Neoformation of clay in lateral root catchments of mallee eucalypts: a chemical perspective

Background and Aims

A previous paper (Annals of Botany 103: 673–685) described formation of clayey pavements in lateral root catchments of eucalypts colonizing a recently formed sand dune in south-west Western Australia. Here chemical and morphological aspects of their formation at the site are studied.

Methods

Chemical and physical examinations of soil cores through pavements and sand under adjacent heath assessed build-up of salts, clay and pH changes in or below pavements. Relationships of root morphology to clay deposition were examined and deposits subjected to scanning electron microscopy and energy-dispersive X-ray analysis. Xylem transport of mineral elements in eucalypt and non-eucalypt species was studied by analysis of xylem (tracheal) sap from lateral roots.

Key Results

The columns of which pavements are composed develop exclusively on lower-tier lateral roots. Such sites show intimate associations of fine roots, fungal filaments, microbiota and clay deposits rich in Si, Al and Fe. Time scales for construction of pavements by eucalypts were assessed. Cores through columns of pavemented profiles showed gross elevations of bulk density, Al, Fe and Si in columns and related increases in pH, Mg and Ca status in lower profiles. A cutting through the dune exhibited pronounced alkalinity (pH 7–10) under mallee woodland versus acidity (pH 5–6·5) under proteaceous heath. Xylem sap analyses showed unusually high concentrations of Al, Fe, Mg and Si in dry-season samples from column-bearing roots.

Conclusions

Deposition of Al–Fe–Si-rich clay is pivotal to pavement construction by eucalypts and leads to profound chemical and physical changes in relevant soil profiles. Microbial associates of roots are likely to be involved in clay genesis, with parent eucalypts supplying the required key mineral elements and carbon sources. Acquisition of the Al and Fe incorporated into clay derives principally from hydraulic uplift from ground water via deeply penetrating tap roots.Key words: Niche construction, eucalypts, root morphology, xylem transport, hydraulic lift, element mining, soil formation, biomineralization, soil pans, duplex soils     

Contemporary biogenic formation of clay pavements by eucalypts: further support for the phytotarium concept

Background and Aims

Clayey (sodosolic) pavements were studied in lateral root catchments of eucalypts where mixed myrtaceous:proteaceous vegetation was colonizing a dune of quartzitic sand blown out from a playa lake during late Pleistocene times. The site at Chillinup in south-west Western Australia provided an opportunity to examine these signature pavements in an unequivocally recent setting, and to assess their effects on competing non-eucalypt vegetation.

Methods

Pavements were located, and their extents and depths assessed by probing with steel rods, followed by corings and pit excavations using an air spade. Listings of plant species, growth forms and root morphologies were assembled for different vegetation zones in a representative transect across the dune. A deep cutting through the dune provided details on pavement morphology and modifications to the sand deposit bioengineered by eucalypt and heath vegetation.

Key Results

Clay pavements comprised closely spaced, round-topped columns whose mean diameters and depths varied between eucalypt species. Incipient pavement formation was characterized by clumps of clay deposited around fine root material. Pavements appeared to have been synthesized in situ from locally accessed and imported constituents. Understoreys on superficial pavements of a tree eucalypt were considerably less dense and biodiverse than on the deeper pavements of two mallee species, whilst most profuse vegetation cover was encountered in heath on unmodified (non-pavemented) sand. Certain species were restricted to superficially located pavements, whilst other ‘generalist’ species occurred widely across the dune. Relict pavements formed by earlier generations of eucalypts were present in certain areas of the transect and in soil profiles of the cutting. Some relict pavements colonized by proteaceous shrubs were overprinted with ferricrete.

