Sap flow and water use in African baobab (Adansonia digitata L.) seedlings in response to drought stress

The African baobab (Adansonia digitata L.) is an important multi-purpose fruit tree with high potential for domestication in drier Africa. Although adult individuals are well-known to be drought resistant, only little has been reported on how young baobab trees can survive drought. Therefore, the aim of this study was to examine short-term soil drought effects on water relations of baobab seedlings. Baobab seedlings used a limited amount of stored water to buffer daily water deficits (~ 8.5 g d^− 1), which contributed up to only ~ 17.5% of daily water use and ~ 6% of total plant water. Under drought, a strong reduction in stomatal conductance (~ 85%) resulted in a midday leaf water potential of − 1 MPa and zero stem sap flow followed by significant leaf loss. Plant anatomy evidenced the presence of water storage tissues and the vulnerability to xylem embolism. The taproot was the most important plant part for water storage (68% of total plant water), suggesting root-succulence rather than stem-succulence. When drought intensified, limitation of leaf transpiration and/or root water uptake led to drought-enforced dormancy. Despite the large amounts of water stored in the taproot (~ 90%) and the stem (~ 75%), only a limited amount of stored water appeared to be used to sustain upper leaves and plant metabolism during the dormant period, and to facilitate recovery following water supply. Drought avoidance, conservative water use and the presence of internal stored water allow baobab seedlings to survive drought.     

Response to drought of Zn-stressed Quercus suber L. seedlings

Zinc is an essential nutrient for higher plants but it becomes toxic as its availability increases. In nature, different stress factors commonly occur concurrently, challenging our ability to predict their impacts. Information on zinc (Zn) effect on plant ability to withstand other sources of stress is scarce. This study examines the effect of zinc supply rate on the response of Quercus suber L. seedlings to water stress. Seedlings were treated with four levels of zinc from 3 to 150 μM, and then exposed to a short severe drought. Zinc concentration in leaves and roots increased with zinc availability. Maximum photosynthetic rate, photochemical efficiency, root length and specific root length decreased as Zn availability increased. The decrease was particularly intense between 50 and 150 μM Zn. The relative effects of drought were less intense in seedlings receiving higher doses of Zn than in those receiving 3 μM. Thus, at severe drought, relative water content of detached leaves decreased to 52% in seedlings receiving the lowest level of Zn. In contrast, relative water content remained above 70% in seedlings exposed to high concentrations of this metal. The pattern was similar for photosynthetic rate and stomatal conductance, as the decrease in these variables resulting from severe drought was 100% and 90% in seedlings receiving 3 μM and 65% and 56% in seedlings receiving the highest Zn dose. Our results suggest that morpho-physiological responses to zinc may foster water conservation strategies and alleviate the effects of drought in the short term, but they may impair seedling ability to root and grow in a longer term.     

Interactive effects of salinity and water depth on the growth of Melaleuca ericifolia Sm. (Swamp paperbark) seedlings

Melaleuca ericifolia Sm. (Swamp paperbark) is a common tree species in freshwater and brackish wetlands in southern and eastern Australia. The survival of this species in many wetlands is now threatened by increased salinity and inappropriate water regimes. We examined the response of 5-month-old M. ericifolia seedlings to three water depths (exposed, waterlogged and submerged) at three salinities (2, 49 and 60 dS m⁻¹). Increasing water depth at the lowest salinity did not affect survival, but strongly inhibited seedling growth. Total biomass, leaf area and maximum root length were highest in exposed plants, intermediate in waterlogged plants and lowest in submerged plants. Although completely submerged plants survived for 10 weeks at the lowest salinity, they demonstrated negative growth rates and were unable to extend their shoots above the water surface. At the higher salinities, M. ericifolia seedlings were intolerant of waterlogging and submergence: all plants died after 9 weeks at 60 dS m⁻¹. Soil salinities increased over time, and by Week 10, exceeded external water column salinities in both the exposed and waterlogged treatments. In exposed sediment, ∼90% of plants survived for 10 weeks at 60 dS m⁻¹ even though soil salinities reached ∼76 dS m⁻¹. No mortality occurred in the exposed plants at 49 dS m⁻¹, and small but positive relative growth rates were recorded at Week 10. We conclude that at low salinities M. ericifolia seedlings are highly tolerant of sediment waterlogging, but are unlikely to tolerate prolonged submergence. However, at the higher salinities, M. ericifolia seedlings are intolerant of waterlogging and submergence and died rapidly after 5 weeks exposure to this combination of environmental stressors. This research demonstrates that salinity may restrict the range of water regimes tolerated by aquatic plants.     

