Reduced Root Growth Research Articles

Soil compaction impacts soybean root growth in an Oxisol from subtropical Brazil

Soil mechanical impedance, hypoxia and water stress are the main soil physical causes of reduced root growth, but they are rarely included in root growth models. The aim of this work was to study the impact of soil compaction on soybean root growth in an Oxisol using extensive field data as well as a mechanistic model that is sensitive to soil physical conditions. Soybean was cultivated under field conditions in a Rhodic Eutrodox in four treatments. The treatments consisted of three soil compaction levels (no-tillage system, areas trafficked by a tractor, and trafficked by a harvester) and soil chiselling management (performed in an area previously cultivated under no-tillage). Soil structural properties (soil penetration resistance, bulk density, total porosity, macroporosity and microporosity), root system parameters (root length density, root dry mass and root anatomy) and crop production components (grain yield, shoot dry biomass) were determined for the four treatments down to 50 cm soil depth. A mechanistic model, sensitive to mechanical and hydric stresses, was applied to simulate soybean root growth. The model was able to simulate the interaction between the soil physical conditions and soybean root growth. Soil compaction differentiated vertical root distribution according to a stress reduction function impeding root elongation. Consequently, root growth was influenced by soil physical conditions during the cropping season, and simulated root length density showed strong agreement to measured data. Soybean grain yield was reduced due to both compaction (caused by harvester traffic) and excessive loosening (promoted by chiselling) relative to the no-tillage system. Soil physical attributes (i.e., soil bulk density, penetration resistance, macroporosity and microporosity) were only weakly correlated with grain yield and root growth. This may be due to the fact that those soil physical attributes are static properties that do not represent the dynamics of mechanical and hydric stresses during the growing season. Soil compaction changed the anatomy, shape and size of roots. Moreover, cortex cells were deformed in the secondary root growth stage. In the compacted soil, mechanical impedance had a major effect on root growth, while in the loose soil, the matric potential (water stress) represented the major soil physical limitation to root growth. Soil chiselling increased the root length density, but it reduced the grain yields due water stress. The study showed that soybean root growth was successfully modelled with respect to soil physical conditions (mechanical impedance, hypoxia and water stress) for different compaction levels of a Rhodic Eutrudox.

Assessment of a 18F-Phenylboronic Acid Radiotracer for Imaging Boron in Maize.

Boron (B) is an essential plant micronutrient. Deficiencies of B have drastic consequences on plant development leading to crop yield losses and reductions in root and shoot growth. Understanding the molecular and cellular consequences of B deficiency is challenging, partly because of the limited availability of B imaging techniques. In this report we demonstrate the efficacy of using 4-fluorophenylboronic acid (FPBA) as a B imaging agent, which is a derivative of the B deficiency mimic phenylboronic acid (PBA). We show that radioactively labelled [18F]FPBA (t½=110 m) accumulates at the root tip, the root elongation zone and at lateral root initiation sites in maize roots, and also translocates to the shoot where it accumulates along the leaf edges. Treatment of maize seedlings using FPBA and PBA causes a shortened primary root phenotype with absence of lateral roots in a dose-dependent manner. The primary root defects can be partially rescued by the addition of boric acid indicating that PBA can be used to induce B deficiency in maize and that radioactively labelled FPBA can be used to image sites of B demand on a tissue level.

Accumulation and phytotoxicity of perfluorooctanoic acid and 2,3,3,3-tetrafluoro-2-(heptafluoropropoxy)propanoate in Arabidopsis thaliana and Nicotiana benthamiana

