An allelopathic substance in red pine needles (Pinus densiflora)

Aqueous methanol extracts of red pine (Pinus densiflora) needles inhibited the growth of roots and shoots of cress (Lepidium sativum), lettuce (Lactuca sativa), alfalfa (Medicago sativa), ryegrass (Lolium multiflorum), timothy (Pheleum pratense), Digitaria sanguinalis and Echinochloa crus-galli. Increasing the extract concentration increased inhibition, suggesting that the pine needles may have growth inhibitory substances and possess allelopathic potential. The aqueous methanol extract of the pine needles was purified, and a main inhibitory substance was isolated and determined by spectral data as 9alpha,13beta-epidioxyabeit-8(14)en-18-oic acid. This substance inhibited root and shoot growth of cress and Echinochloa crus-galli seedlings at concentrations greater than 0.1 mM. The endogenous concentration of the substance was 0.13 mmol/kg pine needle. These results suggest that 9alpha,13beta-epidioxyabeit-8(14)en-18-oic acid may contribute to the growth inhibitory effect of the pine needles and may play an important role in the allelopathy of red pine.     

Isolation and identification of a plant growth inhibitor from Tinospora tuberculata Beumee

Tinospora tuberculata Beumee has been used widely as a folk medicine and several bioactive compounds have been isolated. However, no herbicidal compound has been reported in this species. Therefore, we investigated the presence of phytotoxins in T. tuberculata. The aqueous methanol extracts of T. tuberculata inhibited the growth of roots and shoots of cress (Lepidum sativum L.), lettuce (Lactuca sativa L.), timothy (Phleum pratense L.) and barnyard grass (Echinochloa crus-galli (L.) Beauv.). The extract was then purified by several chromatographic runs with monitoring the inhibitory activity and the main phytotoxic substance was isolated. The chemical structure of the compound was determined by spectral data as syringin (4-[(1E)-3-Hydroxyprop-1-en-1-yl]-2,6-dimethoxyphenyl β-d-glucopyranoside). It inhibited the root and shoot growth of all test plant species at concentrations >10 µM. The concentrations required for 50 % inhibition of root and shoot growth of cress and lettuce ranged from 78.2 to 412 μM, and that of timothy and barnyard grass renged from 9.8 to 73.2 µM. Effectiveness of syringin on monocotyledonous (timothy and barnyard grass) plants was greater than that on dicotyledonous (cress and lettuce) plants. These results suggest that syringin may contribute to the allelopathic effect caused by the T. tuberculata extract.     

Allelopathy of red pine: isolation and identification of an allelopathic substance in red pine needles

Since red pine (Pinus densiflora Sieb. et Zucc.) often forms sparse forest floors where herbaceous plants do not grow well, allelopathy of red pine was investigated. A growth inhibitory substance was isolated from an aqueous methanol extract of red pine needles and determined by spectral data as abscisic acid-β-d-glucopyranosyl ester (ABA-GE). This substance inhibited root and shoot growth of cress and E. crus-galli seedlings at concentrations greater than 0.1 μM. The concentrations required for 50% growth inhibition on roots and shoots of cress were 0.23 and 0.61 μM, respectively, and those of E. crus-galli were 1.1 and 2.8 μM, respectively. The activity of ABA-β-d-glucosidase, which liberates free ABA from ABA-GE, in cress and E. crus-galli seedlings was 13–29 nmol mg⁻¹ protein min⁻¹. Endogenous concentration of ABA-GE in the pine needles was 4.1–21.5 μmol kg⁻¹ and the concentration in soil water of the pine forest was 2.5 μM. The effectiveness of ABA-GE on growth inhibition and the occurrence of ABA-GE in pine needles and soil water suggest ABA-GE may play an important role in the allelopathy of red pine resulting in the formation of sparse forest floors.     

