A new species of Arceuthobium (Viscaceae) from central Mexico

Arceuthobium pendens, a parasite of the pinyonsPinus discolor andP. cembroides, is described from San Luis Potosí and Veracruz, Mexico. This andA. divaricatum Engelm. are the only dwarf mistletoes known to parasitize pinyons.Arceuthobium pendens is distinct fromA. divaricatum Engelm. in its long, slender, greenish shoots, formation of systemic witches brooms, and frequency on various species of pinyons. With the discovery of this new species, 19 dwarf mistletoes are known from Mexico.     

Chemical Composition of Viscin Mucilage from Three Mistletoe Species--A comparison

The viscin of mistletoes is a unique tissue which serves toprovide strong adherence of the seed of the parasite to thehost branch. The viscin mucilage of three species, Phoradendroncalifornicum and Arceuthobium americanum (Viscaceae) and Phthirusapyrifolia (Loranthaceae), was analysed. The major componentof the mucilage of Pho. californicum and A. americanum was polysaccharidic;of this, neutral sugars comprised the largest portion, but substantialamounts of uronic acids and proteins were also present. Xyloseand arabinose were the most abundant of the neutral sugars,and analysis showed glycine to be the most abundant amino acidas well as the presence of higher-than-average amounts of histidine.In contrast, in the mucilage of Pht. pyrifolia glucose was themost abundant neutral sugar and the protein content was substantiallyhigher. Based on the differences in composition observed amongthe different species the future use of viscin components astaxonomic markers is suggested. Viscin mucilage, mistletoes, Arceuthobium americanum, Phthirusa pyrifolia, Phoradendron californicum, g.l.c., amino acid analysis     

Nitrogen concentration and mimicry in some New Zealand mistletoes

Summary The nitrogen concentration in photosynthetic organs of 41 pairs of mistletoes and their respective hosts was compared and found to be higher (in relation to that of their hosts) in cryptic mistletoes and lower in non-cryptic mistletoes. This supports the hypothesis that crypsis may have evolved as a mechanism for avoiding predation by vertebrate herbivores. There were no consistent differences between the nitrogen concentrations of hosts and mistletoes in trees and larger-leaved shrubs (whether native or introduced). The largest differences between mistletoes and their hosts were found on small-leaved divaricate shrubs (Coprosma ssp., Melicope simplex) where the nitrogen concentrations in cryptic mistletoes (Korthalsella spp.) were significantly higher than in their hosts, but significantly lower in prominent mistletoes (Loranthus micranthus, Tupeia antarctica) which apparently advertise their unpalatability. If crypsis in New Zealand mistletoes evolved as a protection against herbivory, then it must have evolved in the absence of mammalian herbivores, and the appropriate selection pressures could have been provided only by moas, extinct ratite birds. Otherwise, alternative explanations, such as differing relationships between water use and nitrogen uptake, must be sought for the observed associations of nitrogen concentration and cryptic mimicry in New Zealand mistletoes.     

Physical and Chemical Properties of Cilia Isolated from Tetrahymena pyriformis*

SYNOPSIS. Cilia devoid of their basal granules and isolated from Tetrahymena pyriformis strain W, by the method of Watson et al. (66) have been characterized in terms of physical, chemical and immunologic properties. The general chemical composition of cilia, based on mean values from these determinations is 66.3% protein, 24.3%“lipid,” 3.7 or 4.8% carbohydrate, and 0.4% nucleic acid. The amino acid composition of cilia includes hydroxyproline and is not significantly different from the amino acid composition of whole cells. Chloroformmethanol extracts of ciliary material contain free amino acids as well as neutral lipids and phospholipids. The lipid composition of cilia also is not significantly different from that of whole cells. Pentose, hexose and small but significant amounts of adenine nucleotide and ribonucleic acid are present. Cilia are a greater stimulus for the production of antibody than an equivalent amount of cellular antigen, and the ciliary antigen is strain specific.     

