Population genetic consequences of extreme variation in sexual and clonal reproduction in an aquatic plant

Most plants combine sexual reproduction with asexual clonal reproduction in varying degrees, yet the genetic consequences of reproductive variation remain poorly understood. The aquatic plant Butomus umbellatus exhibits striking reproductive variation related to ploidy. Diploids produce abundant viable seed whereas triploids are sexually sterile. Diploids also produce hundreds of tiny clonal bulbils, whereas triploids exhibit only limited clonal multiplication through rhizome fragmentation. We investigated whether this marked difference in reproductive strategy influences the diversity of genotypes within populations and their movement between populations by performing two large-scale population surveys (n = 58 populations) and assaying genotypic variation using random amplified polymorphic DNA (RAPDs). Contrary to expectations, sexually fertile populations did not exhibit higher genotypic diversity than sterile populations. For each cytotype, we detected one very common and widespread genotype. This would only occur with a very low probability (< 10-7) under regular sexual recombination. Compatibility analysis also indicated that the pattern of genotypic variation largely conformed to that expected with predominant clonal reproduction. The potential for recombination in diploids is not realized, possibly because seeds are outcompeted by bulbils for safe sites during establishment. We also failed to find evidence for more extensive movement of fertile than sterile genotypes. Aside from the few widespread genotypes, most were restricted to single populations. Genotypes in fertile populations were very strongly differentiated from those in sterile populations, suggesting that new triploids have not arisen during the colonization of North America. The colonization of North America involves two distinct forms of B. umbellatus that, despite striking reproductive differences, exhibit largely clonal population genetic structures.     

Dynamics of genotypic structure in clonal Rhododendron ferrugineum (Ericaceae) populations

Two populations of Rhododendron ferrugineum growing at subalpine level in the Pyrenees (France) were studied in two sites (Bethmale and Mourtis). Identification and delimitation of genets were inferred from amplified fragment length polymorphism (AFLP) markers, along a closure gradient (from meadow to more closed heath) in each site. Surface and age of genets, genotypic diversity (Simpson's index D), 'proportion distinguishable' genotypes and genetic relationships between genets were then estimated. Amplification of the 312 DNA samples with three selective primer pairs gave a mean of 98 detectable peaks (i.e. bands) per sample, with size ranging from 60 to 300 bp. In total 60% (Bethmale) and 70% (Mourtis) of the peaks were polymorphic, and a total of 31 and 23 multilocus genotypes were identified, in Bethmale and Mourtis, respectively. We inferred that pioneer genotypes began arriving 110 years ago mainly over a 40-year period in the Mourtis meadow, and began about 130 years ago over a 100-year period in the Bethmale meadow. After this pioneer stage, populations extended vegetatively. Two different patterns of genotypic dynamics can be identified. At Bethmale, population closure could have led to a dramatic loss of genets and to the selection of highly genetically related genotypes. In contrast, at Mourtis, genotypic diversity and genet density did not change fundamentally along the closure gradient. However the range of genetic diversity diminished from the open to the closed situation, suggesting that thinning could have occurred in the past.     

Comparison of photosynthesis and antioxidative protection in Sedum album and Sedum stoloniferum (Crassulaceae) under water stress

Photosynthetic gas exchange, dry mass production, water relations and inducibility of crassulacean acid metabolism (CAM) pathway as well as antioxidative protection during the C₃-CAM shift were investigated in Sedum album and Sedum stoloniferum from Crassulaceae under water stress for 20 days. Leaf relative water content (RWC), leaf osmotic and water potential decreased with increasing water stress in both studied species. Significant reduction in dry matter production and leaf thickness was detected only in S. stoloniferum after 20-d water stress. Δtitratable acidity and phosphoenolpyruvate carboxylase (PEPC) activity in S. album responded to drought at early stages of stress treatment, continued to increase throughout the entire stress period and reached levels 15 times higher than those in well-watered plants. In S. stoloniferum, however, both parameters responded later and after a transient increase declined again. In S. stoloniferum, in spite of increase by drought stress, net night-time CO₂ assimilation was negative resembling a C₃-like pattern of gas exchange. Catalase (CAT), ascorbate peroxidase (APX), and superoxide dismutase (SOD) activities increased in plants subjected to mild water stress while declined as the stress became severe. Although malondialdehyde (MDA) content was higher in drought-stressed S. stoloniferum, the increase in the concentration of hydrogen peroxide (H2O₂) that may act as a signal for C₃-CAM transition was higher in S. album compared with S. stoloniferum. In drought-stressed plants, SOD activity showed a clear diurnal fluctuation that was more steadily expressed in S. album. In addition, such pattern was observed for CAT only in S. album. We concluded that temporal and diurnal fluctuation patterns in the activity of antioxidant enzymes depended on duration of drought stress and was related to the mode of photosynthesis and degree of CAM induction. According to our results, S. stoloniferum developed a low degree of CAM activity, e.g. CAM-cycling metabolism, under drought conditions.     

