Abiotic edaphic factors affecting the growth of a threatened North American Sunflower, Helianthus paradoxus (Asteraceae)

This study evaluated the relationships among soil moisture, soil salinity, and soil oxygen on the growth of Helianthus paradoxus (Asteraceae), a threatened inland salt marsh species of western North America. The study was conducted in large growth boxes (1×2×0.3 m) tilted at an angle to achieve a saturated to dry water gradient similar to that found in the marsh. This experimental design allowed the evaluation of major abiotic factors (soil moisture and soil salinity) which have been shown to be potentially important for this species, while removing major biotic factors, such as competition from other community dominants. Maximum aboveground biomass occurred in the middle rows of the boxes, where surface soil water was reduced and subsurface soil water was intermediate in the gradient. Regression analyses indicated that H. paradoxus would grow best where surface soil water is approximately 5%, subsurface soil water ranges from 20 to 30%, and where surface soil salinity is less than 0.5 g kg⁻¹. Edaphic variables, particularly soil moisture and soil salinity, affect the growth of H. paradoxus. Data presented here suggest that the survival of this species depends on maintenance of the hydrologic regime.     

The glands of Tamarix aphylla: a system for salt recretion or for carbon concentration?

During periods of high atmospheric humidity, twigs of Tamarix aphylla (L.) Karst. become covered by an alkaline solution. The pH of that solution fluctuates between 8.0 – 8.5 in the dark and 10.5 during the light hours. Such a solution, produced by the glands, constitutes an efficient trap for atmospheric CO₂. Upon the periodic drop in pH, much of the preabsorbed carbon may gradually be released from the solution. This enriches the immediate surroundings of the twigs with CO₂ for prolonged periods of time. The expected concentrations of CO₂, at the boundary layer between the atmosphere and the surfaces of the twigs, are over 1 000 ppm. As net photosynthesis of T. aphylla reaches maximal rates only at CO₂ concentrations of above 500 ppm, the plants may benefit from this extra source of carbon and may exploit it for maximal assimilation during the early morning hours. Thus, the "salt glands'of Tamarix , which are liable for the production of the alkaline recretum, may serve a triple purpose: (a) removal of excess salts out of the twigs, (b) provision of a cover of hygroscopic solutes that moistens the twigs and shortens the duration of transpiration, and (c) providing the plants with an environment enriched in CO₂.     

The effect of behavioural and morphological plasticity on foraging efficiency in the threespine stickleback (Gasterosteus sp.)

We conducted an experiment to assess the change in foraging efficiency resulting from diet-induced morphological and behavioural plasticity in a species of freshwater, threespine stickleback (Gasterosteus sp.). Different degrees of morphological and behavioural change were induced using two prey items commonly found in the diet of this species, allowing us to estimate the relative importance of each type of plasticity. The purpose of the experiment was twofold. First, earlier work had suggested that diet variability might be an important factor in the evolution of trophic morphological plasticity in sticklebacks. The present results extend this work by revealing the adaptive significance of morphological plasticity. The current experiment also qualitatively assessed the compatibility of the time scale of morphological change with that of the natural resource variability experienced by this species. The results indicate that diet-induced plasticity improves foraging efficiency continuously for up to 72 days of prey exposure. This is probably due in part to plasticity of the external trophic morphology but our results also suggest a complex interplay between morphology and behaviour. The time scale appears to be matched to that of natural diet variability although it is possible that some traits exhibit non-labile plasticity. Our discussion highlights the important distinction between conditions favouring the evolution of labile versus non-labile plasticity. The second objective of the experiment was to determine the relative importance of morphological and behavioural plasticity. Few studies have attempted to quantify the adaptive significance of morphological plasticity and no study to our knowledge has separated the effects of morphological and behavioural plasticity. Our experiment reveals that both behavioural and morphological plasticity are important and it also suggests a dichotomy between the two: behavioural plasticity predominately affects searching efficiency whereas morphological plasticity predominately affects handling efficiency.     

Translocation - A key factor limiting the efficiency of nitrogen fixation in legume nodules

A hypothesis is presented that the availability of water for export of nitrogenous products from legume nodules is a major factor limiting the efficiency of symbiotic nitrogen fixation. Water for export of solutes in the xylem probably depends largely on the import of water and reduced carbon in the phloeum, and one function of respiration may be to dispose of reduced carbon in order to increase the supply of water. A second hypothesis presented is that control of gas diffusion in soybean nodules is largely restricted to the cortex nearby the vascular bundles, thus making possible the linkage of solute balances in xylem and phloem with resistance to diffusion. These concepts are used in a re-examination of literature on manipulations of nodules and nodulated plants such as lowering of light levels, water stress, defoliation, stem girdling, and alteration of oxygen supply. The concept of translocation as a major factor limiting efficiency of symbiotic fixation is consistent with the failure of superior rhizobial isolates to improve N input significantly, and this limitation could also prevent exploitation of superior bacterial symbionts in the future     

A new precipitation technique provides evidence for the permeability of Casparian bands to ions in young roots of corn (Zea mays L.) and rice (Oryza sativa L.)