2,3,3,3-Tetrafluoro-2-(heptafluoropropoxy)propanoate (known as GenX) has been used as an alternative to perfluorooctanoic acid (PFOA) which was phased out of formulations for industrial and consumer product applications in 2015. While the effects of GenX on lab animals have been studied, little is known about its effects on plants. This study examined and compared the accumulation and toxicity of GenX and PFOA in the model plants Arabidopsis thaliana and Nicotiana benthamiana. Both plants showed reduction in biomass and root growth following exposure to PFOA or GenX in a dosage-dependent manner. The bioaccumulation factors (BFs) of GenX and PFOA were plant species-dependent, with higher BFs in A. thaliana compared to N. bethanminana. Additionally, GenX and PFOA were more readily accumulated into shoot tissues of A. thaliana than in N. bethanminana. Exposure to GenX also caused a reduction in chlorophyll content (18%) and total phenolic compounds (26%). However, GenX exposure increased superoxide dismutase activity and H2O2 content (1.6 and 2.6 folds increase, respectively) in N. benthamiana. Overall, our result suggest that GenX is bioaccumulative, and that its accumulation likely inhibits plant growth and photosynthesis as well as inducing oxidative stress.

A laser ablation technique maps differences in elemental composition in roots of two barley cultivars subjected to salinity stress.

In saline soils, high levels of sodium (Na+ ) and chloride (Cl- ) ions reduce root growth by inhibiting cell division and elongation, thereby impacting on crop yield. Soil salinity can lead to Na+ toxicity of plant cells, influencing the uptake and retention of other important ions [i.e. potassium (K+ )] required for growth. However, measuring and quantifying soluble ions in their native, cellular environment is inherently difficult. Technologies that allow in situ profiling of plant tissues are fundamental for our understanding of abiotic stress responses and the development of tolerant crops. Here, we employ laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) to quantify Na, K and other elements [calcium (Ca), magnesium (Mg), sulphur (S), phosphorus (P), iron (Fe)] at high spatial resolution in the root growth zone of two genotypes of barley (Hordeum vulgare) that differ in salt-tolerance, cv. Clipper (tolerant) and Sahara (sensitive). The data show that Na+ was excluded from the meristem and cell division zone, indicating that Na+ toxicity is not directly reducing cell division in the salt-sensitive genotype, Sahara. Interestingly, in both genotypes, K+ was strongly correlated with Na+ concentration, in response to salt stress. In addition, we also show important genetic differences and salt-specific changes in elemental composition in the root growth zone. These results show that LA-ICP-MS can be used for fine mapping of soluble ions (i.e. Na+ and K+ ) in plant tissues, providing insight into the link between Na+ toxicity and root growth responses to salt stress.

Duration Limits on Field Storage in Closed Cardboard Boxes before Planting of Norway Spruce and Scots Pine Container Seedlings in Different Planting Seasons

For spring plantings, conifer seedlings are usually packed in closed cardboard boxes and freezer stored over winter. Additionally, seedlings are increasingly being stored in cardboard boxes in spring, summer, and autumn plantings in Finland. The aim of this study was to determine the maximum safe duration for the field storage of Norway spruce (Picea abies (L.) Karst.) and Scots pine (Pinus sylvestris L.) container seedlings in closed cardboard boxes for different planting times (dates) in Nordic boreal conditions. In the first experiment, Norway spruce seedlings (85-cm3 peat plugs) were packed in cardboard boxes in August, September, or October, and, in the second experiment, in the middle of May. In the third Scots pine experiment, mini seedlings (30-cm3 peat plugs) were packed in May. In each experiment, the seedlings were stored in closed cardboard boxes in a nursery for one, three, seven, 14, and 21 days. The control seedlings were stored in open storage in the nursery. After storage, the seedlings were planted in a field. In all of the experiments, increased closed-box storage reduced the maximum photochemical yield of photosystem II (Fv/Fm) in the needles, and reduced root growth after planting. The frost hardiness was weakened in the Norway spruce seedlings that were stored in closed boxes for 21 days in August and October. In the spring experiments, prolonged storage increased the mortality of seedlings. Mortality rates were high in the autumn experiment due to the exceptionally warm and dry weather. Our conclusions, being based on the short term effects of field storage, are that conifer seedlings can be stored in closed boxes for only three days in August and about a week in September, October, and spring.

Virulent Rhodococcus fascians Produce Unique Methylated Cytokinins.