Isolation and identification of two potential phytotoxic substances from the aquatic fern <Emphasis Type="Italic">Marsilea crenata</Emphasis>

Organic agriculture emphasized using of biologically originated herbicides and phytotoxic substances are being considered as a replacement to chemical herbicides. Marsilea crenata is an aquatic perennial fern distributed in the South-East Asian countries and is well known for various biological properties. However, to date, there has been no report that addresses the phytotoxicity of Marsilea crenata. Therefore, we explored phytotoxic properties and phytotoxic substances from Marsilea crenata. An aqueous methanol extracts of Marsilea crenata showed inhibition on the seedling growth of cress, lettuce, alfalfa, barnyard grass, Italian ryegrass, and foxtail fescue. Inhibition increased with increasing extract concentration. The extract was purified by several chromatographic steps and two phytotoxic substances were isolated and identified by spectroscopic analysis as loliolide and isololiolide. At the concentration of 30 μM, loliolide and isololiolide inhibited seedling growth of cress and barnyard grass by 41.3 to 51.1%, and 58.15 to 87.5% of control seedlings, respectively. The concentrations required for 50% inhibition of cress and barnyard grass seedlings ranged from 32.1 to 128.5 μM for loliolide, 37.0 to 176.2 μM for isololiolide. These results suggest these compounds may be responsible for phytotoxic effects of Marsilea crenata extract and could be an important part of organic agriculture.     

Isolation and identification of a potent allelopathic substance in Bangladesh rice

Aqueous methanol extracts of Bangladesh rice (Oryza sativa L. cv. BR17) inhibited the growth of roots and shoots of cress (Lepidium sativum), lettuce (Lactuca sativa), alfalfa (Medicago sativa), timothy (Phleum pratense), Digitaria sanguinalis, Echinochloa crus-galli and Echinochloa colonum. Increasing the extract concentration increased the inhibition, suggesting that the BR17 may have growth inhibitory substances and possess allelopathic potential. The aqueous methanol extract of the BR17 was purified and a main inhibitory substance was isolated and determined by spectral data as 2,9-dihydroxy-4-megastigmen-3-one. This substance inhibited root and shoot growth of cress and E. crus-galli seedlings at concentrations greater than 0.03 and 3 μM, respectively. The concentrations required for 50% growth inhibition on cress roots and shoots were 0.22 and 0.47 μM, respectively, and on E. crus-galli roots and shoots were 36 and 133 μM, respectively. These results suggest that 2,9-dihydroxy-4-megastigmen-3-one may contribute to the growth inhibitory effect of BR17 and may play an important role in the allelopathy of BR17. Thus, Bangladesh rice BR17 may be potentially useful for weed management in a field setting. An erratum to this article can be found at     

Effect of plant growth regulators on plant regeneration of Dioscorea remotiflora (Kunth) through nodal explants

Dioscorea remotiflora (Kunth) is an important wild plant that produces tuberous roots used as a source of food in the Western part of Mexico. Lack of planting material and inefficiency of traditional methods of propagation are the main constraints for implementing large-scale cultivation. In contrast, tissue culture techniques allow increasing multiplication and rapid production of plant material. In this regard, leaves or nodal segments were incubated on MS, B5 and WPM culture media with different PGRs in order to obtain an efficient micropropagation protocol. Leaves explants were unable to inducing shoots or callus. However, nodal segments produced axillary shoots and/or callus in all culture media. MS containing 2.33???M KIN was the most suitable to inducing shoots; an average of 6.6 shoots per segment for 100?% explants was obtained, which displayed also the greater number of nodes (5.0) and leaves (7.9) per segment. A decrease on shoot proliferation was observed combining BA or KIN with 2,4-D or NAA. However, small brownish callus were induced on 100?% of segments using 2.33???M KIN with 5.37???M 2,4-D or 9.30???M KIN plus 2.69???M NAA. In contrast, by adding 2.69???M NAA, 66.4?% of the nodal segments formed shoots and produced also yellowish friable callus on the base of the shoots. Shoots were easily rooted with 8.28???M IBA (96.9?%), displaying the greatest root and shoot biomass, but maximum number of tuberous roots, and root or tuberous root biomass was produced increasing IBA (20.7???M).     