四川大头茶在不同群落中的遗传分化及适合度成分

研究了四川大头茶在 3个群落中的遗传分化和适合度成分。 12个引物的 RAPD分析表明 ,仅有 10 %左右的遗传多样性存在于种群间。四川大头茶纯林结实率 (31.86 % )显著高于针阔混交林 (2 2 .5 3% )和常绿阔叶林 (2 4 .5 2 % )。每果种子数和每果种子均重各种群间差异均显著 ,每果种子数和每果种子均重分别为常绿阔叶林 (2 9.5 0 ,0 .0 15 4 g) ,四川大头茶纯林 (2 8.39,0 .0 172 g) ,针阔混交林 (2 7.4 4 ,0 .0 195 g) ,表明二者间存在着负耦联关系 (trade off)。而以 R =∑lxbx表示的适合度却以常绿阔叶林最大     

Genetic variation in organisms with sexual and asexual reproduction

The genetic variation in a partially asexual organism is investigated by two models suited for different time scales. Only selectively neutral variation is considered. Model 1 shows, by the use of a coalescence argument, that three sexually derived individuals per generation are sufficient to give a population the same pattern of allelic variation as found in fully sexually reproducing organisms. With less than one sexual event every third generation, the characteristic pattern expected for asexual organisms appear, with strong allelic divergence between the gene copies in individuals. At intermediary levels of sexuality, a complex situation reigns. The pair-wise allelic divergence under partial sexuality exceeds, however, always the corresponding value under full sexuality. These results apply to large populations with stable reproductive systems. In a more general framework, Model 2 shows that a small number of sexual individuals per generation is sufficient to make an apparently asexual population highly genotypically variable. The time scale in terms of generations needed to produce this effect is given by the population size and the inverse of the rate of sexuality.     

Reproduction and development of the endangered Sedum integrifolium ssp. leedyi (Crassulaceae)

Information on reproduction and life history is important for the conservation of endangered plants. We investigated rates of flowering, seed set, and germination in populations of the endangered perennial plant Sedum integrifolium ssp. leedyi. Germination and flowering rates differed significantly among populations, but seed set rate did not. We assayed 26 plant clusters (81 stems) from four of the five known populations for evidence of clonal reproduction using 28 randomly amplified polymorphic DNA (RAPD) markers. Of the 81 stems, 75 had unique genotypes and three pairs had identical genotypes, suggesting that clonal reproduction is infrequent. Flowering, seed set, and germination rates were correlated with our estimates of ratios of effective to actual population sizes (Ne/N), but not with Ne. The single formally protected population may be experiencing inbreeding depression. We grew plants from seed to maturity in a greenhouse, with a germination rate of 77% and survival of 98% of the germinants at 6 mo, suggesting that this will be a viable means of ex situ propagation. Plants flowered 4-6 mo after germination and produced mature fruits 1-2 mo later, suggesting that they have the potential to sexually reproduce in their first or second season of growth.     

Genetic variation and clonal diversity in four clonal sedges (Carex) along the Arctic coast of Eurasia

We studied the structure of genetic variation (at both ramet- and genet-level) and clonal diversity within and among populations in the four closely related arctic clonal sedges Carex bigelowii, C. ensifolia, C. lugens and C. stans by use of allozyme markers. Compared to other sedges and arctic plants, the studied taxa all had high levels of genetic variation, both within populations and taxa. These taxa contained most of the total gene diversity (H(T)) within populations and a small part of the diversity among populations (G(ST) ranged 0.05--0.43). Carex bigelowii had genetic variation (H(S) = 0.173, mean for populations) at a comparable level to other outbreeding arctic plants and to other widespread, rhizomatous and mainly outbreeding Carex species. In contrast, C. ensifolia (H(S) = 0.335), C. lugens (H(S) = 0.339) and C. stans (H(S) = 0.294) had within-population variations that were higher than in most other studied Carex species and for arctic plants in general. Genetic variation was not related to any tested environmental variable, but it was lower in areas deglaciated only 10,000 years BP compared to areas deglaciated 60,000 years BP or not glaciated at all during the Weichselian. All the populations were multiclonal, except for two populations of C. stans that were monoclonal. In contrast to genetic variation, clonal diversity decreased with latitude and did not differ between areas with different times of deglaciation. In accordance with previous studies, C. bigelowii and C. lugens were found to be outbreeding, while C. ensifolia and C. stans had mixed mating systems.     