Some strains of Rhodococcus fascians exist only as epiphytes on the plant surface whereas others can become endophytic and cause various abnormalities including the release of multiple buds and reduced root growth. The abnormalities reflect the action of cytokinin. The strains that can become endophytic harbour a linear plasmid that carries cytokinin biosynthesis, activation and destruction genes. However, both epiphytic and endophytic forms can release cytokinin into culture, affect cytokinin metabolism within inoculated plants and enhance the expression of sugar and amino acid transporters and cell wall invertases, but only the endophytic form markedly affects the morphology of the plant. A unique methylated cytokinin, dimethylated N6-(∆2-isopentenyl)adenine (2-MeiP), operating in a high sugar environment, is the likely causative factor of the severe morphological abnormalities observed when plants are inoculated with R. fascians strains carrying the linear plasmid.

Natural Root Cellular Variation in Responses to Osmotic Stress in Arabidopsis thaliana Accessions.

Arabidopsis naturally occurring populations have allowed for the identification of considerable genetic variation remodeled by adaptation to different environments and stress conditions. Water is a key resource that limits plant growth, and its availability is initially sensed by root tissues. The root’s ability to adjust its physiology and morphology under water deficit makes this organ a useful model to understand how plants respond to water stress. Here, we used hyperosmotic shock stress treatments in different Arabidopsis accessions to analyze the root cell morphological responses. We found that osmotic stress conditions reduced root growth and root apical meristem (RAM) size, promoting premature cell differentiation without affecting the stem cell niche morphology. This phenotype was accompanied by a cluster of small epidermal and cortex cells with radial expansion and root hairs at the transition to the elongation zone. We also found this radial expansion with root hairs when plants are grown under hypoosmotic conditions. Finally, root growth was less affected by osmotic stress in the Sg-2 accession followed by Ws, Cvi-0, and Col-0; however, after a strong osmotic stress, Sg-2 and Cvi-0 were the most resilience accessions. The sensitivity differences among these accessions were not explained by stress-related gene expression. This work provides new cellular insights on the Arabidopsis root phenotypic variability and plasticity to osmotic stress.

Soybean Resistance Locus Rhg1 Confers Resistance to Multiple Cyst Nematodes in Diverse Plant Species.

Cyst nematodes consistently threaten agricultural production, causing billions of dollars in losses globally. The Rhg1 (resistance to Heterodera glycines 1) locus of soybean (Glycine max) is the most popular resistance source used against soybean cyst nematodes (H. glycines). Rhg1 is a complex locus that has multiple repeats of an ≈30-kilobase segment carrying three genes that contribute to resistance. We investigated whether soybean Rhg1 could function in different plant families, conferring resistance to their respective cyst nematode parasites. Transgenic Arabidopsis thaliana and potato (Solanum tuberosum) plants expressing the three soybean Rhg1 genes were generated. The recipient Brassicaceae and Solanaceae plant species exhibited elevated resistance to H. schachtii and Globodera rostochiensis and to G. pallida, respectively. However, some negative consequences including reduced root growth and tuber biomass were observed upon Rhg1 expression in heterologous species. One of the genes at Rhg1 encodes a toxic version of an alpha-SNAP protein that has been demonstrated to interfere with vesicle trafficking. Using a transient expression assay for Nicotiana benthamiana, native Arabidopsis and potato alpha-SNAPs (soluble NSF [N-ethylamine sensitive factor] attachment protein) were found to compensate for the toxicity of soybean Rhg1 alpha-SNAP proteins. Hence, future manipulation of the balance between Rhg1 alpha-SNAP and the endogenous wild-type alpha-SNAPs (as well as the recently discovered soybean NSF-RAN07) may mitigate impacts of Rhg1 on plant productivity. The multispecies efficacy of soybean Rhg1 demonstrates that the encoded mechanisms can function across plant and cyst nematode species and offers a possible avenue for engineered resistance in diverse crop species.

Neighbouring plants modify maize root foraging for phosphorus: coupling nutrients and neighbours for improved nutrient-use efficiency.

