Stimulated Root Growth Research Articles

Gain-of-Function Phenotypes of Many CLAVATA3/ESR Genes, Including Four New Family Members, Correlate with Tandem Variations in the Conserved CLAVATA3/ESR Domain

Secreted peptide ligands are known to play key roles in the regulation of plant growth, development, and environmental responses. However, phenotypes for surprisingly few such genes have been identified via loss-of-function mutant screens. To begin to understand the processes regulated by the CLAVATA3 (CLV3)/ESR (CLE) ligand gene family, we took a systems approach to gene identification and gain-of-function phenotype screens in transgenic plants. We identified four new CLE family members in the Arabidopsis (Arabidopsis thaliana) genome sequence and determined their relative transcript levels in various organs. Overexpression of CLV3 and the 17 CLE genes we tested resulted in premature mortality and/or developmental timing delays in transgenic Arabidopsis plants. Overexpression of 10 CLE genes and the CLV3 positive control resulted in arrest of growth from the shoot apical meristem (SAM). Overexpression of nearly all the CLE genes and CLV3 resulted in either inhibition or stimulation of root growth. CLE4 expression reversed the SAM proliferation phenotype of a clv3 mutant to one of SAM arrest. Dwarf plants resulted from overexpression of five CLE genes. Overexpression of new family members CLE42 and CLE44 resulted in distinctive shrub-like dwarf plants lacking apical dominance. Our results indicate the capacity for functional redundancy of many of the CLE ligands. Additionally, overexpression phenotypes of various CLE family members suggest roles in organ size regulation, apical dominance, and root growth. Similarities among overexpression phenotypes of many CLE genes correlate with similarities in their CLE domain sequences, suggesting that the CLE domain is responsible for interaction with cognate receptors.

A central role for the nitrate transporter NRT2.1 in the integrated morphological and physiological responses of the root system to nitrogen limitation in Arabidopsis.

Up-regulation of the high-affinity transport system (HATS) for NO(3)(-) and stimulation of lateral root (LR) growth are two important adaptive responses of the root system to nitrogen limitation. Up-regulation of the NO(3)(-) HATS by nitrogen starvation is suppressed in the atnrt2.1-1 mutant of Arabidopsis (Arabidopsis thaliana), deleted for both NRT2.1 and NRT2.2 nitrate transporter genes. We then used this mutant to determine whether lack of HATS stimulation affected the response of the root system architecture (RSA) to low NO(3)(-) availability. In Wassilewskija (Ws) wild-type plants, transfer from high to low NO(3)(-) medium resulted in contrasting responses of RSA, depending on the level of nitrogen limitation. Moderate nitrogen limitation (transfer from 10 mm to 1 or 0.5 mm NO(3)(-)) mostly led to an increase in the number of visible laterals, while severe nitrogen stress (transfer from 10 mm to 0.1 or 0.05 mm NO(3)(-)) promoted mean LR length. The RSA response of the atnrt2.1-1 mutant to low NO(3)(-) was markedly different. After transfer from 10 to 0.5 mm NO(3)(-), the stimulated appearance of LRs was abolished in atnrt2.1-1 plants, whereas the increase in mean LR length was much more pronounced than in Ws. These modifications of RSA mimicked those of Ws plants subjected to severe nitrogen stress and could be fully explained by the lowered NO(3)(-) uptake measured in the mutant. This suggests that the uptake rate of NO(3)(-), rather than its external concentration, is the key factor triggering the observed changes in RSA. However, the mutation of NRT2.1 was also found to inhibit initiation of LR primordia in plants subjected to nitrogen limitation independently of the rate of NO(3)(-) uptake by the whole root system and even of the presence of added NO(3)(-) in the external medium. This indicates a direct stimulatory role for NRT2.1 in this particular step of LR development. Thus, it is concluded that NRT2.1 has a key dual function in coordinating root development with external NO(3)(-) availability, both indirectly through its role as a major NO(3)(-) uptake system that determines the nitrogen uptake-dependent RSA responses, and directly through a specific action on LR initiation under nitrogen-limited conditions.