Changes in plant growth processes under microgravity conditions simulated by a three-dimensional clinostat

We developed a three-dimensional (3-D) clinostat to simulate a microgravity environment and studied the changes in plant growth processes under this condition. The rate of germination of cress (Lepidium sativum), maize (Zea mays), rice (Oryza sativa), pea (Pisum sativum), or azuki bean (Vigna angularis) was not affected on the clinostat. The clinostat rotation did not influence the growth rate of their roots or shoots, except for a slight promotion of growth in azuki roots and epicotyls. On the contrary, the direction of growth of plant organs clearly changed on the 3-D clinostat. On the surface of the earth, roots grow downward while shoots upward in parallel to the gravity vector. On the 3-D clinostat, roots of cress elongated along the direction of the tip of root primordia after having changed the direction continuously. Rice roots also grew parallel to the direction of the tip of root primordia. On the other hand, roots of maize, pea, and azuki bean grew in a random fashion. The direction of growth of shoots was more controlled even on the 3-D clinostat. In a front view of embryos, shoots grew mostly along the direction of the tip of primordia. In a side view, rice coleoptiles showed an adaxial (toward the caryopsis) while coleoptiles of maize and epicotyls of pea and azuki bean an abaxial curvature. The curvature of shoots became larger with their growth. Such an autotropism may have an important role in regulation of life cycle of higher plants under a microgravity environment.     

Toxicity of Cd to signal grass (Brachiaria decumbens Stapf.) and Rhodes grass (Chloris gayana Kunth.)

Given that Cd accumulates within plant tissues to levels that are toxic to animals, it is necessary to understand the role of plants in highly Cd-contaminated systems and their subsequent impact on the health of animals. A solution culture experiment was conducted to elucidate the effects of increasing Cd²⁺ activity ({Cd²⁺}) on growth of Rhodes grass (Chloris gayana Kunth.) and signal grass (Brachiaria decumbens Stapf.). The shoot and root fresh mass of both Rhodes grass and signal grass was reduced by 50% at ca. 0.5 µM {Cd²⁺}. Elevated {Cd²⁺} resulted in a significant decrease in the tissue Mn concentration for both the shoots and roots, and caused a chlorosis of the veins in the shoots. Root hair growth was prolific even at high {Cd²⁺}, thus root hair growth appeared to be less sensitive to elevated Cd than was root growth per se. The critical shoot tissue concentrations (50% reduction in growth), 230 µg g^?1 for Rhodes grass and 80 µg g^?1 for signal grass, exceeded the maximum level of Cd tolerated in the diet of animals (ca. 5 µg g^?1). When assessing the risk associated with the revegetation of Cd-contaminated sites with Rhodes grass or signal grass, careful consideration must be given, therefore, to the transfer of toxic concentrations of Cd to grazing animals and through the wider food chain.     

Allelopathy and the allelothathic activity of a phenylpropanol from cucumber plants

The growth inhibitory effect of cucumber (Cucumis sativus L.) plants after crop harvested was investigated. Aqueous methanol extracts of the cucumber plants inhibited the growth of roots and shoots of cress (Lepidium sativum L.), lettuce (Lactuca sativa L.), alfalfa (Medicago sativa L.), ryegrass (Lolium multiflorum L.), timothy (Pheleum pratense L.), crabgrass (Digitaria sanguinalis L.), Echinochloa crus-galli (L.) Beauv and Echinochloa colonum (L.) Link, and increasing the extract concentration increased the inhibition. These results suggest that cucumber plants may possess allelopathic activity. The aqueous methanol extract of cucumber plants was divided into ethyl acetate and aqueous fractions, and the growth inhibitory activity of ethyl acetate fraction was greater than that of aqueous fraction. Thus, ethyl acetate fraction was further purified and a main allopathically active substance in the fraction was isolated and determined as (S)-2-benzoyloxy-3-phenyl-1-propanol by spectral data. This substance inhibited root and shoot growth of cress seedlings at concentrations greater than 10 μM, and the concentration required for 50% inhibition of root and shoot growth was 21 and 23 μM, respectively. These results suggest that (S)-2-benzoyloxy-3-phenyl-1-propanol may contribute to the growth inhibitory effect of cucumber plants and may play an important role in cucumber allelopathy. Thus, cucumber plants may be potentially useful for weed management in a field setting. An erratum to this article can be found at     

