Studies on the effect of salinity, drought stress and soil type on nodule activities of Lablab purpureus (L.) sweet (Kashrangeeg)

Lablab purpureus(50 cultivars throughout the world) is a drought-tolerant legume widely grown as a high-protein grain food and forage legume within a wide range of neotropical regions with extensive production in India and similar climatic areas of Asia, Africa, Central and South America. The objective of this work was to study the nitrogen-fixing potentialities of L. purpureus under the effects of the extreme environmental conditions prevailing in the southern part of Egypt aiming at the recommendation of its propagation in areas of the National giant projects.L. purpureusinoculated with Rhizobium sp. Strain I₄ (21 days after planting) grown on Nile valley and Wadi Allaqi soils was relatively tolerant to mild levels of salinity, but the nodule number was reduced to about 35% of the control plants when subjected to a high salt level (120mM NaCl). Lablab plants were similarly affected by different rates of water deficits. This legume was tolerant to moderate levels of drought. The nodule number and weight at 50% of field capacity was about 70% of the control. These values were reduced to 45–55% at a field capacity of 16·5%.Absolute nitrogenase activity, leghaemoglobin content of nodules and protein content of bacteroids and cytosol were moderately affected by mild levels of NaCl and drought but significantly reduced to about 25–35% of the control treatments.The results also indicate that plants grown on the soil of Nile Valley exhibited slightly higher values (nitrogenase, protein, etc.) than those grown on Wadi Allaqi soil. This would encourage the possibility of propagating L. purpureus in the newly reclaimed areas of southern Egypt.     

Photosynthetic responses of Larrea tridentata to a step-increase in atmospheric CO2at the Nevada Desert FACE Facility

Of all terrestrial ecosystems, the productivity of deserts has been suggested to be the most responsive to increasing atmospheric CO₂. The extent to which this prediction holds will depend in part on plant responses to elevated CO₂under the highly variable conditions characteristic of arid regions. The photosynthetic responses ofLarrea tridentata , an evergreen shrub, to a step-increase in atmospheric CO₂(to 550 μmolmol⁻¹) were examined in the field using Free-Air CO₂Enrichment (FACE) under seasonally varying moisture conditions. Elevated CO₂substantially increased net assimilation rate (A_net) in Larrea during both moist and dry periods of the potential growing season, while stomatal conductance (g_s) did not differ between elevated and ambient CO₂treatments. Seasonal and diurnal gas exchange dynamics in elevated CO₂mirrored patterns in ambient CO₂, indicating that elevated CO₂did not extend photosynthetic activity longer into the dry season or during more stressful times of the day. Net assimilation vs. internal CO₂(A/C_i) responses showed no evidence of photosynthetic down-regulation during the dry season. In contrast, after significant autumn rains, A_max(the CO₂saturated rate of photosynthesis) and CE (carboxylation efficiency) were lower in Larrea under elevated CO₂. In situ chlorophyll fluorescence estimation ofLarrea Photosystem II efficiency (F_v/F_m) responded more to water limitation than to elevated CO₂. These findings suggest that predictions regarding desert plant responses to elevated CO₂should account for seasonal patterns of photosynthetic regulatory responses, which may vary across species and plant functional types.     

Water shortage and associated changes in organic nitrogen betweenPappophorum caespitosum(Gramineae) provenances

Three provenances ofPappophorum caespitosum, a forage native grass from the temperate arid region of Argentina, were studied to gain a physiological knowledge of the seasonal organic N status in response to adequate water supply and water stress conditions; and to assess the extent to which variations in the N status between these provenances arise under different water regimes. Plant part samples were analysed for free and bound organic N fractions. Water shortage resulted in a significant depression of plant growth in all provenances. Total N concentration declined along the annual cycle in both water treatments, but it was higher under water stress. The size of the free N pool was very large, suggesting that it may act like an osmoticum. Nitrogen use efficiency increases along the growing period, while the rate of N intake and the specific leaf N content decrease. Variability was found between provenances in most of the physiological and biochemical traits examined under both water regimes.     