Nutrient distribution and neighbours can impact plant growth, but how neighbours shape root-foraging strategy for nutrients is unclear. Here, we explore new patterns of plant foraging for nutrients as affected by neighbours to improve nutrient acquisition. Maize (Zea mays) was grown alone (maize), or with maize (maize/maize) or faba bean (Vicia faba) (maize/faba bean) as a neighbour on one side and with or without a phosphorus (P)-rich zone on the other in a rhizo-box experiment. Maize demonstrated root avoidance in maize/maize, with reduced root growth in 'shared' soil, and increased growth away from its neighbours. Conversely, maize proliferated roots in the proximity of neighbouring faba bean roots that had greater P availability in the rhizosphere (as a result of citrate and acid phosphatase exudation) compared with maize roots. Maize proliferated more roots, but spent less time to reach, and grow out of, the P patches away from neighbours in the maize/maize than in the maize/faba bean experiment. Maize shoot biomass and P uptake were greater in the heterogeneous P treatment with maize/faba bean than with maize/maize system. The foraging strategy of maize roots is an integrated function of heterogeneous distribution of nutrients and neighbouring plants, thus improving nutrient acquisition and maize growth. Understanding the foraging patterns is critical for optimizing nutrient management in crops.

Knockdown of ghAlba_4 and ghAlba_5 Proteins in Cotton Inhibits Root Growth and Increases Sensitivity to Drought and Salt Stresses.

We found 33, 17, and 20 Alba genes in Gossypium hirsutum, Gossypium arboretum, and Gossypium raimondii, respectively. The Alba protein lengths ranged from 62 to 312 aa, the molecular weight (MW) from 7.003 to 34.55 kDa, grand average hydropathy values of −1.012 to 0.609 and isoelectric (pI) values of −3 to 11. Moreover, miRNAs such as gra-miR8770 targeted four genes, gra-miR8752 and gra-miR8666 targeted three genes, and each and gra-miR8657 a, b, c, d, e targeted 10 genes each, while the rests targeted 1 to 2 genes each. Similarly, various cis-regulatory elements were detected with significant roles in enhancing abiotic stress tolerance, such as CBFHV (RYCGAC) with a role in cold stress acclimation among others. Two genes, Gh_D01G0884 and Gh_D01G0922, were found to be highly induced under water deficit and salt stress conditions. Through virus-induced gene silencing (VIGS), the VIGS cotton plants were found to be highly susceptible to both water deficit and salt stresses; the VIGS plants exhibited a significant reduction in root growth, low cell membrane stability (CMS), saturated leaf weight (SLW), chlorophyll content levels, and higher excised leaf water loss (ELWL). Furthermore, the stress-responsive genes and ROS scavenging enzymes were significantly reduced in the VIGS plants compared to either the wild type (WT) and or the positively controlled plants. The VIGS plants registered higher concentration levels of hydrogen peroxide and malondialdehyde, with significantly lower levels of the various antioxidants evaluated an indication that the VIGS plants were highly affected by salt and drought stresses. This result provides a key foundation for future exploration of the Alba proteins in relation to abiotic stress.

An Overview of Important Enzymes Involved in Nitrogen Assimilation of Plants.

Nitrogen (N) is a macro-nutrient that is essential for growth development and resistance against biotic and abiotic stresses of plants. Nitrogen is a constituent of amino acids, proteins, nucleic acids, chlorophyll, and various primary and secondary metabolites. The atmosphere contains huge amounts of nitrogen but it cannot be taken up directly by plants. Plants can take up nitrogen in the form of nitrate, ammonium, urea, nitrite, or a combination of all these forms. In addition, in various leguminous rhizobia, bacteria can convert atmospheric nitrogen to ammonia and supply it to the plants. The form of nitrogen nutrition is also important in plant growth and resistance against pathogens. Nitrogen content has an important function in crop yield. Nitrogen deficiency can cause reduced root growth, change in root architecture, reduced plant biomass, and reduced photosynthesis. Hence, understanding the function and regulation of N metabolism is important. Several enzymes and intermediates are involved in nitrogen assimilation. Here we provide an overview of the important enzymes such as nitrate reductase, nitrite reductase, glutamine synthase, GOGAT, glutamate dehydrogenase, and alanine aminotransferase that are involved in nitrogen metabolism.

