Order Lateral Roots Research Articles

A comparison of root architecture and shoot morphology between naturally regenerated and container-grown seedlings of Quercus ilex

We explored the different mechanisms developed by naturally regenerated seedlings of Quercus ilex L. (Holm oak) under Mediterranean conditions compared to container-seedlings commonly used in plantations. We examined the differences in root architecture (including topology and morphology) and shoot parameters. The results showed that there are many differences in the architecture of the root system as well as in the shoot morphology between the two types of seedlings. The naturally regenerated seedlings were smaller with regard to most of the shoot and root parameters, but they developed a longer taproot, only first order lateral roots, and presented a more herringbone-like root system compared to the container seedlings. Conversely, all types of container seedlings, were larger and had a more extended root system with many orders of lateral roots, while their taproot length was restricted within the container’s depth. The quotient log (α)/ log (μ) for all seedlings, showed a tendency to decrease with plant size. A strict herringbone root system with an elongated taproot may be the optimal root architecture for Quercus ilex L. seedlings in order to survive under Mediterranean conditions.

Post-fire seedlings of Nothofagus alpina in Southern Chile show strong dominance of a single ectomycorrhizal fungus and a vertical shift in root architecture

We investigated belowground responses of Nothofagus alpina seedlings to post-fire conditions during natural regeneration after a wildfire in Chile, focusing on mycorrhizal community and root architecture. The complete root systems of 2-year-old N. alpina seedlings were extracted from a post-fire site with natural regeneration and compared to roots of seedlings from undisturbed forest nearby. Mycorrhizal morphotype richness was determined in each seedling. Morphometric parameters of tertiary root structure and dry biomass of whole root systems were determined in 5 cm vertical intervals and in four lateral root classes. With 43.5% of colonized vital mycorrhizal root tips, the Basidiomycete Descolea antarctica was the most abundant fungal symbiont on post-fire seedlings. Tertiary root morphology of these seedlings was distinct from control plants and characterized by a deep-reaching tap root with rather evenly distributed lateral branches whereas seedlings from the undisturbed site had shallower root systems with most lateral roots concentrated in the upper soil layers. Post-fire seedlings had more mycorrhizal rootlets and mycorrhiza-bearing third order lateral roots than control plants which was expressed in a 34% higher total root number but only a 10% higher total root biomass, although both values were not statistically significant. A major part of these fine roots in seedlings from burnt forest was found in deeper soil horizons, compared to the seedlings from undisturbed forest. According to our results, post-fire conditions clearly favour Descolea antarctica as an early ectomycorrhizal colonizer of Nothofagus seedlings at the studied site. As no significant changes in soil chemistry could be observed at the burnt site, the deep-reaching tertiary root architecture of these seedlings may be interpreted as a response to other abiotic factors like reduced moisture in surface soil.

Root characteristics and growth potential of container and bare-root seedlings of red oak (Quercus rubra L.) in Ontario, Canada

Root characteristics and field performance of container and bare-root seedlings of red oak (Quercus rubra L.) were compared during the first growing season after planting. Sixty seedlings of each stock type were planted on a clearfell and weed-free site near Restoule, Ontario. Twenty-four additional seedlings from each stock type were compared at the start of the study in terms of shoot and root parameters. Measurement of root and shoot parameters were repeated at three dates during the first growing season in the field. The root systems of container stock had a larger number of first order lateral long roots and were significantly more fibrous than bare-root stock. These differences were sustained throughout the first growing season. In terms of field performance, container seedlings had 100% survival and achieved significant increases in both biomass and shoot extension. Bare-root seedlings suffered 25% mortality, significant shoot dieback and more variable growth. The mean relative growth rate (RGR) of container seedlings increased throughout the study period to a maximum of 30 mg/g/day, whereas the mean RGR of bare-root stock remained close to or below zero. Overall, the container seedlings proved less prone to transplanting shock than the bare-root seedlings, most likely due to favourable root architecture and the pattern of root development. Further work may be warranted in container design, growing regimes and root architecture to fully realise the potential of container systems for the production of high quality red oak seedlings across a range of site conditions.

