Interactions between hemiparasitic plants and their hosts: The importance of organic carbon transfer

Hemiparasitic plants display a unique strategy of resource acquisition combining parasitism of other species and own photosynthetic activity. Despite the active photoassimilation and green habit, they acquire substantial amount of carbon from their hosts. The organic carbon transfer has a crucial influence on the nature of the interaction between hemiparasites and their hosts which can oscillate between parasitism and competition for light. In this minireview, we summarize methodical approaches and results of various studies dealing with carbon budget of hemiparasites and the ecological implications of carbon heterotrophy in hemiparasites.Key words: haustorium, heterotrophy, parasitic plant, mistletoe, Rhinanthus, Striga, δ¹³CHemiparasitic plants withdraw resources from the vascular system of their hosts through a specialized transfer organ called haustorium.¹ Hemiparasites attack the host''s xylem, in contrast to the holoparasites that infect both phloem and xylem, and as a consequence, hemiparasitic plants have access to water and mineral nutrients but little carbon.¹ Due to their reduced or non-existing root networks, hemiparasitic plants acquire virtually all mineral nutrients and water from the host while organic carbon is provided, at least in part, by their own photosynthetic activity.^2,3 This is in contrast to holoparasitic plants which rely on the host for the supply of both organic and inorganic nutrients. The location of the attachment to the host and the degree of host dependency represent the most important characters defining the three basic functional types within hemiparasitic plants. Root hemiparasites attack host roots but their above-ground appearance is usually not substantially different from that of a non-parasitic plant. This group can be further divided in two—facultative and obligate hemiparasites consisting of plants that are able (at least sometimes) or unable to complete their life cycle without an attachment to the host respectively. Stem hemiparasites are attached to the host stem (usually trunk or branches) and are all obligate parasites, unable to survive without a host.Hemiparasitic plants have an ambiguous relationship with their hosts which, on the one hand, represent exclusive sources of inorganic nutrients but on the other hand, the co-occurrence of these host plants in the hemiparasite vicinity imposes competition for light. The nature and intensity of this competitive relationship varies across different groups and species of hemiparasites. The ability of hemiparasites to acquire organic carbon (largely in the form of xylem-mobile organic and amino acids) is certainly the key factor affecting this interaction since hemiparasites that are capable of efficient organic carbon abstraction should be minimally affected by shading from their host. The fact that hemiparasites can exhibit substantial carbon heterotrophy is now supported by a large number of studies, although a traditional point of view on hemiparasites that highlights the importance of inorganic resources (mainly nitrogen) acquisition is still prevailing. Therefore, we decided to summarize available information on hemiparasite heterotrophy, outline techniques for assessing the proportion of heterotrophy and estimating the overall carbon budget, and discuss possible implications of this phenomenon on hemiparasite ecology.     

Root xylem transport of amino acids in the root hemiparasitic shrub Olax phyllanthi (Labill) R.Br. (Olacaceaea) and its multiple hosts

The amino acid compositions of the root xylem saps of Olax phyllanthi and a range of its common hosts were examined in native coastal heath in Western Australia and in pot cultures of Olax reliant on single hosts. When hosts specializing in the xylem transport of one major solute (asparagine, glutamine, histidine, arginine or proline) were exploited, the endophytic tissue of haustoria and the xylem sap of Olax showed much lower proportions of this than of other solutes, suggesting pronounced metabolic transformation prior to xylem loading by the parasite. However, the xylem sap of Olax did partly reflect the compositions of its hosts; for example, djenkolic acid and pipecolic acid were present when Olax was parasitic on species of Acacia, and levels of citrulline and aspartic acid were higher than normal when it exploited hosts transporting large amounts of these compounds. Back-flow of S-ethenyl cysteine, a novel amino acid specific to Olax, was observed to another root hemiparasite (Exocarpos sparteus) in native habitat and to certain non-parasitic hosts in water-stressed pot cultures. Haustoria exhibited high levels of glutamine synthetase but showed appreciable in vivo nitrate reductase activity only when on hosts with high xylem levels of nitrate.     

