Leaf isoprene emission declines in Quercus pubescens seedlings experiencing drought – Any implication of soluble sugars and mitochondrial respiration?

Previous studies suggest that the sensitivity of leaf mitochondrial respiration and the pool of soluble sugars to water stress could influence the response of leaf isoprene emission to drought by affecting the availability of extra-chloroplastic carbon for isoprene synthesis. We measured rates of isoprene emission and CO₂ exchange, and the concentration of nonstructural carbohydrates in leaves of Quercus pubescens Willd. seedlings subjected to either normal watering (control plants, C) or drought (droughted plants, D). Stopping of watering caused predawn leaf water potential (Ψ_pd) to decline between −2.3 and −5.1 MPa among D plants, whereas Ψ_pd remained higher than −0.45 MPa in C plants. Isoprene emission (I_s), net CO₂ assimilation (A_n) and dark mitochondrial respiration (R_d) decreased with increasing water deficit, with declines in these variables relative to the respective means of C plants being A_n > I_s > R_d. This resulted in positive pairwise correlations between the three variables. The concentration of nonstructural carbohydrates did not change between treatments, but the concentration of soluble sugars increased and that of starch decreased in D plants as compared with C plants. As a consequence, there was a negative correlation between I_s and the concentration of soluble sugars, which supports a limited use of cytosolic sugars in sustaining isoprene synthesis at high to severe water stress. Our data also indicate that competition between I_s and R_d for the same carbon substrates had little importance for isoprene emission at high to severe water stress, as compared to the overall constraint on isoprene metabolism probably imposed by the shortage of photosynthetic carbon, energy and reducing equivalents.     

Effects of NaCl salinity and water stress on growth and leaf water relations of Asteriscus maritimus plants

Potted plants of Asteriscus maritimus (L.) Less were submitted to water stress (during two consecutive cycles, irrigation water was withheld for 5 days followed by a recovery period of 25 days) and saline stress (150 days of exposure to 0, 70 and 140 mM NaCl daily irrigation) in order to assess the effect on leaf water relations and growth parameters. Plants under saline and water stress conditions showed lower biomass and an early reduction in leaf expansion growth. Both stresses promoted a substantial degree of stomatal regulation; but, in spite of this, the plants showed signs of leaf tissue dehydration, decreases in RWC and Ψ_pd values. However, salt-treated plants, developed a NaCl inclusion mechanisms, underwent osmotic adjustment, which was able to maintain leaf turgor. Under both stress conditions g_l was independent to plant water status in the range between –0.8 and 1.0 MPa. Under water stress conditions, midday leaf water potential showed a threshold value (around −1.1 MPa), below which leaf conductance remained constant. In the salt-treated plants, the gradual closure of the stomata over a wide range of Ψ_md may be important in maintaining some level of photosynthesis.     

Effect of light and nitrogen supply on internal C:N balance and control of root-to-shoot biomass allocation in grapevine

Theoretical plant growth models postulate that the relative rates of shoot and root growth are largely modulated by signals related to carbon and nitrogen status of the plant. To test this experimentally, 6-week-old vegetative cuttings of grapevine (Vitis vinifera L. cv Merlot) were grown aeroponically in different controlled conditions of irradiance (13.8, 8.4 and 5.3 mol PAR m⁻² day⁻¹) and/or nitrogen nutrition (0.15, 1.20 and 7.11 mM N). Total non-structural carbohydrates (TNC) and amino acids (FAA) in leaves and roots were analysed 0, 6 and 28 days after treatment initiation. Both whole-plant biomass accumulation as well as C and N contents were highly responsive to light and N availability. At day 28, plant dry weight was significantly reduced in shaded vines (−35% of that of the control plants) and stimulated under the high irradiance environment (+30%). Deprivation of N enhanced root growth (+51%) at the expense of above-ground growth, whereas leaf dry weight was significantly greater in the high-N treatment than in the control. Vines grown under low-N and high irradiance conditions had the highest root-to-shoot ratios and those grown under low light and high N the lowest. Finally, redistribution of biomass among vegetative vine parts was significantly related to different indicators of the vine C:N status measured either at the whole-plant (N concentration) or at the organ level (TNC:FAA ratio), suggesting that root-to-shoot biomass partitioning was controlled by some aspect of plant C:N balance. Such relationships will be useful to improve allocation rules in a process-based growth model of grapevine.     