EcoMeristem, a model of morphogenesis and competition among sinks in rice. 2. Simulating genotype responses to phosphorus deficiency.

Phenotypic plasticity enables plants to adjust their morphology and phenology to variable environments. Although potentially important for crop breeding and management, the physiology and genetics of plasticity traits are poorly understood, and few models exist for their study. In the previous paper of this series, the structural-functional model EcoMeristem was described and field validated for vegetative-stage rice. This study applies the model to an experimental study on phosphorus deficiency effects on two morphologically contrasting rice cultivars, IR64 and Azucena, grown in controlled environments under hydroponics culture. Phosphorus deficiency caused severe biomass growth reductions in the shoot but not in the root, thus increasing the root / shoot weight ratio. It also inhibited tiller formation and leaf elongation, prolonged the phyllochron, and increased carbohydrate reserve pools in the plant. Analysis aided by the model identified inhibition of leaf extension and tillering as primary effects of the stress. Physiological feedback probably led to longer phyllochron, greater reserve accumulation and root growth stimulation. The main effect of P deficiency appeared to be a reduction in demand for assimilates in the shoot while photosynthetic radiation use efficiency remained nearly constant, resulting in spill-over of excess assimilates into reserve compartments and root growth. The results are discussed in the light of future applications of EcoMeristem for phenotyping and genetic analyses of phenotypic plasticity.

Factors influencing herbicide efficacy when removing lucerne prior to cropping

Farmers often experience inconsistent responses when using herbicides to terminate an established lucerne pasture prior to cropping. In an attempt to redress this problem, a series of field experiments were conducted between 1999 and 2002 at various locations in southern and northern New South Wales, the Australian Capital Territory, and south Western Australia that aimed to identify management guidelines that improved the efficacy of herbicide mixtures commonly used to remove lucerne. Collectively, these studies indicated that herbicides were generally less effective when applied either early (less than 2 weeks) or late (6 weeks or more) in the regrowth cycle of lucerne after defoliation. Herbicide efficacy tended to be greatest if applied to regrowth 3–5 weeks after defoliation, which corresponds to a time when the lucerne crown and root reserves are likely to be in the process of being replenished by photoassimilates transported from the shoot. The impact of timing of herbicide application in relation to season was compared at a number of locations. Across all the sites and years, spring herbicide applications were generally the most effective, removing on average 87% of the lucerne (range 53–100%) compared with 72% in summer (24–100%) and 60% in autumn (7–92%). Spring applications were also more consistent in their effect, removing >80% of the lucerne plants in 9 out of 12 experiments, whereas similar rates of removal occurred on 4 occasions in 9 summer applications and only twice in 8 autumn applications. Some of the seasonal variation could be explained by differences in the amount of rainfall prior to herbicide applications. It was assumed that the relationship between rainfall and herbicide efficacy reflected the stimulation of lucerne shoot and root growth by the additional soil moisture before herbicide treatment. Herbicide mixtures that contained ingredients such as picloram that retain residual activity in the soil tended to be more effective and were less influenced by lucerne growth and season than those herbicides with little or no residual activity. However, such chemicals could potentially restrict which crops can subsequently be grown after a lucerne pasture has been removed. It was concluded that >80% of lucerne plants were likely to be removed using herbicides provided that the herbicide treatment was applied to actively growing lucerne 3–5 weeks after defoliation, and when greater than 70–95 mm rain had fallen in the 6–8 weeks prior to application.