Effect of exogenous application of salicylic acid on the drought stress responses of Gardenia jasminoides

The alleviative effects of exogenous salicylic acid(SA) on plants against drought stress were assessed in Gardenia jasminoides seedlings treated with different concentrations of SA.Drought stress was simulated to a moderate level by 15% polyethylene glycol(PEG) 6000 treatment.Seedlings exposed to 15% PEG for 14 days exhibited a decrease in aboveground and underground dry mass,seedling height,root length,relative water content,photosynthetic pigment content,net photosynthetic rate(Pn),transpiration rate(Tr),stomatal conductance(Gs),and water use efficiency.In PEG-stressed plants,the levels of proline,malondialdehyde(MDA),hydrogen peroxide(H_2O_2),and electrolyte leakage rose significantly,whereas antioxidative activity,including superoxide,peroxidase,and catalase activities,declined in leaves.However,the presence of SA provided an effective method of mitigating PEG-caused physiological stresses on G.jasminoides seedlings,which depended on SA levels.PEG-treated plants exposed to SA at 0.5–1.0 mmol/L significantly eased PEG-induced growth inhibition.Application of SA,especially at concentrations of 0.5–1.0 mmol/L,considerably improved photosynthetic pigments,photosynthesis,antioxidative activity,relative water content,and proline accumulation,and decreased MDA content,H_2O_2 content,and electrolyte leakage.By contrast,the positive effects were not evident,or even more severe,in PEG+SA4 treatment.Based on these physiological and biochemical data,a suitable concentration of SA,potential growth regulators,could be applied to enhance the drought tolerance of G.jasminoides.     

Carbon accumulation, nitrogen and phosphorus use efficiency of Sophora davidii seedlings in response to nitrogen supply and water stress

A greenhouse experiment was conducted to explore whether additional nitrogen (N) supply could enhance carbon (C) accumulation, and phosphorus (P) use efficiency (NUE_P) of Sophora davidii seedlings under dry conditions. Two-month-old seedlings were subjected to a completely randomized design with three water (80, 40 and 20% water field capacity (FC)) and three N supply (N0: 0, Nl: 92 and Nh: 184 mg N kg⁻¹ soil) regimes. Water stress decreased C, N and P accumulation, NUE_P, N and P uptake efficiency (NUtE_N and NUtE_P) regardless of N supply. The S. davidii seedlings exhibited strong responses to N supply, but the responses were not consistent with the various N supply levels. Nl increased C, N and P accumulation, and improved NUE_P, NUtE_N and NUtE_P in the same water treatment. In contrast, Nh did few or even depress effects on C, N and P accumulation, and NUE_P, although NUtE_N and NUtE_P increased with Nh in the same water treatment. Even so, NUE_N decreased with increase of N supply in the same water treatment. The results suggested that appropriate or low N supply should be recommended for S. davidii seedling establishment in dry environment by improving C accumulation and NUE_P.     

Mesquite (Prosopis juliflora (Sw.) DC.), huisache (Acacia farnesiana (L.) Willd.) and catclaw (Mimosa biuncifera Benth.) and their effect on dynamics of carbon and nitrogen in soils of the semi-arid highlands of Durango Mexico

In the northern semiarid and arid part of Mexico, mesquite (Prosopis juliflora (SW.) DC.), huisache (Acacia farnesiana (L.) Willd.) and catclaw (Mimosa biuncifera Benth.), N₂-fixing trees or shrubs, dominate the landscape. It is unknown, however, how much the leaves of those shrubs contribute to dynamics of carbon (C) and nitrogen (N) in soil. We investigated this by adding leaves of each species to soil sampled under the canopy of mesquite, huisache, and catclaw and outside their canopy while monitoring production of carbon dioxide (CO₂), and dynamics of inorganic N (ammonium (NH₄⁺) and nitrate (NO₃⁻)) in an aerobic incubation. The (hemi)cellulose and N content of the catclaw leaves was lower and the lignin and polyphenol content was larger than in the mesquite and huisache leaves. If we considered no priming effect, then 41% of the C added with catclaw leaves, 47% with huisache leaves and 49% with mesquite leaves mineralized within 42 days. The addition of the leaves had little or no effect on N mineralized, and only 6% of organic N of the mesquite leaves was mineralized. It was found that catclaw, huisache and mesquite have a positive effect on the arid and semi-arid ecosystems as they increased soil organic matter and soil N content.     