Time course analysis of ABA and non-ionic osmotic stress-induced changes in water status,chlorophyll fluorescence and osmotic adjustment in Arabidopsis thaliana wild-type (Columbia) and ABA-deficient mutant (aba2)

In the present study, we investigated time course changes of water status including relative water content (RWC), leaf osmotic potential (Ψ_Π), stomatal conductance (g_s), proline (Pro), chlorophyll fluorescence (F_v/F_m) and total chlorophyll content in the Arabidopsis thaliana under PEG-induced drought stress after exogenous ABA treatment. To a better explanation for the role of ABA in the water status of A. thaliana to drought stress, wild-type (Columbia) and ABA-deficient mutant (aba2) of A. thaliana were used in the present study. Moreover, three weeks old Arabidopsis seedlings were applied exogenously with 50 μM ABA and exposed to drought stress induced by 40% PEG8000 (−0.73 MPa) for 6 h, 12 h and 24 h (hours). Our findings indicate that RWC of wild-type and aba2 started to decrease in the first 12 h and 6 h of PEG-induced drought stress, respectively. However, exogenous treatment of 50 μM ABA increased their RWC under drought stress. On the other hand, while Ψ_Π of both genotypes started to decrease in the first 6 h of drought stress, these declines in Ψ_Π were prevented by ABA treatment under stress throughout the experiment; it was more pronounced in aba2 at 24 h. While the highest increase in g_s was obtained in aba2 after 24 h stress, ABA-induced highest decrease in g_s was obtained in the same genotype during 12 h, as compared to PEG-treated group alone. On the other hand, Pro content increased in all treatment groups of ABA-deficient mutant aba2 at 12 h and 24 h. However, Pro content in ABA + PEG treated aba2 plants was higher than in PEG- and ABA-treated plants alone at the end of the 24 h. Drought stress decreased F_v/F_m and total chlorophyll contents of both genotypes while 50 μM ABA alleviated these reductions during drought stress, as compared to PEG stressed plants. On the other hand, 50 μM ABA treatment alone did not create any remarkable effect on F_v/F_m and total chlorophyll contents.These findings indicate that exogenous ABA showed an alleviative effect against damage of drought stress on relative water content, osmotic potential, stomatal conductance, proline, chlorophyll fluorescence and total chlorophyll content of both genotypes during 24 h of drought stress treatment.     

Natural variation of the root morphological response to nitrate supply in Arabidopsis thaliana

Nitrogen fertilization increases crop yield but excessive nitrate use can be a major environmental problem due to soil leaching or greenhouse gas emission. Root traits have been seldom considered as selection criteria to improve Nitrogen Use Efficiency of crops, due to the difficulty of measuring root traits under field conditions. Nonetheless, learning about mechanisms of lateral root (LR) growth stimulation or repression by nitrate availability could help to redesign root system architecture (RSA), a strategy aimed at developing plants with a dense and profound root system and with higher N uptake efficiency. Here, we explored the genetic diversity provided by natural populations of the model species Arabidopsis thaliana to identify potentially adaptive differences in biomass production and root morphology in response to nitrate availability. A core collection of 24 accessions that maximizes the genetic diversity within the species and Col-0 (the reference accession) were grown vertically on agar medium at moderate (N+) nitrate level for 6 days and then transferred to the same condition or to low (N?) nitrate concentration for 7 days. There was a major nutritional effect on the shoot biomass and root to shoot biomass ratio. The variation of the root biomass and RSA traits (primary root length, LRs number, LR mean length, total LRs length and LR densities) was primarily genetically determined. Differences in RSA traits between accessions were somewhat more pronounced at N?. Some accessions produced almost no visible LRs (Pyl-1, N13) at N?, while other produced up to a dozen (Kn-0). Taken together our data illustrate that natural variation exists within Arabidopsis for the studied traits. The identification of RSA ideotypes in the N response will facilitate further analysis of quantitative traits for root morphology.     