Living in isolation – population structure,reproduction, and genetic variation of the endangered plant species Dianthus gratianopolitanus (Cheddar pink)

The endangered plant species Dianthus gratianopolitanus exhibits a highly fragmented distribution range comprising many isolated populations. Based upon this pattern of distribution, we selected a study region in Switzerland with a lower magnitude of isolation (Swiss Jura) and another study region in Germany with a higher degree of isolation (Franconian Jura). In each region, we chose ten populations to analyze population structure, reproduction, and genetic variation in a comparative approach. Therefore, we determined population density, cushion size, and cushion density to analyze population structure, investigated reproductive traits, including number of flowers, capsules, and germination rate, and analyzed amplified fragment length polymorphisms to study genetic variation. Population and cushion density were credibly higher in German than in Swiss populations, whereas reproductive traits and genetic variation within populations were similar in both study regions. However, genetic variation among populations and isolation by distance were stronger in Germany than in Switzerland. Generally, cushion size and density as well as flower and capsule production increased with population size and density, whereas genetic variation decreased with population density. In contrast to our assumptions, we observed denser populations and cushions in the region with the higher magnitude of isolation, whereas reproductive traits and genetic variation within populations were comparable in both regions. This corroborates the assumption that stronger isolation must not necessarily result in the loss of fitness and genetic variation. Furthermore, it supports our conclusion that the protection of strongly isolated populations contributes essentially to the conservation of a species' full evolutionary potential.     

Distribution of bulbil- and seed-producing plants of Poa alpina (Poaceae) and their growth and reproduction in common gardens suggest adaptation to different elevations

• Premise of the study: The European Alps harbor a spatially heterogeneous environment. Plants can be adapted genetically to this heterogeneity but may also respond to it by phenotypic plasticity. We expected the important fodder grass Poa alpina to be adapted to elevation either genetically or plastically. • Methods: We investigated in three elevational common gardens whether growth and reproductive allocation of plants reproducing either by seeds or bulbils suggest adaptation to their elevation of origin and to what extent they can respond plastically to different elevations. Additionally, we analyzed genetic diversity using microsatellites and tested whether seeds are of sexual origin. • Key results: In the field, bulbil-producing plants occurred more often at higher elevations, whereas seed-producing plants occurred more often at lower elevations, but bulbil-producing plants were generally less vigorous in the common gardens. The response of plants to elevational transplantation was highly plastic, and vigor was always best at the highest location. The small genetic differences were not clinally related to elevation of origin, underlining the importance of phenotypic plasticity. Reproductive allocation was, however, independent of elevational treatments. Seed-producing plants had higher genetic diversity than the bulbil-producing plants even though we found that seed-producing plants were facultative apomicts mostly reproducing asexually. • Conclusions: Bulbil-producing P. alpina, showing a fitness cost at lower elevations compared with seed-producing plants, seem better adapted to higher elevations. By means of its two reproductive modes and the capacity to adjust plastically, P. alpina is able to occupy a broad ecological niche across a large elevational range.     

Isozyme variation in Calliandra calothyrsus (Leguminosae): its implications for species delimitation and conservation

Patterns of genetic diversity within and among populations of Calliandra calothyrsus, an important multipurpose tree species, were examined using isozyme analysis. C. calothyrsus is a widespread species distributed throughout Central America and southern Mexico, across a variety of environments. Morphologically and ecologically distinct populations can be identified within this range, but they are currently considered to represent a single species. C. calothyrsus has been introduced to many parts of the tropics, where it is cultivated as a source of fuelwood, animal fodder, green manure, and shade by rural communities. Some of these introductions are known to have originated from Guatemala, but very little is known about the genetic diversity of either the native or naturalized populations. Isozyme electrophoresis of 23 loci across 17 populations of C. calothyrsus indicated that the majority of genetic diversity was partitioned between populations (FST = 0.802) and that within-population heterozygosity was low (mean Ho = 0.057). Naturalized populations had lower than expected heterozygosities and were most similar to material from Santa Maria de Jesus, a natural population in southern Guatemala. Four distinct groups of populations were identified on the basis of Nei's genetic distances and Population Aggregation Analysis (PAA), and correlate with the morphological and ecological differences that can be observed within the species. The results are discussed in relation to species delimitation and conservation.     