Effects on antioxidant enzyme activities and osmolytes in Halocnemum strobilaceum under salt stress

The seedlings of Halocnermum strobilaceum were cultivated in 0.5% hoagland nutrient solution containing 0.0%, 0.9%, 2.7% and 5.4% of NaCl as well as composite salt (Na⁺, Ca²⁺, K⁺, Si⁴⁺) for 20 days; all the contents are in weight ratio. Succulent level, inorganic ions (Na⁺, K⁺), organics such as betaine, proline, malondialdehyde, and antioxidant enzyme activities including superoxide dismutase (SOD), catalase (CAT), peroxidase (POD), betaine aldehyde dehydrogenase (BADH) were measured to reveal its salt tolerance mechanism. When the composite salt concentration reaches 5.4%, SOD activity level, and MDA content is five times the control group; when it reaches 2.7%, the succulent level of seedlings, and the content of K⁺ in roots is nearly two times the NaCl treatment; the dry weight is more than three times the control group; with the NaCl treatment, MDA is three times the contrast; when the salt concentration is 2.7%, POD reaches the maximum. Results indicate that Si⁴⁺, K⁺, and Ca²⁺ from composite salt in the roots of H. strobilaceum improved the water-holding capacity. The activities of antioxidant enzyme were raised by the accumulation of proline and betaine, which increased the salt tolerance. The absorption of K⁺ promoted the high ratio of K⁺/Na⁺ and alleviated the damage of cell membranes of H. strobilaceum, which is associated with osmotic contents such as betaine and proline.     

The diet of the Long-eared Owls, Asio otus, in the desert of northwest China

The contents of regurgitated Long-eared Owls (Asio otus) pellets collected in Minqin Desert Experimental Research Station of northwestern China were analysed. A total of 303 individuals representing five species of rodents and one of bird were identified in the pellets. By frequency, the most common species taken by Long-eared Owls was Meriones meridianus, followed by Cricetulus barabensis and Phodopus roborovskii. By biomass, Meriones meridianus was the most important species taken, followed by C. barabensis and Rhombomys opimus. Our results suggest that the Long-eared Owls were nocturnal hunters and fed on a wide range of animals with respect to their habitats.     

Effects of early season defoliation ofAnthyllis cytisoides(a Mediterranean browse species) on further herbivore attack

To determine if defoliation of a woody plant affects foraging by folivorous insects, we examined the infection rate (number of leaves damaged per total number of leaves sampled on marked stems) ofAnthyllis cytisoidesunder three experimental treatments: 10, 50 and 90% plant defoliation. Observations were made for three age classes, established by trunk base perimeter (equal to or lower than 11 cm, between 11 and 20 cm, larger than 20 cm). Plants respond positively to artificial defoliation by increasing total vegetative length of the stem and total inflorescence length. This response is most evident in young individuals.Response to herbivory was measured as overall infection rate and also as infection rate by different feeding guilds—chewing, mining, or sucking insects. We found that increased defoliation elicited increased resistance of leaves to insect attack. This was particularly evident in young plants. Different insect guilds respond in different ways. Attack by chewing insects declines with defoliation for all plant age classes; only sucking insects which feed on the oldest plants reduce feeding rate with plant defoliation. Finally, mining insects present the opposite trend in young and senescent plants.     

Effects of salinity, temperature and light on germination of invasive Prosopis juliflora (Sw.) D.C.

Effects of salinity, temperature, light and their interactions on the rate and final percentage of germination were evaluated for the invasive shrub Prosopis juliflora, grown under arid environmental conditions of the United Arab Emirates (UAE). Seeds that were not treated with NaCl germinated well in a wide range of temperatures and in both full light and complete dark. Seed germination decreased significantly with the increase in both NaCl concentration and temperature. Optimal germination percentage occurred at 25 °C. The inhibitory effect of high salinity on final germination percentage was greater at 40 °C than at 15 and 25 °C. Germination was completely inhibited in 400 mM NaCl at 40 °C and in 600 mM NaCl at 25 °C. However, germination rate was significantly greater at 40 °C than at 15 and 25 °C in lower salinities and the reverse was true at higher salinities. Germination in light was significantly greater than in the dark at lower salinity levels and high temperature.     

