Gas exchange and water balance of a mistletoe species and its mangrove hosts

Summary The gas exchange and water relations of the hemiparasite Pthirusa maritima and two its mangrove host species, Conocarpus erectus and Coccoloba uvifera, were studied in an intertidal zone of the Venezuelan coast. Carbon uptake and transpiration, leaf osmotic and total water potential, as well as nutrient content in the xylem sap and leaves of mistletoes and hosts were followed through the dry and wet season. In addition, carbon isotope ratios of leaf tissue were measured to further evaluate water use efficiency. Under similar light and humidity conditions, mistletoes had higher transpiration rates, lower leaf water potentials, and lower water use efficiencies than their hosts. Potassium content was much higher in mistletoes than in host leaves, but mineral nutrient content in the xylem sap of mistletoes was relatively low. The resistance of the liquid pathway from the soil to the leaf surface of mistletoes was larger than the total liquid flow resistance of host plants. Differences in the daily cycles of osmotic potential of the xylem sap also indicate the existence of a high resistance pathway along the vascular connection between the parasite pathway along the vascular connection between the parasite and its host. P. maritima mistletoes adjust to the different physiological characteristics of the host species which it parasitizes, thus ensuring an adequate water and carbon balance.     

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.     

Water flows in the parasitic association Rhinanthus minor/Hordeum vulgare

Using the facultative root hemiparasite Rhinanthus minor and its host Hordeum vulgare several aspects of water relations have been measured in this parasitic association. Extraction of xylem sap by the parasite from the host's roots is facilitated by con siderably higher transpiration per leaf area in the parasite than in the host and by the fact that stomata of attached Rhinanthus were open all day and night despite extremely high ABA concentrations in the leaves. By comparison, another root hemiparasite, Melampyrum arvense, parasitizing various grasses in the field, showed normal diurnal stomatal behaviour. The abnormal behaviour of Rhinanthus stomata was not due to anatomical reasons as closure could be induced by applying high external ABA concentrations. Remarkable differences have been detected between the hydraulic conductance of barley seminal roots showing relatively low values and that of Rhinanthus seminal roots showing very high values. The latter could be related to the observed high ABA concentrations in these roots. Whole plant water uptake, transpirational losses, growth-dependent deposition, and the flows of water within the plants have been measured in singly growing Rhinanthus and Hordeum plants and in the parasitic association between the two. Water uptake, deposition and transpiration in Rhinanthus were dramatically increased after attachment to the barley host; most of the water used by the parasite was extracted as xylem sap from the host, thereby scavenging 20% of the total water taken up by the host's roots. This water uptake by the parasitized host, however, due to a parasite-induced reduction in the host's growth, was decreased by 22% as compared to non-parasitized barley. The overall changes in growth-related water deposition in the host and parasite pointed to decreased shoot growth and relatively favoured root growth in the host and to strongly favoured shoot growth in the parasite. These changes in the host became more severe, when more than one Rhinanthus was parasitizing one barley plant.     

A hemiparasite in the forest understorey: photosynthetic performance and carbon balance of Melampyrum pratense

• Melampyrum pratense is an annual root‐hemiparasitic plant growing mostly in forest understorey, an environment with unstable light conditions. While photosynthetic responses of autotrophic plants to variable light conditions are in general well understood, light responses of root hemiparasites have not been investigated.
• We carried out gas exchange measurements (light response and photosynthetic induction curves) to assess the photosynthetic performance of M. pratense in spring and summer. These data and recorded light dynamics data were subsequently used to model carbon balance of the hemiparasite throughout the entire growth season.
• Summer leaves had significantly lower rates of saturated photosynthesis and dark respiration than spring leaves, a pattern expected to reflect the difference between sun‐ and shade‐adapted leaves. However, even the summer leaves of the hemiparasite exhibited a higher rate of light‐saturated photosynthesis than reported in non‐parasitic understorey herbs. This is likely related to its annual life history, rare among other understorey herbs. The carbon balance model considering photosynthetic induction still indicated insufficient autotrophic carbon gain for seed production in the summer months due to limited light availability and substantial carbon loss through dark respiration.
• The results point to potentially high importance of heterotrophic carbon acquisition in M. pratense, which could be of at least comparable importance as in other mixotrophic plants growing in forests – mistletoes and partial mycoheterotrophs. It is remarkable that despite apparent evolutionary pressure towards improved carbon acquisition from the host, M. pratense retains efficient photosynthesis and high transpiration rate, the ecophysiological traits typical of related root hemiparasites in the Orobanchaceae.