Haustoria in action: Case studies of nitrogen acquisition by woody xylem-tapping hemiparasites from their hosts

Summary This review discusses studies conducted by the author and his colleagues on mistletoes and root hemiparasites native to Western Australia. Morphological characteristics of haustoria are described and their anatomical features are discussed in relation to uptake, transfer, and metabolism of xylem-borne nitrogenous solutes derived from a host. Experimental approaches used include comparisons of xylem sap composition of parasite and host(s), solute pool analyses and enzymatic properties of haustoria, host xylem feeding of¹⁵N-labelled solutes to follow the fate of label in haustoria and body of the parasite, and studies using species-specific nonprotein amino acids to validate successful uptake from hosts or occasional backflow of xylem-borne solutes to a host. Field studies on promiscuous root hemiparasites assess frequencies of exploitation of different hosts.¹⁵N natural abundance assays of host and parasite dry matter demonstrate marked preference bySantalum acuminatum for N₂-fixing as opposed to nonfixing hosts. The ability ofOlax phyllanthi to continue to exploit deep-rooted hosts ranks of importance when xylem water potentials of other hosts go out of sucking range during periods of water stress. Comparisons of xylem sap composition of parasites feeding on different hosts indicate remarkable versatility by haustoria in uptake and utilization of the different major nitrogenous solutes received from these hosts. Solute pools in parasites partly reflect metabolic transformations accentuated by haustoria while also indicating direct throughput from xylem of a host. The review concludes by showing how empirically based modelling techniques can be used to estimate proportional gains of N by parasites from single hosts and repercussions on host growth which accompany such exploitation.Dedicated to Professor Brian E. S. Gunning on the occasion of his 65th birthday     

Primary attraction and kairomonal host discrimination in three species of Dendroctonus (Coleoptera: Scolytidae)

Abstract 1 Synthetic blends of bole and foliage volatiles of four sympatric species of conifers were released from pheromone‐baited multiple‐funnel traps to determine if three species of tree‐killing bark beetles (Coleoptera: Scolytidae): (i) exhibited primary attraction to volatiles of their hosts and (ii) discriminated among volatiles of four sympatric species of host and nonhost conifers. 2 Bole and foliage volatiles from Douglas‐fir, Pseudotsuga menziesii (Mirb.) Franco, increased the attraction of coastal and interior Douglas‐fir beetles, Dendroctonus pseudotsugae Hopkins, to pheromone‐baited traps. Primary attraction to bole volatiles was observed in interior D. pseudotsugae. Beetles were significantly less attracted to the pheromone bait when it was combined with volatiles of lodgepole pine, Pinus contorta var. latifolia Engelm. or interior fir, Abies lasiocarpa × bifolia. 3 The monoterpene myrcene synergized attraction of mountain pine beetles, Dendroctonus ponderosae Hopkins, to their aggregation pheromones, but there was no evidence of primary attraction to host volatiles or discrimination among volatiles from the four conifers. 4 There was significant primary attraction of the spruce beetle, Dendroctonus rufipennis Kirby, to bole and foliage volatiles of interior spruce, Picea engelmannii × glauca, but beetles did not discriminate among volatiles of four sympatric conifers when they were combined with pheromone baits. 5 Our results indicate that host volatiles act as kairomones to aid pioneer Douglas‐fir beetles and spruce beetles in host location by primary attraction, and that their role as synergists to aggregation pheromones is significant. For the mountain pine beetle, we conclude that random landing and close range acceptance or rejection of potential hosts would occur in the absence of aggregation pheromones emanating from a tree under attack.     

A comparison between the amino acid composition of an egg parasitoid wasp and some of its hosts

Amino acid compositions of the eggs of five lepidopteran hosts for Trichogramma minutum were compared with each other and with a non-host species, Rhodnius prolixus, in which T. minutum oviposits but does not develop. Host eggs are quite homogeneous, particularly when compared according to groupings of potentially interconvertible amino acids. Combined mole percent values for glycine, serine and alanine were higher in hosts (27.5–29.2 mole%) than in R. prolixus eggs (21.5 mole%), in bovine serum albumin (14.9%), which has been used as a protein source in artificial diets for T. minutum, or in many of the mixtures used in published diets for this species. Since these three amino acids make up 26.3 mole% of the adult amino acid content of T. minutum, their deficiency in diets could require metabolic compensation detrimental to development.Adult T. minutum arising from eggs of Manduca sexta, Choristoneura fumiferana, and Sitotroga cerealella are similar in amino acid composition to each other and, in general, to their hosts. Variability appears greater in hosts than in adult wasp composition, suggesting some interconversion of host amino acids to accommodate inflexible nutritional requirements of T. minutum.In the three host species tested, free amino acids constituted 15.8–19.3% by weight of the amino acid in egg contents. In M. sexta eggs, glycine, serine and alanine together make up 28.4% by weight of the total free amino acid, a much higher proportion than in many published diets. The four free amino acids (isoleucine, leucine, phenylalanine and histidine) reported to be oviposition stimulants in experiments on encapsulated diets are present in sufficient concentrations to induce oviposition in the host species tested and in R. prolixus. S. cerealella egg contents having approximately 1.8 g amino acid, yield one or rarely two adult T. minutum (1g amino acid/insect). In contrast, M. sexta eggs with 94 g amino acid each yield an average of 10–12 adults (8.2g amino acid/insect). This suggests that small hosts are allocated few eggs which can only develop into small adults because of nutrient supply (parasitoid size in metabolically restricted), whereas much larger hosts are allocated proportionately fewer eggs than the former resulting in larger, and presumably more viable and fecund, adults (parasitoid size is established behaviourally).     