Conclusions

Clay pavements formed by eucalypts have pronounced effects on understorey vegetation and may have been instrumental in establishment of the complex mosaics of mallee-woodland and proteaceous heathland observed across semi-arid landscapes of south-west Western Australia. Findings are related to earlier observations on the range of plant-mediated changes in soil profiles discussed in the recently advanced ‘Phytotarium’ concept.Key words: Biomineralization, eucalypt roots, niche construction, soil profiles, woody plants, semi-arid ecosystems, vegetation mosaics, competing phytotaria     

Architectural Tradeoffs between Adventitious and Basal Roots for Phosphorus Acquisition

Adventitious rooting contributes to efficient phosphorus acquisition by enhancing topsoil foraging. However, metabolic investment in adventitious roots may retard the development of other root classes such as basal roots, which are also important for phosphorus acquisition. In this study we quantitatively assessed the potential effects of adventitious rooting on basal root growth and whole plant phosphorus acquisition in young bean plants. The geometric simulation model SimRoot was used to dynamically model root systems with varying architecture and C availability growing for 21 days at 3 planting depths in 3 soil types with contrasting nutrient mobility. Simulated root architectures, tradeoffs between adventitious and basal root growth, and phosphorus acquisition were validated with empirical measurements. Phosphorus acquisition and phosphorus acquisition efficiency (defined as mol phosphorus acquired per mol C allocated to roots) were estimated for plants growing in soil in which phosphorus availability was uniform with depth or was greatest in the topsoil, as occurs in most natural soils. Phosphorus acquisition and acquisition efficiency increased with increasing allocation to adventitious roots in stratified soil, due to increased phosphorus depletion of surface soil. In uniform soil, increased adventitious rooting decreased phosphorus acquisition by reducing the growth of lateral roots arising from the tap root and basal roots. The benefit of adventitious roots for phosphorus acquisition was dependent on the specific respiration rate of adventitious roots as well as on whether overall C allocation to root growth was increased, as occurs in plants under phosphorus stress, or was lower, as observed in unstressed plants. In stratified soil, adventitious rooting reduced the growth of tap and basal lateral roots, yet phosphorus acquisition increased by up to 10% when total C allocation to roots was high and adventitious root respiration was similar to that in basal roots. With C allocation to roots decreased by 38%, adventitious roots still increased phosphorus acquisition by 5%. Allocation to adventitious roots enhanced phosphorus acquisition and efficiency as long as the specific respiration of adventitious roots was similar to that of basal roots and less than twice that of tap roots. When adventitious roots were assigned greater specific respiration rates, increased adventitious rooting reduced phosphorus acquisition and efficiency by diverting carbohydrate from other root types. Varying the phosphorus diffusion coefficient to reflect varying mobilities in different soil types had little effect on the value of adventitious rooting for phosphorus acquisition. Adventitious roots benefited plants regardless of basal root growth angle. Seed planting depth only affected phosphorus uptake and efficiency when seed was planted below the high phosphorus surface stratum. Our results confirm the importance of root respiration in nutrient foraging strategies, and demonstrate functional tradeoffs among distinct components of the root system. These results will be useful in developing ideotypes for more nutrient efficient crops.     

Root system development of Larix gmelinii trees affected by micro-scale conditions of permafrost soils in central Siberia

Spatial distributions of root systems of Larix gmelinii (Rupr.) Rupr. trees were examined in two stands in central Siberia: an even-aged stand (ca. 100 yrs-old) and a mature, uneven-aged (240–280 yrs-old) stand. Five larch trees of different sizes were sampled by excavating coarse roots (diameter > 5 mm) in each stand. Dimensions and ages of all first-order lateral roots were measured. Micro-scale conditions of soil temperature and soil water suction (each 10 cm deep) were also examined in relation to earth hummock topography (mound vs. trough) and/or ground floor vegetation types (moss vs. lichens). All larch trees developed superficial root systems, consisting of the aborted short tap root (10–40 cm in soil depth) and some well-spread lateral roots (n= 4-13). The root network of each tree was asymmetric, and its rooting area reached about four times the crown projection area. Lateral roots generally expanded into the upper soil layers of the mounds where summer soil temperature was 1–6°C higher than inside nearby troughs. Chronological analysis indicated that lateral root expansion started successively from lower to upper parts of each aborted tap root, and some lateral roots occurred simultaneously at several decades after tree establishment. The process of root system development was likely to be primarily linked with post-fire dynamics of rhizosphere environment of the permafrost soils.     