Distribution Pattern of Root Nodules in Relation to Root Architecture in Two Leading Cultivars of Peanut (Arachis hypogaea L.) in Japan

To effectively utilize symbiotic nitrogen fixation, we examined the formation of root nodules along with root system development in two leading peanut cultivars in Japan, Chibahandachi and Nakateyutaka. Differences in the number, size and distribution pattern of root nodules between the two cultivars are discussed in relation to their root architecture. Many root nodules are formed on the 1st-order lateral roots in the peanut. The difference between the two cultivars in the number of nodules on the 1st–order lateral roots and the diameter of the 1st-order lateral roots at the basal part of the taproot increased during secondary thickening period. Those changes were significantly greater in Chibahandachi than in Nakateyutaka at later growth stages. Chibahandachi had fewer, but larger nodules than those in Nakateyutaka. In Nakateyutaka, a larger number of new nodules were formed on the lateral roots at the middle part of the taproot than in Chibahandachi. This suggests that in Chibahandachi nodules grow for a longer period during plant growth, and in Nakateyutaka new nodules are formed even at late stages of plant growth. In addition, there appears to be an optimal diameter of the 1st-order lateral roots for nodulation at each growth stage.

Growth responses and endogenous IAA and iPAs changes of litchi ( Litchi chinensis Sonn.) seedlings induced by arbuscular mycorrhizal fungal inoculation

Seedlings of litchi ( Litchi chinensis Sonn.) were inoculated with two arbuscular mycorrhizal (AM) fungal species, Glomus intraradices and Gigaspora margarita. Plant growth response and morphological changes induced by AM inoculation were investigated. Plant endogenous indoleacetic acid (IAA) and isopentenyl adenosines (iPAs) concentrations were determined. With mycorrhizal infection rate of 9.0–18.8%, plant biomasses increased by 13.5–30.1%. Leaf number, leaflet number, total leaf area and first order lateral root number were significantly increased by AM inoculation. Although G. margarita significantly increased plant P content and uptake, no significant difference in N nutrition was observed between mycorrhizal and non-mycorrhizal plants. Enzyme-linked immunosorbent assay (ELISA) indicated the changes in IAA and iPAs induced by AM inoculation. IAA concentrations in shoots and in roots were 5–7 times and 2–5 times higher in mycorrhizal than in non-mycorrhizal plants, respectively. The iPAs concentrations increased by 1.7 times in shoots and by 1.9–2.5 times in roots, due to mycorrhizal inoculation. We suggest that the changes in endogenous phytohormone level may be responsible for morphological alteration induced by AM inoculation.

Effects of soil bulk density on seminal and lateral roots of young maize plants (Zea mays L.)

AbstractIt is well established that increasing soil bulk density (SBD) above some threshold value reduces plant root growth and thus may reduce water and nutrient acquisition. However, formation and elongation of maize seminal roots and first order lateral (FOL) roots in various soil layers under the influence of SBD has not been documented. Two studies were conducted on a loamy sand soil at SBD ranging from 1.25 g cm–3 to 1.66 g cm–3. Rhizotrons with a soil layer 7 mm thick were used and pre‐germinated plants were grown for 15 days. Over the range of SBD tested, the shoot growth was not influenced whereas total root length was reduced by 30 % with increasing SBD. Absolute growth rate of seminal roots was highest in the top soil layer and decreased with increasing distance from the surface. Increasing SBD amplified this effect by 20 % and 50 % for the top soil layer and lower soil layers, respectively. At the end of the experiment, total seminal roots attributed to approximately 15 % of the total plant root length. Increasing SBD reduced seminal root growth in the lowest soil layer only, whereas FOL root length decreased with SBD in all but the uppermost soil layer. For FOL, there was a positive interaction of SBD with distance from the soil surface. Both, increasing SBD and soil depth reduced root length by a reduction of number of FOL roots formed while the length of individual FOL roots was not influenced. Hence, increasing SBD may reduce spatial access to nutrients and water by (i) reducing seminal root development in deeper soil layers, aggravated by (ii) the reduction of the number of FOL roots that originate from these seminal roots.