Mechanism for the differential translocation of amiben in plants

The proportion of the total plant radioactivity present in shoots at the end of a 24-hour exposure of the roots to 0.5 milligram per liter ¹⁴C-3-amino-2,5-dichlorobenzoic acid (¹⁴C-amiben) ranged from 1.4 to 74.3% in 13 species. When roots of 10-day-old wheat (Triticum aestivum L. em. Thell., Triumph) and 13-day-old barnyard grass (Echinochloa crusgalli L. Beauv.) plants were treated with 0.5 milligram per liter ¹⁴C-amiben for 12 or 24 hours, barnyard grass shoots contained at least eight times more of the total plant radioactivity than did wheat shoots. In similar experiments with ¹⁴C-2-chloro-4-(ethylamino)-6-(isopropylamine)-s-triazine (¹⁴C-atrazine), there were no differences in translocation between these two species.     

Allelopathic Substance Exuded from a Serious Weed, Germinating Barnyard Grass (Echinochloa crus-galli L.), Roots

The allelopathy of a serious weed, barnyard grass (Echinochloa crus-galli L.), was investigated. Root exudates of young barnyard grass showed allelopathic effects and plant-selective activity and inhibited root elongation of all plants tested. With respect to shoot growth, the exudates did not show inhibition of barnyard grass only. The allelopathic substance was isolated and identified as p-hydroxymandelic acid by NMR. p-Hydroxymandelic acid strongly inhibited shoot growth and root elongation of all plants tested. The effects of three congeners of p-hydroxymandelic acid were tested on rice shoot growth. In the biological activity exhibited in rice, shoot growth was related to the hydroxyl groups. Received October 7, 1998; accepted March 29, 1999     

Enhanced in vitro multiplication of Nothapodytes nimmoniana Graham using semisolid and liquid cultures and estimation of camptothecin in the regenerated plants

Nothapodytes nimmoniana (Icacinaceae) yields camptothecin (isoquinoline alkaloid) which is a potent anti-cancer drug. The major objectives of the present study were to develop an efficient protocol for mass propagation of N. nimmoniana using liquid medium and to compare regeneration with semisolid cultures; as also to quantify the amount of camptothecin in regenerated plants. Adventitious shoots were induced from the callus derived from nodal explants on semisolid and liquid Murashige and Skoog (MS) medium supplemented with 1.0, 2.0, 5.0 and 10.0???M 6-benzylaminopurine or kinetin or 2-isopentenyl adenine (2-iP). The highest number of adventitious shoots was regenerated on medium supplemented with 2.0???M BAP. Compared to semisolid medium (41.9 shoots per explant), liquid medium (165.9 shoots per explant) was found suitable for shoot induction and shoot multiplication. Shoots were rooted on MS semisolid medium of one-fourth strength containing IBA (2.4???M) and IAA (5.7???M). The plantlets were acclimatized in a growth chamber at 25°C, 60% relative humidity, with 16-h photoperiod (40???mol?m^?2?s^?1). The camptothecin content was determined in ex vitro plants using HPLC. The analysis revealed that the leaves and stems of ex vitro plants had a considerable amount of camptothecin and these plants could be used as a raw material for camptothecin extraction.     

The chemical-mediated allelopathic interaction between rice and barnyard grass

Aims

The possible involvement of the chemical-mediated interaction in allelopathy between rice and barnyard grass was investigated.

