Root growth and distribution of two short-season rice genotypes

Root growth dynamics of lowland rice (Oryza sativa L.) throughout the growing season are poorly understood. A field experiment was conducted in 1987 to compare root growth and distribution of two rice genotypes at two Arkansas locations on soils with different physical and chemical properties. Two genotypes, Bond and an experimental line (RU8701084), were grown on a Captina silt loam (Typic Fragiudults) at Fayetteville, AR, and on a Crowley silt loam (Typic Albaqualfs) near Stuttgart, Ar. Plots contained minirhizotrons oriented at a 45° angle and extended 55 cm (Captina) and 40 cm (Crowley) vertical to the soil surface. Root measurements were taken several times during the season at specific growth stages. Plant height and tiller number were taken at 9 dates at Fayetteville up to physiological maturity. In general, root length (RL) and root length density (RLD) were greater on the Captina soil. Genotypes at both locations reached maximum root growth rates between active tillering and panicle initiation (PI) and maximum RL by early reproduction. Total RL were similar between genotypes on the Captina. However on the Crowley, the mean RL for Bond between the period of early booting and flood removal was an average of 54% greater than for RU8701084. During early reproductive growth at both locations RL plateaued, but then declined during the grain filling process. There was a trend for RU8701084 to contain a greater percentage of its total RL in the top 20 and 10 cm of soil on the Captina and Crowley, respectively, while Bond tended to be a deeper rooted genotype. Bond had a greater RLD at the 20–30 cm depth increment on the Crowley, which contributed to the greater RL. Less than 15% of the total RL of either genotype was measured below 30 cm on the Crowley. In contrast, nearly 25% of the total RL was found at the 30–40 cm depth increment on the Captina. Results showed that rice root growth varied between soils, that root distribution patterns differed between genotypes, and that patterns of root growth changed with changes in plant development.     

Nutrient uptake relationship to root characteristics of rice

Data on root parameters and distribution are important for an improved understanding of the factors influencing nutrient uptake by a crop. Therefore, a study was conducted on a Crowley silt loam at the Rice Research and Extension Center near Stuttgart, Arkansas to measure root growth and N, P and K uptake by three rice (Oryza sativa L.) cultivars at active tillering (36 days after emergence (DAE)), maximum tillering (41 DAE), 1.25 cm internode elongation (55 DAE), booting (77 DAE) and heading (88 DAE). Soil-root core samples were taken to a depth of 40 cm after plant samples were removed, sectioned into 5 cm intervals, roots were washed from soil and root lengths, dry weights and radii were measured. Root parameters were significantly affected by the soil depth × growth stage interaction. In addition, only root radius was affected by cultivar. At the 0- to 5-cm soil depth, root length density ranged from 38 to 93 cm cm^-3 throughout the growing season and decreased with depth to about 2 cm cm^-3 in the 35- to 40-cm depth increment. The increase in root length measured with each succeeding growth stage in each soil horizon also resulted in increased root surface area, hence providing more exposed area for nutrient uptake. About 90% of the total root length was found in the 0- to 20-cm soil depth throughout the season. Average root radius measured in the 0- to 5-cm and 35- to 40-cm depth increments ranged from 0.012 to 0.013 cm and 0.004 to 0.005 cm, respectively throughout the season. Total nutrient uptake by rice differed among cultivars only during vegetative growth. Differences in total nutrient uptake among the cultivars in the field appear to be related to absorption kinetics of the cultivars measured in a growth chamber study. Published with permission of the Arkansas Agricultural Experiment Station.     