トマトの湿気中根と水中根の外部および内部形態の比較

Effects of root-zone environment, humid atmosphere and nutrient solution with different oxygen levels on external and internal structures of tomato roots were investigated. Young tomato plants were grown in the wet-sheet culture (WSC), in which all roots were developed in a humid atmosphere, or hydroponically in the deep flow technique (DFT) with or without aeration. Dissolved oxygen in the DFT without aeration (DFT) or with aeration (DFT+Air) were 45-62% and 91-96% saturation, respectively. Growth of tomato plants in the DFT+Air and the WSC were more vigorous than those in the DFT. No differences in the total lengths and surface areas were observed among the three plots. The fractal dimension, describing complexity of fractal architectures, of lateral roots in the WSC was higher than those in the DFT+Air. The number and mean length of first order lateral roots were similar among the three plots, whereas the DFT had a large proportion of shorter laterals. Aerenchyma was observed in the stele in the DFT. The WSC had more root hairs, larger cortical cells, metaxylem, stele, and larger deposits of lignin lamellae at exodermis than had the DFT. These changes in external or internal structure of the roots are interpreted to be adaptive responses to the root environment, e.g., anoxia in the DFT and water deficit in the WSC.

Alterações morfológicas no sistema radicular do milho induzidas por fungos micorrízicos e fósforo

O objetivo deste trabalho, conduzido em casa de vegetação, foi avaliar os efeitos da inoculação de fungos micorrízicos arbusculares (Glomus etunicatum e Glomus clarum) e da adição de níveis de P (0, 50, 100 e 200 mg/kg de solo) sobre a morfologia do sistema radicular do milho (Zea mays L.), cultivar BR 201, e a concentração de P na planta, em duas épocas de colheita (18 e 104 dias após semeadura) em solo Latossolo Vermelho-Escuro distrófico desinfestado com Bromex. O experimento foi realizado na Embrapa-Centro Nacional de Pesquisa de Milho e Sorgo, Sete Lagoas, MG. A inoculação de fungos micorrízicos aumentou o peso das raízes secas, o número de raízes laterais primárias e secundárias, e o teor de P na planta, porém reduziu a relação peso das raízes secas/peso da parte aérea seca e o número de pêlos radiculares. Esses efeitos foram dependentes das doses de P aplicadas ao solo e da espécie de fungo micorrízico. O peso das raízes secas das plantas micorrizadas mostrou correlação significativa (P<=0,05) com a colonização micorrízica, nas raízes seminais e adventícias, e o número de raízes laterais primárias e secundárias das plantas micorrizadas mostrou correlação significativa (P<=0,05) com a colonização micorrízica. Glomus etunicatum foi mais eficiente no aumento do peso das raízes secas, do número de raízes laterais primárias e secundárias e no aumento do teor de P na planta, em todas as doses de P; entretanto, doses altas de P reduziram esses efeitos das duas espécies de fungos, como conseqüência da redução da colonização micorrízica.

Effects of Arbuscular Mycorrhizal Fungi on the Morphology of Kidney Bean Root.

The effects of arbuscular mycorrhizal fungus (Glomus R10) on the root morphology and physiology of kidney bean (Phaseolus vulgaris L.) have been investigated. Infection with arbuscular mycorrhizal fungi reduced the length and the number of tap roots, and the 1st and 2nd order lateral roots. However, the number of the 1st order lateral roots per unit length of tap root and that of the 2nd order lateral roots per unit length of the 1st order lateral root were not affected by the infection. Application of phosphorus fertilizer did not change the morphology of the roots. These findings suggest that infection with arbuscular mycorrhizal fungi alters the morphology of the root system of kidney bean. The infection also promoted the bleeding rate and triphenyl-tetrazoliumchioride (TTC) -reducing power per unit length of roots, but did not affect the bleeding rate per plant. It was unclear whether the increase in bleeding rate and TTC-reducing power was due to the physiological changes in the roots or whether the increase in TTC-reducing power and water absorption by the arbuscular mycorrhizal fungi. Our results show that infection with arbuscular mycorrhizal fungi affects the morphology of kidney bean root; however, the effect on the physiology of roots remains to be examined further.