Biofiltering efficiency in removal of dissolved nutrients by three species of estuarine macroalgae cultivated with sea bass (Dicentrarchus labrax) waste waters 1. Phosphate

The potential of three estuarine macroalgae (Ulvarotundata, Enteromorpa intestinalis andGracilaria gracilis) as biofilters for phosphate ineffluents of a sea bass (Dicentrarchus labrax) cultivationtank was studied. These seaweeds thrive in Cádiz Bay and were alsoselected because of their economic potential, so that environmental andeconomicadvantages may be achieved by future integrated aquaculture practices in thelocal fish farms. The study was designed to investigate the functioning of Pnutrition of the selected species. Maximum velocity of phosphate uptake (2.86mol PO₄ g^–1 dry wth^–1) was found in U. rotundata.This species also showed the highest affinity for this nutrient. At low flowrates (< 2 volumes d^–1), the three species efficientlyfiltered the phosphate dissolved in the waste water, with a minimum efficiencyof 60.7% in U. rotundata. Net phosphate uptake rate wassignificantly affected by the water flow, being greatest at the highest rateassayed (2 volumes d^–1). The marked decrease in tissue P shownby the three species during a flow-through experiment suggested that growth wasP limited. However, due to the increase in biomass, total P biomass increasedinthe cultures. A significant correlation was found between growth rates and thenet P biomass gained in the cultures. A three-stage design under low water flow(0.5 volumes d^–1) showed that the highest growth rates (up to0.14 d^–1) and integrated phosphate uptake rates(up to 5.8 mol PO₄ ^3– g^–1dry wt d^–1) were found in E.intestinalis in the first stage, with decreasing rates in thefollowing ones. As a result, phosphate become limiting and low increments oreven losses of total P biomass in these stages were found suggesting thatphosphate was excreted from the algae. The results show the potential abilityofthe three species to reduce substantially, at low water flow, the phosphateconcentration in waste waters from a D. labrax cultivationtank, and thus the quality of effluents from intensive aquaculture practices.     

Heterotrophic gain of carbon from hosts by the xylem-tapping root hemiparasite Olax phyllanthi (Olacaceae)

Heterotrophic gains of carbon from various host species by the root hemiparasitic shrub Olax phyllanthi (Labill) R.Br. were assessed using techniques based on carbon isotope discrimination (¹³C) on C3 and C4 hosts and C:N ratios of xylem sap and dry matter of host and parasite. Heterotrophic benefits (H) to Olax based on ¹³C values were 30% and 19% from two nonnative C4 hosts (Portulaca oleracea and Amaranthus caudatus respectively) compared with 13% and 15% from these hosts when computed on the basis of C:N ratios of host xylem sap and C and N increments of Olax dry matter. Nitrate was the source of N available to pot cultures of the above species and estimates based on C:N ratios assumed that all N accumulated by Olax had come from nitrate absorbed by the host. Equivalent estimates of H for Olax, grown in nitrogen-free pot culture with the native N₂-fixing host Acacia littorea as its sole source of N, indicated 63% and 51% dependence on host carbon when assessed in terms of xylem sap composition of host parasite respectively. Comparisons of xylem sap solutes of Olax and a range of partner hosts indicated marked selectivity in haustorial uptake and transfer of nitrate, amino compounds, organic acids and sugars. Possible implications of variations between hosts in absolute levels of C and inorganic and organic forms of N in xylem are discussed in relation to evidence of much better growth performance of Olax on Acacia littorea and other N₂-fixing legumes than on non-fixers.     

