Random root movements in weightlessness

The dynamics of root growth was studied in weightlessness. In the absence of the gravitropic reference direction during weightlessness, root movements could be controlled by spontaneous growth processes, without any corrective growth induced by the gravitropic system. If truly random of nature, the bending behavior should follow socalled 'random walk' mathematics during weightlessness. Predictions from this hypothesis were critically tested.
In a Spacelab ESA-experiment, denoted RANDOM and carried out during the IML-2 Shuttle flight in July 1994, the growth of garden cress ( Lepidium sativum ) roots was followed by time lapse photography at 1-h intervals.
The growth pattern was recorded for about 20 h. Root growth was significantly smaller in weightlessness as compared to gravity (control) conditions.
It was found that the roots performed spontaneous movements in weightlessness. The average direction of deviation of the plants consistently stayed equal to zero, despite these spontaneous movements. The average squared deviation increased linearly with time as predicted theoretically (but only for 8–10 h).
Autocorrelation calculations showed that bendings of the roots, as determined from the 1-h photographs, were uncorrelated after about a 2-h interval.
It is concluded that random processes play an important role in root growth. Predictions from a random walk hypothesis as to the growth dynamics could explain parts of the growth patterns recorded. This test of the hypothesis required microgravity conditions as provided for in a space experiment.     

Fine root demography in alfalfa (Medicago sativa L.)

In perennial forages like alfalfa (Medicago sativa L.), repeated herbage removal may alter root production and mortality which, in turn, could affect deposition of fixed N in soil. Our objective was to determine the extent and patterns of fine-diameter root production and loss during the year of alfalfa stand establishment. The experiment was conducted on a loamy sand soil (Udorthentic Haploboroll) in Minnesota, USA, using horizontally installed minirhizotrons placed directly under the seeded rows at 10, 20, and 40 cm depths in four replicate blocks. We seeded four alfalfa germplasms that differed in N₂ fixation capacity and root system architecture: Agate alfalfa, a winter hardy commercially-available cultivar; Ineffective Agate, which is a non-N₂-fixing near isoline of Agate; a new germplasm that has few fibrous roots and strong tap-rooted traits; and a new germplasm that has many fibrous roots and a strongly branched root system architecture. Video images collected biweekly throughout the initial growing season were processed using C-MAP-ROOTS software.More than one-half of all fine roots in the upper 20 cm were produced during the first 7 weeks of growth. Root production was similar among germplasms, except that the highly fibrous, branch-rooted germplasm produced 29% more fine roots at 20 cm than other germplasms. In all germplasms, about 7% of the fine roots at each depth developed into secondarily thickened roots. By the end of the first growing season, greatest fine root mortality had occurred in the uppermost depth (48%), and least occurred at 40 cm (36%). Survival of contemporaneous root cohorts was not related to soil depth in a simple fashion, although all survivorship curves could be described using only five rates of exponential decline. There was a significant reduction in fine root mortality before the first herbage harvest, followed by a pronounced loss (average 22%) of fine roots at the 10- and 20-cm depths in the 2-week period following herbage removal. Median life spans of these early-season cohorts ranged from 58 to 131 days, based on fitted exponential equations. At all depths, fine roots produced in the 4 weeks before harvest (early- to mid-August) tended to have shorter median life spans than early-season cohorts. Similar patterns of fine root mortality did not occur at the second harvest. Germplasms differed in the pattern, but not the ultimate extent, of fine root mortality. Fine root turnover during the first year of alfalfa establishment in this experiment released an estimated 830 kg C ha^–1 and 60 kg N ha^–1, with no differences due to N₂ fixation capacity or root system architecture.     

Distribution of mineral nutrient from nodal roots of Trifolium repens: Genotypic variation in intra-plant allocation of 32P and 45Ca

To assess genotypic variability in nutrient supply of shoot branches, the distribution of ³²P and ⁴⁵Ca exported from a source nodal root (24-h uptake period) was measured within a genotype of a large-leaved (Kopu) and a small-leaved (Tahora) cultivar of Trifolium repens. Source-sink relationships of plants were modified by root severance, defoliation, and shade treatments. In control plants of both genotypes distribution of ³²P and ⁴⁵Ca closely followed the pathways that could be predicted from the known phyllotactic constraints on the vascular system. As such there was little allocation of radioisotopes (3.1% and 2.5% of exported ³²P and ⁴⁵Ca, respectively) from the source root to branches on the apposite side of the parent axis (far-side branches). However, genotypic differences in nutrient allocation were apparent, when treatments were imposed to alter intra-plant source-sink relationships. In the large-leaved genotype, the imposed treatments had minor effects on the allocation to far-side branches: whereas, in the small-leaved genotype, root severance and defoliation treatments increased lateral transport to far-side branches to 30% (³²P) and 10% (⁴⁵Ca) of exported radioisotopes. Genotypes with low (8–9) and high (12–13) numbers of vascular bundles were selected from within the large-leaved cultivar. Distribution of ³²P was then measured after plants had been pre-treated by removal of all far-side roots two days prior to labelling. Genotypes with low vascular bundle number allocated 20% and those with high vascular bundle number 3.2% of exported ³²P to far-side branches. It was concluded (1) that genotypic variation exists within T. repens for potential to alter intra-plant allocation of mineral nutrients, in response to treatments that modify source-sink relationships within plants; and (2) that this variation is correlated with differences among genotypes in the organisation of the vasculature of their stolons.     