The inhibition of infection thread development in the cultivar-specific interaction ofRhizobium and subterranean clover is not caused by a hypersensitive response

Summary The cultivar specific interaction ofTrifolium subterranean cv. Woogenellup andRhizobium leguminosarum bv.trifolii strain ANU 794 was examined to establish the basis for nodulation failure on this cultivar. Infections were initiated by strain ANU 794 on cv. Woogenellup. Root hair curling, the initiation of infection threads, and cortical cell divisions were evident on the tap root and appeared normal after microscopic observation. However, in most cases, the infection threads stayed confined to the root hairs. No evidence was found for a hypersensitive response by the plant. The progress of infections on the tap roots was different from that on the lateral roots. This was confirmed by the differential tap and lateral root nodulation patterns of the mutants derived from strain ANU 794, which show enhanced nodulation on cv. Woogenellup. On the lateral roots, cortical cell divisions progressed further than those on the tap root and formed macroscopically visible swellings, which could be divided into two morphological classes. In some cases infection threads developed into these primordia but successful nodules were not established. The inhibition of infection appeared to be manifested at two levels: first, on the tap roots in the root hairs, where many of the infection threads are contained and secondly, in the primordia induced on the lateral roots, where the infection threads sometimes penetrate further than the root hair cell but stop in the primordial cells. It appears that an essential factor or trigger in the communication between plant and bacteria is missing or altered, resulting in an array of primordia-structures, which cease to develop.Abbreviations bv biovar - cv cultivar - Fix⁺ nitrogen fixing - GUS -glucuronidase - Nod⁺ nodulating - HR hypersensitive response - Km kanamycin - LOSs lipo-oligosaccharides - Sm streptomycin - Sp spectinomycin - X-Gluc 5-bromo-4-chloro-3-indonyl--glucuronic acid     

Lateral root formation in pine seedlings

The role of assimilates in lateral root development was studied in Pinus pinea seedlings grown in a nutrient solution. Seedlings were treated with ¹⁴CO₂ for 2 h following removal of the tap root tip at various times prior to the application of ¹⁴CO₂ or removal of a different number of cotyledons at one time. In seedlings with intact root systems most of the radioactivity accumulated in the lower section of the root containing the tap root apex. When the tap root tip was removed, the pattern of radioactivity accumulation along the root was affected by the presence and the stage of lateral root development. Removing the tap root tip of young seedlings (with no lateral roots) resulted in an almost equal distribution of radioactivity along the root. About 50% of the total radioactivity was found in the section showing the highest lateral root growth. Removing the tap root tip of mature seedlings (with lateral roots in the upper section) resulted in an immediate increase in the radioactivity accumulation in the upper section. When lateral roots appeared in the middle section, the pattern of radioactivity distribution was similar to that found in root decapitated young seedlings. Removal of cotyledons of mature seedlings somewhat increased the transport of radioactivity to the lower root section at the expense of the radioactivity in the lateral roots of the upper section. The present study suggests that competition within the root system between the tap root apex and the lateral roots may play an important role in determining the morphology of the root system.     

Is it possible to manipulate root anchorage in young trees?