Comparative ecophysiology of five species of Sedum (Crassulaceae) under well-watered and drought-stressed conditions

Summary Gas exchange patterns, diurnal malic acid fluctuations, and stable carbon isotope ratios of five species of Sedum were investigated to assess the ecophysiological characteristics of three different photosynthetic pathways under well-watered and drought-stressed conditions. All five species have succulent leaves and stems and were examined under identical environmental conditions. When well-watered, Sedum integrifolium (Raf.) Nels. and S. ternatum Michx. displayed C₃ photosynthesis, S. telephioides Michx. and S. nuttallianum Raf. exhibited CAM-cycling, and S. wrightii A. Gray showed CAM. When grown under a less frequent watering regime, S. integrifolium and S. ternatum exhibited CAM-cycling, whereas S. telephioides and S. nuttallianum displayed CAM-cycling simultaneously with low-level CAM. Sedum wrightii retained its CAM mode of photosynthesis. In general, leaf ¹³C values reflected these variations in photosynthetic pathways. While all values of water-use efficiency (WUE) were greater than those reported for most C₃ and C₄ species, no correlation of malic acid accumulation in the CAM and CAM-cycling (including low-level CAM) species with increased WUE was found. Sedum wrightii (CAM) had the highest WUE value at night, yet its 24-h WUE was not different from S. ternatum when the latter was in the C₃ mode. Thus, relative water-use efficiencies of these species of Sedum were not predictable based on photosynthetic pathways alone.     

Disturbance by mowing affects clonal diversity: the genetic structure of Ranunculus ficaria (Ranunculuaceae) in meadows and forests

To study the impact of disturbance by mowing on clonal variation, we compared the genetic structure of Ranunculus ficaria (Ranunculaceae) in meadows and forests located in southeast Germany. We applied random amplified polymorphic DNA (RAPD) analysis to investigate the clonal and genetic diversity and analysed a total of 117 samples from three study plots in each habitat type. Polymerase chain reaction with six primers resulted in 57 fragments. Clonal diversity differed clearly between the two analysed habitat types and was significantly higher in the study plots from meadows than in those from forests. The mean percentage of distinguishable genotypes (PD) was 0.80 in meadow plots and 0.36 in forest plots, and the detected genets were smaller in meadow plots than in forest plots. Mean genetic diversity measured as percentage of polymorphic bands, Shannon’s information index and Nei’s gene diversity was also higher in meadows (44.4, 0.22 and 0.14) than in forests (25.1, 0.09 and 0.05). The higher level of clonal diversity in meadow plots is most likely due to the effects of disturbance by mowing, which increases the dispersal of bulbils and promotes the establishment of new plants in meadows compared to forests.     

Correlated evolution of self-incompatibility and clonal reproduction in Solanum (Solanaceae)

It has been suggested that clonality provides reproductive assurance in cross-fertilizing species subject to pollen limitation, relieving one of the main selective pressures favoring the evolution of self-fertilization. According to this hypothesis, cross-fertilizing species subject to pollen limitation should often be clonal. Here, we investigated the association between clonality and a genetic mechanism enforcing outcrossing, self-incompatibility, in Solanum (Solanaceae). We collected self-incompatibility and clonality information on 87 species, and looked for an association between these two traits. To account for the contribution of shared evolutionary history to this association, we incorporated phylogenetic information from chloroplast (NADH dehydrogenase subunit F) sequence data. We found that self-incompatibility is strongly associated with clonal reproduction: all self-incompatible species reproduce clonally, while the absence of clonality is widespread among self-compatible taxa. The observed correlation persists after taking into account shared phylogenetic history, assumptions about the evolutionary history of self-incompatibility, uncertainty associated with phylogeny estimation, and associations with life history (annual/perennial). Our results are consistent with the hypothesis that clonality provides reproductive assurance, and suggest that the consequences of clonal growth in the evolution of plant reproductive strategies may be more significant than previously thought.     