Effect of salinisation of soil on growth, water status and general nutrient accumulation in seedlings .of Delonix regia (Fabaceae)

Greenhouse experiments were conducted to assess the effect of salinisation of soil on emergence, growth, water content, proline content and mineral accumulation of seedlings of Delonix regia (Hook.) Raf. (Fabaceae). Sodium chloride (NaCl) was added to the soil and salinity was maintained at 0.3, 1.9, 3.9, 6.0 and 7.9 dS m^?1. A negative relationship between seedling emergence and salt concentration was obtained. Salinity caused reduction in water content and water potential of tissues (leaves, stems, tap roots and lateral roots) that resulted in internal water deficit to plants. Consequently, shoot and root elongation, leaf expansion and dry matter accumulation in leaves, stems, tap roots and lateral root tissues of seedlings significantly decreased in response to increasing concentration of salt. Proline content in tissues was very low. There were no effective mechanisms to control net uptake of Na on root plasma membrane and subsequently its transport to shoot tissues. Potassium content significantly decreased in tissues in response to salinisation of soil. This tree species is a moderate salt-tolerant glycophytic plant. Nitrogen and calcium content in tissues significantly decreased as soil salinity increased. Phosphors content in tissues exhibited a declining trend with increase in soil salinity. Changes in tissues and whole-plant accumulation pattern of other elements tested, as well as possible mechanisms for avoidance of Na toxicity in this tree species in response to salinisation, are discussed.     

Ectomycorrhiza affect architecture and nitrogen partitioning of beech (Fagus sylvatica L.) seedlings under shade and drought

The aim of this study was to investigate the influence of ectomycorrhizal fungi (EMF) on the architecture of and nitrogen (N) partitioning in young beech (Fagus sylvatica) plants in response to different light regimes and water deprivation. We hypothesized that EMF modify biomass partitioning and architecture of young beech plants by increased N uptake in comparison with non-mycorrhizal (NM) plants and that therefore, the drought responses of EM and NM plants diverge. We anticipated that full light-exposed plants were more drought tolerant due to improved water status and nutrition, whereas shade-acclimated EM plants were more drought susceptible because of decreased mycorrhizal colonization. To test these hypotheses seedlings were grown in native or sterilized forest soil. To avoid effects of soil pretreatment NM and EM plants were transplanted into sand-peat culture systems and exposed to shade, drought or the combination of both factors. Shade resulted in reduced root biomass production decreasing the root-to-shoot ratio. Mild drought stress (pre-dawn water potential [Ψ_pd] = −1.3 MPa) did not affect biomass partitioning. EMF colonization did not increase plant biomass, but had strong effects on root architecture: the numbers of root tips as well as the absolute and specific root lengths were increased because of formation of thin roots, especially in the diameter classes from 0.2 to 0.8 mm. In contrast to our expectation N uptake of well irrigated EM plants was not increased despite their larger potential for soil exploitation. Overall, EM plants exhibited higher amounts of carbon fixed per unit of N taken up than NM plants and shifted N partitioning towards the roots. Beneficial effects of EMFs were apparent under mild drought but the responses differed depending on the light availability: shaded EM plants showed a delay in the decrease of Ψ_pd; light exposed EM plants showed increased N uptake compared with NM beeches. These results indicate that EMFs are involved in mediating divergent responses of beech to drought depending on the light availability.     