Germination responses of <Emphasis Type="Italic">Caragana korshinskii</Emphasis> Kom. to light,temperature and water stress

Caragana korshinskii Kom. is a very important shrub species for vegetation rehabilitation in northern China for its high ecological and economic values. Experiments were conducted to determine its germination responses to (i) different temperature regimes under light and/or dark conditions, (ii) different light intensities, and (iii) different water potentials combined with varied constant temperatures. Under alternating temperatures (from 5:15 to 25:35°C), final percent germinations of Caragana korshinskii were quite similar. In dark conditions, constant temperatures resulted in lower final percent germinations than alternating temperatures. At a controlled temperature regime of 10:20°C, neither final percent germinations nor germination rates showed significant differences among varied light intensities. As water potentials were reduced from 0 (distilled water) to –0.6MPa, final percent germinations increased slightly and reached the peak at approximately –0.6MPa, however, the increment was not significant. Beyond –0.6MPa, further water potential reduction led to decreased final percent germinations and few seeds could germinate at –1.4MPa. Water stress also strongly inhibited germination at very high or low temperatures. The experimental results suggested that middle May might be a suitable time for aerial seeding for this species.     

The water velocity preferences of Austrosimulium furiosum and Simulium ornatipes (Diptera: Simuliidae) larvae,and the implications for micro-habitat partitioning

The aquatic larvae of two simuliid species, Austrosimulium furiosum (Skuse) and Simulium ornatipes Skuse, which often occur together in Victorian streams, were shown in laboratory experiments to have preferences for different water velocities: larvae of A. furiosum preferred water velocities of 0.2–0.3 m s^–1, and S. ornatipes preferred water velocities of 0.9–1.3 m s ^–1 . Final instar larvae of both species selected slow water speeds of less than 0.25 m s ^–1 prior to pupation. Flow patterns around a cylinder in a laboratory stream were mapped, and the distribution of A. furiosum larvae within the wake, paired vortices and horseshoe vortex was recorded. Larvae selected areas with suitable water velocities and aligned with the flow, providing flow visualization of micro-current speed and direction. The potential for micro-habitat partitioning is discussed in relation to benthic sampling strategies.     

Effects of drought and N level on the interactions of the root hemiparasite Rhinanthus alectorolophus with a combination of three host species

• Increasing nitrogen deposition and more frequent drought events are likely to change plant interactions in natural grasslands. Both factors may also influence the interactions between hemiparasitic plants, regarded as keystone species in many grasslands, and their host species.
• We grew a combination of three suitable hosts, a grass, a forb and a legume, with and without the hemiparasite Rhinanthus alectorolophus at three levels of nitrogen (N) and two levels of water availability in a factorial design.
• Biomass of the hemiparasite and host community increased with N level and was reduced by drought to a similar degree. Larger plants in fertilised pots started to wilt earlier, and the presence of a hemiparasite further increased drought sensitivity. The hemiparasite strongly reduced biomass of the host community and overall productivity, and affected the competitive balance among host plants because it particularly reduced biomass of the dominant grass. These effects were the opposite of those of high N. The hemiparasite increased the root mass fraction of the hosts at all levels of N and water availability, indicating that the effect of the hemiparasite on the hosts was mainly due to loss of belowground resources.
• Our results indicate that hemiparasites will not always respond more strongly to increased N availability and drought than autotrophic plants, and that hemiparasites can have similarly strong effects on grassland communities as soil fertility and drought. By preferentially attacking dominant species the hemiparasites might alleviate the negative effects of nutrient enrichment on grassland diversity.