Effects of indole-3-acetic acid and zinc on the growth, osmotic potential and soluble carbon and nitrogen components of soybean plants growing under water deficit

Three-week old soybean (Glycine max) plants were subjected to a factorial combination of four regimes of soil matric water potential (ψ_m=−0·03, −0·5, −1·0 and −1·5 MPa), two levels of supplementary Zn (O and 20 mgl⁻¹) and two levels of foliar IAA application (O and 10 mgl⁻¹). Under control conditions (no Zn, no IAA), increasing soil drying progressively retarded shoot and root growth (length and dry mass production), reduced leaf relative water content (RWC) and decreased the contents of chlorophyll (Chl) and shoot soluble sugars (SS), but increased soluble sugar content of roots and lowered osmotic water potential of shoots and roots (osmotic adjustment). Total free amino acid (TAA) content increased in shoots but decreased in roots whereas contents of soluble proteins (SP) decreased in shoots and roots. The effect of water stress was statistically significant (p<0·05) and had a major effect (as indicated by η²values) on leaf RWC, shoot and root dry masses and osmotic potential. Supplementary Zn improved root growth at all levels of stress and shoot growth under severe stress. Improvement of growth was positively correlated with the internal tissue Zn concentrations (r=0·91 and 0·86 for shoot and 0·94 and 0·82 for root length and dry mass respectively). Exogenous IAA raised (p<0·05) RWC, Chl, DM (slightly), root SS, and SP, whereas shoot TAA was lowered. Effects on root TAA and shoot SS were more complex: they were lowered at zero stress and raised under severe stress. IAA and Zn in combination had additive effects on Chl, growth and osmotic potential, but their combined effects on SP and TAA were more complex. It is concluded that the treatment of soybean plants grown under conditions of low soil water potentials and Zn deficiency with Zn and IAA solutions counteracted the deleterious effects of stress, especially at high stress levels, and helped stressed plants to grow successfully under these adverse unfavourable conditions.     

Leaf nitrogen productivity as a mechanism driving the success of invasive annual grasses under low and high nitrogen supply

Invasion of the historically perennial-dominated landscapes in the Great Basin by exotic winter annual grasses is one of the most serious plant invasions in North America. Evidence suggests invasive annuals outperform native perennials under N-poor and N-rich conditions. The objective of this study was to identify key traits contributing to the success of invasive annual grasses in these environments. Three invasive annual grasses, two native perennial grasses and one introduced perennial grass were exposed to three levels of N supply. Root biomass, root length, root N uptake rate, root and leaf morphology, leaf nitrogen productivity (leaf NP) as well as biomass and N allocation were quantified over four harvests. Path analysis indicated that leaf NP was the key trait contributing to variation in N capture among the species. Species with a higher leaf NP produced more root length and consequently captured more N under a range of soil N availability. This suggests variation in leaf NP may be one critical trait determining the ability of the resident plant community to resist establishment of these invaders. Restoration programs may be able to increase weed resistance by specifically selecting for this trait in revegetation efforts.     

Eco-physiological mechanisms of Caragana korshinskii Kom. adaptation to extreme drought stress: Leaf abscission and maintaining stem chloroplast integrity

In order to study the eco-physiological mechanisms of C. korshinskii adaptation to extreme drought stress, we investigated the variations of water content in soil, leaves, and stems, the chlorophyll a and b and the carotenoid content in leaves and stems, as well as changes of chloroplast ultrastructure in 2-year-old C. korshinskii specimens during a progressive soil drought process (by ceasing watering until all leaves were shed) and a subsequent rehydration process. During the dehydration process, the chlorophyll a and b and carotenoid contents in the leaves decreased, as did the carotenoid content in the stems. During the 4-day rehydration process, the chlorophyll a and b and carotenoid contents in the leaves and stems increased and gradually returned to normal levels. During ongoing drought stress, chloroplasts in the leaves broke away from cell walls and appeared in the center of cells. Under severe drought stress, the mesophyll ultrastructure and chloroplast configuration in leaves were irreversibly disturbed, as manifested by the inner and outer membranes being destroyed; the thylakoid system disintegrated, the starch grain disappeared, and parts of cell tissue were dismantled into debris. However, the mesophyll ultrastructure and chloroplast configuration in the stems remained complete. This indicates that C. korshinskii utilizes leaf abscission to reduce the surface area to avoid damage from extreme drought stress, and maintains chloroplast integrity and a considerable amount of chlorophyll to enable a rapid recovery of photosynthesis under the rehydration process.     