Cultivar and planting date effects on soybean root growth

To avoid late summer drought, soybean [Gylcine max (L) Merrill] producers in many southern and border states of the USA modify their cropping systems. Options include use of unadapted cultivars and changing planting dates. Because root function is important to plant health and yield, this study was conducted to determine if planting date and soybean cultivar affect root growth and distribution. Seeds of one cultivar from each of four maturity groups (MG III, IV, V, and VI) were sown in mid-April, mid-May, and mid-June in 1992 and 1993 on a Tiptonville silt loam near Portageville, MO. Root observations were performed 30 and 60 days after emergence (DAE) using a minirhizotron system. Cultivars differed for root length density (RLD) only in the 15 to 28 cm depth in 1992 and in the 15 to 28 cm and 29 to 42 cm depths in 1993, but differences were not related to maturity classification of cultivar. Average RLD was 1.02 cm^–3 for MG III and IV cultivars and 1.21 cm cm^–3 for MG V and VI cultivars. Average RLD for the mid-June planting date was 1.65 cm cm^–3 but only 0.73 cm cm^–3 for the mid-April planting date. An increase in RLD between 30 and 60 DAE occurred at all soil depths. For both years, MG V and VI cultivars produced higher yields than the MG III cultivars. Earlier than normal planting dates inhibited early root growth, but did not reduce yield. Cultivars differed only slightly for the rooting characteristics measured in this study. These rooting characteristics may not be important criteria for cultivar selection.Abbreviations MG maturity group - VCR videocassette recorder - DAE days after emergence - RLD root length density - CRLD change in root length density Contribution from the Missouri Agric. Exp. Station Journal Series Number 12, 153Contribution from the Missouri Agric. Exp. Station Journal Series Number 12, 153     

Effects of tall fescue turf on growth and nitrogen fixation potential of the woody legume Lupinus albifrons

The effect of tall fescue turf on growth, flowering, nodulation, and nitrogen fixing potential of Lupinus albifrons Benth. was examined for greenhouse and field grown plants. No allelopathic effect was observed for lupine plants treated with tall fescue leachates. The nitrogen-fixing potential measured by nodule dry weight and acetylene reduction rates was not significantly affected by tall fescue turf.Both the greenhouse and field studies showed that the growth, sexual reproductive allocation and number of inflorescences were significantly reduced when lupine plants were grown with tall fescue. The root-length densities of tall fescue turf and lupine monoculture were measured. The tall fescue turf had 20 times higher root-length density (20 cm cm^-3 soil) than the lupine plant monoculture. This suggests that intense competition at the root zone may be a dominant factor which limits the growth of the lupine plants.The flowering characters of the lupine plants were improved by phosphorus fertilization. Transplanting of older lupine plants into the turf substantially alleviated the tall fescue turf competitive effect.     

Spatial distribution of roots of pearl millet on sandy soils of Niger

Root sampling in crop stands of low planting density requires reliable information on horizontal distribution of roots. This applies particularly to pearl millet in the Sahel, which is sown at a rate of less than two pockets of seed per m². The objective of this study was to investigate the spatial variability of root length density (RLD) among sampling positions in an improved management system with ridging and under traditional sowing. RLD between ridges (bR) was lower compared to sampling positions within ridges (wR) at soil depth layers from 0 to 80 cm soil depth. We found a highly significant, positive correlation between the sum of the root length (RL) of four sampling dates (tillering, booting, flowering, and maturity) with shoot dry mass (SDM) at maturity. The square of the correlation coefficient was highest when calculation of RL was based on RLD at all four sampling positions. While SDM exhibited significant differences among three pearl millet varieties, sole root sampling wR at a lateral distance of 60 cm relative to the pocket would not allow for the detection of varietal differences in RL, while all other sampling positions did. The correlation between RL and SDM was considerably improved if information of RLD bR was included. Under traditional sowing, RLD directly under the plant was lower compared to sampling positions at lateral distance 25 and 50 cm from the centre of the pocket, but this effect of sampling position was not significant. RLD estimates within deeper soil layers were not systematically affected by direction and lateral distance. To obtain accurate information about depth of rooting and RL under traditional sowing, samples should be taken from lateral distances between 20 and 40 cm from the pocket centre.     

Varietal differences in shoot and rooting parameters of pearl millet on sandy soils in Niger

Root parameters are important traits for the acquisition of nutrients and water under resource-limited conditions. In order to investigate the extent of varietal differences in rooting parameters in pearl millet, we compared a total of eight pearl millet varieties in two experiments (ridging and traditional sowing) over four years at the ICRISAT Sahelian Centre, Niger. We found substantial genotypic variation for root length density (RLD), root dry matter, and total root length (RL), but not for specific root length, depth of rooting, or partitioning of roots between topsoil and subsoil (>20 cm depth). RL showed a highly dynamic pattern over the growing season. RL and shoot dry matter were positively correlated over P supplies and contrasting levels of field productivity. The relationship between RL and grain yield as well as harvest index were less clear. The root fraction responded to variation in soil productivity, increasing from roughly 20% under high productivity to more than 40% at low productivity, but we found no evidence of varietal differences in this trait despite pronounced differences in maturity and plant stature. Identification of pearl millet varieties suited for growth under low input conditions in south-west Niger can potentially make use of existing genotypic variability in root parameters, but we suggest that, due to high variability for root traits, indirect selection for shoot parameters (e.g., number of stems) is more promising than direct selection for RLD or RL.     