Studying the effects of heavy metal on chlorophyll and sugar in one year-old seedlings organs of Acer velutinum specie

Pollution is one of the most important factors inhibiting growth in the environment; therefore the effects of zinc pollution were studied in Acer velutinum specie. Two years old seedlings of Acer velutinum specie were prepared from nursery, the concentrations of zero and one hundred and thirty mg per liter of zinc chloride solution were added to the soil of seedlings pots after calculating and after passing a three-month period of seedling growth, the plant organs were removed, then the amount of zinc concentration in the samples was determined and data were analyzed. The results of the analysis showed that the highest amount of accumulation on the stem, root and soil in treatment concentrations is 87.75, 65.68 and 24.78 mg per kg and accumulation of zinc in total chlorophyll and sugar in treatment concentrations is 4.61 and 0.6028 mg per g, respectively, and accordingly Acer velutinum specie is suitable for refinement of soils contaminated with zinc.     

Effects of controlled irrigation on water potential,nitrogen uptake and biomass production in Dalbergia sissoo seedlings

Biomass production, pattern of nodulation, nutrient uptake, net photosynthetic rate (P_n), leaf temperature (T_leaf), leaf nitrate reductase (NR) activity and free proline of Dalbergia sissoo seedlings planted in containers with 120 kg soil were studied under different water stress levels to assess the productive potential of the species in dry areas. Seedlings were irrigated at 20 mm (W₁), 14 mm (W₂), 10 mm (W₃), 8 mm (W₄) throughout the experimental period to maintain the respective treatment up to the lowest soil water content of 7.43%, 5.64%, 4.30% and 3.23%, respectively. There was a treatment (W₅) in which seedling were irrigated once to −0.03 MPa and left without re-irrigation. Decreased irrigation level resulted in lowering of leaf water potential (LWP), net photosynthetic rate (P_n), total number of root nodules and nodule dry mass and nitrogen uptake in the seedling. P_n, leaf nitrate reductase (NR) activity and seedling biomass were highest in W₁ indicating a positive relations of NR activity with CO₂ assimilation and biomass production. The decrease in P_n, leaf NR activity and LWP was sharp at W₃ onwards. Monthly changes in the values of P_n, T_leaf and NR activity indicate environmental effect on these physiological variables. Proline was detected only in the seedlings of W₃, W₄ and W₅ treatments after February and was highest in the seedlings of W₅ treatment. The study suggests that severe water deficit adversely affect physiological and biochemical processes that resulted in reduced growth, nutrient uptake and biomass productivity in D. sissoo seedlings. Re-irrigation above W₃ level is recommended for this species.     

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.     

Windows of opportunity for Prosopis velutina seedling establishment and encroachment in a semiarid grassland

Large increases in the density of woody plants in former grasslands have been documented globally over the past two centuries. Prosopis velutina (velvet mesquite) has recently expanded on over 38 million ha in the grasslands of the American southwest.Mesquite establishment and expansion is potentially influenced by competitive interactions with grasses, particle size distribution (texture) of soil and changes in the amount and/or seasonality of precipitation, amongst other factors.To investigate the relative importance of precipitation seasonality, grass competition, and soil texture on mesquite seedling and establishment, we experimentally manipulated seasonality of precipitation across 72 1.5 m × 1.7 m plots that we planted with two grasses from contrasting functional groups (or left unvegetated) across two highly contrasting soil textures within a semi-desert grassland in Arizona, USA, wherein we followed the demography of 8640 planted mesquite seeds between 2002 and 2006. Up to 70% of the variance in seedling emergence was explained by soil water content, and seedling mortality was more dependent on summer than on winter precipitation.Mortality of less drought tolerant grasses after drought in coarse textured soils created conditions most favorable for seedling establishment, whereas lower rates of seedling establishment were observed in finer textured soils or in plots dominated by more drought tolerant grasses.We conclude that mesquite encroachment is episodic, based on discontinuous patterns of seedling establishment, favored on coarse textured soils and associated with unique conditions of high summer water availability when precipitation is not limiting and when competition from grasses is lowered.     