Growth and root distribution of Vallisneria natans in heterogeneous sediment environments

Plant growth, biomass allocation and root distribution were investigated in the submerged macrophyte Vallisneria natans growing in heterogeneous sediments. Experimentally heterogeneous sediment environments were constructed by randomly placing 4 cm of clay or sandy loam into the top (0–4 cm) or bottom (4–8 cm) layer within an experimental tray, providing two homogeneous and two heterogeneous treatments. Biomass accumulation was significantly affected by the experimental treatments: higher in the homogeneous sediment of clay (32 mg per plant) and the two heterogeneous treatments (about 27 mg per plant), but lower in the homogeneous sediment of sandy loam (15 mg per plant). Root: shoot ratio was also different among the four treatments. Compared with the treatments of clay in the top layer, plants allocated more biomass to roots at the treatments of sandy loam in the top layer. Heterogeneous sediments significantly affected root distribution pattern. Compared with the treatments of sandy loam in the bottom layer, root number (7–8 versus 13–14) and total root length (3.6–4.0 cm versus 29.5–40.0 cm) in the bottom layer were significantly higher in the treatments with clay in the bottom layer. These results indicate that both sediment structure and nutrient availability influence growth and root system distribution of V. natans.     

Soil salinity and drought alter wood density and vulnerability to xylem cavitation of baldcypress (Taxodium distichum (L.) Rich.) seedlings

We investigated the role of hydraulic conductivity, wood density, and xylem cavitation in the response of baldcypress (Taxodium distichum) seedlings to increased soil salinity and drought. One-year-old, greenhouse-grown seedlings were irrigated daily with a 100 mM (≈6‰) salt solution or once per week with fresh water (drought). Controls were irrigated daily with fresh water. Gas exchange rates of stressed plants were reduced by approximately 50% (salt) and 70% (drought), resulting in a 50–60% reduction in diameter growth for both treatments. Stem-specific hydraulic conductivity (K_{S native}) of stressed plants was 33% (salt) and 66% (drought) lower than controls and we observed a strong positive correlation between K_{S native} and gas exchange. In addition, we found a strong relationship between CO₂ assimilation rate (A) and the soil-to-leaf hydraulic conductance (k_L). The relationship was identical for all treatments, suggesting that our moderate salt stress (as well as drought) did not affect the photosynthetic biochemistry of leaves, but rather reduced A via stomatal closure. Lower K_{S native} of stressed plants was associated with increased wood density and greater resistance to xylem cavitation. Xylem pressures causing 50% loss of hydraulic conductivity (P₅₀) were ?2.88 ± 0.07 MPa (drought), ?2.50 ± 0.08 MPa (salt) and ?2.01 ± 0.04 MPa (controls). P₅₀s were strongly correlated with wood density (r = ?0.71, P < 0.01) and K_{S native} (r = 0.74, P < 0.01). These findings support the hypothesis that there is a significant trade-off between a plant's cavitation resistance and its hydraulic efficiency. The results of the present study indicate that stressed plants partitioned their biomass in a way that strengthened their xylem and reduced vulnerability to xylem cavitation. Hence, these seedlings could be better suited to be planted in environments with elevated soil salinity. For most parameters (especially P₅₀), drought had an even more pronounced effect than salinity. This is important as nurseries could produce “stress-acclimated” seedlings simply by reducing irrigation amounts and would not have to contaminate the soils in their nursery beds with salt applications.     

