Xylem-tapping mistletoes: water or nutrient parasites?

Most mistletoes parasitize higher plants by tapping the xylem (a conduction tissue) of their hosts. Field observations of diurnal gas exchange parameters and carbon isotope ratios in xylem-tapping mistletoes from three continents support the hypotheses that water use efficiency and carbon isotope composition are related and that mistletoes which are parasitic for water are also nutrient parasites, differing in their water use efficiency relative to that of their hosts on the basis of host nitrogen supply in the transpiration stream.

Traceless multipole moment densities and transformations in macroscopic electromagnetism.

We consider whether use of traceless multipole moment densities in macroscopic electromagnetism can yield physically acceptable results. For harmonic plane wave fields it is shown that a traceless electric quadrupole density yields linear constitutive relations for which the dynamical material constants (permittivity and magnetoelectric coefficients) and response fields are unphysical. We further show that, within multipole theory, these constitutive relations cannot be transformed into physically acceptable relations. Specifically, the transformed response field D is unphysical for all orders beyond the electric dipole. This contrasts with use of primitive (traced) moment densities, for which unphysical constitutive relations have been successfully transformed up to electric octopole-magnetic quadrupole order, thereby providing also the leading contribution to the ac permeability.

Differences in photosynthetic performance between cyanobacterial and green algal components of lichen photosymbiodemes measured in the field.

Photosymbiodemes are lichens which contain a single mycobiont but have cyanobacteria and green algae as primary photobionts in different parts of the thallus. Members of a photosymbiodeme can be found as separate, free-living lichen species. The photosynthesis of the components of two Pseudocyphellaria photosymbiodemes were studied in the natural forest environment in New Zealand. It was found that the green algal component had a large photosynthetic advantage when thallus water contents were low and the thalli were in equilibrium with atmospheric humidity. The cyanobacterial components were at an advantage when thallus water contents were very high. The environment in which photosymbiodemes are found, forest margins in high humidity areas, seems to provide the correct combination of thallus water contents so that neither component has a major advantage. The photosymbiodemes appear to be models, preserved by the special habitat and showing a possible early stage in the evolution before the definite separation of ecologically specialised green algal and cyanobacterial species.

Moisture content and CO2 exchange of lichens. II. Depression of net photosynthesis in Ramalina maciformis at high water content is caused by increased thallus carbon dioxide diffusion resistance.

Thalli of Ramalina maciformis were moistened to their maximal water holding capacity, thus, simulating actual conditions following a heavy rainfall. Time courses of net photosynthesis at 17° C and 750 µE m-2 s-1 light intensity (PAR) were obtained during drying of the thalli. At ambient CO2 concentrations from 200 to 1,000 ppm, CO2 uptake of the moist lichens was depressed at high water content. After a certain water loss, net photosynthesis increased to a maximal value and decreased again with further drying of the thalli. The degree of initial depression of photosynthesis decreased with increasing ambient CO2 concentration, and it was fully absent at 1,600 ppm ambient CO2. Under these conditions of CO2 saturation, net photosynthesis remained constant at maximum for many hours and decreased only when substantial amounts of water had been lost. We conclude that the carboxylation capacity of the lichen is not affected by high contents of liquid water. Therefore, the depression of CO2 uptake of the water saturated lichen at lower (e.g. natural) ambient CO2 must be due exclusively to increased resistance to CO2 diffusion from the external air to the sites of carboxylation.

Stomata of the CAM plant Tillandsia recurvata respond directly to humidity.

Under controlled conditions, CO2 exchange of Tillandsia recurvata showed all characteristics of CAM. During the phase of nocturnal CO2 fixation stomata of the plant responded sensitively to changes in ambient air humidity. Dry air resulted in an increase, moist air in a decrease of diffusion resistance. The evaporative demand of the air affected the level of stomatal resistance during the entire night period. Due to stomatal closure, the total nocturnal water loss of T. recurvata was less at low than at high humidity. It is concluded that stomata respond directly to humidity and not via bulk tissue water conditions of the leaves. Such control of transpiration may optimize water use efficiency for this almost rootless, extreme epiphyte.

Midday stomatal closure in Mediterranean type sclerophylls under simulated habitat conditions in an environmental chamber : II. Effect of the complex of leaf temperature and air humidity on gas exchange of Arbutus unedo and Quercus ilex.