Methods

Effcts of rice seedlings and rice root exudate on the alleloapthic activity of barnyard grass were determined and a key compound invovled in the allelopathic interaction between rice and barnyard grass was isolated.

Results

Allelopathic activity of barnyard grass was increased by the presence of rice seedlings. Rice root exudates also elevated the allelopahtic activity of barnyard grass. A key compound, which increased the allelopathic activity of barnyard grass, in the rice root exudates was isolated and determined as momilactone B. Momilactone B increased the allelopathic activity of barnyard grass at concentrations greater than 3 μM, and increasing the momilactone B concentration increased the activity.

Conclusions

Momilactone B is known to act as a potent rice allelochemical and to possess strong growth inhibitory activity against barnyard grass. The present research suggests that barnyard grass may response to the presence of neighboring rice by sensing momilactone B in rice root exudates and increase allelopathic activity. Thus, momilactone B may not only act as a rice allelochemical but also play an important role in rice-induced allelopathy of barnyard grass. The induced-allelopathy may provide a competitive advantage for barnyard grass through the growth inhibition of competing plant species including rice. Barnyard grass allelopathy may be one of the inducible defense mechanisms by chemical-mediated plant interaction between rice and barnyard grass. Rice allelopathy was also reported to be increased by the presence of barnyard grass through increasing production and secretion of momilactone B into surrounding environments. During the evolutional process, rice and barnyard grass may have developed the chemical cross talk to activate the defense mechanisms against some biotic stress conditions by detection of certain key compounds.     

Phytotoxic substances with allelopathic activity may be central to the strong invasive potential of Brachiaria brizantha

The grass Brachiaria brizantha, native to eastern Africa, becomes naturalized and dominant quickly in the non-native areas. It was hypothesized that phytotoxic chemical interaction between this plant and native plants may play an important role in the invasion of B. brizantha. However, no potent phytotoxic substance has been reported in this species. Therefore, we investigated possible allelopathic activity and searched for phytotoxic substances with allelopathic activity in B. brizantha. An aqueous methanol extract of B. brizantha inhibited the growth of roots and shoots of garden cress (Lepidium sativum), lettuce (Lactuca sativa), timothy (Phleum pratense) and ryegrass (Lolium multiflorum) seedlings. The extract was purified by several chromatographic runs and three allelopathically active substances were isolated and identified by spectral analysis as (6R,9R)-3-oxo-α-ionol, (6R,9S)-3-oxo-α-ionol and 4-ketopinoresinol. (6R,9R)-3-Oxo-α-ionol and (6R,9S)-3-oxo-α-ionol inhibited root and shoot growth of garden cress at concentrations greater than 30 and 10 μM, respectively. The activity of (6R,9S)-3-oxo-α-ionol was 5.3- to 6.2-fold that of (6R,9R)-3-oxo-α-ionol. The stereochemistry of the hydroxyl group at position C-9 may be important for the inhibitory activities of those compounds. 4-Ketopinoresinol inhibited root and shoot growth of garden cress at concentrations greater than 30 μM. The growth inhibitory activity of (6R,9S)-3-oxo-α-ionol was the greatest and followed by 4-ketopinoresinol and (6R,9R)-3-oxo-α-ionol. These results suggest that those phytotoxic substances may contribute to the allelopathic effect caused by B. brizantha and may be involved in the invasion of B. brizantha.     

Water relations of pine seedlings in relation to root and shoot growth

The effects of water stress on growth and water relations of loblolly and white pine seedlings were studied during series of drying cycles. As mean soil water potential decreased, growth of roots, needles, and buds decreased. Growth of roots during successive severe drying cycles was not uniform, however. A study of needle and root extension showed that of the total growth of roots for 3 7-day drying cycles, only 6% occurred during the third cycle, while needle extension was uniform for the 3 cycles. The difference in response of needles and roots to drying cycles may be attributed primarily to the effect of water stress on the growing region. When subjected to a severe stress, roots matured toward the tip and became dormant, resulting in less growth during subsequent drying cycles. The intercalary growing region of needles, however, was not altered seriously enough by the stress to cause a difference in amount of growth during each drying cycle.