     

Effects of water deficit on N2(C2H2) fixation in cowpea and groundnut

The effect of water deficit on nodulation, N₂ fixation, photosynthesis, and total soluble sugars and leghemoglobin in nodules was investigated in cowpea and groundnut. Nitrogenase activity completely ceased in cowpea with a decrease in leaf water potential ( _leaf) from –0.4 MPa to –0.9 MPa, while in groundnut it continued down to –1.7 MPa. With increasing water stress, the acetylene reduction activity (ARA) declined very sharply in cowpea, but ARA gradually decreased in groundnut. Even with mild water stress ( _leaf of 0.2 MPa), nodule fresh weight declined 50% in cowpea partly due to a severe nodule shedding whereas nodule fresh weight declined in groundnut only when _leaf decreased by 1.0 MPa. No nodule shedding was noticed even at a higher stress level in groundnut. Photosynthesis and stomatal conductance were also more stable in groundnut than in cowpea under water stress. There was a sharp increase in total soluble sugars and leghemoglobin in the nodules of groundut with water stress, but no definite trend could be found in cowpea.     

Elevated CO2 and drought alter tissue water relations of birch (Betula populifolia Marsh.) seedlings

The effect of increasing atmospheric CO₂ concentrations on tissue water relations was examined in Betula populifolia, a common pioneer tree species of the northeastern U.S. deciduous forests. Components of tissue water relations were estimated from pressure volume curves of tree seedlings grown in either ambient (350 l l^–1) or elevated CO₂ (700 l l^–1), and both mesic and xeric water regimes. Both CO₂ and water treatment had significant effects on osmotic potential at full hydration, apoplasmic fractions, and tissue elastic moduli. Under xeric conditions and ambient CO₂ concentrations, plants showed a decrease in osmotic potentials of 0.15 MPa and an increase in tissue elastic moduli at full hydration of 1.5 MPa. The decrease in elasticity may enable plants to improve the soil-plant water potential gradient given a small change in water content, while lower osmotic potentials shift the zero turgor loss point to lower water potentials. Under elevated CO₂, plants in xeric conditions had osmotic potentials 0.2 MPa lower than mesic plants and decreased elastic moduli at full hydration. The increase in tissue elasticity at elevated CO₂ enabled the xeric plants to maintain positive turgor pressures at lower water potentials and tissue water contents. Surprisingly, the elevated CO₂ plants under mesic conditions had the most inelastic tissues. We propose that this inelasticity may enable plants to generate a favorable water potential gradient from the soil to the plant despite the low stomatal conductances observed under elevated CO₂ conditions.     

Gas exchange characteristics and nitrogen relations of two Mediterranean root hemiparasites:Bartsia trixago andParentucellia viscosa

Plant height, light-saturated rates of photosynthesis (A _max) and foliar nitrogen concentration (N ₁) were measured forBartsia trixago under field conditions in Mallorca. All three variables were postively correlated, and were also positively related to the abundance of nitrogen-fixing legumes in the associated vegetation (putative host species).A _max forB. trixago ranged from 7.7 to 18.8 mol m^-2 s^-1; similar rates were measured for a second hemiparasiteParentucellia viscosa, and both species were within the range of rates measured for six putative hosts (10.6–19.2 mol m^-2 s^-1). Fertilization of unattachedB. trixago plants with inorganic nitrogen (ammonium nitrate) elicited neither the growth nor the photosynthetic responses observed in plants considered to be parasitic on legumes and in receipt of an enriched organic nitrogen supply. Both hemiparasites had high diurnal leaf conductances (g _s) (469–2291 mmol m^-2 s^-1) and were at the upper end of the range of those measured in putative hosts (409–879 mmol m^-2 s^-1). In contrast with the latter, high nocturnal rates ofg _s were also recorded for the two hemiparasites (517–1862 mmol m^-2 s^-1). There was no clear relationship between eitherA _max orN ₁ and eitherg _s, transpiration (E) or water use efficiency (A _max/E) inB. trixago plants. The economics of water loss appear to be independent of both the supply of nitrogen from the host and autotrophic carbon fixation.     