     

Host-mediated volatile polymorphism in a parasitic plant influences its attractiveness to pollinators

Host-plants can mediate the interactions between herbivores and their mutualists and also between parasitic plants and their mutualists. The present study reveals how a hemiparasitic plant parasitizing three host species gives rise to three distinct hemiparasite-host neighborhoods which differ in terms of volatile composition and pollinator attractiveness. The study was performed in a population of the mistletoe Tristerix verticillatus infecting three different species of hosts occurring in sympatry within a small area, thus exposing all individuals studied to similar abiotic conditions and pollinator diversity; we assessed the effect of hosts on the hemiparasites’ visual and olfactory cues for pollinator attraction. During the study period, the hemiparasite individuals were flowering but the hosts were past their flowering stage. We collected volatile organic compounds from the hemiparasite and its hosts, measured floral display characteristics and monitored bird and insect visitors to inflorescences of T. verticillatus. We showed that: (1) floral patches did not differ in terms of floral display potentially involved in the attraction of pollinators, (2) hosts and hemiparasites on each host were discriminated as distinct chemical populations in terms of their volatile chemical profiles, (3) insect visitation rates differed between hemiparasites parasitizing different hosts, and (4) volatile compounds from the host and the hemiparasite influenced the visitation of hemiparasite flowers by insects. The study showed that a species regarded as “ornithophilic” by its floral morphology was actually mostly visited by insects that interacted with its sexual organs during their visits and carried its pollen, and that host-specific plant-volatile profiles within the T. verticillatus population were associated with differential attractiveness to pollinating insects.     

The role of hemiparasitic plants: influencing tallgrass prairie quality,diversity, and structure

Wood betony, Orobanchaceae (Pedicularis canadensis) and bastard toadflax, Santalaceae (Comandra umbellata) are two root‐hemiparasitic plant species found in tallgrass prairie communities. Natural resource managers are interested in utilizing these species as “pseudograzers” in grasslands to reduce competitively dominant grasses and thereby increase ecological diversity and quality in prairie restorations and urban plantings. We performed an observational field study at 5 tallgrass prairie sites to investigate the association of hemiparasite abundance with metrics of phylogenetic and ecological diversity, as well as floristic quality. Although no reduction in C₄ grasses was detected, there was a significant association between hemiparasite abundance and increased floristic quality at all 5 sites. Hemiparasite abundance and species richness were positively correlated at one restoration site. In a greenhouse mesocosm experiment, we investigated response to parasitism by P. canadensis in 6 species representing different plant functional groups of the tallgrass prairie. The annual legume partridge pea, Fabaceae (Chamaecrista fasciculata) had the greatest significant dry biomass reduction among 6 host species, but the C₄ grass big bluestem, Poaceae (Andropogon gerardii) had significantly greater aboveground biomass when grown with the hemiparasite. Overall, host species biomass as a total community was significantly reduced in mesocosms, consistent with other investigations that demonstrate influence on community structure by hemiparasitic plant species. Although hemiparasites were not acting as pseudograzers, they have the potential to influence community structure in grassland restorations and remnants.     