Effect of solute and matric potential on in vitro growth and sporulation of strains from a new population of Aspergillus flavus isolated in Italy

The effect of temperature and different solute (Ψ_s) and matric potentials (Ψ_m) on growth and sporulation of three aflatoxigenic strains of Aspergillus flavus isolated from contaminated maize in northern Italy was determined. The Ψ_s of maize-based media were modified ionically (NaCl) and non-ionically (glycerol) and the Ψ_m with PEG 8000 in the range −1.4 to −21.0 MPa at 25 and 30 °C. Both temperature and Ψ_s/Ψ_m stress had statistically significant effects on growth rates of the three strains. Faster growth occurred at 30 °C and −1.4 and −2.8 MPa. A. flavus strains were more sensitive to Ψ_m than Ψ_s stress with limits of −9.8 MPa and −14 to−18 MPa, respectively. Sporulation was significantly influenced by Ψ_s potential, solute type and temperature. This suggests that these aflatoxigenic strains of A. flavus isolated from aflatoxin-contaminated maize are probably able to colonise crop debris rapidly at prevailing temperatures and water stress conditions. This type of information on the ecology of aflatoxin producing A. flavus strains isolated in Italy will contribute to the development of a systems model to predict their activity in crop residue and colonisation of maize grain.     

UVC-priming mediated modulation of forskolin biosynthesis key genes against Macrophomina root rot of Coleus forskohlii⿿A tissue culture based sustainable approach

Coleus forskohlii is susceptible to root rot disease that reduces yield of root specific metabolite forskolin. In this communication, we first time reported sustainable management of Macrophomina phaseolina root rot in C. forskohlii via Ultraviolet-C (UVC)-hormesis using tissue culture. Compact calli were developed using 2,4-dichlorophenoxy acetic acid (2.0 mg l^⿿1) and tolerance level of UVC exposure was optimized. Calli exhibited 88% viability, 8.4% relative growth in biomass and higher absorbance in 2,3,5-triphenyl tetrazolium chloride analysis subsequent to 10 min of UVC exposure. Pathogenicity of M. phaseolina was reduced in UVC-primed calli-regenerated plants compared with un-primed ones under controlled condition. Plant height, leaf number, shoot and root biomass significantly improved in UVC-primed calli-regenerated plants over un-primed ones when challenged by M. phaseolina in glasshouse. Significant elevation of economically important forskolin via up-regulation of key forskolin biosynthesis genes GGPP synthase, CfTPS2 and CfTPS3 highlighted the potentiality of UVC-priming to boost plant⿿s defence mechanism and protect from drastic economic loss.     

The functional divergence of biomass partitioning,carbon gain and water use in Coffea canephora in response to the water supply: Implications for breeding aimed at improving drought tolerance

Robusta coffee (Coffea canephora) is widely cultivated in regions where water availability is the major environmental constraint affecting crop production. The functional divergence associated with biomass partitioning, carbon gain and water use in response to water supply was examined in 10 one-year-old clones of robusta coffee with varying degrees of drought tolerance. The plants were grown outdoors in 24 L pots and either irrigated or subjected to a four-month water deficit. Under conditions of ample irrigation, clones with superior water use ability (i.e., a higher water potential, transpiration rate, apparent hydraulic conductance and biomass partitioning into roots and a lower wood density) displayed enhanced carbon gains. In contrast, under drought conditions, clones that postponed dehydration via more conservative water use rates showed lower relative decreases in stomatal conductance, photosynthetic rates and biomass accumulation. Isotopic signatures (δ¹³C) might be useful for identifying clones with improved performance under drought conditions. Our results suggest that combining useful morphological and physiological traits facilitates the successful assessment of coffee clonal performance in response to drought at the seedling stage. This strategy may be valuable when exploring a large number of genotypes in coffee-breeding programs because it reduces the time and resource costs that would otherwise be wasted on potentially undesirable genotypes.     