Ethylene signaling plays a pivotal role in mechanical-stress-induced root-growth cessation in Arabidopsis thaliana

ABSTRACTPlant roots show growth cessation as a primary response to mechanical stress. To clarify the molecular basis of this response, we have previously established an assay system to monitor the root growth response of Arabidopsis seedlings to mechanical stimuli using dialysis membrane-covered agar media. Here we examined the effect of plant hormones and their related molecules on this response. Amino-cyclopropane carboxylate, a precursor of ethylene, remarkably enhanced the growth reduction while silver ions, which block ethylene perception, nullified the response. Furthermore, salicylic acid, which inhibits ethylene biosynthesis, alleviated the root growth reduction, whereas methyl jasmonate had no apparent effect on the response. These results suggest that the root-growth cessation observed in response to mechanical stress involves ethylene signaling; however, this response may be independent from the pathway that integrates signals from ethylene and jasmonate.

Can methyl jasmonate treatment of conifer seedlings be used as a tool to stop height growth in nursery forest trees?

A plant's induced defense system can be triggered by the application of the plant hormone methyl jasmonate (MeJA), and recent research suggest that MeJA treatment may become a tool for protection of conifer seedlings against insect herbivory (e.g. by the pine weevil Hylobius abietis). A side-effect of MeJA application is temporarily reduced height growth. This has generally been considered as negative, but in forest tree nurseries this could instead be beneficial since it is commonly desired to stop the growth of nursery seedlings in late summer. Artificially longer dark periods (long nights/short days) are widely used in high-latitude nurseries to terminate height growth and induce freezing tolerance. However, long night treatment requires specialized nursery equipment and are labor intensive. Therefore alternatives are sought after. We compared long-night and MeJA treatments by following the growth of Norway spruce (Picea abies) seedlings throughout one season. The regulatory effect of MeJA on height growth was similar if not even better than that of long nights, i.e. it was terminated faster. However, MeJA treatment also reduced root growth and delayed the development of freezing tolerance. MeJA may therefore not replace long-night treatments, but it could facilitate a more flexible application of long nights by gaining a longer time interval during which this treatment can be used without risking the seedlings growing too large.

Para-aminobenzoic acid synthase from mushroom Agaricus bisporus enhances UV-C tolerance in Arabidopsis by reducing oxidative DNA damage

UV exposure is harmful to plants. Increasing resistance against UV light is thus of great importance to their growth. Para-Aminobenzoic acid (PABA) has major roles in many biological processes, involving nucleotide biosynthesis, DNA repair, and DNA methylation, which contributed to UV irradiation. However, no study reports the effect of PABA on UV tolerance, or details of the underlying molecular mechanisms are explored. Hence, the objective of the research is to study the protective effect of PABA on UV in Arabidopsis and explored the molecular mechanisms. We overexpressed PABA synthase gene (Pabs) from Mushroom Agaricus bisporus in Arabidopsis and observed reduced root growth and UV-C hyposensitivity exposed to 2500 J m−2 UV-C light. UV-C-induced DNA damage was significantly reduced and the expression of decreased DNA methylation 1 (DDM1) was remarkably higher in the Pabs lines, suggesting that overexpression of Pabs may protect against UV-induced DNA damage. In addition, overexpression of Pabs leads to an elevated reactive oxygen species production at root tips and enhanced catalase and superoxide dismutase activity, which may correlate with the enhanced UV tolerance of the Pabs overexpression lines. In summary, overexpression of Pabs from A. bisporus enhances UV-C tolerance of Arabidopsis, suggesting that Pabs takes an important part in defence against DNA damage.