The optimal root system architecture for increased tree anchorage has not yet been determined and in particular, the role of the tap root remains elusive. In Maritime pine (Pinus pinaster Ait.), tap roots may play an important role in anchoring young trees, but in adult trees, their growth is often impeded by the presence of a hard pan layer in the soil and the tap root becomes a minor component of tree anchorage. To understand better the role of the tap root in young trees, we grew cuttings (no tap root present) and seedlings where the tap root had (?) or had not (+) been pruned, in the field for 7 years. The force (F) necessary to deflect the stem sideways was then measured and divided by stem cross-sectional area (CSA), giving a parameter analogous to stress during bending. Root systems were extracted and root architecture and wood mechanical properties (density and longitudinal modulus of elasticity, E _L ) determined. In seedlings (?) tap roots, new roots had regenerated where the tap root had been pruned, whereas in cuttings, one or two lateral roots had grown downwards and acted as tap roots. Cuttings had significantly less lateral roots than the other treatments, but those near the soil surface were 14% and 23% thicker than plants (+) and (?) tap roots, respectively. Cuttings were smaller than seedlings, but were not relatively less resistant to stem deflection, probably because the thicker lateral roots compensated for their lower number. Apart from stem volume which was greater in trees (+) tap roots, no significant differences with regard to size or any root system variable were found in plants (?) or (+) tap roots. In all treatments, lateral roots were structurally reinforced through extra growth along the direction of the prevailing wind, which also improved tap root anchorage. Predictors of log F/CSA differed depending on treatment: in trees (?) tap roots, a combination of the predictors stem taper and %volume allocated to deep roots was highly regressed with log F/CSA (R ² = 0.83), unlike plants (+) tap roots where the combined predictors of lateral root number and root depth were best regressed with log F/CSA (R ² = 0.80). In cuttings, no clear relationships between log F/CSA and any parameter could be found. Wood density and E _L did not differ between roots, but did diminish with increasing distance from the stem in lateral roots. E _L was significantly lower in lateral roots from cuttings. Results showed that nursery techniques influence plant development but that the architectural pattern of Maritime pine root systems is stable, developing a sinker root system even when grown from cuttings. Anchorage is affected but the consequences for the long-term are still not known. Numerical modelling may be the only viable method to investigate the function that each root plays in adult tree anchorage.     

Differential Two-Dimensional Protein Patterns as Related to Tissue Specificity and Water Conditions in Brassica napus var oleifera Root System

Differential two-dimensional protein patterns as related to tissue specificity and water conditions were investigated within Brassica napus var oleifera root system. The different parts of the root system (tap root, lateral roots, and drought-induced short roots) were analyzed under various moisture regimes (regular watering at field capacity, progressive drought stress, and rewatering). Tissue specificity was evident from 25 differences in protein patterns (qualitative and quantitative) between well-watered lateral and tap roots. Twice as many polypeptides (52) were drought-affected and the response to the water stress was shown to be similar in both root types. In addition, more than half of the polypeptides detected as organ-specific were affected by drought. Based upon the trend of variation observed under drought and rehydration, three categories of polypeptides could be defined that might be differently involved in drought susceptibility or tolerance. A highly differentiated protein pattern characterized the drought-induced short roots. This pattern appeared as far from the watered as from the water-stressed normal roots. In particular, 13 unique polypeptides were detected which could be relevant to their adaptive morphogenesis and/or their specific drought tolerance induction. Upon rehydration, their polypeptide pattern and their specific morphology returned to a normal well-watered lateral root type.     

Seasonal changes in adventitious root formation in hypocotyl cuttings of Pinus sylvestris: Influence of Photoperiod during stock plant growth and of indolebutyric acid treatment of cuttings

Seedlings of Pinus sylvestris were grown for 6 weeks under natural light conditions in a temperature controlled environment room. Cuttings from these plants were rooted in tap water or in indolebutyric acid (IBA) solutions for 60 days at an irradiance of 16 W m^-2. Experiments were performed at 3-week intervals during two growth seasons. — Seasonal changes in root formation were found in control cuttings as well as in IBA treated cuttings. The number of roots and the percentage of cuttings that rooted were high during early spring and autumn. During the summer period hardly any roots were formed. Stimulation of root formation by IBA occurred manily during spring and autumn when cuttings already possessed the ability to form roots. — The influence of photoperiod during stock plant growth was also investigated. Shorter photoperiod resulted in an increase in the number of roots and rooting percentage. The period during summer where rooting was inhibited under natural light conditions was considerably shortened when stock plants were grown at a photoperiod of only 4 h. The results demonstrate the importance of the growing conditions for stock plants for subsequent root formation. The results are discussed with special reference to the role of irradiance.     