Seasonal dynamics of fine root biomass, root length density, specific root length, and soil resource availability in a Larix gmelinii plantation

Fine root turnover is a major pathway for carbon and nutrient cycling in terrestrial ecosystems and is most likely sensitive to many global change factors. Despite the importance of fine root turnover in plant C allocation and nutrient cycling dynamics and the tremendous research efforts in the past, our understanding of it remains limited. This is because the dynamics processes associated with soil resources availability are still poorly understood. Soil moisture, temperature, and available nitrogen are the most important soil characteristics that impact fine root growth and mortality at both the individual root branch and at the ecosystem level. In temperate forest ecosystems, seasonal changes of soil resource availability will alter the pattern of carbon allocation to belowground. Therefore, fine root biomass, root length density (RLD) and specific root length (SRL) vary during the growing season. Studying seasonal changes of fine root biomass, RLD, and SRL associated with soil resource availability will help us understand the mechanistic controls of carbon to fine root longevity and turnover. The objective of this study was to understand whether seasonal variations of fine root biomass, RLD and SRL were associated with soil resource availability, such as moisture, temperature, and nitrogen, and to understand how these soil components impact fine root dynamics in Larix gmelinii plantation. We used a soil coring method to obtain fine root samples (⩽2 mm in diameter) every month from May to October in 2002 from a 17-year-old L. gmelinii plantation in Maoershan Experiment Station, Northeast Forestry University, China. Seventy-two soil cores (inside diameter 60 mm; depth intervals: 0–10 cm, 10–20 cm, 20–30 cm) were sampled randomly from three replicates 25 m × 30 m plots to estimate fine root biomass (live and dead), and calculate RLD and SRL. Soil moisture, temperature, and nitrogen (ammonia and nitrates) at three depth intervals were also analyzed in these plots. Results showed that the average standing fine root biomass (live and dead) was 189.1 g·m⁻²·a⁻¹, 50% (95.4 g·m⁻²·a⁻¹) in the surface soil layer (0–10 cm), 33% (61.5 g·m⁻²·a⁻¹), 17% (32.2 g·m⁻²·a⁻¹) in the middle (10–20 cm) and deep layer (20–30cm), respectively. Live and dead fine root biomass was the highest from May to July and in September, but lower in August and October. The live fine root biomass decreased and dead biomass increased during the growing season. Mean RLD (7,411.56 m·m⁻³·a⁻¹) and SRL (10.83 m·g⁻¹·a⁻¹) in the surface layer were higher than RLD (1 474.68 m·m⁻³·a⁻¹) and SRL (8.56 m·g⁻¹·a⁻¹) in the deep soil layer. RLD and SRL in May were the highest (10 621.45 m·m⁻³ and 14.83m·g⁻¹) compared with those in the other months, and RLD was the lowest in September (2 198.20 m·m⁻³) and SRL in October (3.77 m·g⁻¹). Seasonal dynamics of fine root biomass, RLD, and SRL showed a close relationship with changes in soil moisture, temperature, and nitrogen availability. To a lesser extent, the temperature could be determined by regression analysis. Fine roots in the upper soil layer have a function of absorbing moisture and nutrients, while the main function of deeper soil may be moisture uptake rather than nutrient acquisition. Therefore, carbon allocation to roots in the upper soil layer and deeper soil layer was different. Multiple regression analysis showed that variation in soil resource availability could explain 71–73% of the seasonal variation of RLD and SRL and 58% of the variation in fine root biomass. These results suggested a greater metabolic activity of fine roots living in soil with higher resource availability, which resulted in an increased allocation of carbohydrate to these roots, but a lower allocation of carbohydrate to those in soil with lower resource availability. __________ Translated from Acta Phytoecologica Sinica, 2005, 29(3): 403–410 [译自: 植物生态学报, 2005, 29(3): 403–410]     