Comparative water use and nitrogen relationships in a mistletoe and its host

Summary The impact of the xylem-tapping mistletoe Phoradendron juniperinum on the nitrogen and water relations of its host Juniperus osteosperma was investigated under natural field conditions. Leaf conductance, leaf water potential, and leaf Kjeldahl nitrogen contents were followed through the growing season on mistletoes, infected junipers (separating infected from uninfected stems) and uninfected junipers. Infected trees experienced lower leaf water potentials than uninfected trees and also had lower leaf conductances and lower leaf nitrogen contents. Infected juniper stems had higher conductances than uninfected stems. Mistletoes had higher leaf nitrogen contents than their hosts and much of this nitrogen appeared as arginine, a potential nitrogen storage compound. Photosynthetic rates (per unit leaf area) were significantly higher in junipers than in the mistletoe, and higher in the uninfected than infected junipers. Water use efficiencies as estimated by carbon isotope ratios were significantly lower in mistletoes than in their hosts. Increased mistletoe infestation appeared to increase absolute water use efficiency of both host and mistletoe.Dedicated to Professors D. Wiens (Salt Lake City) and H. Ziegler (München) for initiating our curiosity in mistletoes     

Arceuthobium (Viscaceae) in Mexico and Guatemala: Additions and range extensions

Three new dwarf mistletoes are described:Arceuthobium globosum subsp.grandicaule (Mexico and Guatemala),A. aureum subsp.aureum (Guatemala) andA. aureum subsp.petersonii (Chiapas, Mexico).Arceuthobium guatemalense is recorded for the first time in Mexico. Significant range extensions are recorded forA. abietisreligiosae, A. divaricatum,A. gillii subsp.nigrum, andA. rubrum. New hosts are reported for several taxa. Nineteen members of the genus are presently known from Mexico, and three (possibly four) from Guatemala. Chromosome counts are reported for the first time for 3 taxa.     

Host-preference and density of woodrose-forming mistletoes (Loranthaceae) on savanna vegetation,South Africa

In the Bushbuckridge region of South Africa host preference and density of two woodrose-forming mistletoes, Erianthemum dregei (Eckl. & Zeyh.) V. Tieghem and Pedistylis galpinii (Schinz ex Sprague) was quantified in relation to land-use (harvested or unharvested), rainfall (high > 660 or low < 660 mm year^–1) and catenal position (top or lower slope). These two mistletoes are generalist hemi-parasites of savanna trees and shrubs occurring on 25 and 17 hosts respectively, seven of which are shared. Thirty-six percent of woody plant species recorded were found to be hosts. Although Sclerocarya birrea (A. Rich.) Hochst. comprised only 4% of woody plant density in the environment, it was the principal host for both mistletoes, accounting for 71% of total E. dregei and 42% of P. galpinii infection. Mistletoe infection relative to density of Ficus stuhlmanii, Trichilia emetica and Cassine transvaalensis indicated that these were preferential hosts to S. birrea. Mistletoe host preference was negatively correlated with host wood density. Mistletoe number per tree had a weak relationship to canopy size. Mistletoes of all size classes were denser at high rainfall relative to low rainfall sites. Interestingly, the overall mistletoe size class distribution was similar between harvested and unharvested sites. The ratio of living to dead mistletoe was 2 to 1 for E. dregei and 1.5 to 1 for P. galpinii. There are sufficient dead mistletoes in unharvested and harvested areas to satisfy present market demand. Living E. dregei predominated in harvested rather than unharvested areas suggesting that current-harvesting levels had little or no negative effect on the population. In contrast, P. galpinii was denser in unharvested areas possibly owing to its higher market value and thus higher harvesting levels.     