An ecological study ofPedicularis dendrothauma, an arboreal hemiparasitic epiphyte from Nepal

Pedicularis dendrothauma is an arboreal epiphytic hemiparasite that was recently discovered in Nepal. We present results of preliminary studies on its habitat, demography, hemiparasitism, flower development, and pollination. While showing a strong preference forAbies spectabilis trunks and branches as a substrate,P. dendrothauma also grows on other species of trees and shrubs in the understorey as well as on dead wood on the forest floor, and only very rarely is rooted in soil or on rocks, rooted in a bryophyte layer. Demographic monitoring of the population in 1999 and 2001 allows us to begin to predict longevity, average size of flowering plants, and survivorship and flowering probability on different substrates. We demonstrate that individual plants ofP. dendrothauma live at least four, and probably at least five years. Studies of haustoria show that the species has retained its hemiparasitic habit despite having moved from a terrestrial to an arboreal habitat. Flowering probability increases with plant size; an average flowering plant has five leaves and a longest leaf that measures 21.5 cm. Survivorship varies from substrate to substrate, with the greatest survivorship over a two year period occurring in plants growing on living trees. Flowering probability was the same for plants growing on living trees as it was for plants on dead wood. Studies of flower development suggest that plants growing high in the canopy proceeded through the stages of floral development more rapidly than plants nearer ground level. Despite many hours of field observation, the method of pollination remains a mystery; we suggest different possible pollination mechanisms.     

Host Dependence and Preference of the Root Hemiparasite, Pedicularis cephalantha Franch. (Orobanchaceae)

The interaction between parasitic plants and their hosts is an important topic in both agriculture and ecology. Little, however, is known about that of the hemiparasite Pedicularis. It is essential to understand the host dependence and preference of Pedicularis for successful cultivation of plants in this genus and for understanding the roles they play in natural communities. We tested the effects of two herbaceous host species (Trifolium repens and Polypogon monspeliensis) on the survival and growth performance of Pedicularis cephalantha. Five P. cephalantha seedlings and two host plants were planted in each pot. In the control, no host plants were planted (treatment 1). Host plants were planted in three combinations: only T. repens (treatment 2) or P. monspeliensis (treatment 3) or a mixture of both (treatment 4). The results showed that P. cephalantha performed better in the presence of host plants, and host plants are more essential to P. cephalantha for proper development than for survival. The grass host P. monspeliensis proved to be a better host plant for P. cephalantha than the legume host T. repens. The high dependence of P. cephalantha on host plants and its host preference were demonstrated in this study. This is the first report of the performance of Chinese Pedicularis species in cultivation throughout all life stages (from seeds to seeds).     

Pollination Ecology and Endemic Adaptation of Pedicularis howellii Gray (Scrophulariaceae)

Abstract Pedicularis howellii , endemic to the Siskiyou Mountains of California-Oregon, is a root hemiparasite obligately dependent upon bumblebee pollinators that remove pollen by vibration from its short-tubed, nectarless, rostrate flowers, which reflect visible and ultraviolet (360nm) light attractive to Bombus foragers. All six Bombus species in the study area pollinated the plant, but only on P. howellii and P. racemosa were B. mixtus workers the most abundant pollinators among the seven bumblebee-pollinated plants studied, including Delphinium decorum, Dodecatheon jeffreyi, Penstemon newberryi, P. shastensis, and Phacelia heterophylla. Analysis of corbicular pollen loads of Bombus pollinators indicated that pollen foragers on Pedicularis species were less pollen-constant than all other bumblebee pollinators. Although P. howellii and P. racemosa blooming periods overlapped slightly, phenological isolation of blooming periods of plants sharing the same pollinators was not evident. Chemical soil analysis of its habitat and quadrat analysis of the population structure of Pedicularis howellii indicated that the plant grows in a moderately fertile forest soil and is restricted to the edge of forest canopy openings where sunlight favors development of plants to the flowering stage. The endemism of P. howellii is related to a similar "edge effect" survival of P. furbishiae in a boreal forest riparian habitat previously studied.     

