-grown), illuminated (light-grown), and treated with BA and illuminated (BA-treated light-grown), for additional 4, 8 or 12 hr. The trnL transcripts and tRNA(Leu) levels in BA-treated dark-grown seedlings were 5 and 3 times higher, respectively after 4 hr BA treatment, while in the BA treated light-grown seedlings the level of trnL transcripts was only 3 times higher and had no detectable effect on mature tRNA(Leu) when compared to the time-4 hr dark-grown seedlings. However, the level of mature tRNA(Leu) did not show marked changes in the light-grown seedlings, whereas the level of trnL transcripts increases 3 times after 8 hr illumination of dark-grown seedlings. These data indicate that both light and cytokinin can signal changes in plastid tRNA gene expression. The possible regulatory mechanisms for such changes are discussed.

Base Sequence↗
PubMed2006

ABA-responsive RNA-binding proteins are involved in chloroplast and stromule function in Arabidopsis seedlings.

The phytohormone abscisic acid (ABA) regulates essential growth and developmental processes in plants. Recently, RNA-binding proteins have been described as components of ABA signaling during germination. We have identified ten ABA-regulated RNA-binding proteins in Arabidopsis seedlings. Among those genes, AtCSP41B and cpRNP29 are highly expressed in seedlings. Using promoter:reporter gene analyses, we showed that both AtCSP41B and cpRNP29 were in particular expressed in photosynthetically active organs like green cotyledons, leaves, and petioles. The analysis of CFP-fusion proteins demonstrates that cpRNP29 localized to chloroplasts and AtCSP41B to chloroplasts and stromules. Whereas RNA-binding of cpRNP29 has previously been shown, we demonstrated through in vitro RNA-binding assays that recombinant AtCSP41B binds to RNA, and that chloroplast petD RNA can serve as a target of AtCSP41B. Developmental or environmental stimuli affected the expression of AtCSP41B and cpRNP29 in seedlings. Both genes were repressed during senescence, but only AtCSP41B was significantly repressed upon water stress. In addition, AtCSP41B and cpRNP29 exhibited low expression in etiolated seedlings compared to green seedlings, and cpRNP29 was regulated during the day photoperiod. Homozygous T-DNA insertion lines were isolated, characterized on the molecular level, and monitored for phenotypic changes. Taken together, the data show that both proteins are regulated during processes that are known to involve ABA signaling. Their localization in chloroplasts and RNA-binding activity suggest a role in chloroplast RNA metabolism in Arabidopsis seedlings.

Abscisic Acid↗
PubMed2006

Patterns of peroxidative ethane emission from submerged rice seedlings indicate that damage from reactive oxygen species takes place during submergence and is not necessarily a post-anoxic phenomenon.

Using ethane as a marker for peroxidative damage to membranes by reactive oxygen species (ROS) we examined the injury of rice seedlings during submergence in the dark. It is often expressed that membrane injury from ROS is a post-submergence phenomenon occurring when oxygen is re-introduced after submergence-induced anoxia. We found that ethane production, from rice seedlings submerged for 24-72 h, was stimulated to 4-37 nl gFW(-1), indicating underwater membrane peroxidation. When examined a week later the seedlings were damaged or had died. On de-submergence in air, ethane production rates rose sharply, but fell back to less than 0.1 nl gFW(-1) h(-1) after 2 h. We compared submergence-susceptible and submergence-tolerant cultivars, submergence starting in the morning (more damage) and in the afternoon (less damage) and investigated different submergence durations. The seedlings showed extensive fatality whenever total ethane emission exceeded about 15 nl gFW(-1). Smaller amounts of ethane emission were linked to less extensive injury to leaves. Partial oxygen shortage (O(2) levels <1%) imposed for 2 h in gas phase mixtures also stimulated ethane production. In contrast, seedlings under anaerobic gas phase conditions produced no ethane until re-aerated: then a small peak was observed followed by a low, steady ethane production. We conclude that damage during submergence is not associated with extensive anoxia. Instead, injury is linked to membrane peroxidation in seedlings that are partially oxygen deficient while submerged. On return to air, further peroxidation is suppressed within about 2 h indicating effective control of ROS production not evident during submergence itself.