Interactions between the soilborne root pathogenPhytophthora nicotianae var.parasitica and the arbuscular mycorrhizal fungusGlomus mosseae in tomato plants

In order to study the influence of Arbuscular Mycorrhiza (AM) on the development of root rot infection, tomato plants were raised with or withoutGlomus mosseae and/orPhytophthora nicotianae var.parasitica in a sand culture system. All plants were fed with a nutrient solution containing one of two phosphorus (P) levels, 32µM (I P) or 96µM (II P), to test the consequence of enhanced P nutrition by the AM fungus on disease dynamics. Mycorrhizal plants had a similar development to that of control plants. Treatment withPhytophthora nicotianae var.parasitica resulted in a visible reduction in plant weight and in a widespread root necrosis in plants without mycorrhiza. The presence of the AM fungus decreased both weight reduction and root necrosis. The percentage reduction of adventitious root necrosis and of necrotic root apices ranged between 63 and 89% The enhancement of P nutrition increased plant development, but did not appreciably decrease disease spread. In our system, mycorrhiza increased plant resistance toP. nicotianae var.parasitica infection. Although a contribution of P nutrition by mycorrhiza cannot be excluded, other mechanisms appear to play a crucial role.     

Turnover of organic nitrogen in soils and its availability to crops

The root development of barley seedlings grown for one week in an aerated nutrient solution was studied in the presence of dissolved organic matter from an aqueous chestnut leaf litter extract. In particular, the different effects of low and high molecular weight fractions (small molecules: molecular weight <1000; large molecules: >10,000) of the leaf litter extract were examined. In the presence of large molecules root growth was inhibited, an irregular root tip morphology was observed, and Ca and Mg concentrations in the shoots were lower than in control plants. These phytotoxic effects were not caused by the formation of an impermeable layer of large molecules on the root surfaces that lower accessibility for nutrient cations as inferred from voltammetric experiments. A germination assay using spruce seeds, however, indicated allelochemical effects of large molecules, which exhibit a higher aromaticity than the small molecules as indicated by spectroscopic characterisation. In the growth experiments with small molecules, no influence on the root development of barley was evident, but an increase of Ca and Mg in the shoots was detected. During these growth experiments, a large amount of the small molecules, mainly simple phenols and amino acids, disappeared from the nutrient solution. The loss of small molecules was most likely the effect of mineralisation.Abbreviations DOC dissolved organic carbon - DOM dissolved organic matter - LLE leaf litter extract - MW molecular weight - HMDE hanging mercury drop electrode     

Root growth dynamics of Brussels sprouts (Brassica olearacea var.gemmifera) and leeks (Allium porrum L.) as reflected by root length,root colour and UV fluorescence

Minirhizotron observations of roots of leeks and Brussels sprouts grown in the Wageningen Rhizolab were used to study the dynamics of root length. Day of appearance and the time of decay were assessed for individual root segments visible on the minirhizotron surface.A Brussels sprouts crop produced much more root length than leeks, but the average longevity of these roots was about half that of leek roots.To investigate whether root colour or UV fluorescence could be used as a quantitative index of root functionality or root age, changes in root colour (on a scale of greys) over time were measured with interactive image analysis. In both crops a gradual change towards black was found with ageing. Measurements of the intensity of the UV fluorescence showed that leek roots fluoresced more than Brussels sprouts roots. Over time, UV fluorescence decreased in Brussels sprouts roots but increased in leek roots. It is concluded that UV fluorescence cannot be used as a universal indicator of root age or root functionality, but in some plant species it may be used to separate (transparent) roots from the background with image analysis techniques.     

Root,shoot and soil parameters required for process-oriented models of crop growth limited by water or nutrients

A review is given of the prospects for using process-oriented models of water and nutrient uptake in improving integrated agriculture. Government-imposed restrictions on the use of external inputs will increase the likelihood of (temporary) nutrient or water stress in crop production in NW Europe and thus a better understanding is required of shoot-root-soil interactions than presently available. In modelling nutrient and water uptake, three approaches are possible: 1) models-without-roots, based on empirically derived efficiency ratios for uptake of available resources, 2) models evaluating the uptake potential of root systems as actually found in the field and 3) models which also aim at a prediction of root development as influenced by interactions with environmental factors. For the second type of models the major underlying processes are known and research can concentrate on model refinement on the one hand and practical application on the other. The main parameters required for such models are discussed and examples are given of practical applications. For the third type of models quantification of processes known only qualitatively is urgently needed.     