Aspects of tissue water relations and seasonal changes of leaf water potential components of evergreen and deciduous species coexisting in tropical dry forests

Summary This study compared the tissue water relations and seasonal changes in leaf water potential components of an evergreen tree,Morisonia americana, and two evergreen shrubs,Capparis verrucosa andC. aristiquetae, with two deciduous trees,Humboltiella arborea andLonchocarpus dipteroneurus, and the deciduous vineMansoa verrucifera. All these species coexist in a tropical dry forest in Venezuela. Leaves of the evergreen species are sclerophyllous, while those of the deciduous species are mesophytic. Leaf area to leaf weight ratios of fully mature leaves were about 75 and 170 cm² g^–1 in evergreen and deciduous species, respectively. Seasonal fluctuations of leaf water content per unit of dry weight, water potential, and turgor pressure were smaller in evergreen than in deciduous species. The analysis of tissue water relations using pressurevolume curves showed that evergreen species could develop a higher leaf turgor and lose turgor at lower leaf water potentials than deciduous species. This was related to a lower osmotic potential at full turgor in evergreen (-3.0 MPa)_than in deciduous (-2.0 MPa) species, rather than to the elastic properties of leaf tissue. The volumetric modulus of elasticity was 14 MPa in evergreen compared with 7–10 MPa in deciduous species. Thus, leaf characteristics are important in determining the drought resistance of evergreen species of this tropical dry forest.     

Photosynthesis and growth rates of Laurencia brongniartii J. Agardh (Rhodophyta, Ceramiales) in preparation for cultivation

Laurencia brongniartii is usually found at depths below 4 m, but can be found in shallow subtidal areas in crevices and on the walls of a coral reef in Amami Oshima Island, Kagoshima Prefecture, Japan, where irradiances were significantly lower than those at similar depths in open water. In preparation for the possible cultivation of this species for its antibiotic compounds, the effects of temperature and irradiance on photosynthesis and growth were measured. Photosynthesis and growth rates of L. brongniartii explants were highest at 26 and 28 °C, which closely corresponded to temperatures found during August to late December when it was most abundant. The estimated maximum photosynthesis rate (P _max) was 4.41 mol photon m^–2 s^–1 at 26 °C and 4.07 mol photon m^–2 s^–1 at 28 °C. Saturating irradiance occurred at 95 mol photon m^–2 s^–1 at 26 °C and 65 mol photon m^–2 s^–1 at 28 °C. In contrast, growth experiments at 41.7 mol photon m^–2 s^–1 caused bleaching of explants and the maximum growth rate observed during the study was 3.02 ± 0.75% day^–1 at 28 °C and 25 mol photon m^–2 s^–1. The difference in the saturating irradiance for photosynthesis and the irradiance that caused bleaching in growth experiments suggests that long-term exposure to high irradiance was detrimental and should be addressed before the initiation of large scale cultivation.     

Biofiltering efficiency in removal of dissolved nutrients by three species of estuarine macroalgae cultivated with sea bass (Dicentrarchus labrax) waste waters 2. Ammonium

Three estuarine macroalgae (Ulva rotundata,Enteromorpha intestinalis, Gracilariagracilis) of economic potential were cultivated in the laboratory toassess their biofiltering capacities for ammonium in waste effluents from a seabass (Dicentrarchus labrax) cultivation tank. The studywasdeveloped to investigate the functioning of N nutrition of the three species.Atlow water flow (< 2 volumes d^–1) the three species strippedefficiently the ammonium dissolved in the waste water from the fish tank, withaminimum biofiltering efficiency estimate of 61% in unstarved cultures ofG. gracilis at a water flow of 2 volumesd^–1. Maximum velocity for ammonium uptake (89.0 molNH₄ ⁺ g^–1 dry wth^–1) was found in U. rotundata,whereas G. gracilis showed the highest affinity for thisnutrient. The net ammonium uptake rate was significantly affected by the waterflow, being greatest at the highest flow assayed (2 volumesd^–1). Variations of tissue N and C:N ratios during aflow-through experiment suggested that N was not limiting macroalgal growth.However, when ammonium was supplied at a flow rate of 0.5 volumesd^–1, specially in a three-stage design, the marked reductionintissue N and the biomass C:N:P ratios suggested a more general nutrientdeficiency. A significant correlation was found between growth rates and the Nbiomass gained in the cultures. The three-stage design under low water flow(0.5volumes d^–1) showed that the highest ammonium uptake rates (upto 80.9 mol NH₄ ⁺ g^–1 dry wtd^–1 in U. rotundata) were found inthe first stage, with decreasing rates in the following ones. As a result, lowincrements or even losses of total N biomass in these stages were found,suggesting that ammonium was excreted from the algae. We conclude that thesespecies present a potential ability to biofilter the ammonium dissolved inwastewater from a D. labrax cultivation tank, suggesting thatscaling up the biofiltration designs, future practises using these macroalgaemay be implemented in the local fish farms, resulting in both environmental andeconomical advantages.     