Effects of soil nitrogen:phosphorus ratio on growth rate of Artemisia ordosica seedlings

To address how the ratios of nitrogen and phosphorus (N:P ratios) in soil affect plant growth, we performed a two-factor (soil available N:P ratios and plant density) randomized block pot experiment to examine the relationships between soil N:P ratios, and the N:P ratios and growth rate of Artemisia ordosica seedlings. Under moderate water stress and adequate nutrient status, both soil N:P and plant density influenced the N:P ratios and growth rates of A. ordosica. With the increase of soil N:P ratios, the growth rates of A. ordosica seedlings decreased significantly. With the increase of soil N:P ratios, N:P ratios in A. ordosica seedlings increased significantly. While the nitrogen concentrations in the plant increased slightly, the phosphorus concentrations significantly decreased. With the increase of plant density, the shoot N:P ratios and growth rates significantly decreased, which resulted from soil N:P ratios. Thus, soil N:P ratios influenced the N:P ratios in A. ordosica seedlings, and hence, influenced its growth. Our results suggest that, under adequate nutrient environment, soil N:P ratios can be a limiting factor for plant growth.     

Effects of climatic change on biomass and biomass allocation in Leymus chinensis (Poaceae) along the North-east China Transect (NECT)

A field experiment was conducted to examine the effects of large-scale climatic changes on biomass and biomass allocation in Leymus chinensis on an 11 longitude gradient along the North-east China Transect (NECT) in 2000. Ten sites, selected for plant sampling along the gradient, experienced approximately uniform theoretical radiation, but differed in precipitation and geographical elevation. The total biomasses, vegetative shoot biomass and reproductive shoot biomass of the species increased from the west to the east with the decrease of aridity or the increase of precipitation, but that of rhizome biomass decreased in the spatial ranging from 116° to 120° E. Vegetative shoot biomass allocation increased from the west to the east along the gradient; rhizome allocations, however, dropped significantly. Unlike those of rhizome and vegetative shoot, reproductive shoot biomass allocations at the two ends were remarkably lower than that in the middle of the gradient. In general, the total and component biomass and their allocations showed strong correlation with precipitation or aridity index along the gradient.     

Influence of understory removal, thinning and P fertilization on N2fixation in a mature mesquite (Prosopis glandulosavar.glandulosa) stand

The natural abundance¹⁵N/¹⁴N method was used to estimate the influence of silvicultural and P fertilization treatments on N accretion, N₂fixation and N partitioning among tissues in a mature mesquiteProsopis glandulosavar.glandulosastand in Texas. The silvicultural treatments consisted of understory removal, herbicide treatment of brushy resprouts, thinning trees to single stems and 100 kg ha⁻¹P fertilization. The trees had a mean basal diameter of 17·8 cm with 8 to 35 cm range. The stand was slow growing with the increase in dry matter ranging from 0·465 Mg ha⁻¹year⁻¹to 0·701 Mg ha⁻¹year⁻¹for the 8 years after the treatments were applied. N accretion after 8 years ranged from 3·1 kg ha⁻¹year⁻¹to 4·4 kg ha⁻¹year⁻¹.Due to the range in δ¹⁵N of the leaves, twigs, branches and trunk, we used the weighted (by biomass) average δ¹⁵N per tree in calculations of the percent N derived from N₂fixation (%Ndfa). There was considerable variability in δ¹⁵N of the reference plants, i.e. from 3·3 to 5·9. In contrast there was low variability in the background δ¹⁵N of nearby soils (7·0±1·0). As the total above-ground biomass δ¹⁵N of a grass grown outside the influence of mesquite (7·8±0·58) had the same δ¹⁵N as the soil (7·5±1·0), we used the grass outside the influence of mesquite and the weighted tree mean δ¹⁵N to calculate % of N derived from N₂fixation.The decrease in intraspecific competition by thinning multistemed trees to single stemmed trees was the only treatment that significantly (p= 0·0001) increased growth. Interspecific competition, i.e. understory removal, did not increase growth. There were no significant differences in total N production or N fixation among treatment means. The most striking result was the highly positive correlation between tree δ¹⁵N and total N per tree and biomass per tree (R²= 0·90,F= 164·4, df. = 18, mean square error (MSE) = 0·155,p= 0·0001). This implies that the younger trees colonizing infertile soils relied more heavily on N₂fixation than larger trees which accumulated 1200 kg ha⁻¹more N under their canopies. The percentage N derived from N₂fixation ranged from 63 to 73% in the various treatments. Despite the high percentage of N derived from N₂fixation, the N₂fixation of the stand was very low, i.e. 1·98 to 2·80 kg N ha⁻¹year⁻¹, due to the low growth of the stand. We believe that comparisons of the whole tree weighted δ¹⁵N to background soil δ¹⁵N provides a more reasonable approach to estimate % N₂fixation than comparisons of leaves of fixers and reference plants.