Factors affecting production of COS and CS2 in Leucaena and Mimosa species

Carbon disulfide (CS₂) and carbonyl sulfide (COS) are colorless, foul-smelling, volatile sulfur compounds with biocidal properties. Some plants produce CS₂ or COS or both. When used as an intercrop or forecrop, these plants may have agronomic potential in protecting other plants. Most of the factors which affect production of these plant-generated organic sulfides are unknown. We determined the effects of sulfate concentration, plant age, nitrogen fixation, drought stress, root injury (through cutting), and undisturbed growth on COS production in Leucaena retusa or Leucaena leucocephala and the effect of some of these factors on CS₂ production in Mimosa pudica. In addition, we determined if organic sulfides were produced in all Leucaena species. When L. retusa and M. pudica seedlings were grown in a plant nutrient medium with different sulfate concentrations (50 to 450 mg SL^-1), COS or CS₂ from crushed roots generally increased with increasing sulfate concentration. COS production was highest (74 ng mg^-1 dry root) for young L. retusa seedlings and declined to low amounts (<5 ng mg^-1 dry root) for older seedlings. Nitrogen fixation reduced the amounts of COS or CS₂ produced in L. leucocephala and M. pudica. Under conditions of undisturbed growth, root cutting, or drought stress, no COS production was detected in 4-to 8-weeks-old L. retusa plants. COS or CS₂ or both was obtained from crushed roots or shoots of all 13 known Leucaena species.     

Modelling crop growth and biomass partitioning to shoots and roots in relation to nitrogen and water availability,using a maximization principle

Many crop models relate the allocation of dry matter between shoots and roots exclusively to the crop development stage. Such models may not take into account the effects of changes in environment on allocation, unless the allocation parameters are altered. In this paper a crop model with a dynamic allocation parameter for dry matter between shoots and roots is described. The basis of the model is that a plant allocates dry matter such that its growth is maximized. Consequently, the demand and supply of carbon, nitrogen, and water is maintained in balance. This model supports the hypothesis that a functional equilibrium exists between shoots and roots.This paper explains the mathematical computation procedure of the crop model. Moreover, an analysis was made of the ability of a crop model to simulate plant dry matter production and allocation of dry matter between plant organs. The model was tested using data from a greenhouse experiment in which spring wheat (Triticum aestivum L.) was grown under different soil moisture and nitrogen (N) levels.Generally, the model simulations agreed well with data recorded for total plant dry matter. For validation data the coefficient of determination (r²) between simulated and measured shoot dry weight was 0.96. For the validation treatments r² was slightly lower, 0.94. In addition to dry matter production the model succeeded satisfactorily in simulating the dry weight of different plant organs. The response of simulated root to shoot ratio to the level of soil moisture was mainly in accordance with the measured data. In contrast, the simulated ratio seemed to be insensitive to the changes in the levels soil N concentration used in the experiment.The data used in the present study were not extensive, and more data are needed to validate the model. However, the results showed that the model responses to the changes in soil N and water level were realistic and mostly agreed with the data. Thus, we suggest that the model and the method employed to allocate dry matter between roots and shoots are useful when modelling the growth of crops under N and water limited conditions.     

Influence of drought on competition between selected Rhizobium meliloti strains and naturalized soil rhizobia in alfalfa