Resource limitation and the role of a hemiparasite on a restored prairie

Hemiparasitic plants tend to thrive in and significantly affect plant communities in low-nutrient, high-light environments. Hemiparasites are assumed to be weak competitors for light but strong parasites, leading to the prediction that effects on hosts and communities should be a function of resource supply. We investigated the effects of light and mineral nutrients on hemiparasite-host relations in two experiments. Removal of the hemiparasite, addition of fertilizer, and full sun significantly increased total aboveground dry mass in small plots on a restored tallgrass prairie. After 3 years, removal of Pedicularis canadensis almost doubled the mass of grasses and had smaller effects on forb species, but the impact of the parasite was independent of resource level. Fertilizer increased grass growth only in full sun, increased non-legume forb growth only when shade was applied, and tended to depress legume growth when shaded. Light manipulation did not affect the hemiparasite across 4 years of manipulation but fertilizer increased P. canadensis shoot mass. A complementary greenhouse experiment with Andropogon gerardii as host produced qualitatively similar effects and showed that shade reduced root growth of both the host and the parasite. These results do not support common assumptions regarding hemiparasite-host relations under field conditions but indicate that a small hemiparasite can significantly affect prairie productivity regardless of resource supply.     

Carbon,water and nutrient relations of two mistletoes and their hosts: A hypothesis*

Abstract. The carbon, water and nutrient relations of the xylem parasites Loranthus europaeus and Viscum laxum and their respective hosts. Quercus robur and Pinus sylvestris, were followed throughout clear days in July in order to study water and nutrient interactions in a simple system in which the plant growth depends on the host for its water and nutrients. At similar quantum flux densities, temperatures and vapour pressure deficits, the mistletoes had higher rates of transpiration and lower leaf water potentials than their hosts, but similar rates of CO₂ assimilation. Based on measurements of the nutrient content of the xylem and on seasonal measurements of the biomass and the tissue nutrient content, the present study suggests that the high rates of transpiration may be necessary for the parasites to take up sufficient nitrogen from the xylem of the host for production of biomass (leaves, fruits and stems).     

甘肃马先蒿不同寄生密度对紫花针茅内生真菌共生体生理特性的影响

【目的】根寄生植物持续掠夺禾草体内营养物质成为禾草生长过程中的生物逆境,禾草内生真菌提高冷季型禾草对生物和非生物逆境耐受能力。然而,有关禾草内生真菌对根寄生逆境下禾草生理过程调控作用的研究鲜有报道。【方法】开展温室盆栽试验,以带菌(E+)和不带菌(E-)紫花针茅为研究对象,研究甘肃马先蒿不同寄生密度对紫花针茅抗氧化酶活性、渗透调节物质和根系活力影响的动态变化规律。【结果】甘肃马先蒿寄生显著增加紫花针茅抗氧化酶活性、丙二醛和脯氨酸含量,而根系活力却快速降低;高密度寄生紫花针茅植株生理特性指标显著高于低密度寄生或自然生长植株;同时,E+紫花针茅抗氧化酶活性、脯氨酸含量和根系活力显著高于E-植株,而E-植株丙二醛含量显著高于E+植株。【结论】禾草内生真菌通过增强抗氧化酶活性、调节细胞膜透性和增强根系生长能力的途径提高紫花针茅对根寄生逆境的耐受能力,利用植物替代方法带菌紫花针茅可以作为一种生物防治手段用于防控根寄生杂草。     

The role of local adaptation in the relationship between an endangered root hemiparasite Euphrasia rostkoviana, and its host, Agrostis capillaris