Reproduction and pathogenicity of Helicotylenchus multicinctus, Meloidogyne incognita and Pratylenchus coffeae, and their interaction with Radopholus similis on Musa

Abstract The reproduction and pathogenicity of Helicotylenchus multicinctus, Meloidogyne incognita and Pratylenchus coffeae was studied on Musa AAA cv. Grande Naine pot plants and microplot plants. First, separate experiments using different inoculation numbers (between 218 and 4144) and different exposure times (between 8 and 22 weeks) were executed in pots for each nematode species. Helicotylenchus multicinctus suppressed root growth by 13%, comparing inoculated with uninoculated plants. In contrast, M. incognita stimulated root growth by 6.7%, while P. coffeae had no effect on root growth. The interaction between R. similis and each of these three nematode species was investigated in a concomitancy experiment in pots. Plants without concomitant species yielded the highest R. similis population densities but did not differ from plants previously inoculated with H. multicinctus and P. coffeae. Meloidogyne incognita was the only concomitant species that reduced R. similis population densities. Inoculation with R. similis affected the recovered numbers of P. coffeae albeit with large variability. In the microplot experiment, all four nematode species except H. multicinctus reduced the bunch weight of bananas, compared to uninoculated plants. Pratylenchus coffeae and R. similis increased root damage by 129 and 262%, respectively, while only the latter species suppressed root weight with 66%.

Sugarcane Straw and its Phytochemicals as Growth Regulators of Weed and Crop Plants

The present study investigates the effect of soil amended with sugarcane straw leachate and its constituents on root elongation of weed and crop plants. The influence of soil biotic and abiotic factors on plant growth were also evaluated through assays in both autoclaved soil and sand. In unsterile soil, straw leachates stimulated root growth of some test plants at 6 g dry straw ls−1 and inhibited root elongation at higher concentrations. A bioassay guided process of the bioactive leachate constituents led to the isolation of vanillic, syringic and ferulic acids. These compounds were also assayed on the test plants in the three mentioned substrates. In unsterile soil, phenolics stimulated the growth of some species at 19 mg l−1. Higher phenolics concentrations were inhibitory. The concentration needed to inhibit 25% root elongation (EC25) was calculated from the dose–response curves. The concentration of phenolics in the leachate (64 g dry straw l−1) was estimated to be 187 mg l−1 (ferulic acid), 131 mg l−1 (vanillic acid) and 20 mg l−1 (Syringic acid). In general, these concentrations were below the EC25 values determined in unsterile soil indicating that these compounds cannot completely explain the strong inhibitory activity of sugarcane straw leachates. The role of soil factors on phytotoxicity of sugarcane straw leachate and its identified growth regulators is also discussed.

Involvement of Quinolinate Phosphoribosyl Transferase in Promotion of Potato Growth by a Burkholderia Strain

Burkholderia sp. strain PsJN stimulates root growth of potato explants compared to uninoculated controls under gnotobiotic conditions. In order to determine the mechanism by which this growth stimulation occurs, we used Tn5 mutagenesis to produce a mutant, H41, which exhibited no growth-promoting activity but was able to colonize potato plants as well as the wild-type strain. The gene associated with the loss of growth promotion in H41 was shown to exhibit 65% identity at the amino acid level to the nadC gene encoding quinolinate phosphoribosyltransferase (QAPRTase) in Ralstonia solanacearum. Complementation of H41 with QAPRTase restored growth promotion of potato explants by this mutant. Expression of the gene identified in Escherichia coli yielded a protein with QAPRTase activities that catalyzed the de novo formation of nicotinic acid mononucleotide (NaMN). Two other genes involved in the same enzymatic pathway, nadA and nadB, were physically linked to nadC. The nadA gene was cotranscribed with nadC as an operon in wild-type strain PsJN, while the nadB gene was located downstream of the nadA-nadC operon. Growth promotion by H41 was fully restored by addition of NaMN to the tissue culture medium. These data suggested that QAPRTase may play a role in the signal pathway for promotion of plant growth by PsJN.

Nitrogen regulation of root branching.