Root System Development of Japonica-Indica Hybrid Cultivars. 2. Branching habit.

Root system development of japonica-indica hybrid rice cultivars (J-IH : Milyang 23, Suwon 258) was compared with that of japonica-type cultivars (JT: Nipponbare, Koshihikari) with reference to branching habit from the viewpoint of developmental morphology. The four cultivars were grown in 45-liter polyethylene containers with paddy field soil and the plants with their root systems were carefully dug up at the grain-filling stage. After careful washing, the main stems of each cultivar were divided into each phytomer and the number, diameter and length of both crown and lateral roots were recorded together with their node position. From these data, the root dimensions (total number of lateral roots, total roots surface area and total root volume) were estimated for each crown root. The results indicated that lateral root development of J-IH was more vigorous than that of JT, especially in the higher phytomers formed after the panicle initiation stage. Such vigorous root branching in J-IH was mainly attributed to an increase in the number of second-order lateral roots. Detailed examination showed that the L-type first-order lateral roots were longer and the density of both L-type first-order and S-type second-order lateral roots were higher in J-IH than in JT. Thus, the root system of J-IH was characterized by long and dense L-type first-order lateral roots and large volume of S-type second-order lateral roots due to their high density. Results of this study suggest that the root system of J-IH increases the total length and surface area, by developing many S-type sedond order lateral roots, which may be related to their high-yield of J-IH.

Root and Shoot Characteristics of Peach Trees with Different Growth Habits

Shoot and root characteristics of four peach tree [Prunus persica (L.) Batsch (Peach Group)] growth habits (compact, dwarf, pillar, and standard) were studied. In compact trees, leaf number (1350/tree) was twice, but leaf area (6 cm2/leaf) was half that of pillar and standard trees. The number of lateral branches in compact trees (34) was nearly three times more than in pillar and standard trees. Leaf area index (total one-side leaf area per tree divided by the canopy cross-sectional area of the tree) of pillar trees was greater than compact, dwarf, and standard trees (13 compared with 4, 4, and 3, respectively) due to a narrower crown diameter. Dwarf trees were distinct with few leaves (134/tree) and less than half the roots of the other growth habits. Compact trees produced more higher order lateral (HOL) roots than pillar and standard trees. More second order lateral (SOL) roots were produced by compact than standard trees (1.2 vs. 0.8 SOL roots per centimeter first order lateral root). Pillar trees had higher shoot: root dry weight (DW) ratios (2.4) than compact and standard trees (1.7 for both) due to lower root DWs. Root topology was similar among compact, pillar, and standard peach trees but root axes between branch junctions (links) were significantly longer in compact trees. Compact trees had more and longer HOL roots in roots originating near the root collar (stem-root junction) (i.e., more fibrous roots) and this appeared to correlate with more lateral branches in the canopy. These results indicate significant differences in root as well as shoot architecture among growth habits that can affect their use as scion or rootstock cultivars.

Bacteria associated with Pinus sylvestris-Lactarius rufus ectomycorrhizas and their effects on mycorrhiza formation in vitro.