Ethane↗
PubMed2003

Patterns of dynamic irradiance affect the photosynthetic capacity and growth of dipterocarp tree seedlings.

In the deeply shaded understorey of S.E. Asian rain forests the growth and survival of dipterocarp seedlings is limited by their ability to maintain a positive carbon balance. Photosynthesis during sunflecks is an important component of carbon gain in understorey plants. To test the sensitivity of photosynthesis and growth to variation in the pattern of dynamic irradiance, dipterocarp tree seedlings (Shorea leprosula and Hopea nervosa) were grown for 370 days under shaded forest light treatments of equal total daily photosynthetic photon flux density (approximately 3.3 mol m(-2) day(-1)), but characterised by either long flecks (LF) or short flecks (SF). Seedling growth was more than 4-fold greater under LF, compared with SF, in both species. Variation in the relative growth rates (RGR) and light saturated rates of photosynthesis (A(max)) were strongly positively correlated with the mean duration of sunflecks. Variation in RGR was strongly correlated with greater unit leaf rate growth, indicating that photosynthetic carbon gain per unit leaf area was greater under LF. The accumulation of starch in leaves over the diurnal period was 117% greater in both species under LF, compared with SF. Greater carbon gain in seedlings under LF is likely to have resulted from the combination of (1) greater A(max) (S. leprosula 35%, H. nervosa 40%), (2) more efficient dynamic photosynthesis, and (3) greater incident photosynthetic quantum yield, compared with seedlings receiving the SF irradiance treatment. The pattern of dynamic irradiance received by seedlings may significantly impact their growth and survival to a previously unrecognised extent, with important consequences for regeneration processes and hence forest structure and composition.

Adaptation, Physiological↗
PubMed2003

Leaf display and photosynthesis of tree seedlings in a cool-temperate deciduous broadleaf forest understorey.

To examine a possible convergence in leaf photosynthetic characteristics and leaf display responses to light environment in seedlings of three canopy and two shrub tree species in understorey of cool-temperate deciduous broadleaf forest, relationships between light environment, leaf orientation and leaf light-photosynthetic response were measured. Light capture of the seedlings (17-24 individuals with 2-12 leaves for each species) was assessed with a three dimensional geometric modeling program Y-plant. Leaf photosynthetic characteristics of the five species were found to have acclimated to the understorey light environment, i.e., low light compensation point and high apparent quantum yield. In addition, light-saturated photosynthetic rates were higher in seedlings inhabiting microsites with higher light availability. Efficiencies of light capture and carbon gain of the leaf display were evaluated by simulating the directionalities of light capture and daily photosynthesis for each seedling using hemispherical canopy photography. The results showed that most of the seedlings orientated their leaves in a way to increase the daily photosynthesis during the direct light periods (sunflecks) rather than maximize daily photosynthesis by diffuse light. Simulations also showed that daily photosynthesis would increase only 10% of that on actual leaf display when the leaves orientated to maximize the diffuse light interception. Simulations in which leaf orientations were varied showed that when the leaf display fully maximized direct light interception, the time that leaves were exposed to excessive photon flux density of >800 mumol photons m(-2) s(-1) were doubled. The understorey seedlings studied responded to the given light environments in a way to maximize the efficiency of acquisition and use of light during their short (approximately 3 month) seasonal growth period.

Computer Simulation↗
PubMed2004

Effects of dung and seed size on secondary dispersal, seed predation, and seedling establishment of rain forest trees.