Cytotoxic and genotoxic assessment of 2-chloropyridine using Allium cepa ana-telophase and comet test

2-Chloropyridine (2-CPY) is an important precursor of cosmetics, pesticides and other pharmaceutical products and it is also defined as trace chemical in industrial wastewater as the products of the metabolites of agricultural chemical products and river pollutants. In this study, the effects of 2-CPY on mitotic index (MI), mitotic phase frequencies, chromosome aberrations (CAs) and DNA damage in Allium cepa root cells were investigated with Allium ana-telophase and comet assay. Concentrations of 0.5xEC50 (25 ppm), EC50 (50 ppm) and 2xEC50 (100 ppm) of 2-CPY, Methyl methanesulfonate (MMS-10 ppm, positive control) and distilled water (negative control) were applied to A. cepa roots for 24, 48, 72 and 96 h. 2-CPY showed a cytotoxic effect by reducing root growth and MI, but also showed genotoxic effect by increasing CAs (disturbed ana-telophase, chromosome laggards, stickiness, bridges and polyploidy) and DNA damage at substantial levels. The amount of 2-CPY was shown to be increased statistically in both duration and dose by liquid chromatography-tandem mass spectrometry (LC-MS/MS). 2-CPY should be used carefully and investigated its cyto-genotoxic effects with other toxicology test systems.

Effects of iron and root zone pH on growth and physiological responses of paper birch (Betula papyrifera), trembling aspen (Populus tremuloides) and red-osier dogwood (Cornus stolonifera) seedlings in a split-root hydroponic system

Iron deficiency that is induced by high soil pH is a major factor affecting plant growth in calcareous soils and in some areas that have been reclaimed following industrial activities. Since the effects of high soil pH commonly involve Fe deficiency, in this study, we examined whether Fe provided to part of the root system exposed to low pH would alleviate high pH stress in paper birch (Betula papyrifera), trembling aspen (Populus tremuloides) and red-osier dogwood (Cornus stolonifera) seedlings. The plants were grown in a controlled environment growth room in mineral nutrient solution at pH 5 and 9 and provided with either 0 or 40 µM Fe in a split-root system for 8 weeks. At the end of the treatments, plant dry weights, net photosynthesis, transpiration rates, root ferric-chelate reductase activity, and leaf chlorophyll concentrations were measured, and elemental analyses were carried out in young leaves. The results demonstrated high root zone pH affected Fe, P and Zn concentrations in young leaves. In the three considered species, plants with part of their root system exposed to pH 5 had higher dry weights, net photosynthesis, and transpiration rates compared with the plants with the whole root system immersed in pH 9 solution. High root zone pH reduced photosynthesis, transpiration rates, leaf chlorophyll concentrations and the uptake of Fe, P, and Zn in plants. Partial exposure of the root system to low pH and Fe supply reduced leaf chlorosis and partly alleviated the high pH stress in the studied plants by improving Fe uptake, but did not alleviate root growth reductions.

Cytotoxicity and Genotoxicity in Allium cepa L. Root Meristem Cells Exposed to the Herbicide Penoxsulam

Penoxsulam is a new sulfonamide herbicide used on rice crops for annual grasses, broadleaf weeds, and sedges control, either grown through transplanting or direct dry or direct seeding methods of planting. In this study, Allium cepa ana-telophase and comet assay were used to examine the cyto-genotoxic effects of herbicide penoxsulam on A. cepa roots. A. cepa bulbs were exposed to ½xEC50 (12.5 µg/L), EC50 (25 µg/L) and 2xEC50 (50 µg/L) concentrations of penoxsulam for 24, 48, 72 and 96 h. Distilled water and 10 ppm of methyl methanesulfonate (MMS) were used as negative and positive control groups, respectively. Penoxsulam showed a cytotoxic effect by reducing root growth and mitotic index (MI), a genotoxic effect because it statistically increased chromosome aberrations (CAs, anaphase bridge, chromosomal laggards, polyploidy, disturbed ana-telophase and stickiness) and DNA damage as compared to control. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) was also used for quantitative analysis of penoxsulam in A. cepa root meristem cells. Further molecular toxicological evaluations associated in the cyto-genotoxicity of penoxsulam on plants are needed to confirm these results

Involvement of Phenolic Acids in Short-Term Adaptation to Salinity Stress is Species-Specific among Brassicaceae.