Photosynthetic gas exchange characteristics in three different almond species during drought stress and subsequent recovery

Three different drought stress levels (water potential of the nutrient solution, Ψ_s = −0.6, −1.2 and −1.8 MPa, respectively), and a control treatment (Ψ_s = −0.1 MPa), were applied during 2 weeks to three almond species, followed by 3 weeks of recovery. The selected test species were Prunus dulcis (Miller) D.Webb (bitter almond) and two wild almond species, P. lycioides (Spach) C.K. Schneider and P. scoparia (Spach) C.K. Schneider. All three are species native to Iran, and can be used as rootstock, but only P. dulcis is actually used for commercial almond production.In the absence of drought stress, maximum net assimilation rate (A_max) is highest for P. scoparia and lowest for P. dulcis. For all species A_max was above 16 μmol CO₂ m⁻² s⁻¹. A similar relationship between A_max and dark respiration rate (R_d), was observed for all species. This relationship suggests that optimisation of the carbon budget is independent of species.The three investigated species seem to have a different reaction to a similar stress, indicating different drought stress coping strategies. P. scoparia lost all its leaves during the experiment, while P. lycioides only kept some leaves, however, the remaining leaves were almost totally wilted and did not allow for any photosynthesis measurement. P. scoparia did not recover during the experiment, as no new leaves were developed once Ψ_s was restored to pre-drought stress levels. However, this species has green stems, indicating that stem photosynthesis might play an important role in the plants’ overall carbon balance. This species is an opportunistic one (sensu [Higgins, S.S., Larsen, F.E., Bendel, R.B., Radamaker, G.K., Bassman, J.H., Bidlake, W.R., Alwir, A., 1992. Comparative gas-exchange characteristics of potted, glasshouse-grown almond, apple, fig, grape, olive, peach and Asian pear. Sci. Hortic. 52 (4), 313–329]), where assimilation is seriously limited by non-stomatal processes as evidenced by measurements of intercellular CO₂ concentration, eventually resulting in total leaf loss. All P. lycioides leaves almost completely wilted during the experiment, but this species recovered rather quickly. Leaves, newly formed at the end of the experiment, obtained maximal assimilation rates under control Ψ_s levels, equivalent to those measured in the control treatment. Finally, P. dulcis did keep at least part of its leaves during drought stress. However, assimilation rates after 2 weeks of drought treatment and 3 weeks of recovery were only about half of those measured in the control treatment. Of the three investigated species, non-stomatal limitation of assimilation seems to be the least important in P. dulcis.Intrinsic water use efficiency, defined as the ratio of assimilation rate over stomatal conductance, increased for P. dulcis with increasing drought stress, while a different pattern was observed for P. lycioides and P. scoparia, indicating non-stomatal processes prevail over stomatal limitations of the assimilation process. It was concluded that P. dulcis is the species most tolerant to drought. P. scoparia tries to avoid drought, whereas P. lycioides has an intermediate behaviour. Besides P. dulcis, also P. lycioides seems to have some potential for use as rootstock for commercial almond production.     

Short-time response in root morphology of Vallisneria natans to sediment type and water-column nutrient

Rooted submerged macrophytes can absorb significant amounts of nutrients from both sediment and water. We investigated root morphology of Vallisneria natans in mesocosm plastic bins, in response to three types of sediment (sandy loam, clay, and a 50:50 (v/v) mixture of the two sediments) and two levels of water-column nutrient (well water and nutrient medium). Compared to the plants grown in the clay or mixed sediments, root diameter decreased (0.39–0.41 versus 0.36–0.37 mm) but total root length per plant increased (0.87–1.27 versus 1.14–1.62 m) when grown in sandy loam. Increase of nutrient availability in water column led to decreased specific root length (306–339 versus 258–281 m g⁻¹). However, both sediment type and water-column nutrient had no impacts on root number (ranged from 19 to 24 number of roots per plant). Root weight ratio, root:leaf mass ratio and root:leaf length ratio generally decreased with enhanced nutrient availability in sediment or water. Plant growth was affected by sediment type alone (P < 0.05), rather than water-column nutrient (P > 0.05). However, plant N and P contents were significantly impacted by both sediment type (P ≤ 0.001) and water-column nutrient (P < 0.05). Increase of nutrient availability in sediment or water led to increased plant N (ranged from 2.47 to 4.77 mg g⁻¹) and P concentrations (ranged from 42.8 to 62.0 mg g⁻¹). These results indicate that considerable variation in root morphology of V. natans exists in response to the fertility of the sediment it is rooted in.     