Many plant species can modify their root architecture to enable them to forage for heterogeneously distributed nutrients in the soil. The foraging response normally involves increased proliferation of lateral roots within nutrient-rich soil patches, but much remains to be understood about the signalling mechanisms that enable roots to sense variations in the external concentrations of different mineral nutrients and to modify their patterns of growth and development accordingly. In this review we consider different aspects of the way in which the nitrogen supply can modify root branching, focusing on Arabidopsis thaliana. Our current understanding of the mechanism of nitrate stimulation of lateral root growth and the role of the ANR1 gene are summarized. In addition, evidence supporting the possible role of auxin in regulating the systemic inhibition of early lateral root development by high rates of nitrate supply is presented. Finally, we examine recent evidence that an amino acid, L-glutamate, can act as an external signal to elicit complex changes in root growth and development. It is clear that plants have evolved sophisticated pathways for sensing and responding to changes in different components of the external nitrogen supply as well as their own internal nitrogen status. We speculate on the possibility that the effects elicited by external L-glutamate represent a novel form of foraging response that could potentially enhance a plant's ability to compete with its neighbours and micro-organisms for localized sources of organic nitrogen.

Colonization of Tomato Root Seedling by Pseudomonas fluorescens 92rkG5: Spatio–temporal Dynamics, Localization, Organization, Viability, and Culturability

The localization, viability, and culturability of Pseudomonas fluorescens 92 rkG5 were analyzed on three morphological root zones (root tip + elongation, root hair, and collar) of 3-, 5-, and 7-day-old tomato plants. Qualitative information about the localization and viability was collected by confocal laser scanning microscopy. Quantitative data concerning the distribution, viability, and culturability were obtained through combined dilution plating and flow cytometry. Colonization by P. fluorescens affected root development in a complex way, causing a general increase in the length of the collar and early stimulation of the primary root growth (3rd day), followed by a reduction in length (7th day). The three root zones showed different distribution, organization, and viability of the bacterial cells, but the distribution pattern within each zone did not change with time. Root tips were always devoid of bacteria, whereas with increasing distance from the apex, microcolonies or strings of cells became more and more prominent. Viability was high in the elongation zone, but it declined in the older parts of the roots. The so-called viable but not culturable cells were observed on the root, and their proportion in the distal (root tip + elongation) zone dramatically increased with time. These results suggest the existence of a specific temporal and spatial pattern of root colonization, related to cell viability and culturability, expressed by the plant-beneficial strain P. fluorescens 92 rkG5.

Root-associated bacteria containing 1-aminocyclopropane-1-carboxylate deaminase improve growth and nutrient uptake by pea genotypes cultivated in cadmium supplemented soil

The effect of inoculation with Pseudomonas brassicacearum Am3, Pseudomonas marginalis Dp1 and Rhodococcus sp. Fp2 containing 1-aminocyclopropane-1-carboxylate deaminase (ACCD) on growth and uptake of N, P, K, Ca, S, Fe and Cd in shoots of pea (Pisum sativum) genotypes VIR188, VIR1658, VIR3429 and VIR4488 was studied in pot experiment with non-polluted and Cd-supplemented (10 mg Cd kg−1) sod-podzolic soil. The growth-promoting effect of bacteria depended on plant genotype and bacterial strain. Only Rhodococcus sp. Fp2 had no ACCD activity in vitro in the presence of Cd and did not stimulate pea growth in Cd-supplemented soil. Inoculation with bacteria counteracted the Cd-induced inhibition of nutrient uptake by plants probably through stimulation of root growth and enhancement of nutrient uptake processes. Nutritional effects of the bacteria were specific with respect to the nutrient.

Root responses of Quercus ilex L. seedlings to drought and fire

Drought treatments in holm-oak (Quercus ilex) seedlings induce variations in total root length, number of root apices, shoot/root dry weight, and root electrolyte leakage. When drought treatments last for more than 50 days a considerable number of fine lateral roots die, irrespective of branching order or distribution within the root system. Scorching of drought-treated seedlings induces a transient stimulation of root growth. These results indicate that root turnover is deeply affected during treatments, with survival of seedlings being entrusted to the tolerance of a number of roots situated in the deeper region of the root system. Activity of the meristematic tissue present within the apices of these surviving roots supports regeneration of above-ground lost organs during recovery. Knowledge of the mechanisms ensuring the survival of Mediterranean tree seedlings following drought and fire is useful for developing models of vegetation dynamics.

Is root growth under phosphorus deficiency affected by source or sink limitations?