Drought is an important environmental factor that can affect rhizobial competition and N₂ fixation. Three alfalfa (Medicago sativa L. and M. falcata L.) accessions were grown in pots containing soil from an irrigated (Soil 1) and a dryland (Soil 2) alfalfa field in northern Utah, USA. Mutants of three strains of Rhizobium meliloti Dang. from Pakistan (UL 136, UL 210, and UL 222) and a commercial rhizobial strain 102F51a were developed with various levels of resistance to streptomycin. Seeds inoculated with these individual streptomycin-resistant mutants were sown in the two soils containing naturalized rhizobial populations. Soils in the pots were maintained at −0.03, −0.5, and −1.0 MPa. After 10 weeks, plants were harvested and nodule isolates were cultured on agar medium with and without streptomycin to determine nodule occupancy (proportion of the nodules occupied by introduced rhizobial strains). Number of nodules, nodule occupancy, total plant dry weight, and shoot N were higher for Soil 1 than Soil 2. Number of nodules, plant dry weight, and shoot N decreased as drought increased from −0.03 to −1.0 MPa in the three alfalfa accessions. Rhizobial strains UL 136 and UL 222 were competitive with naturalized alfalfa rhizobia and were effective at symbiotic N₂ fixation under drought. These results suggest that nodulation, growth, and N₂ fixation in alfalfa can be improved by inoculation with competitive and drought-tolerant rhizobia and may be one economically feasible way to increase alfalfa production in water-limited environments. Joint contribution from USDA-ARS and the Utah Agric. Exp. Sta., Utah State Univ., Logan, UT 84322-4810, USA. Journal Paper No. 4931. Joint contribution from USDA-ARS and the Utah Agric. Exp. Sta., Utah State Univ., Logan, UT 84322-4810, USA. Journal Paper No. 4931.     

Exchange of NO and NO2 between wheat canopy monoliths and the atmosphere

The fluxes of NO and NO₂ between wheat canopy monoliths and the atmosphere were investigated with the dynamic chamber technique. For this purpose monoliths were dug out at different plant growth stages from a field site, transported to the institute, and placed in an environmental growth chamber. The wheat canopy monoliths were exposed over a period of four days to the average ratios of atmospheric NO₂ and NO measured at the field site, i.e. NO₂ concentration of about 18 mL L^-1 plus NO concentration lower than 0.5 nL L^-1. Under these conditions NO emission into the atmosphere and NO₂ deposition into canopy monoliths was observed. Both fluxes showed diurnal variation with maximum rates during the light and minimum rates during darkness. NO₂ fluxes correlated with soil temperature as well as with light intensity. NO fluxes correlated with soil temperature but not with light intensity. From the investigation performed the diurnal variation of the NO and NO₂ compensation points, the maximum rates of NO and NO₂ emission, and the total resistances of NO and NO₂ fluxes were calculated. Under the assumption that the measured data are representative for the whole vegetation period, annual fluxes of NO and NO₂ were estimated. Annual NO emission into the atmosphere amounted to 87 mg N m^-2 y^-1 (0.87 kg ha^-1 y^-1), annual NO₂ deposition into canopy monoliths amounted to 1273 mg N m^-2 y^-1 (12.73 kg ha^-1 y^-1). Apparently, the uptake of atmospheric nitrogen by the wheat field from NO₂ deposition is about 15 times higher than the loss of nitrogen from NO emission. It can therefore be assumed that even in rural areas wheat fields are a considerable sink for atmospheric nitrogen. The annual sink strength estimated in the present study is ca. 12 kg N ha^-1 y^-1. The possible origin of the NO emitted and the fate of atmospheric NO₂ taken up by the wheat canopy monoliths are discussed.Preliminary results of this paper were presented at the Joint Workshop COST 611/Working Party 3 and EUROTRAC in Delft, The Netherlands (Ludwig et al., 1991).     

Growth and foraging consequences of facultative paedomorphosis in the tiger salamander,Ambystoma tigrinum nebulosum