Accumulation of selenium by different plant species grown under increasing sodium and calcium chloride salinity

High levels of naturally occurring selenium (Se) are often found in conjunction with different forms of salinity in central California. Plants considered for use in phytoremediation of high Se levels must therefore be salt tolerant. Selenium accumulation was evaluated for the following species under increasing salt (NaCl and CaCl) conditions:Brassica napus L. (canola),Hibiscus cannibinus L. (kenaf),Festuca arundinacea L. (tall fescue), andLotus tenuis L. (birdsfoot trefoil). The experimental design was a complete randomized block with four salt treatments of <1, 5, 10, and 20 dS m^-1, four plant species, three blocks, and six replicates per treatment. Ninety days after growing in the respective salt treated soil with a Se concentration of 2 mg Se kg^-1 soil, added as Na₂SeO₄, all plant species were completely harvested. Among the species tested, shoot and root dry matter yield of kenaf was most significantly (p<0.001) affected by the highest salt treatment and tall fescue and canola were the least affected species. Generally there was a decrease in tissue accumulation of Se with increasing salt levels, except that low levels of salinity stimulated Se accumulation in canola. Canola leaf and root tissue accumulated the highest concentrations of Se (315 and 80 mg Se kg^-1 DM) and tall fescue the least (35 and 7 mg Se kg^-1 DM). Total soil Se concentrations all harvest were significantly (p<0.05) lower for all species at all salt treatments. Removal of Se from soil was greatest by canola followed by birdsfoot trefoil, kenaf and tall fescue. Among the four species, canola was the best candidate for removing Se under the tested salinity conditions. Kenaf may be effective because of its large biomass production, while tall fescue and birdsfoot trefoil may be effective because they can be repeatedly clipped as perennial crops.     

Fine Root Distribution in Dehesas of Central-Western Spain

A Dehesa is a structurally complex agro-silvo-pastoral system where at least two strata of vegetation, trees and herbaceous plants coexist. We studied the root distribution of trees (Quercus ilex L.) and herbaceous plants, in order to evaluate tree and crops competition and complementarity in Dehesas of Central Western Spain. 72 soil cores of 10 cm diameter (one to two metre deep) were taken out around 13 trees. Seven trees were intercropped with Avena sativa L. and six trees were in a grazed pasture dominated by native grasses. Soil coring was performed at four distances from the tree trunks, from 2.5 (beneath canopy) till 20 m (out of the canopy). Root length density (RLD) of herbaceous plants and trees was measured using the soil core-break method. Additionally, we mapped tree roots in 51 profiles of 7 recently opened road cuts, located between 4 and 26 m of distance from the nearest tree. The depth of the road cuts varied between 2.5 and 5.5 m. Herbaceous plant roots were located mostly in the upper 30 cm, above a clayey, dense soil layer. RLD of herbaceous plants decreased exponentially with depth until 100 cm depth. Holm-oak showed a much lower RLD than herbs (on average, 2.4 versus 23.7 km m⁻³, respectively, in the first 10 cm of the soil depth). Tree RLD was surprisingly almost uniform with depth and distance to trees. We estimated a 5.2 m maximum depth and a 33 m maximum horizontal extension for tree roots. The huge surface of soil explored by tree roots (even 7 times the projection of the canopy) could allow trees to meet their water needs during the dry Mediterranean summers. The limited vertical overlap of the two root profiles suggests that competition for soil resources between trees and the herbaceous understorey in the Dehesa is probably not as strong as usually assumed.     

Temporal and spatial patterns of soil water following wildfire-induced changes in plant communities in the Great Basin in Nevada,USA