EXPERIMENTAL EVIDENCE FOR HOST RACES IN MISTLETOE (PHORADENDRON TOMENTOSUM)

In order to test the hypothesis that mistletoes (Phoradendron tomentosum) are differentially adapted for the host species that they occur on, mistletoe seeds from the three most common hosts in central Texas (hackberry, Celtis occidentalis, elm, Ulmus crassifolia, and mesquite, Prosopsis glandulosa) were planted onto different individuals of each of the three hosts. Germination of seeds and formation of haustorial disks by seedlings were followed in the subsequent 17 months. Germination of seeds was very high for all nine combinations of maternal (source) and seed (experimental) hosts (range 82%–98%). There were no significant differences in seed germination between the two groups when source and experimental hosts were the same species and when they were different species. In contrast, development of haustorial disks when source and experimental hosts were the same species was significantly greater than when experimental host and source host species were different. The data suggest that populations of mistletoes are genetically differentiated such that early seedling development is greatest when there is correspondence between maternal and seedling host species.     

Carbon and nitrogen isotope ratios, nitrogen content and heterotrophy in New Zealand mistletoes

The carbon isotope ratio ('¹³C) of New Zealand mistletoes (-29.51ǂ.10‰) and their hosts (-28.89ǂ.12‰) is generally more negative, and shows less difference between mistletoes and their hosts, than found in previous studies. In 37% of the examined pairs, the '¹³C of mistletoes was less negative than that of their hosts. These reversals were not associated with the relative position (proximal or distal) of the host material with regard to the mistletoe. Differences between host and mistletoe tended to be greater on hosts with less negative '¹³C. Both nitrogen content and isotope ratio ('¹⁵N) of the mistletoe leaves were strongly correlated with those of their hosts. Nitrogen contents of mistletoe leaves were similar to those of their hosts at low nitrogen contents but proportionately less on hosts with a high nitrogen content, whereas '¹⁵N of mistletoes was consistently similar to that of their hosts. The '¹³C of mistletoes was related to both host nitrogen content and '¹⁵N, but '¹³C in host tissue was related to neither, suggesting that the mistletoes derived both nitrogen and carbon from their hosts. The '¹³C of both hosts and mistletoes were significantly related to leaf conductance and carbon dioxide concentration but relationships with transpiration and water use efficiency were not significant. In all cases there was no clear separation between the responses of hosts and mistletoes. This may be related to the similarity of stomatal conductance, transpiration and photosynthesis in the studied mistletoes and their hosts and is consistent with the small differences in '¹³C between mistletoes and hosts found in this study. Consequently, the estimation of mistletoe heterotrophy from carbon discrimination is confounded, as the small difference between host and mistletoe carbon discrimination could equally well result from either similarities in photosynthesis and water relations or heterotrophic assimilation of host-derived carbon. The differences between our study and previous studies (which are mostly from seasonally dry or semi-arid to arid environments) may be related to the temperate environment in which these mistletoes grow. Water is freely available so that the mistletoe is able to obtain sufficient water and dissolved nutrients without having to maintain the high transpiration rate and low water potentials that are needed to extract water from a water-stressed host. Similarly, mistletoe photosynthesis is less inhibited by water stress. The physiological similarities between mistletoe and hosts from a temperate environment are reflected in their similar '¹³C values.     

The effect of nitrogen supply on growth and water-use efficiency of xylem-tapping mistletoes

Xylem-tapping mistletoes are known to have normally a higher rate of transpiration and lower water-use efficiency than their hosts. The relationships between water relations, nutrients and growth were investigated for Phoradendron juniperinum growing on Juniperus osteosperma (a non-nitrogen-fixing tree) and for Phoradendron californicum growing on Acacia greggii (a nitrogen-fixing tree). Xylem sap nitrogen contents were approximately 3.5 times higher in the nitrogen-fixing host than in the non-nitrogen-fixing host. The results of the present study show that mistletoe growth rates were sevenfold greater on a nitrogen-fixing host. At the same time, however, the differences in water-use efficiency between mistletoes and their hosts, which were observed on the non-nitrogen-fixing host did not exist when mistletoes were grown on hosts with higher nitrogen contents in their xylem sap. Growth rates and the accumulation of N, P, K, and Ca as well as values for carbon-isotope ratios of mistletoe tissues support the hypothesis that the higher transpiration rates of mistletoes represent a nitrogen-gathering mechanism.Abbreviation ¹³C carbon-isotope ratio Dedicated to Professor Dr. Hubert Ziegler on the occasion of his 60th birthday     

Carbon and nitrogen parasitism by a xylem‐tapping mistletoe (Tapinanthus oleifolius) along the Kalahari Transect: a stable isotope study