• The structure and functioning of mycorrhizosphere bacterial communities from the soil organic fermentation horizon (FH) of a Pinus sylvestris stand was investigated. • Bacteria were isolated from P. sylvestris-Lactarius rufus mycorrhizas, and their effect on ectomycorrhiza formation, localization within the mycorrhizosphere, and phenotypic and genotypic characteristics determined. • The isolates could be divided into seven distinct phenotypic and physiological groups. Two Paenibacillus isolates stimulated infection of both first and second order lateral roots, resulting in a doubling of the percentage root conversion to mycorrhizas, relative to the control treatment, after 8wk. Two Burkholderia isolates, and a Rhodococcus sp., had no effect on numbers of first order mycorrhizal lateral roots, but increased formation of secondary mycorrhizal lateral roots. There was evidence that the Burkholderia, but not the Paenibacillus or Rhodococcus isolates, associated preferentially with mycorrhizal roots, and that the Burkholderia strains were capable of more effective spread to the root tip than the other isolates. • Ectomycorrhizosphere bacteria appear to promote mycorrhiza formation by means of a variety of mechanisms, and there are similarities in the nature of bacteria inhabiting ecto- and arbuscular mycorrhizospheres.

The effect of neighbors on the root system of the desert annualEremobium aegyptiacum

Intra-specific competition with a primary focus on root competition between plants living in an area with low resource levels, was studied using the natural monotypic population of a desert annual plantEremobium aegyptiacum (Cruciferae). We tested the effect of neighboring distance on shoot and root biomass, and such root parameters as root length, diameter of root neck, number of first order lateral roots and number of lateral roots per unit of main root length. Our results indicate a strong negative relationship between neighboring plant density and such plant parameters as shoot and root biomass, and root neck diameter. The number of first order lateral roots and the number of lateral roots per unit of main root length were negatively related to the distance between neighboring plants. Contrary to predictions, there was no influence of competition on node distribution: neither root overlap nor root avoidance was observed in pairs of adjacent plants.

169 Root Characteristics of Peach Trees with Different Shoot Growth Habits

Peach trees (Prunus persica L.) with diverse shoot growth habits have been developed, but little is known about their root systems. Characterizing shoot and root systems can improve basic understanding of peach tree growth and be important in the development of rootstocks and own-rooted trees. This research determined shoot and root characteristics of four peach tree growth habits (compact, dwarf, pillar, and standard). Seed from four peach growth habits were planted in 128-L containers, grown outside during the 1998 growing season, and then harvested. Compact tree leaf number (1350/tree) was twice, but leaf area (6 cm2/leaf) was half, that of pillar and standard trees. The number of lateral branches in compact trees (34) was nearly three-times more than in pillar and standard trees. The leaf area index (LAI) of pillar trees was greater than compact and standard trees (13 compared with 4 and 3, respectively) due to a narrower crown diameter. Dwarf tree shoots were distinct with few leaves (134 per tree) and a large LAI of 76. Compact trees grew more higher-order lateral roots than pillar and standard trees. More second-order lateral (SOL) roots were produced by compact than standard trees (1.2 vs. 0.8 SOL roots/cm first-order lateral root). Pillar trees had higher shoot-to-root dry weight ratios (2.4) than compact and standard trees (1.7 for both) due to smaller root dry weights. The results indicate fundamental differences in root characteristics among the peach tree growth habits. Compact trees had more higher order lateral roots in roots originating near the root collar (i.e., more fibrous roots), and this correlated with more lateral branches in the canopy. Shoot weights were the same among pillar, compact, and standard trees but root weights were less in pillar trees, resulting in greater shoot-to-root dry weight ratios. These results indicate significant differences in root as well as shoot architecture among growth habits that can affect their use as scion or rootstock varieties.

Differences in numbers of nodules and lateral roots between soybean (Glycine max L. Merr.) cultivars, Kitamusume and Toyosuzu

Two cultivars of soybean (Glycine max L. Merr .), Kitamusume and Toyosuzu were grown with commercially-purchased granulated soil in a greenhouse. Kitamusume formed a larger number of nodules per g shoot dry weight and its nodules showed a smaller average diameter and average dry weight per nodule than Toyosuzu regardless of plant age or rhizobial strains (Bradyrhizobium japonicum (AI017, J5033, 646, J1B140), B. elkanii (USDA94), and Rhizobium fredii (MAFF210054)) inoculated at the rate of 108-109 cells per 3 L pot. These differences were observed in the nodules formed on both primary and lateral roots. With a lower inoculation dose of rhizobia (102 and 105 cells per pot), Kitamusume formed a smaller number of larger nodules than Toyosuzu inoculated with 108-109 cells per pot. This result indicated that the number of nodules was the factor directly controlled by host, because the size of the nodules was not determined by the genetic background, but changed depending on the number of nodules formed. The number of the first order lateral roots of 21 d old Kitamusume was 1.49 times larger than that of Toyosuzu. The relationship between the number of nodules and the number of first order lateral roots of 14 cultivars showed a significant positive correlation. These results indicated that the formation of nodules and lateral roots may be similarly controlled by a certain factor in different cultivars.