Infection of tall fescue (Festuca arundinacea Schreb.) with its endemicNeotyphodium coenophialum-endophyte (Morgan-Jones and Gams) Glenn, Bacon and Hanlin appears to reduce copper (Cu) concentrations in forage and serum of grazing animals, contributing to a range of immune-related disorders. A greenhouse experiment was conducted to identify effects of novel endophyte strains on Cu acquisition by tall fescue (Festuca arundinacea Schreb.) varieties Grasslands Flecha and Jesup infected with a novel, non ergot producing endophyte strain AR542, and two perennial ryegrass (Lolium perenne L.) varieties Aries and Quartet infected with a novel, non lolitrem B producing strain AR1, and their noninfected (E−) forms. Individual endophyte/grass associations were cultivated in nutrient solutions at 1.0 (P+) and 0.0 mM (P−) phosphorus concentrations. The Cu²⁺-binding activity of extracellular root exudates, and concentrations of Cu and other heavy metals in roots and shoots were measured. Extracellular root exudates of AR542-infected vs. E− tall fescue had higher Cu²⁺-binding activity only in P− nutrient solution as shown by lower concentration of free Cu²⁺ (0.096 vs. 0.188 mmol Cu²⁺ g⁻¹ root DM, respectively). The Cu²⁺-binding activity by root exudates of perennial ryegrass was not affected by endophyte infection, but was higher (i.e., lower concentration of free Cu²⁺) in P− vs. P+ nutrient solution (0.068 vs. 0.114 mmol Cu²⁺ g⁻¹ root DM). In this hydroponic experiment, Cu concentrations in shoots of both grasses were not a function of Cu²⁺-binding activity and endophyte effects on heavy metal concentrations in shoots and roots were specific for each variety. The Cu²⁺-binding activity of extracellular root exudates may affect Cu accumulation by field-grown, endophyte-infected tall fescue under P-limiting growth conditions and warrants verification by more specific methods.     

Sorghum root length density and the potential for avoiding striga parasitism

Striga hermonthica is a serious root parasite of sorghum in the semiarid tropics. Successful parasitism is dependent on interactions of Striga seeds and host roots. Several sorghum cultivars have been found which resist parasitism. The basis of resistance is not well known. One possible method for reducing the chances of parasitism is by restricted host root development. This research was conducted to evaluate this hypothesis in sorghum known to possess resistance to parasitism by Striga.Root length density of 21-day-old pot-grown resistant cultivars, Framida, N-13, IS-9830, Tetron and P-967083, were compared to that of the susceptible check, Dabar, using the line intercept method of measuring root length. There was no significant difference between resistant cultivars and the susceptible cultivar Dabar. The RLD of resistant P-967083 however was significantly less than Framida, another resistant cultivar.The RLD of Dabar was compared to that of Framida and P-967083 in USA and Niger field trials. Root length density was determined on soil cores taken at flowering with a Giddings Soil Sampler. Each core was divided into 10-cm fractions for estimating RLD by the line intercept method. In the USA Dabar had significantly greater RLD than the two resistant cultivars in the upper 10-cm portion of the soil profile, but only significantly greater than P-967083 in the 10–20-cm portion. Significant differences in RLD between susceptible and resistant cultivars were not found at depths between 20–60 cm. In field trials in Niger, RLD of Dabar was significantly greater than either resistant cultivar in the (0 to 30 cm) portion of the soil core. These results suggest that part of the Striga resistance of P-967083 and perhaps Framida may be a result of avoiding interactions between parasitic seeds and host roots.     

Comparison of tomato root distributions by minirhizotron and destructive sampling

Calibration of minirhizotron data against root length density (RLD) was carried out in a field trial where three drip irrigation depths: surface (R0) and subsurface, 0.20 m (RI) and 0.40 m depth (RII) and two processing tomato cultivars: `Brigade' (CI) and `H3044' (CII) were imposed. For each treatment three minirhizotron tubes were located at 10, 37.5 and 75 cm of the way from one plant row to the next. Roots intersecting the minirizotrons walls were expressed as root length intensity (L _a) and number of roots per unit of minirhizotron wall area (N _ra). Root length density (RLD) was calculated from core samples taken for each minirhizotron tube at two locations: near the top of the minirhizotron (BI) and 15 cm apart from it, facing the minirhizotron wall opposite the plant row (BII). Minirhizotron data were regressed against RLD obtained at BI and BII and with their respective means. The results show that for all the situations studied, better correlations were obtained when RLD was regressed with L _a than with N _ra. Also was evident that the relationship between L _a and RLD was strongly influenced by the location of soil coring. RLD was correlated with L _a trough linear and cubic equations, having the last ones higher determination coefficients. For instance at 10 cm from the plant row when values from the top layer (0–40 cm) were analysed separately, L _a was significantly regressed with RLD measured at BII and described by the equations: RLD = 0.5448 + 0.0071 L _a (R ² = 0.51) and RLD = 0.4823 + 0.0074L _a + 8×10^–5 L _a ² – 5×10^–7 L _a ³ (R ² = 0.61). Under the 40 cm depth the highest coefficients of determination for the linear and cubic equations were respectively 0.47 and 0.88, found when L _a was regressed with RLD measured at BI. For minirhizotrons located at 75 cm from the plant row and for location BI it was possible to analyse jointly data from all depths with coefficients of determination of 0.45 and 0.59 for the linear and cubic equations respectively.     