Summary Facultative paedomorphosis in salamanders occurs when larvae respond to varying environmental conditions by either metamorphosing into terrestrial metamorphic adults or retaining their larval morphology to become sexually mature paedomorphic adults. Several hypotheses have been proposed for the evolutionary maintenance of this environmentally induced dimorphism, but few data are available to assess them adequately. We studied a montane population of the tiger salamander,Ambystoma tigrinum nebulosum, and measured the adult growth rate and body condition across three growing seasons to assess the relative costs and benefits of each morph. Metamorphic adults grew more than paedomorphic adults in terms of snout—vent length across years and in weight within years. Dietary analyses and foraging experiments revealed some of the proximate factors that may underlie these differential growth patterns. Across all prey, metamorphs had significantly higher biomass and calories per stomach sample than paedomorphs. Metamorphic diets primarily consisted of the fairy shrimpBranchinecta coloradensis, whereas paedomorphic diets contained a variety of benthic and terrestrial invertebrates. Foraging experiments revealed that both morphs are more successful at capturing fairy shrimp relative to other prey types and both show high electivity toward this prey. However, fairy shrimp occurred only in non-permanent ponds and thus are inaccessible to paedomorphs, which can survive only in permanent ponds. Paedomorphs also experience higher levels of intraspecific competition with large larvae in permanent ponds than metamorphs do in non-permanent ponds. Thus, metamorphs obtain a growth advantage over paedomorphs by foraging in non-permanent ponds that contain fairy shrimp and have reduced intraspecific competition. These results suggest that paedomorphs should have decreased fitness relative to metamorphs, primarily because metamorphs can move into the best habitats for growth. The net fitness effect of morph-specific differences in dispersal depend on whether there are trade-offs with other life history traits. Nonetheless, because the relative benefit of metamorph dispersal will change with environmental conditions in permanent ponds and the surrounding habitat, the relative fitness payoff to each morph should track changes in the environment. Thus, facultative paedomorphosis may be maintained in part by variable, environmentally-specific fitness payoffs to each morph.     

The evolution of actinorhizal symbioses: Evidence for multiple origins of the symbiotic association

According to morphologically based classification systems, actinorhizal plants, engaged in nitrogen-fixing symbioses with Frankia bacteria, are considered to be only distantly related. However, recent phylogenetic analyses of seed plants based on chloroplast rbcL gene sequences have suggested closer relationships among actinorhizal plants. A more thorough sampling of chloroplast rbcL gene sequences from actinorhizal plants and their nonsymbiotic close relatives was conducted in an effort to better understand the phylogenetic relationships of these plants, and ultimately, to assess the homology of the different actinorhizal symbioses. Sequence data from 70 taxa were analyzed using parsimony analysis. Strict consensus trees based on 24 equally parsimonious trees revealed evolutionary divergence between groups of actinorhizal species suggesting that not all symbioses are homologous. The arrangement of actinorhizal species, interspersed with nonactinorhizal taxa, is suggestive of multiple origins of the actinorhizal symbiosis. Morphological and anatomical characteristics of nodules from different actinorhizal hosts were mapped onto the rbclL-based consensus tree to further assess homology among rbcL-based actinorhizal groups. The morphological and anatomical features provide additional support for the rbcL-based groupings, and thus, together, suggest that actinorhizal symbioses have originated more than once in evolutionary history.     

Prediction of a low dose herbicide effect from studies on binding of metribuzin to the chloroplasts of Chenopodium album L.

Chenopodium album L. seedlings at the 4- and 8-leaf stage were exposed to low concentrations metribuzin [4-amino-6-(l, l-dimethyl)-3-(methylthio)-l,2,4-triazin (4 H )-one] in nutrient solution to study herbicide uptake and the effects of low-dose rates. Chlorophyll fluorescence was measured to relate the inhibition of photosynthesis to herbicide dose. The minimum rate at which metribuzin fully inhibited photosynthesis was less than 1 μM for seedlings at the 4-leaf stage of development, and between 1 and 5 μM for the 8-leaf stage seedlings. With isolated chloroplasts, experiments were conducted to establish the relationship between the amount of herbicide molecules bound to each chloroplast and the inhibition of photosynthesis. From the dose-response curves obtained it was calculated that photosynthesis was fully inhibited when 7.5 10⁵ molecules metribuzin were bound to each chloroplast. This amount of binding was used to estimate minimum-lethal dose rates of metribuzin required for seedlings differing in fresh weight of leaves and amounts of chloroplasts present. It is suggested that prediction of a low dose herbicide effect from studies on binding of photosystem-II inhibitors in combination with chlorophyll fluorescence measurements may lead to the development of a new weed management strategy.     