Biocontrol of Rhizoctonia solani damping-off and promotion of tomato plant growth by endophytic actinomycetes isolated from native plants of Algerian Sahara

Thirty-four endophytic actinomycetes were isolated from the roots of native plants of the Algerian Sahara. Morphological and chemical studies showed that twenty-nine isolates belonged to the Streptomyces genus and five were non-Streptomyces. All isolates were screened for their in vitro antifungal activity against Rhizoctonia solani. The six that had the greatest pathogen inhibitory capacities were subsequently tested for their in vivo biocontrol potential on R. solani damping-off in sterilized and non-sterilized soils, and for their plant-growth promoting activities on tomato seedlings. In both soils, coating tomato seeds with antagonistic isolates significantly reduced (P < 0.05) the severity of damping-off of tomato seedlings. Among the isolates tested, the strains CA-2 and AA-2 exhibited the same disease incidence reduction as thioperoxydicarbonic diamide, tetramethylthiram (TMTD) and no significant differences (P < 0.05) were observed. Furthermore, they resulted in a significant increase in the seedling fresh weight, the seedling length and the root length of the seed-treated seedlings compared to the control. The taxonomic position based on 16S rDNA sequence analysis and phylogenetic studies indicated that the strains CA-2 and AA-2 were related to Streptomyces mutabilis NBRC 12800^T (100% of similarity) and Streptomyces cyaneofuscatus JCM 4364^T (100% of similarity), respectively.     

Role of mycorrhizal networks and tree proximity in ectomycorrhizal colonization of planted seedlings

The importance of mycorrhizal network (MN)-mediated colonization under field conditions between trees and seedlings was investigated. We also determined the combined influences of inoculum source and distance from trees on the ectomycorrhizal (EM) community of seedlings. On six sites, we established trenched plots around 24 residual Pseudotsuga menziesii var. glauca trees and then planted seedlings at four distances (0.5, 1.0, 2.5, and 5.0 m) from the tree into four mesh treatments that served to restrict inoculum access (i.e., planted into mesh bags with 0.5, 35, 250 μm pores or directly into soil). Ectomycorrhizal communities were identified after two growing seasons using morphological and molecular techniques. Mesh treatments had no effect on seedling mycorrhizal colonization, richness, or diversity, suggesting that MN-mediated colonization, was not an essential mechanism by which EM communities were perpetuated to seedlings. Instead, wind-borne and soil inoculum played an important role in seedling colonization. The potential for MNs to form in these forests was not dismissed, however, because trees and seedlings shared 83 % of the abundant EM. Seedlings furthest from trees had a simpler EM community composition and reduced EM richness and diversity compared to seedlings in closer proximity.     

Nitric oxide enhances salt tolerance in cucumber seedlings by regulating free polyamine content

Nitric oxide (NO), an endogenous signaling molecule in plants and animals, mediates responses to abiotic and biotic stresses. This study was conducted in nutrient solution to investigate the effects of exogenous sodium nitroprusside (SNP), an NO donor, on plant growth and free polyamine content in cucumber leaves and roots under NaCl stress. The results showed that 100 μM SNP in solution significantly improved the growth of cucumber seedlings under NaCl stress for 8 days, as indicated by increased, plant height, stem thickness, fresh weight and increased dry matter accumulation. Further analysis demonstrated that the content of free polyamines and the activity of polyamine oxidase (PAO) in cucumber seedling leaves and roots initially increased dramatically under NaCl stress, although they decreased over a longer period of stress. Throughout the treatment period, the value of (spermine + spermidine)/putrescine [(Spd + Spm)/Put] also decreased under NaCl stress compared to the control. In contrast, the application of 100 μM SNP in the nutrient solution decreased the content of free Put, Spd, total free polyamines and PAO activity under NaCl stress. It also caused an increase in the content of Spm and the value of (Spd + Spm)/Put, adjusted the ratio of three kinds of free polyamines (Put, Spd, Spm) in cucumber seedlings. The high (Spd + Spm)/Put value and the accumulation of Spm were beneficial to improving the salt tolerance of plants. Therefore, NO alleviated the damage to cucumber seedlings caused by salt stress. NO enhanced the tolerance of cucumber seedlings to NaCl stress by regulating the content and proportions of the different types of free polyamines.     