Genotype and Planting Density Effects on Rooting Traits and Yield in Cotton （Gossypium hirsutum L,）

Root density distribution of plants is a major Indicator of competition between plants and determines resource capture from the solh This experiment was conducted in 2005 at Anyang, located in the Yellow River region, Henan Province, China. Three cotton （Gossyplum hlrsutum L.） cultivars were chosen： hybrid Btcultlvar CRI46, conventional Btcultlvars CRI44 and CRI45. Six planting densities were designed, ranging from 1.5 to 12.0 plants/m^2. Root parameters such as surface area, diameter and length were analyzed by using the DT-SCAN Image analysis method. The root length density （RLD）, root average diameter and root area Index （RAI）, root surface area per unit land area, were studied. The results showed that RLD and RAI differed between genotypes; hybrid CRI46 had significantly higher （P 〈0.05） RLD and RAI values than conventlonal cultlvars, especially under low planting densities, less than 3.0 plants/m^2. The root area index （RAI） of hybrid CRI46 was 61% higher than of CRI44 and CRI45 at the flowering stage. The RLD and RAI were also significantly different （P = 0.000） between planting densities. The depth distribution of RAI showed that at Increasing planting densities RAI was Increasingly distributed in the soil layers below 50 cm. The RAI of hybrid CRI46 was for all planting densities, obviously higher than other cultivars during the flowering and boll stages. It was concluded that the hybrid had a strong advantage in root maintenance preventing premature senescence of roots. The root diameter of hybrid CRI46 had a genetically higher root diameter at planting densities lower than 6.0 plants/m^2. Good associations were found between yield and RAI In different stages. The optimum planting density ranged from 4.50 plants/m^2 to 6.75 plants/m^2 for conventional cultlvars and around 4.0-5.0 plants/m^2 for hybrids.     

A comparison of minirhizotron,core and monolith methods for quantifying barley (Hordeum vulgare L.) and fababean (Vicia faba L.) root distribution

Root research has been hampered by a lack of good methods and by the amount of time involved in making measurements. The use of the minirhizotron as a quantitative tool requires comparison with conventional destructive methods. This study was conducted in the greenhouse to compare the minirhizotron technique with core and monolith methods in quantifying barley (Hordeum vulgare L.) and fababean (Vicia faba L.) root distribution. Plants were grown in boxes (80 cm long × 80 cm wide × 75 cm deep) in a hexagonal arrangement to minimize the effects of rooting anistrophy. Minirhizotron observations and destructive sampling to a depth of 70 cm using core and monolith methods were performed at the ripening growth stage. Total root length for the entire depth interval was generally higher in barley (159–309 m) than fababean (110–226 m). Significant correlation coefficients between monolith and core methods for root length density (RLD, cm cm^–3) was observed in both crops (p 0.01). A method and depth interaction showed no significant differences in fababean RLD distribution measured by core and monolith methods. However, the RLD was different for the uppermost 40 cm depth in barley. The relationship for RLD between minirhizotron and core methods was significant only in barley (r=0.77^*). For both crops, estimates of RLD in the top 10-cm layer by the minirhizotron technique were lower than those by core and monolith techniques. In contrast, estimates of RLD were higher in fababean at a depth >30 cm. Destructive sampling still remains the method to quantify root growth in the 0–10 cm soil layer. ei]B E Clothier     

Effects of fungal endophytes on the seed and seedling biology of Lolium perenne and Festuca arundinacea