Root System Development of Cassava and Sweetpotato during Early Growth Stage as Affected by High Root Zone Temperature

Root zone temperature (RZT) is an important factor that affects the establishment of crops in the field. This study was conducted to determine the effect of high RZT on the root system development of cassava and sweetpotato and to assess the response of each root system component of cassava and sweetpotato to high RZTs. Cassava and sweetpotato are widely grown crops in soils that are prone to reach a high temperature especially during the dry season. The plant root system was the primary object of investigation in this study. Cassava and sweetpotato cuttings were grown in pots for 20 days in a growth chamber where two different soil temperature regimes were maintained : 40°C (high RZT) and 25°C (normal). High RZT significantly decreased the total length of the adventitious roots (ARs) and the number and total length of the first order lateral roots (LRs) in both cassava and sweetpotato. Reduction in these root growth parameters was greater in the latter than in the former. High RZT caused a marked increase in the number of the second order LRs in sweetpotato and tended to enhance the formation of the third order LRs. Under high RZT, in cassava, a higher proportion of the total number of ARs and the first order LRs, and thus, a greater fraction of the total root length came from the lower nodes of the cuttings that were buried deep in the soil at planting. In sweetpotato, a greater proportion of the total number and total length of the ARs and the first order LRs was observed in the nodes of the cuttings that were buried closer to the soil surface than those buried deeper. An opposite trend was evident in both cassava and sweetpotato grown under a normal RZT. In the screenhouse experiment where plants were grown for 12 days, root growth of cassava was significantly reduced by the high RZT that prevailed when the soil was not covered with mulch materials. The root development in the mulched soil was similar to that under normal RZT in the growth chamber.

Quantitative Analysis of Soil Sheath Distribution in Maize Root Systems

SummaryThe soil sheath (SS) distribution and its change with the passage of time in maize root systems were investigated. Maize seeds were planted in deep pots, and the roots were sampled at 2, 3, 4 and 6 weeks after sowing. The distribution of SS on the axile roots and the 1st order lateral roots was examined quantitatively. On axile roots, as classified by the nodes from which they emerged, younger roots were more thickly sheathed than older ones. A similar tendency was observed in the 1st order lateral roots. Significant correlations were observed between the thickness of SS, and the root length or the diameter in one part of the axile roots. L-type 1st order lateral roots (branched 1st order laterals) were sheathed more than S-type lateral roots (non-branched). Quantitative analysis revealed that the amount of SS was closely related to the root length, thickness, branching capacity, and root age.

The influence of pH on root morphology and mineral content of Pinus pinaster Ait. seedlings

ABSTRACT Two-week old seedlings of Pinus pinaster Ait. were grown in nutrient solutions at pH values ranging from 3.5 to 6.5. Acidity exerted a different influence on the growth of the main and lateral roots, thus changing the morphology of the whole root system. Roots grown at pH 3.5 had the shortest length, but the highest biomass and were markedly thickened. The total number of 2nd order lateral roots per plant was not significantly influenced by pH, whereas their length and the development of 3rd order roots were the highest at pH 3.5 and the lowest at pH 6.5. Mineral nutrition was affected by the pH of the culture solution. The P and Fe concentrations in roots were higher in the pH 3.5 and 4.5 solutions than in pH 5.5 and 6.5. In contrast, lower K, Ca, Mg and especially Mn contents were found in the roots grown at pH 3.5. However, needle dry weight was unaffected by low pH and neither chlorosis nor critical levels of mineral elements were observed. These results suggest that the compensatory growth of lateral roots allows P. pinaster seedlings to maintain an adequate nutritional status in the shoot even under highly acidic conditions.