Shrubs of the Mediterranean sclerophyllous species Arbutus unedo and Quercus ilex were studied under simulated habitat conditions in an environmental chamber. Temperature, humidity, and light intensity were altered stepwise to simulate diurnal changes in conditions similar to those measured in an evergreen macchia in Sobreda, Portugal. Leaves were enclosed in cuvettes which reproduced the growth chamber climate and which allowed measurement of gas exchange. Increasing atmospheric stress in the form of higher temperature and lower humidity on successive days gradually results in midday depression of transpiration rate and net photosynthesis rate of leaves due to midday stomatal closure.

A digital registration system for net photosynthesis and transpiration measurements in the field and an associated analysis of errors.

A digital registration system used with temperature- and humidity-controlled cuvettes for net photosynthesis and transpiration measurements in the field is described. The associated errors of the measured parameters and calculated data are estimated. The digitalization is based on an analogue registration which is of primary importance in the control of experimental conditions in the cuvettes. The digital system is connected to the analogue registration in series. The error associated with digitalization is 0.1% across 70% of the scale. This error increases to 0.2% between 3 and 30% on the scale due to a minor lack of linearity. The reproducibility of the digitalization is ±0.024%.The error associated with data transfer in the digitalization and the errors of the analogue registration are estimated for temperature and humidity measurements (error of air and leaf temperature is ±0.1° C; error of the dew point temperature is ±1.1° C dew point). The effect of these errors on the calculation of relative humidity and the water vapour difference between the leaf and the air is determined using the progressive error law. At 30° C and 50% relative humidity, the error in relative humidity is ±7.4%, the error for the water vapour difference is ±6.6%. The dependence of these errors on temperature and humidity is shown.The instrument error of the net photosynthesis measurement is calculated to be ±4.2%. Transpiration measurements have an average inaccuracy of ±8.3%. The total diffusion resistance which is calculated from values of transpiration and the water vapour difference has an average error of ±10.9%. The sizeable influence of errors in humidity and temperature measurements on the calculated diffusion resistance is demonstrated. The additional influence of biological errors associated with field measurements is discussed.

[Eco-physiological investigations on wild and cultivated plants in the Negev Desert : II. The influence of climatic factors on carbon dioxide exchange and transpiration at the end of the dry period]. / Ökophysiologische Untersuchungen an wild- und Kulturpflanzen der Negev-Wüste : II. Die wirkung der außenfaktoren auf CO2-Gaswechsel und transpiration am Ende der Trockenzeit.

The influence of climatic factors on net photosynthesis, dark respiration and transpiration was investigated in the Negev Desert at the end of the dry summer period when plant water stress was at a maximum. Species studied included: dominant species of the natural vegetation (Artemisia herba-alba, Hammada scoparia, Noaea mucronata, Reaumuria negevensis, Salsola inermis, Zygophyllum dumosum), cultivated plants receiving rainfall and run-off water during the winter season in the run-off farm Avdat (Prunus armeniaca, Vitis vinifera), and irrigated cultivated plants receiving additional water during the summer season (Citrullus colocynthis, Datura metel). 1. Light saturation of net photosynthesis was reached at 60-90 klx conforming to the high solar radiation intensities of the desert. 2. Maximum rates of CO2 uptake per unit of dry weight for the irrigated mesomorphic plants was ten times that of the wild plants. However, in comparison to the other species, maximal rates of CO2 uptake for wild plants were higher when calculated on a leaf area basis than when represented on a dry weight basis. Maximum rates of net photosynthesis per unit chlorophyll content for some of the wild plants (Salsola and Noaea) were comparable to those of the cultivated Vitis and irrigated Citrullus and Datura, Hammada exhibited even higher rates than Prunus. This demonstrates the great photosynthetic capacity of the wild plants even at the end of the dry season. 3. The upper temperature compensation point for net photosynthesis of the wild plants was unusually high as an adaptation to the temperatures of the habitat. Compensation points higher than 49°C exceed the maxima known so far for other flowering species. Maximum rates of net photosynthesis of Hammada were measured when the temperature of the photosynthetic organs was 37°C; at 49°C photosynthesis was only reduced by 50%. 4. Leaf temperature affects plant gas exchange by influencing stomatal aperture. Diffusion resistance of leaves to water vapour was reduced at low temperatures and increased at high temperatures. Reduction of net photosynthesis and transpiration of desert plants at midday may, therefore, be the result of temperature-induced stomatal closure. The possible influence of peristomatal transpiration on stomatal aperture is also discussed. Peristomatal transpiration is directly related to the vapour pressure gradient between the leaf mesophyll and the ambient air which increases with increasing temperatures. 5. Diffusion resistance to water vapour was reduced at high temperatures approaching the limits of heat resistance, due to increased stomatal aperture. This resulted in greater transpirational cooling. 6. Under conditions of increased leaf water stress, diffusion resistance increased, either by sudden stomatal closure at specific threshold values of water stress or through a continuous increase in resistance. This increased resistance is coupled with decreases in transpiration and photosynthesis. 7. In several plant species increased diffusion resistance during the course of the day caused decreased transpiration without a corresponding decrease in photosynthesis. Under these conditions, the ratio of CO2 uptake to transpiration became more favourable as the day progressed. The possibility that this favourable gas exchange response is the result of an increased mesophyll resistance to water vapour loss is discussed.