Summary Many grasses are infected by endophytic fungi that grow intercellularly in leaves, stems, and flowers and are transmitted maternally by hyphal growth into ovules and seeds. The seed biology and seedling growth of endophyte-infected and uninfected perennial ryegrass (Lolium perenne) and tall fescue (Festuca arundinacea) were investigated under controlled environmental conditions. The percentage of filled seeds produced by infected tall fescue was over twice of uninfected tall fescue; infected and uninfected perennial reegrass had similar percentages. Weights of seeds from infected and uninfected plants were similar in both species. Seeds from infected plants of both species exhibited a higher rate of germination than seeds from uninfected plants. Shoot growth in the greenhouse was compared by making three sequential harvests of above-ground plant parts from infected and uninfected plants of both species. Infected perennial ryegrass plants produced significantly more biomass and tillers than uninfected plants after 6 and 10 weeks of growth and significantly more biomass after 14 weeks of growth. Infected tall fescue plants produced significantly more biomass and tillers than uninfected plants after 10 and 14 weeks of growth. The physiological mechanism of enhancement of growth is not known. The results of this study suggest that infected plants may have a selective advantage in populations with uninfected members.     

Invasive Plants can Inhibit Native Tree Seedlings: Testing Potential Allelopathic Mechanisms

The mechanisms by which invasive species affect native communities are not well resolved. For example, invasive plants may influence other species through competition, altered ecosystem processes, or other pathways. We investigated one potential mechanism by which invasive plants may harm native species, allelopathy. Specifically, we explored whether native tree species respond differently to potential allelopathic effects of two invasive plant species. We assessed the separate effects of Lolium arundinaceam (tall fescue) and Elaeagnus umbellata (autumn olive) on three common successional tree species: Acer saccharinum (silver maple), Populus deltoides (eastern cottonwood), and Platanus occidentalis (sycamore). Tall fescue and autumn olive are widely planted and highly invasive or persistent throughout North America where they often grow in forest edges, old fields, and other sites colonized by pioneering tree species. In an exploratory greenhouse experiment, we applied aqueous extracts derived from soil, leaf litter, or live leaves to native trees. We compared these treatments to a sterile water control and also to minced leaves leached in water, a common, but potentially less realistic method of testing for allelopathy. For all tree species, minced leaves from tall fescue reduced the probability that seedlings emerged, and minced leaves of autumn olive reduced the number of days to emergence. During other demographic stages, the three native tree species diverged in their responses to the invasive plants. Platanus occidentalis exhibited the widest range of responses, with reduced root biomass due to minced tissue from both invasive species, reduced days to emergence and marginally reduced survival from minced tall fescue, and reduced leaf biomass from tall fescue leaf litter. Populus deltoides appeared insensitive to most extracts, although survival was marginally increased with application of minced or fresh leaf extracts from autumn olive. In addition, minced tall fescue shortened the time to seedling emergence for Acer saccharinum, potentially a positive effect. Overall, results suggest that allelopathy may be one mechanism underlying the negative impacts of tall fescue and autumn olive on other plant species, but that effects can depend strongly upon the source of allelochemicals and the tree species examined.     

Field performance of Solanum sisymbriifolium, a trap crop for potato cyst nematodes. II. Root characteristics

Hatching of potato cyst nematodes is induced by root exudates of Solanaceae, such as Solanum sisymbriifolium, and is therefore related to root length distribution of this crop. A mathematical model was derived to relate the hatching potential to root length density (RLD). A series of field experiments was carried out to study actual root length distribution of S. sisymbriifolium in relation to shoot properties and to provide input into the model. Using a modified Poisson distribution formula for the three‐dimensional distribution of roots in a volume of soil, the relation between the zone of influence of hatching agents and the RLD could be derived. On this basis, the minimal RLD was estimated, which is needed to expose 75%, 90% or 95% of cysts to root exudates, as a function of the length of the zone of influence of hatching agents on cysts. The logarithm of the total root length showed a linear relation with the logarithms of above‐ground biomass and with leaf area index. Root diameter distribution was the same for all crops examined and independent of soil depth. Fine roots (<0.4 mm in diameter) constituted around 50% of total root length. Using a zone of influence of 1.00, 0.75 and 0.50 cm around the centre of each root, a minimal RLD for sufficient soil exploration (75%) was estimated. Depth at which that minimal RLD was exceeded was linearly related to total root length (km m^?2) and to above‐ground crop biomass, enabling estimations being made of the potential hatching efficacy as related to measurable properties of S. sisymbriifolium crops. The proposed approach to derive potential hatching effects from crop properties needs further validation; particularly, the distance of influence of root exudates is a critical factor.     