Subnanomolar concentrations of membrane chitolipooligosaccharides from Rhizobium leguminosarum biovar trifolii are fully capable of eliciting symbiosis-related responses on white clover

Axenic seedling bioassays were performed on white clover, vetch, and alfalfa to assess the variety and dose responses of biological activities exhibited by membrane chitolipooligosaccharides (CLOSs) from wild type Rhizobium leguminosarum bv. trifolii ANU843. Subnanomolar concentrations of CLOSs induced deformation of root hairs (Had) and increased the number of foci of cortical cell divisions (Ccd) in white clover, some of which developed into nodule meristems. In contrast, ANU843 CLOSs were unable to induce Had in alfalfa and required a 10⁴-fold higher threshold concentration to induce this response in vetch. Also, ANU843 CLOSs were not mitogenic on either of these non-host legumes. In addition, CLOS action also increased chitinase activity in white clover root exudate. Thus, the membrane CLOSs from wild type R. leguminosarum bv. trifolii are fully capable of eliciting various symbiosis-related responses in white clover in the same concentration range as extracellular CLOSs of other rhizobia on their respective legume hosts. These results and our earlier studies indicate that membrane CLOSs represent one of many different classes of bioactive metabolites made by R. leguminosarum bv. trifolii which elicit more intense symbiosis-related responses in white clover than in other legumes. Therefore, CLOSs evidently play an important role in symbiotic development, but they may not be the sole determinant of host-range in the Rhizobium-clover symbiosis.Abbreviations Ccd cortical cell division - CLOS chitolipooligosaccharide - Had root hair deformation     

The effects of humic substances within the Ah horizon of a Calcic Luvisol on morphological changes related to invertase and peroxidase activities in wheat roots

The chemical and biological characteristics of humus within the Ah horizon (Calcic-Luvisol) have been studied. Attention was paid to variation in the NMR spectra of humic fractions and ¹³C values and to how these changes are related to different biological humic fraction activities.The chemical changes in particular involve the decrease of the aromatic component and the increase of the non-aromatic component within the horizon and the different ¹³C value not only within the horizon but also among the humic fractions distinctive of different molecular sizes.An attempt has been made to explain the vertical chemical changes in terms of processes affecting the biological characteristics of the high and low molecular size humic fractions. The main conclusions are that the low molecular size humic fractions, in the upper part of the horizon, are of greater importance with respect to the other humic fractions in influencing the enzyme activities linked to growth metabolism. The biological role of the high molecular size humic fractions characterised by a relevant content of peptidic- and carbohydratic-C is also presented.     

Pre-inoculation of Ri T-DNA-transformed pea roots with Pseudomonas fluorescens inhibits colonization by Pythium ultimum Trow: an ultrastructural and cytochemical study

The influence exerted by Pseudomonas fluorescens, strain 63-28R, in stimulating plant defense reactions was investigated using an in-vitro system in which Ri T-DNA-transformed pea (Pisum sativum L.) roots were subsequently infected with Pythium ultimum. Cytological investigations of samples from P. fluorescens-inoculated roots revealed that the bacteria multiplied abundantly at the root surface and colonized a small number of epidermal and cortical cells. Penetration of the epidermis occurred through the openings made by the disruption of the fibrillar network at the junction of adjacent epidermal cell walls. Direct cell wall penetration was never observed and bacterial ingress into the root tissues proceeded via an intercellular route. Striking differences in the extent of fungal colonization were observed between bacterized and non-bacterized pea roots following inoculation with P. ultimum. In non-bacterized roots, the pathogen multiplied abundantly through most of the tissues while in bacterized roots, pathogen growth was restricted to the epidermis and the outer cortex. At the root surface, the bacteria interacted with the pathogen, in a way similar to that observed in dual culture tests. Most Pythium cells were severely damaged but fungal penetration by the bacteria was never observed. Droplets of the amorphous material formed upon interaction between the bacteria and the host root were frequently found at the fungal cell surface. Incubation of sections with a -1,4-exoglucanase-gold complex revealed that the cell wall of markedly altered Pythium hyphae was structurally preserved. Successful penetration of the root epidermis was achieved by the few hyphae of P. ultimum that could escape the first defensive line in the rhizosphere. Most hyphae of the pathogen that penetrated the epidermis exhibited considerable changes. The unusual occurrence of polymorphic wall appositions along the host epidermal cells was an indication that the host plant was signalled to defend itself through the elaboration of physical barriers.Abbreviations AGL Aplysia gonad lectin - PGPR plant growth-promoting rhizobacteria The authors wish to thank Sylvain Noël for excellent technical assistance. This study was supported by grants from the Fonds Québécois pour la formation de chercheurs et l'Aide à la Recherche (FCAR), the Natural Sciences and Engineering Council of Canada (NSERC) and the Ministère de l'Industrie, du Commerce, de la Science et de la Technologie (SYNERGIE).