We experimentally studied the role of local adaptation and the co-evolutionary relationship between an annual, endangered root hemiparasite Euphrasia rostkoviana and its main host Agrostis capillaris. According to our hypothesis, the existence of local adaptation in hemiparasites should be observable in better hemiparasite performance when attached to A. capillaris hosts originating From Euphrasia populations. After one month of growth, the height and the number of leaves of hemiparasites were not affected by the origin of their hosts. The differences in growth were due to between population effects. The situation remained constant after three months. Hemiparasite biomass was not affected by the origin of the hosts. The percentage of hemiparasites surviving after one. two and three months was not affected by the origin of the hosts although there was a weak tendency towards better survival of hemiparasites with familiar hosts than with unfamiliar hosts. All variables used to measure hemiparasite performance during its complete life-cycle gave only limited support for the local adaptation hypothesis. Nevertheless, the familiar hosts suffered less from parasitism as indicated by their higher biomass after the experiment. This suggests that there may be some interactions between hemiparasites and their hosts based on their spatial population structure and common history as competitors.     

云南哀牢山两种常见半寄生植物的生态化学计量特征及其与寄主的关系

为深入探究半寄生植物与其寄主间的养分关系, 在云南哀牢山徐家坝地区选取两种常见半寄生植物椆树桑寄生(Loranthus delavayi)和柳叶钝果寄生(Taxillus delavayi), 研究其寄主枝条-吸器-寄生枝条-寄生叶片间的碳(C)、氮(N)、磷(P)生态化学计量特征关系。结果表明: 1)两种半寄生植物的寄主枝条-吸器-寄生枝条-寄生叶片这一连续体的C、N、P生态化学计量特征变化趋势并不完全相同, 具有物种自身的特性。2)同一半寄生植物的寄主枝条具有相似的C、N、P生态化学计量特征, 寄主物种对半寄生植物的生态化学计量特征没有显著影响。3)寄主枝条的C、N、P生态化学计量特征间具有紧密的相关关系, 吸器弱于寄主枝条, 寄生枝条弱于吸器, 寄生叶片的N、P含量相关关系不显著。4)寄主枝条与寄生叶片间的C含量存在极显著负相关关系。5)吸器与寄主枝条间的C、N、P生态化学计量特征存在紧密的相关关系, 但在生态化学计量特征的数值上吸器与寄生枝条更为接近。吸器作为连接寄主与寄生植物的关键部位, 其与寄主枝条极为显著的相关关系体现了寄主枝条养分对寄生植物的重要性, 而吸器在元素含量、计量比的数值以及相互关系上与寄生枝条更为相似, 则体现了吸器作为寄生植物器官具有与寄生枝条相似的生理功能。通过对寄主枝条-吸器-寄生枝条-寄生叶片C、N、P生态化学计量特征的分析, 为深入研究半寄生植物的养分利用策略与生态适应性提供了重要的基础资料。     

The Distribution of Nine Elements in Shoots of Lupinus albus L. and Lupinus angustifolius L. Compared with that of Silicon as a Measure of Passive Transport

The Ca, Mg, K, Na, P, Fe, Mn, Zn, Cu and opaline-Si contentsof leaves, stems and inflorescences from each order of shootaxis was determined in Lupinus albus L. and Lupinus angustifoliusL. The distribution of Si was used as a base for passive transportin the transpiration stream. All of the elements investigatedwere redistributed among the leaves, stems and inflorescencesof either L. albus or L. angustifolius. Most of the elementsinvestigated were enriched in the inflorescences and depletedin either the leaves or the stems. Sodium was enriched in thestems whereas Ca and Mn were redistributed only in L. albus.The pattern of element redistribution was similar in each ofthe lateral shoot axes except for the youngest. For the elementsenriched in the inflorescences, more than half was suppliedby redistribution. Calcium redistribution was similar to thatfor K, which is regarded as phloem mobile, but a mechanism forCa redistribution is uncertain. Lupinus albus L., Lupinus angustifolius L., mineral transport, leaves, inflorescences, transpiration, silicon, calcium     

Transpiration, CO2 assimilation, WUE, and stomatal aperture in leaves of Viscum album (L.): Effect of abscisic acid (ABA) in the xylem sap of its host (Populus x euamericana).