Root Distribution of Two Tree Species Under a Heterogeneous Nutrient Environment

Potted sweetgum Liquidambar styracflua and loblolly pine Pinus taeda seedlings were grown for 4 months in a greenhouse to test the effects of soil nutrient heterogeneity on plant root development. The same quantity of fertilizer was applied either uniformly to the entire pot soil surface or to just one quarter of it. Pots were placed in full sunlight or underneath shade cloth that attenuated light to 30% of full sunlight. The spatial development of root systems of both species was strongly influenced by the fertilization arrangements, but they reacted to soil nutrient heterogeneity differently; sweetgum roots concentrated in the fertilized patch, while loblolly pine roots extended much more to the unfertilized patches. Root system morphology was also affected by nutrient arrangements. For both species, root interbranch length increased and fine roots were thinner in nutrient-rich patches. Root branching angles were either unaffected by nutrient patchiness or too variable to detect a trend. Root proliferation in the rich patches was primarily due to fine root elongation and development of higher order lateral roots. Fertilization arrangements under full light did not significantly affect most aspects of total plant growth or allometry, including total plant mass, above-ground mass, leaf mass, total fine root mass, total root mass, fine root mass/shoot mass and fine root mass/leaf mass. However, interspecific differences were usually significant. In shade, large fertilization treatment effects appeared in total plant mass, above-ground mass, leaf mass and height. Root development in heterogeneous soils differed between light treatments. Sweetgum was more plastic than loblolly pine in its reaction to soil nutrient patchiness under full light, but both had similar plasticity in shade. This study suggests that sweetgum and loblolly pine might have different nutrient foraging strategies and that the strategies shift as light availability changes. These results suggest that competitive interactions between sweetgum and loblolly pine may be affected by the spatial heterogeneity of soil resources. Since sweetgum is a more efficient soil forager, silvicultural methods that encourage uniform distribution of soil resources may also increase the competitive ability of loblolly pine, the more commercially valuable of the two species.

Effects of soil compaction on plant growth phosphorus uptake and morphological characteristics of vesicular—arbuscular mycorrhizal colonization of Trifolium subterraneum

SUMMARYWe investigated the effect of soil compaction and phosphorus (P) application on morphological characteristics of mycorrhizal colonization and growth responses, to determine the reasons for reduced responses observed in our previous work with compacted soil. Growth, phosphorus (P) uptake and intensity of vesicular–arbuscular (VA) mycorrhizal colonization were studied in clover plants (Trifolium subterraneum L.) with and without VA mycorrhizal colonization at two P applications and three levels of soil compaction. Phosphorus was supplied either at constant mass concentration (mg P kg−1 soil) or at constant volume concentration (mg P dm−3 soil). Increasing bulk density of the soil from 1·1 to 1.6 Mg m−3 significantly decreased root length and shoot d. wt, but increased the diameter of both main axes and first order lateral roots regardless of P application. Total P uptake and shoot d. wt of clover plants colonized by Glomus intraradices (Schenck & Smith) were significantly greater than those of non‐mycorrhizal plants at all levels of soil compaction and both P applications. However, soil compaction to a bulk density of 1.6 Mg m−3 (penetrometer resistance = 3.5 MPa at a matric potential of − 33 kPa) significantly decreased mycorrhizal growth response. There was no evidence that the increased volume concentration of P at high bulk densities was responsible for the reduced responses. Soil compaction had no significant effect on the fraction of root length containing arbuscules and vesicles, but total root length colonized by arbuscules, vesicles or by any combination of arbuscules, vesicles and intra‐radical hyphae significantly decreased as soil compaction was increased. The air‐filled porosity of highly compacted soil, which varied from 0.07 to 0.11 over the range of matric potentials encountered (− 33 and − 100 kPa), had no significant effect on the intensity of internal colonization.