The present study explores the xylem‐tapping parasitism by mistletoe (Tapinanthus oleifolius) on native tree species along the Kalahari Transect (KT) using the stable isotopes of carbon and nitrogen. Mistletoe‐host pairs were collected at three geographical locations along the KT rainfall gradient in the 2005 and 2006 wet seasons. Foliar total carbon, total nitrogen and their stable isotope compositions (δ¹³C and δ¹⁵N) were measured. Heterotrophy (H) was calculated using foliar δ¹³C values of mistletoes and their hosts as an indicator of proportion of carbon in the mistletoes derived from host photosynthate. Based on the mistletoe H‐value and relationship between the mistletoe foliar δ¹⁵N and their host foliar δ¹⁵N, the results showed that mistletoes along the KT derived both nitrogen and carbon from their hosts. Mistletoes may regulate water use in relation to nitrogen supply. The proportion of carbon in the mistletoes derived from host photosynthate was between 35% and 78%, and the degree of heterotrophy was species‐specific with only limited annual variation. The study emphasizes the importance of incorporating parasitic associations in future studies on studying carbon, water and nutrient cycling along the Kalahari.     

Biochemical Changes in Terminal Root Galls Caused by an Ectoparasitic Nematode,Longidorus africanus: Amino Acids

The amino acids of terminal root galls caused by Longidorus africanus on bur marigold (Bidens tripartita L.) and grapevine (Vitis vinifera L.) were studied. The galled roots of bur marigold contained 73% more cell-wall protein and 184% more free amino acids. The main changes among the free amino acids of the galled tissue were a large increase (1900%) in proline and a decrease in aspartic acid (56%) compared with the respective check tissue. Hydroxyproline decreased in the wall protein fraction from 5.6% in the healthy tissue to 3.6% in the infected tissue.Percent of hydroxyproline in total amino acids of the wall protein fraction of grapevine roots decreased from 0.7% in the healthy tissue to 0.3% in the galled tissue, and total proteins of this fraction decreased from 9.5 mg to 4.5 rag, respectively. Total protein in the protoplasmic fraction also decreased from 3.0 mg in healthy to 1.0 mg in infected roots. No change was noticed in total proteins in the free amino acids fraction but free proline decreased 40% in the infected roots.The relationship of these differences to the specific reactions of the hosts to nematode feeding is discussed.     

Investigations on the endogenous levels of abscisic acid in a range of parasitic phanerogams

The endogenous levels of abscisic acid in the parasitic phanerogamsArceuthobium oxycedri (DC) Bieb.,Cassytha filiformis L.,Lathraea squamaria L.,Melampyrum pratense L.,Orobanche hederae Duby., andViscum album L. were investigated. In general, the content of abscisic acid was high in parasites which deprive their hosts of both phloem- and xylem-transported substances and much lower in those that deprive their hosts of sap from the xylem only. Within the parasites studied in more depth (e.g.,Orobanche hederae, Lathraea squamaria, andMelampyrum pratense) the highest abscisic acid levels were found in their sink regions, especially in their inflorescences. It is suggested that a high concentration of abscisic acid is associated with plant tissues showing a high demand for phloem-transported substances. The possible role of abscisic acid in such tissues is discussed.     

Reduced mycorrhizae on Juniperus monosperma with mistletoe: the influence of environmental stress and tree gender on a plant parasite and a plant-fungal mutualism

Summary We examined how an important plant mutualist (fungal mycorrhizae) interacted with a common tree parasite, a xylem-tapping mistletoe (Phoradendron juniperium Engelm.) growing on one-seeded juniper (Juniperus monosperma Engelm.). We also examined how host tree gender and environmental stress might be involved in this interaction. Four major patterns were observed. First, the mycorrhizal levels of trees of both sexes were negatively correlated with mistletoe density. In comparisons of heavily and lightly infested trees at the stressful site, high mistletoe levels were associated with 27% less mycorrhizae on male trees and 38% less mycorrhizae on the roots of female trees. Second, the reduction of mycorrhizae on trees with high mistletoe levels was slightly but significantly greater for female trees than male trees. These results are consistent with the hypothesis that severe mistletoe infestation suppresses mycoresis and that this suppression is more severe in female trees because of their greater energetic investment in reproduction. Third, female junipers growing in the stressful ash and cinder fields averaged three-fold higher levels of mistletoe infestation than male trees. Fourth, no differences in mistletoe infestation were observed between male and female trees growing in the more favorable soils. Comparisons with other systems suggest that both mistletoes and herbivores have similar interactions with mycorrhizae.     