Seeds dispersed by tropical, arboreal mammals are usually deposited singly and without dung or in clumps of fecal material. After dispersal through defecation by mammals, most seeds are secondarily dispersed by dung beetles or consumed by rodents. These post-dispersal, plant-animal interactions are likely to interact themselves, as seeds buried by dung beetles are less likely to be found by rodents than unburied seeds. In a series of three experiments with seeds of 15 species in central Amazonia (Brazil), we determined (1) how presence and amount of dung associated with seeds influences long-term seed fate and seedling establishment, (2) how deeply dung beetles bury seeds and how burial depth affects seedling establishment, and (3) how seed size affects the interaction between seeds, dung beetles, and rodents. Our overall goal was to understand how post-dispersal plant-animal interactions determine the link between primary seed dispersal and seedling establishment. On average, 43% of seeds surrounded by dung were buried by dung beetles, compared to 0% of seeds not surrounded by dung ( n=2,156). Seeds in dung, however, tended to be more prone than bare seeds to predation by rodents. Of seeds in dung, probability of burial was negatively related to seed size and positively related to amount of dung. Burial of seeds decreased the probability of seed predation by rodents three-fold, and increased the probability of seedling establishment two-fold. Mean burial depth was 4 cm (0.5-20 cm) and was not related to seed size, contrary to previous studies. Probability of seedling establishment was negatively correlated with burial depth and not related to seed size at 5 or 10 cm depths. These results illustrate a complex web of interactions among dung beetles, rodents, and dispersed seeds. These interactions affect the probability of seedling establishment and are themselves strongly tied to how seeds are deposited by primary dispersers. More generally, our results emphasize the importance of looking beyond a single type of plant-animal interaction (e.g., seed dispersal or seed predation) to incorporate potential effects of interacting interactions.

Animals↗
PubMed2004

Microhabitat associations and seedling bank dynamics in a neotropical forest.

We conducted a rigorous test of tropical tree seedling microhabitat differentiation by examining microhabitat associations, survival and growth of established seedlings of ten tropical tree species representing a four-factor gradient in seed size. Eight microhabitat variables describing soil and light conditions were measured directly adjacent to each of 588 seedlings within twelve 10x100 m belt transects at Paracou, French Guiana, and at 264 reference points along the transects. From these measurements, we defined three principal components describing soil richness, soil softness and canopy openness. Six of ten species (in 9 of 30 total cases) were distributed non-randomly with respect to microhabitat along at least one principal component. However, few species demonstrated clear microhabitat specialization. All shifts in distribution relative to reference points were in the same direction (richer, softer soil). Furthermore, of 135 pairwise comparisons among the species, only 7 were significantly different. More than three-fourths of all seedlings (75.3%) survived over the 2-year monitoring period, but survival rates varied widely among species. In no case was the probability of survival influenced by any microhabitat parameter. Relative height growth rates for the seedlings over 2 years varied from -0.031 cm cm(-1) year(-1) (Dicorynia guianensis, Caesalpiniaceae) to 0.088 cm cm(-1) year(-1) (Virola michelii, Myristicaceae). In only 4 of 30 cases was height growth significantly associated with one of the three principal components. Because the conditions in this study were designed to maximize the chance of finding microhabitat differentiation among a group of species differing greatly in life history traits, the lack of microhabitat specialization it uncovered suggests that microhabitat partitioning among tropical tree species at the established seedling stage is unlikely to contribute greatly to coexistence among these species.

Ecosystem↗
PubMed2005

Interaction between cadmium and atrazine during uptake by rice seedlings (Oryza sativa L.).

The uptake of atrazine by rice seedlings (Oryza sativa L.) through plant roots from nutrient solution was investigated in the presence and absence of Cd2+ over an exposure period of four weeks. It was found that both atrazine and Cd2+ were toxic to rice seedlings. Both shoot and root biomasses decreased when the seedlings were exposed to increasing atrazine or Cd2+ concentrations in nutrient solutions. In the absence of Cd2+, a linear relationship was observed between atrazine concentrations in roots/shoots and in external solution, and more atrazine is concentrated in roots than in shoots. When atrazine and Cd2+ concentrations in solution were maintained at mole ratio of 1:1, the accumulation of atrazine by seedlings was less and the seedling biomass was greater than found with other ratios, such as 1:2 or 2:1. Therefore, the formation of the complex between atrazine and Cd2+ reduced the individual toxicities. Analyses of data with the quasi-equilibrium partition model indicated that the atrazine concentrations in rice seedlings and external water were close to equilibrium. In the presence of Cd2+, however, the measured bioconcentration factor (BCF) of atrazine with roots and shoots were considerably greater. The latter findings resulted presumably from the atrazine-Cd2+ complex formation that led to a large apparent BCF.

Atrazine↗