Leaves of the mistletoe Viscum album (L.) show a high rate of transpiration, even when the host is under severe drought stress. The hypothesis that a strong control of ABA influx from the xylem sap of the host into the mistletoe prevents stomatal closure in mistletoe leaves was tested under the following conditions: sections of poplar twigs carrying a mistletoe were perfused with artificial xylem sap that contained different ABA concentrations and both transpiration and ABA levels were analysed in mistletoe leaves. Despite variation by a factor of 10(4), the ABA content of the host xylem did not affect ABA levels, leaf transpiration, CO(2) assimilation, WUE, or the degree of stomatal aperture in mistletoe leaves. These observations support the hypothesis of a strong control of ABA influx from the host of the xylem into the mistletoe, although degradation of ABA before it enters the mistletoe leaves cannot be excluded. This mechanism may ensure a water and nutritional status favourable for the mistletoe, even if the water status of the host is impaired. Despite the lack of short-term sensitivity of ABA levels in mistletoe leaves to even strong changes of ABA levels in the xylem sap of the host, ABA levels in mistletoe leaves were relatively high compared to ABA levels in the leaves of several tree species including poplar. Since significant transpiration of the mistletoe leaves was observed despite high ABA levels, a diminished sensitivity of the stomata of mistletoe leaves to ABA has to be concluded. The stomatal density of adaxial Viscum leaves of 89+/-23 stomata per mm is lower than those reported in a study performed at the end of the 19th century.     

Pollination Ecology of Pedicularis megalantha D. Don (Scrophulariaceae) in the Himachal Himalaya

Abstract In the summer of 1990, the pollination ecology of Pedicularis megalantha was studied in the montane-subalpine spruce-fir forest zone (2750-3050 m) on the north slope of Mt. Huttoo at Narkanda, Himachal Pradesh, in the Indian Himalaya. Its yellow, long-tubed, nectarless flower with a curled rostrum overarched by a broad, inverted lower corolla lip was pollinated exclusively by Bombus albopleuralis and B. tunicatus workers hanging inverted from the corolla and vibrating pollen from introrse anthers concealed within the galea and releasing pollen through a small ventral opening in the galea base. The stigma, protruding from the tip of the rostrum, contacted pollen deposited on the ventral side of the insect's thorax. Corbicular pollen loads from P. megalantha pollinators indicated equal numbers of monolectic and oligolectic foragers. P. megalantha appeared to suffer from competition for pollinators by Cynoglossum wallichii at one site but to be favored in a mixed plant community with nectariferous species offering a forage resource complementary to Pedicularis pollen. As in P. punctata , the long, nectarless corolla tube of P. megalantha appears to function in extending the rostrate vibration pollination mechanism beyond the plant's foliage, which would interfere with its function. It is not an adaptation for nectar-foraging lepidopteran pollinators. P. megalantha was also found to be a root hemiparasite.     

Differential response of stomata to air humidity in the parasitic mistletoe (Phthirusa pyrifolia) and its host,mandarin orange (Citrus resitulata)

Measurements of CO₂ and H₂O exchange rate and the calculated leaf conductance of attached leaves were conducted over a range of leaf-to-air vapour pressure difference (VPD) (1.5 to 5.5 kPa) to compare the response of the parasitic mistletoe, Phthirusa pyrifolia, with that of its host, the mandarin orange, Citrus reticulata. Seedlings of the host infected with the parasite were grown in well-watered and adequately fertilized large pots outdoors at the CIAT headquarters, Palmira, Colombia, South America. Observations of leaf anatomy of the parasite and nutrient analysis of young tissues of both the parasite and host were made. The photosynthetic rate of the host decreased linearly with increased VPD, whereas the parasite showed a constant rate. This trend coincided with similar responses in leaf conductance. Due to the insensitivity of the parasite stomata, the transpiration rate increased linearly with VPD as compared with an initial increase and then a decrease in the host transpiration rate. The higher photosynthetic rate and the closure of stomata of the host resulted in high water use efficiency as compared with that of the parasite. The parasite accumulated in its leaves more N, P, K and less Ca and Mg than the host. The significance of the host-parasite differential response to air humidity is discussed in relation to mechanism underlying stomatal sensitivity and in the context of host-parasite association.Visiting Scientist, Coordinator, and Research Assistant of the Cassava Physiology Program.     