Bioengineering of soil profiles in semiarid ecosystems: the ‘phytotarium’ concept. A review

This review draws attention to information from the literature and our own observations supporting the view that higher plants and micro-organisms display an intrinsic capacity to be proactively involved in pedogenetic processes. ‘Bioengineering’ of this kind is deemed to be spearheaded by principal deep-rooted tree and shrub species and to result in optimisation of command and conservation of water and nutrients within an ecosystem. Specific examples discussed in the paper include, the formation of silicon- or iron-based linings of vertical channels and pores, binding of sand on roots, generation of organically derived hydrophobicity, development of clay-based hardpans and texture-contrast seals, precipitation of silcrete, calcrete and ferricrete pavements, effective accessing and conservation of P resources, including mining by microbes and the biological cycling of Si and Al via plants and micro-organisms. In each case, definitive roles and mechanisms are suggested for the organisms involved, particularly in relation to formative effects relating to secretion of organic acids, dispersing agents and other classes of exudate. We introduce the term ‘phytotarium’ to connote the collective outcomes of the above biotic influences in construction and maintenance of niches peculiar to specific vegetation types and then review the evidence of imprinting of soil profiles due to operation of phytotaria. Examples given relate to the lateral and vertical facies encountered in certain contemporary soil profiles and paleosols with which we are familiar and are described in a companion paper.     

Seasonal water uptake and movement in root systems of Australian phraeatophytic plants of dimorphic root morphology: a stable isotope investigation

A natural abundance hydrogen stable isotope technique was used to study seasonal changes in source water utilization and water movement in the xylem of dimorphic root systems and stem bases of several woody shrubs or trees in mediterranean-type ecosystems of south Western Australia. Samples collected from the native treeBanksia prionotes over 18 months indicated that shallow lateral roots and deeply penetrating tap (sinker) roots obtained water of different origins over the course of a winter-wet/summer-dry annual cycle. During the wet season lateral roots acquired water mostly by uptake of recent precipitation (rain water) contained within the upper soil layers, and tap roots derived water from the underlying water table. The shoot obtained a mixture of these two water sources. As the dry season approached dependence on recent rain water decreased while that on ground water increased. In high summer, shallow lateral roots remained well-hydrated and shoots well supplied with ground water taken up by the tap root. This enabled plants to continue transpiration and carbon assimilation and thus complete their seasonal extension growth during the long (4–6 month) dry season. Parallel studies of other native species and two plantation-grown species ofEucalyptus all demonstrated behavior similar to that ofB. prionotes. ForB. prionotes, there was a strong negative correlation between the percentage of water in the stem base of a plant which was derived from the tap root (ground water) and the amount of precipitation which fell at the site. These data suggested that during the dry season plants derive the majority of the water they use from deeper sources while in the wet season most of the water they use is derived from shallower sources supplied by lateral roots in the upper soil layers. The data collected in this study supported the notion that the dimorphic rooting habit can be advantageous for large woody species of floristically-rich, open, woodlands and heathlands where the acquisition of seasonally limited water is at a premium.     

Rooting and establishment of Calabrian pine plantlets propagated in vitro: influence of growth substances, rooting medium and origin of explant

The effect of auxins and a cytokinin on induction of roots in cultured axillary shoots of Pinus brutia Ten, has been tested. Auxin was crucial for root initiation and the rooting response varied according to the type and concentration of auxin applied. Both auxin and cytokinin and the interactions between them affected the quantity and quality of the induced roots. Aerated non-sterile tap water was an effective rooting medium, comparable to agar. After planting out into soil, some of the influences of auxin and cytokinin could still be seen after six months. However, roots developed normally in the soil, displaying dichotomous lateral branches. The results draw attention to the need for care in the choice and application of the medium for the initial induction of roots. Results of greenhouse trials indicated that the most vigorous plants were obtained via the axillary shoots.     