Salinity is a major abiotic stress negatively affecting plant growth and consequently crop production. The effects of short-term salt stress were evaluated on seedlings of three globally important Brassica crops—Chinese cabbage (Brassica rapa ssp. pekinensis), white cabbage (Brassica oleracea var. capitata), and kale (Brassica oleracea var. acephala)—with particular focus on phenolic acids. The physiological and biochemical stress parameters in the seedlings and the levels of three main groups of metabolites (total glucosinolates, carotenoids, and phenolics) and individual phenolic acids were determined. The salt treatments caused a dose-dependent reduction in root growth and biomass and an increase in stress parameters (Na+/K+ ratio, reactive oxygen species (ROS) and glutathione (GSH)) in all seedlings but most prominently in Chinese cabbage. Based on PCA, specific metabolites grouped close to the more tolerant species, white cabbage and kale. The highest levels of phenolic acids, particularly hydroxycinnamic acids, were determined in the more tolerant kale and white cabbage. A reduction in caffeic, salicylic, and 4-coumaric acid was found in Chinese cabbage and kale, and an increase in ferulic acid levels was found in kale upon salinity treatments. Phenolic acids are species-specific among Brassicaceae, and some may participate in stress tolerance. Salt-tolerant varieties have higher levels of some phenolic acids and suffer less from metabolic stress disorders under salinity stress.

Chemical root-pruning of Moringa oleifera for improved seedling growth

Low survival rates in greenhouse raised Moringa oleifera seedlings after transplanting had been ascribed to rapid growth rates and poorly formed and etiolated stems. The objective of the study was to determine the effect of indole acetic acid (IAA) on above-ground vegetative growth of M. oleifera seedling when used as root pruning agent. Hardened-off uniform one-month-old M. oleifera seedlings were transplanted in 20-cm-diameter plastic pots containing pasteurised loam soil and Hygromix at 3:1 (v/v) ratio. A week after transplanting, IAA concentrations of 0.0, 0.2, 0.4, 0.8, 1.6, 3.2, 6.4, 12.8, 25.6 and 51.2 mg mL−1 distilled water with G49 wetting agent and sticker were applied weekly to the respective plants. The effects of IAA concentrations were highly significant (P ≤ .01) on plant height, leaf number, dry root mass and significant (P ≤ .05) on stem diameter, which were further subjected to the lines of the best fit. Leaf number, plant height and stem diameter over increasing IAA concentrations exhibited positive quadratic relations, with the relationships being explained by 99, 98 and 98%, respectively. However, dry root mass and increasing concentrations of IAA exhibited negative quadratic relations, with the relationship being explained by 98%. At low concentrations, IAA increased plant height, number of leaves and stem diameter, whereas, at high concentration, the material reduced plant variables. However, all concentrations used reduced dry root mass. In conclusion, IAA was able to reduce root growth and promote shoot growth, with optimum for M. oleifera seedling growth established at a concentration level of 1.98 mg mL−1.

Variability of leaf photosynthetic characteristics in rice and its relationship with resistance to water stress under different nitrogen nutrition regimes.

The negative effects of water stress on rice can be alleviated by NH4 + nutrition. However, the effects of mixed nitrogen (N) nutrition (NO3 - + NH4 + ) on resistance to water stress are still not well known. To investigate the response of rice growth to water stress and its relationship with photosynthetic characteristics, a hydroponic experiment supplying different N forms was conducted. Compared with NO3 - nutrition, mixed-N and NH4 + nutrition greatly alleviated the reduction of leaf area, chlorophyll content, and photosynthesis under water stress, whilst subsequently maintaining higher biomass. In contrast, water stress inhibited the root-shoot ratios in NH4 + - and mixed-N-supplied plants, indicating reduced root growth and higher photosynthate availability to shoots. The following key observations were made: (1) a similar stomatal limitation and low proportion of activated Rubisco were observed among the three different N nutrition regimes; (2) increased mesophyll conductance in NH4 + - and mixed-N-supplied plants simultaneously stimulated leaf photosynthesis and improved the water use efficiency and (3), the maximum carboxylation rate and actual photochemical efficiency of photosystem II in NH4 + - and mixed-N-supplied plants were significantly higher than that in NO3 - -supplied plants, thus resulting in higher photochemical efficiency under water stress. In conclusion, mixed-N and NH4 + nutrition may be used to develop strategies for improved water stress resistance and stimulated biomass production under conditions of osmotic stress and possibly drought.