Transpiration of loblolly pine was lower in the second drying cycle than in the first. Needle water potential after rewatering was as high as that of control plants watered daily; root resistance was apparently not important in restricting transpiration during a second drying cycle. Needle diffusion resistance of loblolly pine, measured with a low-resistance diffusion porometer, was slightly higher during the second drying cycle than during the first. In addition, many primary needles were killed during the first period of stress. These factors contributed to the reduction of transpiration during the second drying cycle. Diffusion resistance of Coleus increased and transpiration ceased during the first drying cycle while water potential remained relatively high. After rewatering, both leaf resistance and transpiration returned to the control level, presumably because the stress during the first period of drying was not severe. The diffusion resistances observed for well-watered plants were 30 to 50 sec·cm⁻¹ for loblolly pine, 3 to 5 sec·cm⁻¹ for Coleus, and 4 to 6 sec·cm⁻¹ for tomato. These values agree closely with those reported by other workers.

     

香茅天然挥发物的化感作用及其化学成分分析

在野外香茅草丛地表和密闭容器中进行了香茅天然挥发物对玉米和稗草种子萌发和幼苗生长影响的试验,并采用固相微萃取和气相色谱质谱联用技术,对香茅挥发物的化学成分进行了分析.生物测定表明,野外香茅草丛和密闭容器中香茅挥发物对玉米和稗草种子萌发率影响不显著,但对玉米和稗草幼苗的生物量、根长及苗高均产生显著抑制影响,表明香茅挥发物中存在潜在的化感物质.对挥发物的分析结果表明,根挥发物有10种成分,主要成分是长叶松烯,含量为56.67%,其次为芹子烯内酯(20.03%),其余成分含量都在10%以下.茎叶挥发物有12种成分,主要成分是柠檬醛,含量达53.98%,其次是z柠檬醛,含量为34.40%,其余成分含量都在4%以下.研究表明,在香茅挥发物中存在较多的萜类化合物,茎叶挥发物中有2个单萜,9个倍半萜,根挥发物全部为倍半萜.因此,种植香茅时不应忽视它的化感作用.     

Two novel phytotoxic substances from Leucas aspera

Leucas aspera (Lamiaceae), an aromatic herbaceous plant, is well known for many medicinal properties and a number of bioactive compounds against animal cells have been isolated. However, phytotoxic substances from L. aspera have not yet been documented in the literature. Therefore, current research was conducted to explore the phytotoxic properties and substances in L. aspera. Aqueous methanol extracts of L. aspera inhibited the germination and growth of garden cress (Lepidum sativum) and barnyard grass (Echinochloa crus-galli), and the inhibitory activities were concentration dependent. These results suggest that the plant may have phytotoxic substances. The extracts were then purified by several chromatographic runs. The final purification was achieved by reversed-phase HPLC to give an equilibrium (or inseparable) 3:2 mixture of two labdane type diterpenes (compounds 1 and 2). These compounds were characterized as (rel 5S,6R,8R,9R,10S,13S,15S,16R)-6-acetoxy-9,13;15,16-diepoxy-15-hydroxy-16-methoxylabdane (1) and (rel 5S,6R,8R,9R,10S,13S,15R,16R)-6-acetoxy-9,13;15,16-diepoxy-15-hydroxy-16-methoxylabdane (2) by spectroscopic analyses. A mixture of the two compounds inhibits the germination and seedling growth of garden cress and barnyard grass at concentrations greater than 30 and 3 μM, respectively. The concentration required for 50% growth inhibition (I₅₀) of the test species ranges from 31 to 80 μM, which suggests that the mixture of these compounds, are responsible for the phytotoxic activity of L. aspera plant extract.     