Evaluation of diverse soybean germplasm for root growth and architecture

Root characteristics of soybean (Glycine max (L.) Merr.) improve drought avoidance by increasing water uptake from the soil profile. Screening genotypes for improved root architecture without breaking the taproots or losing lateral roots is a challenge. Due to difficulty in separating roots from field or potting soil, a rapid and effective screening method with a suitable growth medium to assess root characteristics under controlled conditions needs to be established. We describe two screening techniques “the cone system” and “the tube system” using turface:sand medium. In the cone system thirty four soybean lines including cultivars and exotic plant introduction (PI) lines were evaluated for tap root length and root biomass, 12 days after sowing. Eight replications per line were grown in a growth chamber. Significant differences among genotypes for tap root length were detected by the cone system. Validity of results from the cone system was tested by evaluating root growth 21 days after planting for eight lines in the tube system. A coefficient of determination of 0.72 indicated good agreement between the two screening systems for evaluating genotypes for rooting depth. The cone system will be a useful method to easily and rapidly assess soybean genotypes for root growth.     

Relationships between cluster root-bearing taxa and laterite across landscapes in southwest Western Australia: an approach using airborne radiometric and digital elevation models

As part of a general soil and regolith mapping exercise across cleared and remnant bush land, radiometric data for distribution of potassium (K), uranium (U) and thorium (Th) were examined alongside relief models and floristic and soil surveys in test catchments at Elashgin and Toolibin in the Western Australian wheat-belt. The Elashgin survey showed that highly weathered low K soils co-concentrated U and Th and were vegetated mainly by cluster root-bearing Proteaceae and Casuarinaceae. In granitic soils ratios of U to Th were higher and cluster root bearing taxa much less prominent, except where ferricrete gravels were concentrated. Draping of radiometric imagery over a digital elevation model showed spiral waveforms of high and low U and Th signal which were largely independent of topography but demarcated different oligotrophic communities. General observations and a detailed 900-m transect along an aeolian sand plume at Toolibin showed very high U and Th in ferricrete gravels where Proteaceae were dominant, but failed to separate proteaceous versus myrtaceous shrublands on deep sands due to truncation of signal. Augering along the transect and examination of floristic, soil and signal composition of 32 sites in the Lake Toolibin catchment confirmed presence and degree of development of ferricretes in the rhizosphere of Proteaceae-dominated communities and showed high reactivity scores for Al in `Bs' horizons in profiles carrying such vegetation. Highly specific associations between Proteaceae and very high U and Th were generally evident on exposed ferricrete gravels. The data are discussed in relation to the effects of root exudates on ferricrete formation and destruction and how the broader spatial pattern of such processes might relate to competition for soil phosphorous.     

Differential Al resistance and citrate secretion in the tap and basal roots of common bean seedlings

The common bean root system is composed of several types of root (e.g. tap, basal, and lateral roots), whose physiological functions may be of great difference. However, we do not know if the root system of common bean differs in organic acid secretion and thus aluminium (Al) resistance. In the present study, the tap and basal roots of three common bean genotypes (i.e. G19842, SQ12 and BAT881) from different origins were compared for their citrate secretion and Al resistance. Grown in a simple solution containing 30 µM Al³⁺ for 24 h, genotype G19842 maintained 75% relative tap root length [RTRL = (tap root length with Al)/(tap root length without Al)], 48% relative basal root length [RBRL = (basal root length with Al)/ (basal root length without Al)], genotype SQ12 maintained 62% RTRL and 57% RBRL, while BAT881 only maintained 31% RTRL and 19% RBRL, indicating differential sensitivity of bean genotypes and root types to Al stress. The amounts of Al‐induced citrate secretion by the tap/basal roots were 9.8/5.1, 8.2/5.9 and 5.4/4.1 nmol cm^?2 FR (fresh root) [12 h]^?1 for G19842, SQ12 and BAT881, respectively, indicating that both bean genotypes and root types differ in organic acid secretion. In G19842, the root surface area was 25% higher in tap root apex than that in basal root apex, and the amounts of citrate secretion per unit surface area and per root apex were 29 and 62% higher in tap root apex than those in basal root apex, respectively, suggesting that the higher citrate secretion in the tap root apex could be attributed to the larger surface area and the higher secretion activity. Stronger inhibition of Al‐induced citrate secretion in the basal than tap roots by anthracene‐9‐carboxylic acid, an inhibitor of anion channel and K‐252a, a broad range inhibitor of protein kinase may also imply the differences in the activities of anion channels and K‐252a‐sensitive protein kinases on the plasma membrane between the tap and basal roots, resulting in differential citrate secretion. We propose that the higher Al resistance in the tap root than in basal roots might be attributed to both greater number and higher activity of the anion channels in the former, thus allowing more citrate secretion in this root type.     