Growth,biomass allocation,and autofragmentation responses to root and shoot competition in Myriophyllum spicatum as a function of sediment nutrient supply

Plant growth, biomass allocation and autofragmentation were investigated in response to root and shoot competition in the submersed macrophyte Myriophyllum spicatum L. growing in two sediment environments. Biomass accumulation and allocation were significantly affected by sediment fertility, with a higher total biomass observed in fertile sediment (average: 4.69 g per plant vs. 1.12 g per plant in infertile sediment). Root-to-shoot ratios were 0.34 and 0.06 in the infertile and fertile sediments, respectively, reflecting the high investment placed on roots under infertile conditions. In the presence of root, shoot, and full competition, whole plant biomass decreased by 18%, 12% and 24% in the infertile sediments, and 23%, 25% and 33% in the fertile sediments, respectively. Root weight ratios (RWRs) increased with root competition by 38% (P < 0.001) and 12% (P = 0.002), while leaf weight ratios (LWRs) decreased with shoot competition by 6% (P = 0.042) and 5% (P = 0.001) in the infertile and fertile sediments, respectively. A total of 406 autofragments were harvested in the fertile sediments, but none were obtained from the infertile sediments. In the control, autofragment number and biomass was 166% and 175% higher compared to the competition treatment. Root and shoot competition resulted in a 21% (P = 0.043) and 18% (P = 0.098) decrease in the autofragment biomass, respectively. These results indicated that M. spicatum responds to different sediment fertility by changing its allocation patterns. Moreover, both root and shoot competition influenced plant growth and autofragmentation, while sediment nutrient availability played an important role in M. spicatum autofragmentation.     

Herbivory,mowing, and herbicides differently affect production and nutrient allocation of Alternanthera philoxeroides

This glasshouse study examined the effect of three damage types on plant growth and nutrient allocation of the invasive aquatic plant, alligator weed (Alternanthera philoxeroides). The damage included: repeated leaf removal, a single application of herbicide, and one-time shoot removal. Damage types were meant to simulate the effects of insect herbivory, chemical, and mowing/grazing, respectively. Response variables included plant biomass and both the concentration and abundance of nutrients. Complete shoot removal and herbicide treatments caused an initial decline in growth rate, followed by several weeks of increasing rates and finally a second decline during the fourth week. Plants from control and repeated leaf removal treatments showed a steady increase in growth rate from the treatment application to the final harvest, but control plants were accumulating biomass three times faster than repeated defoliation plants by the fifth week (9.7 and 3.5 g week⁻¹, respectively). Not surprisingly, all treatments led to lower total cumulative biomass 5 weeks after treatment application (mean 30.8 g) when compared with controls (49.0 g). However, despite the repeated leaf removal and complete shoot removal treatments removing similar quantities of biomass (mean 8.0 and 7.5 g respectively), repeated removal of leaves produced less total biomass (26.2 g) and led to less cumulative above ground biomass (20.1 g) than the other treatments (mean total = 33.1 g, mean above ground = 25.7 g). Repeated leaf removal also produced less below ground biomass (6.1 g) than the shoot removal treatment (8.5 g) and had the greatest negative effect on nitrogen and potassium abundance in plant tissues after 5 weeks. In addition, it reduced the amount of phosphorous to a lower level than herbicide treated or control plants. These results indicate that repeated leaf removal was the treatment most effective in reducing biomass and depleting nutrients in A. philoxeroides plants.     