A novel substance with allelopathic activity in Ginkgo biloba

Ginkgo (Ginkgo biloba) is one of the oldest living tree species and has been widely used in traditional medicine. Leaf extracts of ginkgo, such as the standardized extract EGb761, have become one of the best-selling herbal products. However, no bioactive compound directed at plants has been reported in this species. Therefore, we investigated possible allelopathic activity and searched for allelopathically active substances in ginkgo leaves. An aqueous methanol leaf extract inhibited the growth of roots and shoots of garden cress (Lepidium sativum), lettuce (Lactuca sativa), timothy (Phleum pratense) and ryegrass (Lolium multiflorum) seedlings. The extract was purified by several chromatographic runs and an allelopathically active substance was isolated and identified by spectral analysis to be the novel compound 2-hydroxy-6-(10-hydroxypentadec-11-enyl)benzoic acid. The compound inhibited root and shoot growth of garden cress and timothy at concentrations greater than 3 μM. The activity of the compound was 10- to 52-fold that of nonanoic acid. These results suggest that 2-hydroxy-6-(10-hydroxypentadec-11-enyl)benzoic acid may contribute to the allelopathic effect caused by ginkgo leaf extract. The compound may also have potential as a template for the development of new plant control substances.     

Effects of exogenous nitric oxide on cadmium toxicity, element contents and antioxidative system in perennial ryegrass

The effects of sodium nitroprusside (SNP, a donor of NO) on cadmium (Cd) toxicity in ryegrass seedlings (Lolium perenne L.) were studied by investigating the symptoms, plant growth, chlorophyll content, lipid peroxidation, H⁺-ATPase enzyme and antioxidative enzymes. Addition of 100???M CdCl₂ caused serious chlorosis and inhibited the growth of ryegrass seedlings, and dramatically increased accumulation of Cd in both shoots and roots, furthermore, the absorption of macro and micronutrients were inhibited. Addition of 50, 100, 200???M SNP significantly decreased the transport of Cd from roots to shoots, alleviated the inhibition of K, Ca, Mg and Fe, Cu, Zn absorption induced by Cd, reduced the toxicity symptoms and promoted the plant growth. The accumulation of reactive oxygen species (ROS) significantly increased in ryegrass seedlings exposed to Cd, and resulted in the lipid peroxidation, which was indicated by accumulated concentration of thiobarbituric acid-reactive substances. Addition of 50, 100, 200???M SNP significantly decreased the level of ROS and lipid peroxidation. Activities of antioxidant enzymes also showed the same changes. Addition of 50, 100, 200???M SNP increased activities of superoxide dismutase, peroxidase, catalase and ascorbate peroxidase in ryegrass seedlings exposed to Cd. Addition of 100???M SNP had the most significant alleviating effect against Cd toxicity while the addition of 400???M SNP had no significant effect with Cd treatment.     

Soil physical strength rather than excess ethylene reduces root elongation of Eucalyptus seedlings in mechanically impeded sandy soils

Seedling establishment in heavily compact soils is hampered by poor root growth caused by soil chemical or physical factors. This study aims to determine the role of ethylene in regulating root elongation through mechanically impeded sandy soils using Eucalyptus todtiana F. Muell seedlings. Concentrations of ethephon (1, 10, and 100???M) were added to non-compact soils, and endogenous ethylene production from seedling roots was compared to ethylene production of roots grown in physically compacted field soils (98.6?% sand). The ethylene-inhibitor 3,5-diiodo-4-hydroxybenzoic acid (DIHB) (0.1???M) was included for each treatment to counteract the negative effects of excess ethylene or compact soils on root elongation. Root elongation was reduced in high ethylene soils by 49?% and high bulk density soils by 44?%. Root ethylene production increased ninefold in roots grown in the high ethylene environment (100???M), but decreased 80?% in compact soils. The use of DIHB did not alter root length and produced varying results with respect to ethylene production, suggesting an interaction effect involving high amounts of soil ethylene. While ethylene regulates root growth, the physical strength of sandy soils is the major factor limiting root elongation in mechanically impeded soils.