Root development and configuration in intensively managed radiata pine plantations

Summary In south-east Australia, where radiata pine (Pinus radiata D. Don) is grown on sandy soils low in nutrients and short of water, early establishment, and rapid growth to canopy closure lead to increased productivity. At this stage demands for nutrients and water are high, and trees respond vigorously to silvicultural inputs.For several months after transplanting in winter roots are confined within a narrow planting wedge, low temperature restricts new root growth and slows recovery from water stress in plants. From spring, depending upon the configuration and vigour of the roots transplanted, lateral roots extend radially throughout the soil.Although there were small decreases in concentration of roots radially from the stems of very young trees, such spatial differences disappeared between ages 2 and 3, so that rooting density was independent of distance from the stem. The pattern of vertical distribution of lateral roots was not influenced by age and 80–90% of the lateral roots were within the top 30 cm soil. Roots developed rapidly as the trees grew towards canopy closure, but in general the rooting densities of these pines are among the lowest reported for plants. In rapidly growing trees approaching canopy closure, the secondary thickening of the lateral roots was sufficient to double the weight of roots without altering root length.Knowledge about root growth and root configuration during the early phase of plantation development will assist management decisions where intensive silviculture is practiced, and hence ensure the most efficient use of nutrients and water.     

Initial growth of Brazilian pine (Araucaria angustifolia) under equal soil volumes but contrasting rooting depths

Araucaria angustifolia is a critically endangered tall tree species of valuable wood, and field observations led to the suggestion that limitations imposed to the vertical growth of its tap root system greatly restrict the height of mature individuals. However, experimental studies dealing with the effects of soil depth on the species growth are mostly lacking. This study evaluated and compared the growth responses of young plants of A. angustifolia to distinct rooting depths but same soil volumes. Seeds were planted in pots of different heights and diameters, all containing 3 liters of soil mixture. Plants were submitted to four available rooting depths: 65 (T1), 35 (T2), 20 (T3), and 10 (T4) cm. There were eight experimental units in each treatment, arranged in a randomized complete block design, each block containing two units per treatment. Contrary to what was expected, the T3 and T4 plants had accumulated more mass and attained the same height as the other two groups, after a 10-month growth period in a green house. Those plants also had thicker stems, longer shoot branches, and thicker and longer lateral roots, which were interpreted as compensatory responses to increase plant anchorage and stability. The inverse relationship between rooting depth and plant mass was attributed to a down-regulation of shoot growth because or restricted lateral space and/or poor soil aeration of the longer and narrower pots. This experiment allowed us to demonstrate that is not the possibility of the tap root to grow deep into the soil that ensures a better growth to plants of A. angustifolia: provided that the offer of soil volume and resources are the same, the vertical extension of the tap root does not result in greater growth of the plants. In fact, much greater growth impairment is expected from lateral than from vertical restriction to root growth.     