Reduced net photosynthesis (Pn) and decreasing shoot and root biomass are typical effects of phosphorus deficiency in plants. Lower biomass accumulation could be the result of reduced Pn (source limitation), but may also be due to direct negative effects of low P availability on growth (sink limitation). Because of the principal importance of root growth for P uptake, this study specifically examined the question whether source or sink limitations were responsible for reduced root growth rates under P deficiency. Rice plants were grown in nutrient solutions with four levels of P supply and at two light treatments and the effect of Pxlight treatments on growth and carbohydrate distribution was observed. Plants had up to 70% higher Pn when grown with natural (high) light compared with low light. Higher Pn, however, did not lead to additional growth under P deficiency, suggesting that assimilate supply from source leaves to roots was not a limiting factor under P deficiency. This was supported by observations that root starch concentrations increased in P-deficient roots. The comparison of two genotypes with different tolerance to P deficiency showed that the more tolerant one preferentially distributed P to roots where the additional P stimulated root growth and, ultimately, P uptake. The results therefore suggest that source limitation is of little importance under P deficiency. Even at highly sub-optimal tissue P concentrations of below 0.7 mg P g(-1) dry weight, plants were able to produce enough assimilates to sustain growth rates that were directly limited by low P availability.

Cowpea reactions to Striga gesnerioides I. effect on growth

Field trials were conducted in 1993, 1994 and 1995 wet seasons to determine the effect of Striga gesnerioides on the vegetative growth of 16 cowpea varieties. These included 11 resistant varieties, 2 tolerant varieties and 3 susceptible varieties. The growth of S. gesnerioides on each of the cowpea varieties was also evaluated. The highest Striga infestation was observed on several cowpea varieties notably all the susceptible and several tolerant varieties. Cowpea varieties generally had root and shoot growth stimulation at 5 and 7 WAP under low Striga infestation in the late planted trial (LPT) in 1993 and the early planted trial (EPT) in 1994. There was no clear-cut difference in the change in the root dry weight of the resistant and susceptible varieties due to Striga infestation. However, heavy Striga infestation greatly reduced the root and shoot dry weight of most varieties, between 1994 and 1995 irrespective of sowing dates. Greater reduction was observed in the leaf area and root nodules of the susceptible than resistant cowpea varieties. The susceptible varieties had conspicuous reduction in shoot dry weight at 9 WAP in each trial. The reduction in cowpea shoot dry weight due to Striga infestation was slightly greater in the LPT than EPT, and there was an increasing tendency towards greater reduction of the shoot dry weight from the trial in 1993–1995, due to increasing level of infestation. The losses usually observed in the cowpea host productivity could be due to the reduction in the root nodulation, leaf area, root and shoot growth under Striga infestation.

Combining a modelling with a genetic approach in establishing associations between genetic and physiological effects in relation to phosphorus uptake

The Pup1 locus confers tolerance to phosphorus (P) deficiency in rice (Oryza sativa L.). Transferring the Pup1 locus to an intolerant genotype increased P uptake by a factor 3 to 4. Lines with the Pup1 locus maintained higher root growth rates under P deficiency, but only as they started to diverge from intolerant lines in P uptake. It was thus not possible to determine if differences in root growth preceded and caused differences in P uptake or whether high root growth was the result of higher external P uptake efficiency (P influx per root size). The purpose of this paper is to review experimental evidence on the effect of Pup1 in light of recent results in modelling cause-and-effect relations between root growth, external efficiency and P uptake. Model simulations suggested that only very small changes in factors enhancing root growth were needed to explain the effect of Pup1 on P uptake. A 22% increase in root fineness or in internal P utilization efficiency (root dry matter per root P) was sufficient to triple P uptake . External root efficiency had to increase by 33 to account for the effect of Pup1. However, the most noticeable effect of increases in external efficiency was a subsequent stimulation of root growth that contributed eight times more to final P uptake compared to the change in external efficiency. Comparisons of model simulations with empirical data suggested that measured differences in external efficiency between Nipponbare and NIL-Pup1 were sufficiently large to account for the increase in P uptake. A segregation analysis using several pairs of contrasting NILs (at the Pup1 locus) further confirmed this as Pup1 co-segregated with external efficiency but not with seedling root growth or internal efficiency.