Responses to drought of five Brachiaria species. II. Water relations and leaf gas exchange

Neotropical savannas are exposed to recurrent dry periods of varied duration, and forage grasses must be able to cope with such temporal stresses to maintain productive pastures. This study compared leaf water relations and net photosynthesis under drought of five perennial Brachiaria species: the tufted B. brizantha (CIAT 6780), the semi-stoloniferous B. decumbens (CIAT 606) and B. mutica, and the stoloniferous B. humidicola (CIAT 679) and B. dictyoneura (CIAT 6133). Plants of the five grasses were grown in large pots and subjected to drought by suspending watering until first wilting symptoms (14 days for B. brizantha, B. decumbens and B. mutica, and 29 days for B. humidicola and B. dictyoneura). Afterwards, they were re-watered and a second soil dry cycle was imposed. Time trends in leaf water potential (_l), relative water content (RWC), osmotic potential at full turgor (₀ ¹⁰⁰), stomatal conductance (Gs) and net photosynthesis (A) of stressed (DT) plants were compared to those of well-irrigated (CT) plants. Predawn _l in DT plants decreased to a minimum of –1.5 and –2.0 MPa in B. brizantha and B. mutica, compared to –2.5 to –3.0 MPa in B. decumbens, B. humidicola and B. dictyoneura. RWC decreased up to 50% in B. brizantha, compared to 75% in the other species. In B. humidicola, B. dictyoneura and in a lesser extent, B. decumbens, leaves of DT plants adjusted osmotically, by an apparent accumulation of nutrient solutes, at a rather constant ratio of turgid to dry weight of the tissue. Calculated osmotic adjustment ranged between 0.38 (B. decumbens) to 0.87 MPa (B. humidicola). This adjustment in ₀ ¹⁰⁰ was in some cases maintained 7 days after re-watering. In B. brizantha and B. mutica, Gs and A were significantly affected by drought, with maximum reduction percentages at the second drought period of 65 and 80%, respectively. The corresponding reduction in B. decumbens was 53 and 55%, respectively; whereas in B. humidicola and B. dictyoneura Gs and A were reduced less than 20%. In all species, re-watering allowed for the water relations (except ₀ ¹⁰⁰) and photosynthetic activity of leaves of DT plants to reach values comparable to those of CT plants. Results are discussed in term of root morphology and soil water extraction pattern, as well as leaf traits that may contribute to withstand drought under moderate soil water stress.     

Seasonal Dynamics of the Diatoms of the Genus Chaetoceros Ehrenberg in Amursky Bay (Sea of Japan)

The qualitative and quantitative composition of the Chaetoceros Ehr. species was studied in Amursky Bay (Sea of Japan) from January 1996 until May 1998. In all, 30 species, 1 variety, and 1 form of this genus were registered. The species Chaetoceros occurred in plankton throughout the year at a water temperature of –1.8–25°C and a salinity of 11–35. The numbers of Chaetoceros species varied between 100 and 1071000 cells/l, and the biomass varied between 0.9 × 10^–3 and 3.3 g/m³. The numbers were maximum in summer and minimum in the beginning of spring. The Chaetoceros species comprised 45–70 and 5–18% (winter), 68–98 and 65–95% (spring), 50% (summer), and 20% (autumn) of the total phytoplankton numbers and biomass. Six dominant species and 1 variety of Chaetoceros were found. Seasonal complexes formed by the Chaetoceros species were identified and described.     