Contrasting changes in vegetation structure and diversity with time since fire in two Australian Mediterranean‐climate plant communities

Mallee‐heath and mallee communities occur in a mosaic across large areas of south‐western Australia, in topographically subdued and fire‐prone landscapes. Consequently, it could be expected that these communities would have historically experienced similar fire regimes, and would respond similarly to variation in aspects of the fire regime. We studied the response of mallee‐heath and mallee to time since the last fire, measuring species density, species–area relationships, diversity indices and vegetation structure. Floristic responses to time since fire accorded with the initial floristic composition model of plant succession, with declining species density and Shannon diversity with age in mallee‐heath. Mallee‐heath exhibited structural senescence when > approximately 45–55 years since fire, with increasing standing dead vegetation, bare ground and stagnating or declining size in sprouting Eucalyptus spp. Mallee showed no such evidence of senescence, and indeed continued to increase in stature beyond the mean fire interval but without the compositional change required to provide support for the relay floristic model of plant succession. These results indicate that mallee‐heath is a fire maintained community and as such is reliant upon periodic burning to maintain diversity and vigour. Mallee, in contrast, is modified but not maintained by fire (at least over the period of time since fire examined) and hence is less susceptible to fire interval effects. Indeed, structural attributes likely to be significant for fauna habitat and carbon sequestration continue to develop in mallee unburnt for 55 years or more. Different responses to time since fire will create challenges for management, particularly in fragmented landscapes where fire potentially interacts with other threatening processes.     

硝态氮(NO-3)对水稻侧根生长及其氮吸收的影响

侧根是植物吸收利用土壤养分的重要器官,其生长发育受内部遗传因子和外部环境矿质养分的影响.通过琼脂分层培养发现:局部供应NO-3可以诱导水稻( Oryza sativa L.)主根或不定根上侧根的生长.为研究旱种条件下NO-3对水稻侧根发育及其N吸收的影响,设置了3个蛭石培养实验:分根处理、全株缺N、全株供N处理.分根处理(一半根系供应3 mmol/L KNO3,另一半根系供应3 mmol/L KCl)结果表明:局部供应NO-3 能够促进水稻侧根生长.而在全株处理下,N饥饿诱导了侧根的伸长.水稻根系对NO-3的这两种反应都存在着显著的基因型差异.同时对地上部N浓度、可溶性总糖含量及N含量分析表明,这些生理指标在分根处理与全株加N处理中的差异均不显著,表明分根处理也能基本满足植株正常生长对N的需求.在分根处理中,水稻的N含量与分根处理中供N一侧的平均侧根长度存在显著正相关,这表明在养分不均一的介质中,侧根长度对水稻N素吸收具有十分重要的作用.而在N素充足的条件下,两者之间的相关性并不显著,这暗示在养分充足的环境下,侧根长度可能并不是决定根系吸收N素的主要因素.     

Histological analysis of adventitious rooting in Arabidopsis thaliana (L.) Heynh seedlings

ABSTRACT

Adventititous rooting is essential for the post-embryonic growth of the root apparatus in various species. In Arabidopsis thaliana, adventitious rooting has been reported in some mutants, and auxin seems to be the inducer of the process. The objective of the study was to identify the tissues involved in adventitious rooting in the most commonly used ecotypes for molecular and genetic studies (i.e. Columbia, Wassilewskija and Landsberg erecta) both in the presence and absence of exogenous auxin. Seedlings of the three ecotypes were grown under various conditions. When grown under 16 hours light/day for 11 days, all seedlings showed adventitious roots, both with and without auxin, however, both adventitious and lateral rooting were enhanced by exogenous auxin (2 µM naphthaleneacetic acid). Independently of the presence of auxin and of the ecotype, the hypocotyl pericycle produced adventitious roots directly (i.e., according to the same pattern of lateral root formation by the pericycle cells in the primary root). However, in the presence of auxin, roots of indirect origin also, and mainly, formed and their formation was preceded by the exfoliation of the tissues external to the stele. Exfoliation was caused by cell hypertrophy, separation, and disintegration, which mainly involved the endodermis. At the exfoliation site, the pericycle, with a minor contribution of a few endodermal cells, produced the callus from which indirect roots arose. The finding that adventitious rooting occurs in the absence of auxin (all ecotypes) indicates that this process is part of the normal root apparatus in Arabidopsis, with the hypocotyl pericycle as the target tissue of the process. Exogenous auxin alters adventitious rhizogenesis mainly affecting the endodermis response.