Changes in intracellular and apoplastic peroxidase activity, ascorbate redox status, and root elongation induced by enhanced ascorbate content in Allium cepa L.

Onions (Allium cepa L.) treated with external ascorbic acid or with the immediate precursor of its synthesis L-galactono-gamma-lactone show a stimulated elongation rate of the roots and an increase in the number of new radicles appearing at the bulb base. Treatment with both molecules resulted in an enhanced accumulation of ascorbate and dehydroascorbate along the root axis, but the distribution of these redox forms was not uniform along the root, as detected in intracellular (symplastic) and extracellular (apoplastic) compartments. Thus, those radicular zones metabolically more active, such as the meristem and the elongation zone, accumulated the highest amount of both redox forms of ascorbate. On the other hand, ascorbate and L-galactono-gamma-lactone also stimulated cytosolic glucose-6-phosphate dehydrogenase activity and inhibited peroxidase activity as deduced from in vivo and in vitro experiments. Differences were also found when comparing apoplastic and symplastic activities. These results are compatible with the idea of an ascorbate-mediated stimulation of root growth by inhibiting cell wall stiffening and increasing root metabolism.

EFFECTS OF N-FORM AND SALINE PRIMING ON GERMINATION AND VEGETATIVE GROWTH OF GALIA-TYPE MELON (CUCUMIS MELO L. CV. PRIMAL) UNDER SALINITY

EFFECTS OF N-FORM AND SALINE PRIMING ON GERMINATION AND VEGETATIVE GROWTH OF GALIA-TYPE MELON (CUCUMIS MELO L. CV. PRIMAL) UNDER SALINITY

Peroxidase mediated hydrogen peroxide production in barley roots grown under stress conditions

All applied metals (Co, Al, Cu, Cd) and NaCl inhibited barley root growth. No root growth inhibition was caused by drought exposure, in contrast to cold treatment. 0.01 mM H2O2 stimulated root growth and GA application did not affect root growth at all. Other activators and inhibitors of H2O2 production (SHAM, DTT, 10 mM H2O2, 2,4-D) inhibited root growth. Loss of cell viability was most significant after Al treatment, followed by Cd and Cu, but no cell death was induced by Co. Drought led to slight increase in Evans blue uptake, whereas neither NaCl nor cold influenced this parameter. DTT treatment caused slight increase in Evans blue uptake and significant increases were detected after 2,4-D and 10 mM H2O2 treatment, but were not induced by others stressors. Metal exposure increased guaiacol-POD activity, which was correlated with oxidation of NADH and production of H2O2. Exposure to drought caused a minor change in NADH oxidation, but neither H2O2 production nor guaiacol-POD activity was increased. Cold and NaCl application decreased all monitored activities. Increase in NADH oxidation and guaiacol-POD activity was caused by 10 mM H2O2 and 0.01 mM 2,4-D treatment, which also caused enhancement of H2O2 production. Slight inhibition of all activities was caused by 0.01 mM H2O2, GA, DTT; more pronounced inhibition was detected after SHAM treatment. The role of H2O2 production mediated by POD activity in relation to root growth and cell viability under exposure to some abiotic stress factors is discussed.

Bacillus spp. as Biocontrol Agents of Root Rot and Phytophthora Blight on Ginseng

Ginseng (Panax ginseng) is one of the most widely cultivated medicinal herbs in Korea. However, yield losses reached up to 30-60％ due to various diseases during 3 or 5 years of ginseng cultivation in the country. Therefore, successful production of ginseng roots depends primarily on the control of diseases. The objective of this study was to select potential biocontrol agents from rhizobacteria isolated from various plant internal root tissues for the control of multiple ginseng diseases as an alternative to fungicides. Among 106 Bacillus strains, two promising biocontrol agents, Bacillus pumilus strain B1141 and Paenibacillus lentimobus strain B1146, were selected by screening against root rot of ginseng caused by Cylindrocarpon destructans in a greenhouse. Pre-inoculation of selected isolates to seed or l-year-old root of ginseng resulted in stimulation of shoot and/or root growth of seedlings, and successfully controlled root rot caused by C. destructans (P<0.05). Furthermore, drenching of cell suspension of the selected isolates on seedling-growing pots reduced the incidence of Phytophthora blight after the seedlings were challenged with zoospores of Phytophthora cactorum (P<0.05). P. lentimorbus strain B1146 showed antifungal activity against various soil-borne pathogens in vitro, while B. pumilus strain B1141 did not show any. Results of this study suggest that some rhizobacteria can induce resistance against various plant diseases on ginseng.