Effect of NaCI on nitrate reductase,glutamate dehydrogenase and glutamate synthase inVigna radiata calli

The effect of NaCI stress on the activities of nitrate reductase (NR), glutamate dehydrogenase (GDH) and glutamate synthase (GOGAT) in callus lines ofVigna radiata which differ in salt resistance, was studied at weekly intervals upto 28 d of growth. After 28 d, the NaCI resistant callus (selected at 300 mM NaCI) at NaCI concentrations higher than 200 mM maintained higher NR activity than non-selected line. NaCI stress also affects aminating and deaminating activities of GDH. The NADH-GDH activity in the presence of NaCI was higher in the resistant than non-selected line. On the other hand, NAD-GDH activity in both the lines was completely inhibited after 7 d of growth. The increased activity of NADH-GDH in resistant calli may play a vital role in protecting the cells from toxic effect of increased endogenous level of ammonia which probably accumulates due to efficient NO₃ ⁻ reduction. NADH-GOGAT activity was found to decrease under salt stress in both the callus lines. Nitrogen assimilation in salt-resistant calli under salt stress was found to be characterized by high NR and NADH-GDH activities, concomitantly with low GOGAT activity. The authors are grateful to DST and CSIR for financial assistance.     

Gas exchange in leaves of the root hemiparasitemelampyrum arvense L. before and after attachment to the host plant

Gas exchange characteristics of a hemiparasiteMelampyrum arvense L. before and after attachment to the hostCapsella bursa pastoris (L.) Med. were compared. The net photosynthetic rates (P_N) on a leaf area basis were extremely low and in comparison to the value obtained for the host were about 15 % and 23 % for the unattached and attached hemiparasite, respectively. Also the concentration of photosynthetic pigments was low (as compared with the host the content of chlorophylls was about 33 % and 49 % and of carotenoids about 38 % and 36 % in the unattached and attached hemiparasite, respectively). On the other hand the rates of respiration were high (about 1.8 and 2.6 times higher in the unattached and attached hemiparasite, respectively, than in the host). In darkness stomatal conductance (g_S) of the host and the unattached hemiparasite was rapidly reduced to 10 % of the value obtained in light, g_S of the attached hemiparasite was decreased only by about 30%. A total reduction of g_S occurred at relative water content (RWC) of 85 %, 75% and 45 % for the unattached hemiparasite, the host, and the attached hemiparasite, respectively. The transpiration (E) rate in the preparasitic stage was very low, being 2.6 and 4.5 times smaller than in the host and the attached hemiparasite, respectively. In the attached hemiparasite WUE was 7.5 and 3 times poorer than in the host and in the preparasitic stage, respectively.     

Growth and fecundity of Daphnia after diapause and their impact on the development of a population

Laboratory and field investigations revealed that the life history traits of exephippial and parthenogenetic generations of Daphnia differ substantially. Daphniids hatching from resting eggs grow faster and their definitive body sizes are bigger than of hatchlings from subitaneous eggs. Size at maturity for exephippial animals is significantly larger. In spite of this, they mature a few days earlier than parthenogenetic females. In this study, the difference was 3–4 days for the laboratory experiments and 1–3 days for the field. Fecundity of the exephippial generation is markedly higher. Here, the clutch size for this generation was up to 3.5–4.0 times as large as for the parthenogenetic generation. Moreover, obtained results suggest that the relationship between clutch size and body length for both generations differ significantly.Estimates of the intrinsic rate of increase for field Daphnia populations demonstrated that life history traits of exephippial animals lead to a two or threefold higher rate of increase in the conditions of invertebrate predation pressure. Under moderate fish pressure, obtained r values for the daphniids hatching from resting eggs were larger than those from subitaneous. High growth rate of exephippial females is disadvantageous only under the conditions of severe pressure by fish. Obtained results suggest that hatchlings from diapausing eggs an acceleration of population increase by several times during the beginning of the development of a population with periodical re-establishment from resting eggs.