[Eco-physiological investigations on wild and cultivated plants in the Negev Desert : III. Daily courses of net photosynthesis and transpiration at the end of the dry period]. / Ökophysiologische Untersuchungen an Wild- und Kulturpflanzen der Negev-Wüste : III. Tagesläufe von Nettophotosynthese und Transpiration am Ende der Trockenzeit.

The daily course of net photosynthesis and transpiration was measured with temperature and humidity controlled cuvettes at the end of the dry summer season in the Negev Desert. Species studied included: dominant species of the natural vegetation, cultivated plants in the run-off farm Avdat and permanently irrigated plants. An analysis of the influence of single climatic factors on gas exchange was given in part II of this publication. The reactions of the plants to complex changes in all the environmental parameters is the subject of this present study. 1. One-peaked daily courses of net photosynthesis occur in the irrigated species Citrullus colocynthis and Datura metel. After a high rate of net photosynthesis and transpiration before noon CO2 uptake is gradually reduced through stomatal closure even under good soil water conditions. Stomatal closure on C. colocynthis is controlled by the leaf temperatures whereas D. metel closes its stomata due to increasing water stress. 2. Without additional irrigation one-peaked daily courses are only possible with special constitutional adaptations to the extreme climate together with a balanced regulation of water loss. The annual Salsola inermis shows over the whole day no reduction in transpiration. Related to chlorophyll content, CO2 uptake almost attains the rates observed in the irrigated C. colocynthis. It is still unknown what type of water sources are at the disposal of this plant. The perennial chamaephytes Hammada scoparia and Reaumuria negevensis attain a stabilization of net photosynthesis at a lower level through reduction of stomatal water loss and through increased mesophyll resistance to water vapour. This reduces transpiration to a greater degree than CO2 uptake. The stomatal reactions of H. scoparia seem to be mainly controlled by the evaporation conditions in the atmosphere. Related to chlorophyll content in the assimilatory organs, net photosynthesis of both species is higher at noon than in all other chamaephytes. 3. Two-peaked daily courses of net photosynthesis are shown by plants of the run-off farm (Prunus armeniaca and Vitis vinifera). Both have a very high metabolic activity during the morning which is comparable even with that of D. metel and of H. scoparia. At noon, CO2 and H2O exchange is reduced through stomatal closure and falls below the level of all the other plants in the natural vegetation. In apricot, net photosynthesis of vertically oriented leaves does not drop to the compensation point at noon as was found with horizontally oriented leaves which had leaf temperatures some 6-8°C higher. 4. In the natural vegetation, only Noaea mucronata shows gas exchange reactions similar to those of apricot and grapvines. Also N. mucronata has high rates of net photosynthesis in the morning followed by a great reduction of gas exchange through stomatal closure at noon. At a higher water stress this type of an asymmetric two-peaked daily course is changed into a more flat symmetric two peaked curve with low metabolic activity during the morning. In Zygophyllum dumosum and in Artemisia herba-alba the reduction in CO2 uptake at noon is not caused by stomatal closure, but through temperatures above the optimum. The diffusion resistance for water vapour increases steadily during the day. 5. The daily balance of the CO2 exchange is calculated and is compared with the metabolic activity under optimal conditions. The potential photosynthetic capacity of wild plants under optimal conditions is more nearly met under the prevailing desert conditions than in the case with cultivated plants. The effects of the different types of daily courses of gas exchange on the distribution of plants of the Saharo-Arabian region (Reaumuria negevensis, Zygophyllum dumosum) and of plants of the Irano-Turanian vegetation (Hammada scoparia, Artemisia herbaalba) are discussed.