Growth response of four turfgrass species to salinity

The need for salinity tolerant turfgrasses is increasing because of the increased use of effluent or other low quality waters for turfgrass irrigation. Greenhouse container and hydroponic experiments were conducted to determine the relative salinity tolerance and growth responses of ‘Challenger’ Kentucky bluegrass ( Poa pratensis L.) (KBG), ‘Arid’ tall fescue ( Festuca arundinacea Schreb) (TF), ‘Fults’ alkaligrass ( Puccinellia distans (L.) Parl.) (AG), and a saltgrass ( Distichlis spicata (Torr.) Beetle) collection (SG). In the container experiments, irrigation waters of different salinity levels were applied to experimental plants grown in plastic pots filled with a mix of sand and Isolite. The results indicated that KBG, TF, AG, and SG experienced a 50% shoot growth reduction at 4.9, 10.0, 20.0, and 34.9 dS m −1, respectively, and a 50% root growth reduction at 5.8, 19.6, 24.9 and 41.0 dS m −1, respectively. In the hydroponic experiment, grasses were grown in saline solution at 2.0, 4.7, 9.4, 14.1, 18.8, and 23.5 dS m −1. Kentucky bluegrass, TF, AG, and SG experienced a 50% shoot growth reduction at 5.5, 14.2, 23.0, and 34.5 dS m −1, respectively, and a 50% root growth reduction at 7.9, 21.5, 30.4 and 40.8 dS m −1, respectively. Root to shoot ratio of KBG remained constant, whereas those of TF, AG, and SG increased at all salinity levels. Salinity caused root cortex cells to collapse, in KBG at 14.1 dS m −1 and in TF at 23.5 dS m −1. Alkaligrass and SG only had a few cell collapses even at 23.5 dS m −1. Bi-cellular salt glands were observed only on leaves of SG. The ranking for salinity tolerance of selected grasses was: SG>AG>TF>KBG. Salt glands present in SG, root growth stimulation of SG and AG, and maintenance of relatively high root to shoot ratio in TF are apparent adaptive mechanisms exhibited by these grasses for salinity tolerance.

How do plants achieve tolerance to phosphorus deficiency? Small causes with big effects.

Genotypic differences in phosphorus (P) uptake from P-deficient soils may be due to higher root growth or higher external root efficiency (micrograms of P taken up per square centimeter of root surface area). Both factors are highly interrelated because any additional P provided by externally efficient roots will also stimulate root growth. It will be necessary to separate both factors to identify a primary mechanism to formulate hypotheses on pathways and genes causing genotypic differences in P uptake. For this purpose, a plant growth model was developed for rice (Oryza sativa) grown under highly P-deficient conditions. Model simulations showed that small changes in root growth-related parameters had big effects on P uptake. Increasing root fineness or the internal efficiency for root dry matter production (dry matter accumulated per unit P distributed to roots) by 22% was sufficient to increase P uptake by a factor of three. That same effect could be achieved by a 33% increase in external root efficiency. However, the direct effect of increasing external root efficiency accounted for little over 10% of the 3-fold increase in P uptake. The remaining 90% was due to enhanced root growth as a result of higher P uptake per unit root size. These results demonstrate that large genotypic differences in P uptake from a P-deficient soil can be caused by rather small changes in tolerance mechanisms. Such changes will be particularly difficult to detect for external efficiency because they are likely overshadowed by secondary root growth effects.