Dinitrogen fixation in a water-stressed Alnus clone is limited by host xerotolerance

The osmotolerance, rather than the halotolerance, of the endosymbiont predicted the xerotolerance of acetylene reduction by Alnus nodulated withFrankia ARgP5 ^AG . Cloned plants ofAlnus glutinosa (L.) Gaertn. AG8022-16 were subjected to water stress under controlled conditions in an environmental growth chamber. Transpiration, stomatal conductance, and leaf water potential had decreased after successive 10 day periods of moderate (75% of water demand) and severe (50% of water demand) water stress. After severe stress had wilted the plants, reducing leaf water potential to –2.10 MPa, nitrogenase activity had fallen to 2.51 M per plant per hour. The reported rapid turnover of nitrogenase implies thatFrankia mycelium was metabolically active at this low water potential, a water potential at which no Alnus-derivedFrankia has been reported active. Although ARgP5 ^AG was similar to other such strains in halotolerance (lower limitca.–1.25 MPa), the low water potential limit for growth with glucose (a non-assimilated osmoticum) wasca.–2.53 MPa. Nitrogenase activity was apparently more limited by host xerotolerance than by endophyte xerotolerance.Journal article J-5400 of the Oklahoma Agriculture Experiment Station, Oklahoma State University, Stillwater, OK 74078, USA.     

Stomatal Reactions of Two Different Maize Lines to Osmotically Induced Drought Stress

Two maize lines differing in drought resistance were grown at different drought stress induced by polyethylene glycol (PEG 10 000) solutions with osmotic potentials of –0.20, –0.40 and –0.80 MPa in the semipermeable membrane system. During the five days soil water content decreased (from 0.43 to 0.29, 0.25 and 0.23 g cm^–3 for three PEG solutions, respectively) as well as leaf water potentials (_w; from – 0.54 to –0.76, –1.06 and –1.46 MPa). These values were not significantly different between the investigated lines, indicating that a controlled and consistent soil moisture stress was achieved. Soil drying induced an increase in the ABA content of leaves and xylem of both lines and the effects on stomatal conductance were greater in drought susceptible line (B-432) compared to drought resistant line (ZPBL-1304). To test possible difference in stomatal sensitivity to xylem ABA between lines and to assess any ABA vs. _w interaction, roots were fed with 10, 50 and 100 mmol m^–3 ABA solutions in another set of experiments. These results showed that manipulation of xylem ABA affected stomata of both lines similarly. Comparison of stomatal sensitivity to drought-induced and applied ABA demonstrated that drought treatment affected stomata of investigated lines by differentially increasing their sensitivity to xylem ABA, thus confirming an interaction between chemical signalling and hydraulic signalling.     

Stomatal conductance in a tropical xerophilous shrubland at a lava substratum

Diurnal variation in leaf stomatal conductance (g _s) of three xerophilous species (Buddleia cordata, Senecio praecox and Dodonaea viscosa) was measured over a 10-month period during the dry and wet seasons in a shrubland that is developing in a lava substratum in Mexico. Averaged stomatal conductances were 147 and 60.2 (B. cordata), 145 and 24.8 (D. viscosa) and 142.8 and 14.1 mmol m^–2 s^–1 (S. praecox) during the wet and dry season respectively. Leaf water potential () varied in a range of –0.6 to –1.2 (S. praecox), –0.6 to –1.8 (B. cordata) and –0.9 to –3.4 MPa (D. viscosa) during the same measurement periods. Stomata were more sensitive to changes in irradiance, air temperature and leaf–air vapour pressure difference in the rainy season than the dry season. Although stomatal responses to were difficult to distinguish in any season (dry or rainy), data for the entire period of measurement showed a positive correlation, stomata tending to open as increased, but there is strong evidence of isohydric behaviour in S. praecox and B. cordata. A multiplicative model relating g _s to environmental variables and to accounted for 79%–83% of the variation of g _s in three sites (pooled data); however, the performance of the model was poorer (60%–76%) for individual species from other sites not included in the pooled data.     