A phylogeny of all species of Arceuthobium (Viscaceae) using nuclear and chloroplast DNA sequences

The genus Arceuthobium (dwarf mistletoes, Viscaceae) comprises 42 species that parasitize hosts in Pinaceae and Cupressaceae in the Old and New Worlds. Maximum parsimony analyses were conducted on two data partitions (separately and combined): nuclear ribosomal internal transcribed spacer (ITS) sequences for all 42 currently recognized species and chloroplast trnT-L-F sequences for 34 New World species. The Old and New World species were phylogenetically distinct using ITS, thus making subgenus Arceuthobium paraphyletic. Arceuthobium pendens and A. guatemalense comprise the basalmost clade of subgenus Vaginata, characterized by the presence of flabellate secondary branching. The trnT-L-F sequences, which vary widely in length depending upon taxon, contain three times less phylogenetic signal than ITS, although homoplasy for this partition is lower. Several of the clades obtained from analysis of nuclear ITS sequences are also recovered using trnT-L-F sequences such as A. guatemalense and A. pendens, the A. rubrum group, the A. vaginatum group, and the A. campylopodum group. The ITS + trnT-L-F tree is well resolved except for four internal nodes. A revised classification of the genus is discussed that recognizes only monophyletic species that are well differentiated by molecular data.     

THE ROLE OF BACTERIAL SYMBIONTS IN AMINO ACID COMPOSITION OF BLACK BEAN APHIDS

To evaluate the role of bacterial symbionts (Buchnera spp.) in the black bean aphids (Aphis craccivora Koch), the aphids were treated with the antibiotic, rifampicin, to eliminate their intracellular symbiotic bacteria. Analysis of protein and amino acid concentration in 7‐day‐old of aposymbiotic aphids showed that the total protein content per mg fresh weight was significantly reduced by 29%, but free amino acid titers were increased by 17%. The ratio of the essential amino acids was in general only around 20% essential amino acids in phloem sap of broad bean, whereas it was 44% and 37% in symbiotic and aposymbiotic aphids, respectively, suggesting that the composition of the free amino acids was unbalanced. For example, the essential amino acid, threonine represented 21.6% of essential amino acids in symbiotic aphids, but it was only 16.7% in aposymbiotic aphids. Likewise, two nonessential amino acids, tyrosine and serine, represented 8.9% and 5.6% of total amino acids in symbiontic aphids, respectively, but they enhanced to 21.1% and 13.6% in aposymbiotic aphids. It seems likely that the elevated free amino acid concentration in aposymbiotic aphids was caused by the limited protein anabolism as the result of the unbalanced amino acid composition.     

Integrated nitrogen,carbon, and water relations of a xylem-tapping mistletoe following nitrogen fertilization of the host

Xylem-tapping mistletoes transpire large volumes of water (E) while conducting photosynthesis (A) at low rates, thus maintaining low instantaneous wateruse efficiency (A/E). These gas-exchange characteristics have been interpreted as a means of facilitating assimilation of nitrogen dissolved at low concentration in host xylem water; however, low A/E also results in substantial heterotrophic carbon gain. In this study, host trees (Juniperus osteosperma) were fertilized and gas exchange of mistletoe (Phoradendron juniperinum) and host were monitored to determine whether mistletoe A/E would approach that of the host if mistletoes were supplied with abundant nitrogen. Fertilization significantly increased foliar N concentrations (N), net assimilation rates, and A/E in both mistletoe and host. However, at any given N concentration, mistletoes maintained lower A and lower A/E than their hosts. On the other hand, when instantaneous water-use efficiency and A/N were calculated to include heterotrophic assimilation of carbon dissolved in the xylem sap of the host, both water-use efficiency and A/N converged on host values. A simple model of Phoradendron carbon and nitrogen budgets was constructed to analyze the relative benefits of nitrogen- and carbonparasitism. The model assumes constant E and includes feedbacks of tissue nitrogen concentration on photosyn-thesis. These results, combined with our earlier observation that net assimilation rates of mistletoes and their hosts are approximately matched (Marshall et al. 1994), support part of the nitrogen-parasitism hypothesis: that high rates of transpiration benefit the mistletoe primarily through nitrogen gain. However, the low ratio of A/E is interpreted not as a means of acquiring nitrogen, but as an inevitable consequence of an imbalance in C and N assimilation.This research was supported by the National Science Foundation (grants BSR-8706772 and 8847942).