Expression of the bacteriophage T4 lysozyme gene in tall fescue confers resistance to gray leaf spot and brown patch diseases

Tall fescue (Festuca arundinacea Schreb.) is an important turf and forage grass species worldwide. Fungal diseases present a major limitation in the maintenance of tall fescue lawns, landscapes, and forage fields. Two severe fungal diseases of tall fescue are brown patch, caused by Rhizoctonia solani, and gray leaf spot, caused by Magnaporthe grisea. These diseases are often major problems of other turfgrass species as well. In efforts to obtain tall fescue plants resistant to these diseases, we introduced the bacteriophage T4 lysozyme gene into tall fescue through Agrobacterium-mediated genetic transformation. In replicated experiments under controlled environments conducive to disease development, 6 of 13 transgenic events showed high resistance to inoculation of a mixture of two M. grisea isolates from tall fescue. Three of these six resistant plants also displayed significant resistance to an R. solani isolate from tall fescue. Thus, we have demonstrated that the bacteriophage T4 lysozyme gene confers resistance to both gray leaf spot and brown patch diseases in transgenic tall fescue plants. The gene may have wide applications in engineered fungal disease resistance in various crops.     

The effect ofAcremonium coenophialum on the growth and nematode infestation of tall fescue

The presence of the endophytic fungusAcremonium coenophialum Morgan-Jones et Gams in tall fescue (Festuca arundinacea Schreb.) induces toxicity when this grass is grazed by cattle; however, there is evidence that removing the endophyte reduces the stand vigor and longevity of fescue. A field trial was conducted to determine the effects of water supply and the presence of the endophytic fungus on plant growth, drought tolerance, and soil nematode populations in Kentucky 31 tall fescue. The design included two factors, level of endophyte infection (0 and 75%) and irrigation regime (none, low, and high). Where water deficits occurred, herbage yield and leaf area were lower, and percentage dead tissue and canopy minus air temperature were greater in endophyte-free compared with endophyte-infected fescue. Soil populations ofPratylenchus scribneri andTylenchorhynchus acutus were substantially higher in the noninfected than in the endophyte-infected plots. The endophyte apparently confers drought tolerance to Kentucky 31 tall fescue, and this effect may be at least partially mediated through enhanced resistance to soil-borne nematodes.Published with the approval of the Director of the Ark. Agric. Exp. Stn.     

The Vertical Pattern of Rooting and Nutrient Uptake at Different Altitudes of a South Ecuadorian Montane Forest

The vertical pattern of root length densities (RLD) of fine roots (<2 mm in diameter) and nitrogen (N) uptake potential were determined at different altitudes (1,900, 2,400, and 3,000 m a.s.l.) of a tropical montane forest in order to improve our knowledge about the depth distribution of nutrient uptake in this ecosystem. At higher altitudes, precipitation rate and frequency of fog were higher than at lower altitudes while mean annual air temperature decreased with increasing altitude. Soils were always very acid with significantly lower pH at a depth of 0.0–0.3 m in mineral soil at 3,000 m (2.8–2.9) than at 1,900 and 2,400 m (3.1–3.5). The vertical distribution of RLD was very similar both during the dry and the rainy season. During the dry season the percentage of root length in the organic layer increased from 51% at 1,900 m to 61% at 2,400 m and 76% at 3,000 m. At 3,000 m, RLD was markedly higher in the upper 0.05 m than in the remaining organic layer, whereas at 1,900 m and 2,400 m RLD were similar in all depths of the organic layer. In mineral soil, RLD decreased to a greater degree with increasing soil depth at the upper two study sites than at 1,900 m. The relative N uptake potential from different soil layers (RNUP) was determined by ¹⁵N enrichment of leaves after application of ¹⁵N enriched ammonium sulphate at various soil depths. RNUP closely followed fine root distribution confirming the shallower pattern of nutrient uptake at higher altitudes. RNUP was very similar for trees, shrubs and herbs, but shallower for saplings which obtained N only from the organic layer at both altitudes. Liming and fertilizing (N, P, K, Mg) of small patches in mineral soil had no significant impact on fine root growth. We conclude that the more superficial nutrient uptake ability at higher altitudes may be partly related to increased nutrient input from canopy by leaching. However, the specific constraints for root growth in the mineral soil of tropical montane forests warrant further investigations.