Accumulation of free amino acids during exposure to drought in three springtail species

Springtails are closely related to insects, but they differ from these with respect to water balance, in particular because springtails are small and have high integumental permeability to water. Here we report a series of experiments addressing the dynamics of osmoregulation, water content and accumulation of free amino acids (FAAs) in three springtail species during exposure to a gradually increasing environmental desiccation simulating conditions in drought exposed soil. Folsomia candida and Protaphorura fimata (both living in the deeper soil layers; euedaphic species) were active throughout the 3 week exposure, with the developing drought regime ending at −3.56 MPa (the soil water activity at the permanent wilting point of plants is −1.5 MPa) and remained hyperosmotic (having an body fluid osmolality higher than the corresponding environment) to their surrounding air. Sinella curviseta (living in upper soil/litter layers; hemiedaphic species) also survived this exposure, but remained hypoosmotic throughout (i.e. with lower osmolality than the environment). The body content of most FAAs increased in response to drought in all three species. Alanine, proline and arginine were the most significantly upregulated FAAs. By combining our results with data in the literature, we could account for 82% of the observed osmolality at −3.56 MPa in F. candida and 92% in P. fimata. The osmolality of S. curviseta was only slightly increased under drought, but here FAAs were considerably more important as osmolytes than in the two other species. We propose that FAAs probably have general importance in drought tolerance of springtails.     

Germination responses of Croton macrostachyus (Euphorbiaceae) to various physico-chemical pretreatment conditions

Croton macrostachyus Hochst. ex Del. (Euphorbiaceae) is a multipurpose, deciduous, and medium sized tree of pantropic occurrence. Because the species has numerous useful qualities (e.g., establishment and growth in disturbed sites, drought tolerance, fast growth rate, copious litter/necromass production, suitability for agroforestry, and ability to attract avian frugivores), its speedy restoration has become increasingly critical. Germination studies were therefore conducted on seeds pooled from five widely located provenances with a view to supporting efforts geared toward the speedy propagation and restoration of this valuable tree species. Seed pretreatments were achieved using various dilution levels of plant-derived smoke–water (1:1, 1:10, 1:100 and 1:1000), as well as gibberellic acid (GA₃) or potassium nitrate (KNO₃) ranging in concentration from 0.1 to 100 μmol. The control was to use distilled water for seed pretreatment. Seeds were germinated under either illuminated (ca 60 μmol m^− 2 s^− 1; cool-white fluorescent lamp) or non-illuminated conditions. Experiments on the impact of seed storage durations, as well as storage temperatures were also conducted. The study found that germination percentage (GP: ca 90%), and mean germination time (MGT: 14 days) were significantly (P < 0.001) better when seeds were pretreated with smoke–water and germinated under non-illuminated conditions, than when these were pretreated with various concentrations of GA₃ or KNO₃ (GP and MGT of ca 65% and 20 days, respectively). Germination percentage (GP) and germination vigor (GV) declined with increasing storage-time for all storage temperatures, but GV's decline was faster for seeds stored at 22 °C than for those stored at 5 and 15 °C. On the other hand, mean germination time (MGT) increased significantly (P < 0.01) with seed storage-time of up to 8 months at 5, 15, and 22 °C, but the increase was more marked for seeds stored at 22 °C than for those stored at 5 and 15 °C. From these investigations, it is concluded that germination of C. macrostachyus seeds through use of smoke–water is faster, cheaper, and technically less demanding, compared to that of either GA₃ or KNO₃. The study also concludes that C. macrostachyus is intermediate between orthodox and recalcitrant seeds, and that it is non-photoblastic.     