CO2-Gaswechsel-Untersuchungen zur SO2-Resistenz von Flechten.

The SO2 resistance of 12 lichen species with different growth forms and taken from different sites was investigated. The thalli were either exposed to different concentrations of SO2 gas (concentration at entry into the cuvette: 0.5; 1.0; 2.0 and 4.0 mg SO2/m3 air) or treated with Na2S2O5 solutions of differing concentration and pH. As a viability criterion the CO2 exchange of the thalli was measured with an infrared gas analyzer before and immediately after SO2 exposure and subsequently at intervals of several weeks. In some cases the chlorophyll content was also determined. 1. Species-specific differences were clearly apparent in the SO2 gas-exposure experiments. The influence of SO2 on net photosynthesis and dark respiration in the most sensitive species was detectable after exposure to 0.5 mg SO2/m3 for 14 h in a fully hydrated state. The photosynthetic intensity of Lobaria pulmonaria was actually irreversibly damaged. In contrast, the most resistant species survived a treatment with 4 mg SO2/m3 for the same length of time with little or no permanent impairment of their CO2 exchange. The reaction of the lichen species investigated to the above treatment allows us to arrange them in decreasing order of resistance: Xanthoria parietina (most resistant), Parmelia scortea, Parmelia acetabulum, Hypogymnia physodes, Parmelia saxatilis, Platismatia glauca, Labaria pumonaria, Parmelia stenophylla, Evernia prunastri. The most sensitive species, Evernia prunastri, is characteristically a fruticose lichen. Lichens with this growth form are known from field studies to be especially sensitive. 2. Examples of the same species (Parmelia saxatilis, Lobaria pulmonaria) can vary in their SO2 resistance according to their growing site. Morphological and anatomical characteristics (thallus and cortex thickness) may cause these differences. 3. The sensitivity of the lichens to SO2 is closely dependent upon their moisture status. When the water potential is lowered the SO2 uptake is reduced and with it the injury. Dried thalli survive high SO2 concentrations in their surroundings without damage. 4. The treatment with Na2S2O5 solutions also brought out species-specific differences in lichen resistance. However, the sequence of decreasing resistance is not the same as that to SO2 gas treatment. Irreversible damage of photosynthesis is not necessarily correlated with destruction of chlorophyll. 5. The damage caused to the lichens by the Na2S2O5 solutions (of the same concentration) is closely dependent upon the pH of the medium. At a low pH the effect is much more pronounced than at a high pH. This can be interpreted as due to the concentration of damaging ions, which changes according to the degree of dissociation of the solution; this is pH dependent. The results are discussed on the basis of Levitt's resistance concept that the total resistance of lichens to SO2 in the air is dependent upon two components, "avoidance" and "tolerance" (see Fig. 15). Resistance to a specific SO2 concentration in the air depends upon how much SO2 is taken up by the thallus, which is conditioned among other things by thallus organization (life form, surface characteristics) and by the degree of hydration of the poikilohydric organism. The toxicity of the SO2 taken up by the lichen can also be reduced; the pH of the thallus and its buffering capacity (dependent among other things upon site and substrate) play a dominant role in this process. In addition to these "avoidance" factors the total resistance of lichens is also dependent upon the plasmatic resistance of sensitive systems to SO2 ("tolerance"). This type of resistance, due to the influence of Na2S2O5 solution, is subject to considerable deviation (for example due to the developmental state of the lichen). The differences in the sequence of resistance for the investigated lichen species in terms of total resistance (SO2 treatment) and plasmatic resistance (Na2S2O5 solution treatment) show the significance of the "avoidance" component for the total resistance of the organisms.In ecological terms the investigation supports the view that lichens are highly sensitive to SO2, even in concentrations which occur due to real immisions. The study also shows the complexity of an ecological interpretation of experimentally determined resistance phenomena.