Salt tolerant Methylobacterium mesophilicum showed viable colonization abilities in the plant rhizosphere

The source of infection has always been considered as an important factor in epidemiology and mostly linked to environmental source such as surface water, soil, plants and also animals. The activity of the opportunistic pathogens associated with plant root, their adaptation and survival under hostile environmental condition is poorly understood. In this study the salt tolerance ability of Methylobacterium mesophilicum and its colonization in the root and shoot of plants under severe drought and salt stress conditions were investigated. The colonization of plant by M. mesophilicum was investigated in a gnotobiotic sand system, and their survival in pots with saline soil. Bacterial strain was found to colonize rhizosphere of cucumber, tomato and paprika grown under normal and salt stress condition and reached up to 6.4 × 10⁴ and 2.6 × 10⁴ CFU/g root. The strain was resistant to Gentamicin, Ampicillin, Amoxicillin plus Clavulanic acid, Cefotaxime, neomycin, penicillin and was also tolerant to salinity stress (up to 6% NaCl). These abilities play important roles in enabling persistent colonization of the plant surface by M. mesophilicum strains. In conclusion, this study provides background information on the behaviour of opportunistic pathogen M. mesophilicum on plants and their survival in harsh environmental conditions.     

Response of Potamogeton crispus root characteristics to sediment heterogeneity

Plant growth, biomass allocation, root distribution and plant nutrient content were investigated in the submerged macrophyte Potamogeton crispus growing in heterogeneous sediments. Three experimental sediments heterogeneous in nutrient content and phosphorus release capacity were used: sandy loam with low nutrient content (A), clay with intermediate nutrient content (B), and clay with high nutrient content (C). Biomass accumulation was significantly affected by the sediment type, and was highest in clay C (1.23 mg per plant dry weight) but lowest in sandy loam (0.69 mg per plant dry weight). The root:shoot ratios in treatments A, B and C were 0.30, 0.14 and 0.09, respectively. P. crispus allocated more biomass to roots in sandy loam compared with the other sediments. The average root numbers in sediments A, B and C were 16, 19 and 20, respectively, and the total root lengths in sediments A, B and C were 238.84, 200.36 and 187.21 cm, respectively. Almost 90% of the root biomass was distributed in the 0–15 cm depth in sediments B and C, compared with 64.53% in sediment A. The rank order of plant nitrogen and phosphorus concentrations in the sediment types was C > B > A. These results indicate that both sediment structure and nutrient availability influence the growth and distribution of the root system of P. crispus.     

Short and long-term uptake of Hg in white lupin plants:Kinetics and stress indicators

The intrinsic characteristics of white lupin regarding biomass production and tolerance to abiotic stresses could make it a good candidate to be used in degraded mine soils containing mercury (Hg), but white lupin behaviour in response to Hg has to be previously evaluated. With this aim, kinetic parameters of Hg uptake in short and long-term experiments, and Hg resistance of white lupin plants using several stress indicators were studied. The plants were grown with increasing Hg doses in nutrient solutions (0, 5 and 10 μM). Hg uptake showed an active component in Hg influx, suggesting the existence of a low affinity root transporter that can be used for Hg uptake into white lupin root cells. K_m and V_max values obtained for the saturable component were 217.7 ± 27.6 μM and 3.78 ± 0.18 μmol Hg g FW^?1 h^?1. Hg accumulation was concentration–time-dependent, showing a saturable behaviour for the lower doses but a linear behaviour for the highest ones. A high ability of Hg absorption by white lupin was observed both in short and long-term uptake experiments. The highest Hg dose reduced biomass production especially in the shoots. Moreover, increases in chlorophylls, malondialdehyde, total thiols and phenols were observed in Hg-stressed plants. The enhancement of total thiols and phenols levels in roots reduced oxidative stress for the 5 μM dose, but not for higher Hg levels. The deleterious effect of Hg was less marked in root tissues, in spite of their accumulation of very high Hg amounts (99%) because of, at least in part, a combined increase in total thiols and phenolics able to minimize oxidative stress. Our results suggested that phenolic content in roots could be used as a new and easy-to-use indicator of Hg presence. On the whole, white lupin showed a certain ability to survive in Hg-contaminated media and it would be possible to include it in combined decontamination strategies.     