Diurnal courses of leaf conductance and transpiration of mistletoes and their hosts in Central Australia

Summary In Australia, diurnal courses of leaf conductance and transpiration of hemiparasitic mistletoes (Loranthaceae) and their hosts were measured using steady-state porometers under conditions of partial drought and high evaporative demand. The sites spanned a diversity of climatic regions ranging from the subtropical arid zone with winter rainfall, through the subtropical arid zone with summer rainfall to the tropical summer rainfall zone. With one exception (Acacia farnesiana with deciduous leaves), the hosts were trees or shrubs with evergreen, sclerophyllous leaves or phyllodes.The measurements confirm previous observations that mistletoes transpire at higher rates than their hosts. For adult leaves from all of the 18 different host/mistletoe pairs investigated, the daily average leaf conductances were higher in the parasites than in their hosts. The ratios ranged from 1.5 to 7.9. In the most extreme case,Amyema maidenii had a daily rate of water loss 8.9 times higher than its hostAcacia cowleana. Hoever, the parasites did not exhibit unlimited transpiration. Despite high water loss rates, leaf conductance showed large and consistent changes during the course of the day, indicating definite stomatal regulation. The typical diurnal pattern of conductance in both mistletoes and hosts consisted of an early morning peak followed by a continuous decrease throughout the remainder of the day. There was no abrupt decrease in leaf conductance of the parasites that might be interpreted as a threshold response with respect to internal water potential. In most cases, the continuous stomatal closure occurred without substantial changes in leaf water potential over a time span of several hours. The decrease in leaf conductance was correlated with an increase in leaf-to-air water vapor difference, which was associated with increasing leaf temperatures. It seems probable that external humidity plays a major role in the stomatal response. Diurnal courses of leaf conductance of the host/parasite pairs usually showed similar general patterns, even when the absolute rates were quite different. Thus, mistletoes not only control their water loss by stomatal action but this regulation seems to occur in coordination with the stomatal response of their hosts.The integrated mistletoe/host system must also endure severe drought conditions. Controlled water use is necessary for long-term survival of the host. Assuming stomatal behavior in the host is well adapted to ensure its existence, then similar performance in the mistletoe would promote survival of both host and parasite.     

The influence of the parasitic angiosperm Striga gesnerioides on the growth and photosynthesis of its host, Vigna unguiculata

Cowpea plants (Vigna unguiculata) infected with the root hemiparasiticangiosperm Striga gesnerioides accumulated less biomass thanuninfected plants over a growth period of 60 d. The allometricrelationship between shoot and root dry weight was similar inparasitized plants relative to control plants, as was the proportionof dry matter partitioned into leaf, stem and root tissue. However,infected plants failed to make any significant investment ofdry matter in pods. The rate of photosynthesis of the youngestfully expanded leaf of parasitized plants was significantlylower than for control plants. The lower rates of photosynthesiswere not attributable to stomatal limitation, a loss of chlorophyllor to an accumulation of carbohydrate. The depression of photosynthesisin the young leaves was transient. As control leaves aged, photosynthesisdeclined. This also occurred in Striga infected plants, butto a lesser extent resulting in higher rates of photosynthesisin mature leaves when compared to those of uninfected plants.The foliar nitrogen content of parasitized plants was higherthan control plants consistent with the slower rate of photosyntheticdecline of older leaves. The data are discussed with respectto the influence of parasitic weeds on host growth and photosynthesis. Key words: Cowpea, hemiparasite, allometry, nitrogen