Ecophysiological investigations on wild and cultivated plants in the Negev Desert : I. Methods: A mobile laboratory for measuring carbon dioxide and water vapour exchange.

Carbon dioxide exchange and transpiration measurements of various wild and cultivated plants were carried out during the dry summer period in 1967 in the Central Negev Desert (Israel). A mobile laboratory used for these investigations is described. Measurements were carried out with conditioned plant chambers which followed either the ambient temperature and humidity or else allowed the experiments to be carried out under constant conditions. The accuracy of the measurements was estimated. The mean error of the determination of the CO2 exchange rate amounts to ±0.07 mg CO2·g-1·h-1. Transpiration rate is measured with an error of ±0.15 g H2O·g-1·h-1. The response time of the instrumentation to reach 90% equilibrium after a change in photosynthesis or transpiration is 7 to 9 minutes. Errors which are caused by changes of quality of incident radiant energy and altered turbulence conditions for the leaves enclosed in the chamber, are discussed.

Determination of leaf heat resistance: comparative investigation of chlorophyll fluorescence changes and tissue necrosis methods.

Heat tolerance limits for a variety of vascular plant leaves were determined both with the conventional post-culture necrosis method and by measurements of the heat-induced increase in chlorophyll fluorescence (F-T curves). The reliability of the fluorescence test was improved with the addition of far-red background light which counteracts dark reduction of the Photosystem II acceptor pool by heat-stimulated endogenous electron donors. This was of particular importance in the case of xeromorphic leaves in which the diffusion barrier for oxygen is high. A satisfactory correlation was found between T L50, the temperature at which a 30 min exposure results in 50% necrotic leaf area following post culture, and the critical temperature, T c ,the temperature at which the dark fluorescence level begins to increase during slow heating of a leaf sample at a rate of 0.7 K min-1, in the fluorescence test. The correlation can be described by a linear function, T L50=1.12 T c -5.37,with a correlation coefficient, r=0.87. Maximal deviation of the regression line from the line T L50=T c was 1.2 K, with 22 determinations for leaves with widely varying heat tolerance limits. This shows that heat-induced fluorescence changes within the thylakoid membrane may be connected with the irreversible leaf tissue damage which occurs following prolonged exposure to high temperature. On the basis of the heat dosage equation of Lepeschkin, a more general expression can be obtained which allows calculation of the accumulated heat dosage under the experimental conditions of the standard fluorescence test (slow heating, 0.7 K min-1). Such calculations reveal that for a given species the 'fraction of critical dosage' begins to increase, i.e. accumulating heat reaches an injurious level, at a temperature which approximately coincides both with T L50, obtained with the necrosis method, and with T c ,the critical temperature derived from the fluorescence test. Hence, the increase in fraction of critical dosage and the rise in chlorophyll fluorescence seem to concur. It is concluded that the fluorescence assay provides a rapid and reliable means of determining the heat tolerance limit of leaf tissue.

The control by atmospheric factors and water stress of midday stomatal closure in Arbutus unedo growing in a natural macchia.

Midday closure of stomata of well-watered (ψ between-10 and-25 bar) or moderately stressed (ψ between-25 and-35 bar) Arbutus unedo plants occurs when midday leaf temperatures increase above 30°C and vapor pressure difference between leaf air spaces and the external air increases above approximately 30 mbar/bar. Moderate water stress decreases maximum conductance and may result in greater sensitivity to high leaf temperature and vapor pressure dificit, which results in earlier closure and later reopening of stomata. Severe water stress (ψ of-50 bar) changes the form of the daily pattern observed for leaf conductance. A single morning peak in conductance occurs followed by decrease in conductance over the remainder of the day. Morning fog in Portugal during the dry season may facilitate stomatal opening and may allow improvement of carbon balances of the leaves for short periods, but contributes little to improvement of plant water balances over the longer term.

Water vapor uptake and photosynthesis of lichens: performance differences in species with green and blue-green algae as phycobionts.