Comparison of salinity tolerance of three Atriplex spp. in well-watered and drying soils

Members of the Chenopodiaceae are well adapted to both salt and drought stress and can serve as model species to understand the mechanisms of tolerance in plants. We grew Atriplex hortensis (ATHO), A. canescens (ATCA), and A. lentiformis (ATLE) along a NaCL salinity gradient under non-water-limited conditions and in drying soils in greenhouse experiments. The species differed in photosynthetic carbon fixation pathway, capacity for sodium uptake, and habitat preferences. Under non-water-limited conditions, ATLE (C4) maintained high growth rates up to 30 g L⁻¹ NaCl. ATHO (C3) had lower growth than ATLE at high salinities, while ATCA (C4) grew more slowly than either ATLE or ATHO and showed no net growth above 20 g L⁻¹ NaCl. ATHO and ATLE accumulated twice as much sodium in their shoots as ATCA, but all three species had increasing sodium levels at higher salinities. Potassium, magnesium and calcium levels were relatively constant over the salinity gradient. All three species showed marked accumulation of chloride across the salinity gradient, whereas nitrate, phosphorous and sulfate decreased with salinity. The effect of drought was simulated by growing plants in sealed pots with an initial charge of water plus NaCl, and allowing them to grow to the end point at which they no longer were able to extract water from the soil solution. Drought and salinity were not additive stress factors for Atriplex spp. in this experiment. NaCl increased their ability to extract water from the soil solution compared to fresh water controls. ATLE showed increased shoot dry matter production and increased water use efficiency (WUE) as initial salinity levels increased from 0 to 30 g L⁻¹ NaCl, whereas dry matter production and WUE peaked at 5 g L⁻¹ for ATHO and ATCA. Final soil moisture salinities tolerated by species were 85 g L⁻¹, 55 g L⁻¹ and 160 g L⁻¹ NaCl for ATHO, ATCA and ATLE, respectively. C4 photosynthesis and sodium accumulation in shoots were associated with high drought and salt tolerance.     

A novel optical interference technique to measure minute root elongations of Japanese red pine (Pinus densiflora Seibold & Zucc.) seedlings infected with ectomycorrhizal fungi

This study presents a new technique to measure root elongation of Japanese red pine (Pinus densiflora Seibold & Zucc.) seedlings with very high sensitivity in the order of sub-nanometer by using a novel optical interference method called statistical interferometry. The principle of the statistical interferometry is based on the statistics of a speckle field, which is generated when a rough surface is illuminated by a laser light. The technique facilitates to obtain minute root elongation measurements in the order of sub-seconds. The root elongation behavior of Pinus densiflora seedlings infected with ectomycorrhizal fungi, Pisolithus sp. (Ps) and Cenococcum geophilum Fr. (Cg), was investigated in comparison with that of an uninfected control. In the experiments, two points on a root with the separation of 3 mm were illuminated by laser beams and the elongation was measured continuously by analyzing speckle patterns successively taken by a CCD camera. The root elongation rate (RER), measured as the length of root elongation per second per millimeter (mean ± S.D.) for Ps-infected, Cg-infected and uninfected seedlings were 10.85 ± 2.41, 5.54 ± 1.43, and 2.41 ± 1.01 nm s⁻¹ mm⁻¹, respectively. We found that the RERs of seedlings infected with ectomycorrhizal fungi were significantly higher than that of the uninfected seedlings, and the seedlings infected with Ps fungi showed the highest RER. We conducted another experiment to observe two-dimensional root growth, in which the growth measurements were obtained for 4 months. From this experiment, we observed that root growth of ectomycorrhizal fungi infected seedlings were higher than that of the uninfected seedlings. The evaluation of results from these two techniques proves that the proposed statistical interferometry is much faster and very sensitive technique, where the time required for growth monitoring is 10⁷ times less than the other. We can conclude that, at the scale of either very short time or relatively long time, the symbiotic relationship between root and ectomycorrhiza has a positive effect steadily on the root elongation.     