Effects of dichromate on growth and root system architecture of Arabidopsis thaliana seedlings

The hexavalent form of chromium [Cr(VI)] is toxic for most organisms; however, very little information is available regarding the effects of this metal on plant morphogenesis. In this work, we investigated the effects of Cr(VI) on the growth and development of Arabidopsis thaliana, a species widely used as a model for studying the diverse physiological and cellular processes in plants. Elongation of root hairs and biomass production were stimulated by relatively low concentrations (100 μM) of Cr(VI) as potassium dichromate. Concentrations of Cr(VI) greater than 200 μM were toxic to plants as revealed both by arrested growth of roots and shoots and the development of chlorosis in leaves. At 200 μM the primary root growth was totally inhibited but the plants continued their growth manifesting different alterations in root development. These alterations correlated with changes in mitotic activity and in cellular expansion. The analyses of A. thaliana transgenic plants that express the auxin-inducible marker DR5:uidA, and the response of the auxin-resistant mutants axr2 and aux1–7 to dichromate suggest that auxins do not participate as mediators in the cellular and physiological responses to this metal. The primary root growth inhibition by 200 μM dichromate was alleviated by more than 70% by increasing the sulfate, phosphate or nitrate concentration in the media, which suggests a relation of dichromate with these mineral nutrients.     

Effects of overexpressing photosynthetic carbon flux control enzymes in the cyanobacterium Synechocystis PCC 6803

Synechocystis PCC 6803 is a model unicellular cyanobacterium used in e.g. photosynthesis and CO₂ assimilation research. In the present study we examined the effects of overexpressing Ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO), sedoheptulose 1,7-biphosphatase (SBPase), fructose-bisphosphate aldolase (FBA) and transketolase (TK), confirmed carbon flux control enzymes of the Calvin-Bassham-Benson (CBB) cycle in higher plants, in Synechocystis PCC 6803. Overexpressing RuBisCO, SBPase and FBA resulted in increased in vivo oxygen evolution (maximal 115%), growth rate and biomass accumulation (maximal 52%) under 100 μmol photons m⁻² s⁻¹ light condition. Cells overexpressing TK showed a chlorotic phenotype but increased biomass by approximately 42% under 100 μmol photons m⁻² s⁻¹ light condition. Under 15 μmol photons m⁻² s⁻¹ light condition, cells overexpressing TK showed enhanced in vivo oxygen evolution. This study demonstrates increased growth and biomass accumulation when overexpressing selected enzymes of the CBB cycle. RuBisCO, SBPase, FBA and TK are identified as four potential targets to improve growth and subsequently also yield of valuable products from Synechocystis PCC 6803.     

Effects of phosphorus and nitrogen amendments on the growth of Egeria najas

The effect of nutrient addition on the growth of E. najas was evaluated in a dose response experiment using sand amended with phosphorus (P) and nitrogen (N), and in enrichment trials with N and P amendments to natural sediments. Plants, water and sediment came from lagoons of the Upper Paraná River Floodplain and from Itaipu Reservoir (Brazil). Relative growth rates (RGRs) of E. najas shoots, based on dry mass (DM), varied from 0.03 to 0.060 d⁻¹ for both nutrients. Root:shoot biomass ratios were related to sediment exchangeable P (r = −0.419; P = 0.03) and N (r = −0.54; P = 0.006), however root RGR was not related to sediment nutrient concentrations. When natural sediments were amended with N and P, neither shoot nor root RGRs differed among treatments for substrata from either the reservoir or the floodplain lagoons (P > 0.05). Comparison of nutrient concentrations measured in natural sediments collected from several sites in both the Upper Paraná River Floodplain (range 49–213 μg P g⁻¹ DM; 36–373 μg N g⁻¹ DM) and Itaipu Reservoir (range 43–402 μg P g⁻¹ DM; 7.9–238 μg N g⁻¹ DM) showed that sediment N and P from these systems usually exceeded minimum requirements necessary for E. najas growth, as measured in the dose response experiment. Together, these results indicate that E. najas, at least in early stages of development, responds to sediment nutrient amendments and relies upon bottom sediments to meet its N and P requirements and that for at least two Brazilian ecosystems, growth of this species is not limited by insufficient sediment N or P. Thus, reducing N and P in water is not enough to control E. najas growth in short time periods in these ecosystems.