Dry lichen thalli were enclosed in gas exchange chambers and treated with an air stream of high relative humidity (96.5 to near 100%) until water potential equilibrium was reached with the surrounding air (i.e., no further increase of weight through water vapor uptake). They were then sprayed with liquid water. The treatment took place in the dark and was interrupted by short periods of light. CO2 exchange during light and dark respiration was monitored continuously. With no exception water uptake in all of the lichen species with green algae as phycobionts lead to reactivation of the photosynthetic metabolism. Further-more, high rates of CO2 assimilation were attained without the application of liquid water. To date 73 species with different types of Chlorophyceae phycobionts have been tested in this and other studies. In contrast, hydration through high air humidity alone failed to stimulate positive net photosynthesis in any of the lichens with blue-green algae (Cyanobacteria). These required liquid water for CO2 assimilation. So far 33 species have been investigated, and all have behaved similarly. These have included gelatinous as well as heteromerous species, most with Nostoc phycobionts but in addition some with three other Cyanophyceae phycobionts. The same phycobiont performance differences existed even within the same genus (e.g. Lobaria, Peltigera) between species pairs containing green or blue-green phycobionts respectively. Free living algae also seem to behave in a similar manner. Carbon isotope ratios of the lichen thalli suggest that a definite ecological difference exists in water status-dependent photosynthesis of species with green and blue-green phycobionts. The underlying biochemical or biophysical mechanisms are not yet understood. Apparently, a fundamental difference in the structure of the two groups of algae is involved.

Carotenoid composition and metabolism in green and blue-green algal lichens in the field.

The carotenoid composition of 33 species of green algal lichens and 5 species of blue-green algal lichens was examined and compared with that of the leaves of higher plants. As in higher plants, green algal lichen species which were found in both shade and full sunlight exhibited higher levels of the carotenoids involved in photoprotective thermal energy dissipation (zeaxanthin as well as the total xanthophyll cycle pool) in the sun than in the shade. This was particularly true when thalli were moist during exposure to high light, or presumably became desiccated in full sunlight. However, the reverse trend in the carotenoid composition of green algal lichens was also observed in those species which were found predominantly either in the shade or in full sunlight. In this case sun-exposed lichens often possessed lower levels of zeaxanthin and of the components of the xanthophyll cycle than lichens which were found in the shade. In contrast to higher plants, the lichens from all habitats exhibited a relatively high ratio of carotenoids to chlorophylls (more characteristic of sun leaves), very low levels of α-carotene (similar to that found in sun leaves), and a level of ß-carotene similar to that found in shade leaves. Zeaxanthin, but not the expoxides of the xanthophyll cycle, was also frequently found in blue-green algal lichens. A trend for increasing levels of zeaxanthin with increasing growth light regime was observed inPeltigera rufescens, the species which was found to occur over the widest range of light environments. The level of zeaxanthin per chlorophylla in these blue-green algal lichens was in a range similar to that per chlorophylla+b in green algal lichens. However, zeaxanthin was also absent in one species,Collema cristatum, in full sunlight. Thus, the zeaxanthin content of the blue-green algal lichens can be similar to that of higher plants, or it can be rather dissimilar, as was also the case in the green algal lichen species. The presence of large amounts of ketocarotenoids in blue-green algal lichens is also noteworthy.

Pseudocyphellaria dissimilis: a desiccation-sensitive, highly shade-adapted lichen from New Zealand.

Pseudocyphellaria dissimilis, a foliose, cyanobacterial lichen, is shown not to fit into the normal ecological concept of lichens. This species is both extremely shade-tolerant and also more intolerant to drying than aquatic lichens previously thought to be the most desiccation-sensitive of lichens. Samples of P. dissimilis from a humid rain-forest site in New Zealand were transported in a moist state to Germany. Photosynthesis response curves were generated. The effect of desiccation was measured by comparing CO2 exchange before and after a standard 20-h drying routine. Lichen thalli could be equilibrated at 15° C to relative humidities (RH) from 5% to almost 100%. Photosynthesis was saturated at a photosynthetically active radiation (PAR) level of 20 µmol m-2 s-1 (350 µbar CO2) and PAR compensation was a very low 1 µmol m-2 s-1. Photosynthesis did not saturate until 1500 µbar CO2. Net photosynthesis was relatively unaffected by temperature between 10° C and 30° C with upper compensation at over 40° C. Temporary depression of photosynthesis occurred after a drying period of 20 h with equilibration at 45-65% relative humidity (RH). Sustained damage occurred at 15-25% RH and many samples died after equilibration at 5-16% RH. Microclimate studies of the lichen habitat below the evergreen, broadleaf forest canopy revealed consistently low PAR (normally below 10-20 µmol m-2 s-1) and high humidities (over 80% RH even during the day time). The species shows many features of an extremely deep shade-adapted plant including low PAR saturation and compensation, low photosynthetic and respiratory rates and low dry weight per unit area.