A novel optical interference technique to measure minute root elongations of Japanese red pine (Pinus densiflora Seibold & Zucc.) seedlings infected with ectomycorrhizal fungi

This study presents a new technique to measure root elongation of Japanese red pine (Pinus densiflora Seibold & Zucc.) seedlings with very high sensitivity in the order of sub-nanometer by using a novel optical interference method called statistical interferometry. The principle of the statistical interferometry is based on the statistics of a speckle field, which is generated when a rough surface is illuminated by a laser light. The technique facilitates to obtain minute root elongation measurements in the order of sub-seconds. The root elongation behavior of Pinus densiflora seedlings infected with ectomycorrhizal fungi, Pisolithus sp. (Ps) and Cenococcum geophilum Fr. (Cg), was investigated in comparison with that of an uninfected control. In the experiments, two points on a root with the separation of 3 mm were illuminated by laser beams and the elongation was measured continuously by analyzing speckle patterns successively taken by a CCD camera. The root elongation rate (RER), measured as the length of root elongation per second per millimeter (mean ± S.D.) for Ps-infected, Cg-infected and uninfected seedlings were 10.85 ± 2.41, 5.54 ± 1.43, and 2.41 ± 1.01 nm s^?1 mm^?1, respectively. We found that the RERs of seedlings infected with ectomycorrhizal fungi were significantly higher than that of the uninfected seedlings, and the seedlings infected with Ps fungi showed the highest RER. We conducted another experiment to observe two-dimensional root growth, in which the growth measurements were obtained for 4 months. From this experiment, we observed that root growth of ectomycorrhizal fungi infected seedlings were higher than that of the uninfected seedlings. The evaluation of results from these two techniques proves that the proposed statistical interferometry is much faster and very sensitive technique, where the time required for growth monitoring is 10⁷ times less than the other. We can conclude that, at the scale of either very short time or relatively long time, the symbiotic relationship between root and ectomycorrhiza has a positive effect steadily on the root elongation.     

The Effects of Drought and Shade on the Performance,Morphology and Physiology of Ghanaian Tree Species

In tropical forests light and water availability are the most important factors for seedling growth and survival but an increasing frequency of drought may affect tree regeneration. One central question is whether drought and shade have interactive effects on seedling growth and survival. Here, we present results of a greenhouse experiment, in which seedlings of 10 Ghanaian tree species were exposed to combinations of strong seasonal drought (continuous watering versus withholding water for nine weeks) and shade (5% irradiance versus 20% irradiance). We evaluated the effects of drought and shade on seedling survival and growth and plasticity of 11 underlying traits related to biomass allocation, morphology and physiology. Seedling survival under dry conditions was higher in shade than in high light, thus providing support for the “facilitation hypothesis” that shade enhances plant performance through improved microclimatic conditions, and rejecting the trade-off hypothesis that drought should have stronger impact in shade because of reduced root investment. Shaded plants had low biomass fraction in roots, in line with the trade-off hypothesis, but they compensated for this with a higher specific root length (i.e., root length per unit root mass), resulting in a similar root length per plant mass and, hence, similar water uptake capacity as high-light plants. The majority (60%) of traits studied responded independently to drought and shade, indicating that within species shade- and drought tolerances are not in trade-off, but largely uncoupled. When individual species responses were analysed, then for most of the traits only one to three species showed significant interactive effects between drought and shade. The uncoupled response of most species to drought and shade should provide ample opportunity for niche differentiation and species coexistence under a range of water and light conditions. Overall our greenhouse results suggest that, in the absence of root competition shaded tropical forest tree seedlings may be able to survive prolonged drought.     

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.     

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.