Selective feeding by larval dytiscids (Coleoptera:Dytiscidae) and effects of fish predation on upper littoral zone macroinvertebrate communities of arctic lakes

Carbon and nitrogen stable isotopic data from the primary producers in mangrove ecosystems are needed to investigate trophic links and biogeochemical cycling. Compared with other mangrove species (e.g. Rhizophora mangle) very few measurements have been conducted on the white mangrove, Laguncularia racemosa. The carbon and nitrogen stable isotopic and elemental compositions of L. racemosa were analyzed and compared from Florida and Belize. ¹³C values of L. racemosa from Florida (mean = –26.4) were slightly higher than those from Twin Cays, Belize (mean = -27.4), which may be due to higher salinity in some parts of the Florida site. There was no difference between the ¹⁵N values from L. racemosa from these two sites (Florida mean = 0.6; Belize mean = 0.3), which are indicative of nitrogen derived from nitrogen fixation in a planktonic marine system. However, higher ¹⁵N values from L. racemosa at Man of War Cay in Belize (11.4 and 12.3), which is fertilized by roosting marine birds (14.0), illustrate that L. racemosa can sensitively reflect alternative nitrogen sources. Although the isotopic data could not distinguish between Avicennia germinans, R. mangle and L. racemosa in Belize the L. racemosa had considerably higher C/N ratios (46.5 – 116.1) compared with the Florida samples (42.2 – 76.0) or the other mangrove species. Unlike some previous findings from R. mangle, substrate characteristics (e.g. salinity, NH₄ ⁺, and H₂S) were not related to the isotopic or elemental composition of L. racemosa. ¹³C, ¹⁵N and C/N were analyzed for ecosystem components from L. racemosa habitats at Twin Cays, including other plants (e.g. R. mangle, A. germinans and seagrass), detritus, microbial mats and sediments. Results from mass-balance calculations show that mangrove detritus composes very little of the sediment, which is principally composed of microbial biomass (80 – 90%). Detritus at some sites is also influenced by sources other than that from L. racemosa, including seagrass leaves.     

Filtering impacts of larval and sessile zebra mussels (Dreissena polymorpha) in western Lake Erie

Summary We assessed the feeding biology of veliger larvae of the introduced zebra mussel (Dreissena polymorpha Pallas) in laboratory experiments using inert microspheres as food analogues. Mean clearance rate on 2.87-m beads ranged between 247 and 420 L veliger^–1 day^–1. Clearance rate was unrelated to bead concentration up to 100 beads L^–1, but was positively correlated with veliger shell length. Clearance rates of Dreissena veligers are within the range of those reported for marine bivalve veligers of similar size and for herbivorous Great Lakes microzooplankton, but are orders of magnitude lower than those of settled, conspecific adults. The impact of settled zebra mussel grazing activities on phytoplankton stocks may be up to 1162 times greater than that exerted by veliger populations in western Lake Erie. Based on 1990 size-frequency distributions and associated literature-derived clearance rates, reef-associated Dreissena populations in western Lake Erie (mean depth 7 m) possess a tremendous potential to filter the water column (up to 132 m³ m^–2 day^–1) and redirect energy from pelagic to benthic foodwebs. Preliminary analyses indicate that chlorophyll a concentration is strongly depleted (<1 g L^–1) above Dreissena beds in western Lake Erie.