Carbon and nitrogen isotope ratios of mistletoes growing on nitrogen and non-nitrogen fixing hosts and on CAM plants in the Namib desert confirm partial heterotrophy.

Xylem-tapping mistletoe species growing on Mimosaccae, non-Mimosaceae and hosts performing Crassulacean acid metabolism (CAM) were studied along an aridity gradient in the Namib desert. °13C-values of mistletoes became more negative with decreasing nitrogen (N)-concentration in their leaves, while the host plants showed no such relationship. This might suggest that mistletoes regulate their water use efficiency according to the nitrogen supply from the host. However, further inspection of the data indicates that the relations of δ13C-values with leaf nitrogen in mistletoes may result from carbon input from the host. This is especially true for mistletoes growing on CAM plants which exhibit a very high δ13C-value, but show no evidence of CAM. It is calculated that about 60% of the carbon in mistletoes growing on C3 and on CAM hosts originated from the host. The hypothesis of Marshall and Ehleringer (1990) that xylem tapping mistletoes are also carbon parasites could explain the change in δ13C-values with N-supply and the difference in δ13C-values between mistletoes growing on C3 and CAM hosts.

Estimates of nitrogen fixation by trees on an aridity gradient in Namibia.

Nitrogen (N2) fixation was estimated along an aridity gradient in Namibia from the natural abundance of 15N (δ15N value) in 11 woody species of the Mimosacease which were compared with the δ15N values in 11 woody non-Mimosaceae. Averaging all species and habitats the calculated contribution of N2 fixation (N f ) to leaf nitrogen (N) concentration of Mimosaceae averaged about 30%, with large variation between and within species. While in Acacia albida N f was only 2%, it was 49% in Acacia hereroensis and Dichrostachys cinerea, and reached 71% in Acacia melifera. In the majority of species N f was 10-30%. There was a marked variation in background δ15N values along the aridity gradient, with the highest δ15N values in the lowland savanna. The difference between δ15N values of Mimosaceae and non-Mimosaceae, which is assumed to result mainly from N2 fixation, was also largest in the lowland savanna. Variations in δ15N of Mimosaceae did not affect N concentrations, but higher δ15N-values of Mimosaeae are associated with lower carbon isotope ratios (δ13C value). N2 fixation was associated with reduced intrinsic water use efficiency. The opposite trends were found in non-Mimosaceae, in which N-concentration increased with δ15N, but δ13C was unaffected. The large variation among species and sites is discussed.

Development of a photosynthesis model with an emphasis on ecological applications : V. Test of the applicability of a steady-state model to description of net photosynthesis of Prunus armeniaca under field conditions.

A physiologically based steady-state model of whole leaf photosynthesis (WHOLEPHOT) is used to describe net photosynthesis daily time courses in Prunus armeniaca. Net photosynthesis rates are calculated in response to incident light intensity, leaf temperature, air carbon dioxide concentration, and leaf diffusion resistance measured at five minute intervals. The steady-state calculations closely approximate the observed net photosynthesis rates for a broad range of weather conditions and leaf stomatal behavior.

Ecophysiological investigations on lichens of the Negev desert : V. A model to simulate net photosynthesis and respiration ofRamalina maciformis.

Previous publications have reported on investigations of CO2 exchange in the desert lichenRamalina maciformis both in its natural habitat in the Negev and in the laboratory. Utilizing laboratory data, net photosynthesis and dark respiration were expressed as mathematical functions of the most important environmental factors. Based on these relationships, a model is developed that allows one to predict CO2 exchange of the plant. Input data are light intensity, temperature, and water content of the thallus, together with a measure of the rate of the seasonal change of photosynthetic and respiratory activity. The validity of the model is tested by comparing simulated daily courses of CO2 uptake and release of the lichen with independent results of CO2 exchange measurements conducted in the field during and after the condensation of dew. The sensitivity of the model is shown by simulating changes in the input data of temperature and water content of the lichen.