Using an insoluble inorganic salt precipitation technique, the permeability of cell walls and especially of endodermal Casparian bands (CBs) for ions was tested in young roots of corn (Zea mays) and rice (Oryza sativa). The test was based on suction of either 100 µm CuSO₄ or 200 µm K₄[Fe(CN)₆] into the root from its medium using a pump (excised roots) or transpirational stream (intact seedlings), and subsequent perfusion of xylem of those root segments with the opposite salt component, which resulted in precipitation of insoluble brown crystals of copper ferrocyanide. Under suction, Cu²⁺ could cross the endodermis apoplastically in both plant species (although at low rates) developing brown salt precipitates in cell walls of early metaxylem and in the region between CBs and functioning metaxylem vessels. Hence, at least Cu²⁺ did cross the endodermis dragged along with the water. The results suggested that CBs were not perfect barriers to apoplastic ion fluxes. In contrast, ferrocyanide ions failed to cross the mature endodermis of both corn and rice at detectable amounts. The concentration limit of apoplastic copper was 0.8 µm at a perfusion with 200 µm K₄[Fe(CN)₆]. Asymmetric development of precipitates suggested that the cation, Cu²⁺, moved faster than the anion, [Fe(CN)₆]^4–, through cell walls including CBs. Using Chara cell wall preparations (‘ghosts’) as a model system, it was observed that, different from Cu²⁺, ferrocyanide ions remained inside wall-tubes suggesting a substantially lower permeability of the latter which agreed with the finding of an asymmetric development of precipitates. In both corn and rice roots, there was a significant apoplastic flux of ions in regions where laterals penetrated the endodermis. Overall, the results show that the permeability of CBs to ions is not zero. CBs do not represent a perfect barrier for ions, as is usually thought. The permeability of CBs may vary depending on growth conditions which are known to affect the intensity of formation of bands.     

Photosynthesis, transpiration and salt fluxes through leaves of Leptochloa fusca L. Kunth

Abstract Salt excretion by glands on the leaves of Leptochloa fusca was studied. The rate of excretion was strongly dependent on temperature up to 39°C, which is near the optimum for photosynthesis in this thermophilic C₄ grass. The concentration of salt in the xylem required to sustain the observed rate of excretion was low (about two orders of magnitude less than the external concentration). Salt excretion is concluded to be a secondary mechanism of salt tolerance, with exclusion at the roots being the major mechanism. The rate of salt excretion was strongly dependent on temperature.     

Rice cultivar evaluation for phosphorus use efficiency

Phosphorus deficiency is one of the most growth-limiting factors in acid soils in various parts of the world. The objective of this study was to screen 25 rice cultivars (Oryza sativa L.) at low, medium, and high levels of soil P. Number of tillers, root length, plant height, root dry weight and shoot dry weight were related to tissue P concentrations, P uptake and P-use efficiency. Shoot weight was found to be the plant parameter most sensitive to P deficiency. Significant cultivar differences in P use efficiency were found. Phosphorus use efficiency was higher in roots than shoots and decreased with increasing levels of soil P. Positive correlations were found among growth parameters such as plant height, tillers, root and shoot weight, and P content of roots and shoots. These results indicate selection of rice cultivars for satisfactory performance under low P availability can be carried out using shoot and root dry weight as criteria.     

Water transport parameters and regulatory processes inEremosphaera viridis

Summary The water relations parameters and the osmoregulatory response ofEremosphaera viridis were investigated both by using the pressure probe technique and by analyzing the intracellular pool of osmotically active agents. In the presence of various concentrations of different salts a biphasic osmoregulatory response was recorded, consisting of a rapid decrease in turgor pressure due to water loss followed by an increase in turgor pressure to the original turgor pressure value (depending on the salt). The values of turgor pressure, volumetric elastic modulus and hydraulic conductivity depended on the composition of the media. Nonelectrolytes did not cause a turgor recovery after the initial water efflux. The second phase of turgor regulation in the presence of salts was characterised by the intracellular accumulation of ions and sugars and required at least 24 hr. Analysis of the cell sap showed that the increase in the internal osmotic pressure was mainly achieved by accumulation of sucrose. Additionally, accumulation of glucose was observed in illuminated cells in the presence of Rb and K. Electron micrographs suggested that the sucrose was produced by degradation of starch granules. Turgor pressure recovery after salt stress seemed to be dependent on temperature and is well correlated with the according photosynthetic activity. The data suggest that a temperature-dependent enzyme which is activated by potassium or rubidium is involved in the regulatory response.     

Bias in leaf area — sapwood area ratios and its impact on growth analysis in Pinus contorta

Summary Two alternative estimators of individual tree leaf area (A₁) area are used to derive estimates of leaf-area index (L) for 40 plots in Pinus contorta Dougl. stands. One estimator of A₁ is based on the common assumption of a constant ratio between A₁ and sapwood cross-sectional area at breast height (A_s). The second estimator of A₁ accounts for tree-to-tree variation in the relation between A₁ and A_s. The apparent relationship between stand growth and leaf-area index is strongly dependent on the way leaf area is estimated. When L is derived from a constant A₁A_s ratio, stand growth appears to be strongly correlated with L. However, when L is based on estmates of A₁ that account for tree-to-tree variation in the A₁ — A_s relation, stand growth is seen to be only weakly related to L. Stand structure, quantified as percent live-crown, accounts for a great deal of the observed variation in leaf-area efficiency. These contrasting relationships illustrate the importance of unbiased estimates of L in interpreting the link between stand-level processes and leaf area.Utah Aggriculural Experiment Station Journal Paper No. 3333     

Zinc tolerance in populations ofDeschampsia cespitosa (Gramineae) beneath electricity pylons

Patterns of zinc tolerance were examined in eightDeschampsia cespitosa (L.)Beauv. populations from normal and zinc-contaminated soils, using solution culture methods. Zinc-tolerant populations have evolved beneath pylons, and their tolerance patterns (degree, variance, heritability) vary. Tolerance is genetically based in all of them. In contrast to the cases of other species previously reported, some within-population differences are considerable, and may be due to both heterogeneity of the soils zinc contents, various gene combinations and gene recombinations between genotypes. One control population contains many tolerant plants, an original result which is discussed.