Genetic variation and evolutionary trade-offs for sexual and asexual reproductive modes in Allium vineale (Liliaceae)

Populations of Allium vineale commonly include individuals with very different allocation patterns to three modes of reproduction: sexual flowers, aerially produced asexual bulbils, and belowground asexual offsets. If selection is currently acting to maintain these different allocation patterns there must be a genetic basis for variation in allocation to these three reproductive modes. In addition, negative genetic correlations between reproductive traits would imply evolutionary trade-offs among reproductive strategies. We evaluated the heritability of these allocation patterns by growing 16 clones from a single population in the greenhouse at two levels of fertilization. Bulb mass and the mass and number of bulbils, offsets, and flowers were used as response variables, in addition to the proportion allocated to each reproductive mode. We found evidence of substantial heritable variation in allocation to sexual reproduction and in allocation within the two modes of asexual reproduction, indicating high sensitivity of these allocation patterns to natural selection. We also found evidence of strong negative genetic correlations between bulbil and flower traits, as well as between bulbil and offset traits, with one group of genotypes allocating greater resources to aerial asexual bulbils and the second group allocating more resources to belowground asexual offsets and aerial flowers. Phenotypic plasticity in allocation to above- vs. belowground asexual reproduction and sexual vs. asexual aerial reproduction was limited, indicating that plants are unlikely to change reproductive mode in response to nutrient availability. Together, then, we have demonstrated strong heritability for, and trade-offs in, the reproductive allocation patterns within this plant population.     

Quantitative proteomics analysis of leaves from two Sedum alfredii (Crassulaceae) populations that differ in cadmium accumulation

Due to their extraordinary capacity to hypertolerate and hyperaccumulate heavy metals in above‐ground tissues, hyperaccumulator plant species have gained wide attention from researchers seeking biotechnologies to manage environmental heavy metal pollution. However, the molecular basis of hyperaccumulation is still far from being fully understood. Here, we used iTRAQ to perform a quantitative proteomics study of the leaves of Sedum alfredii (Crassulaceae) from hyperaccumulating population (HP) and non‐hyperaccumulating population (NHP). A total of 248 proteins had constitutively higher levels in HP leaves than in NHP leaves. Cadmium (Cd) treatment led to the induction of 13 proteins in HP leaves and 33 proteins in NHP leaves. Two proteins were induced by Cd in both HP leaves and NHP leaves. The annotations for many of the proteins that were higher in HP leaves and proteins that were induced by Cd treatments were associated with vacuolar sequestration, cell wall/membrane modification, and plant defense. In addition to establishing a global empirical foundation for the study of proteins in S. alfredii, our findings relating to the differential constitutive and inducible expression of proteins open potential new research avenues and bolster previously reported suppositions about Cd hyperaccumulation in hyperaccumulator plants.     

真鲷野生群体和人工繁殖群体的同酶遗传差异

采用水平淀粉凝胶电泳技术对采集于黄海海州湾海域的真鲷野生群体和经过 2代人工繁育的养殖群体进行了同工酶遗传变异研究。分析检测了 2个群体各 50个样本肌肉和肝脏组织的 1 3种同工酶共 2 0个基因位点 ,其中MDH_a、GPI、PGM等 1 0个位点为多态位点 ,ME和EST_b为变异程度比较高的位点。野生群体和人工繁殖群体的多态位点比例分别为 45%和 2 5% (P0 .95) ;群体平均观察杂合度分别为 0 .1 41± 0 .0 4 4和 0 .0 95± 0 .0 4 3。结果表明 ,真鲷的野生群体和养殖群体拥有较高程度的遗传变异水平 ,但是养殖群体的遗传变异水平比野生群体有一定程度的降低。养殖群体遗传变异水平的降低在一定程度上是由于亲鱼数量少所致。比较了同工酶分析和RAPD分析的结果。将此两种技术相结合在鱼类群体遗传多样性分析中具有比较实际的意义。     

Isozyme variation and phylogenetic relationships in Vicia subgenus Cracca (Fabaceae)

BACKGROUND AND AIMS: The phylogenetic relationships among 27 vetch species belonging to the subgenus Cracca of the genus Vicia were studied in comparison with three species of Lathyrus section Lathyrus on the basis of isozyme variation. METHODS: Isozymes encoded by 15 putative loci of ten enzymes were resolved by polyacrylamide gel electrophoresis and isozyme variation was analysed by using parsimony and neighbour-joining methods. KEY RESULTS: The analyses revealed 63 parsimony-informative and 36 species-specific orthozymes. Of the latter, 23 are monomophic and are suitable for identification of V. benghalensis, V. palaestina, V. dumetorum, V. pisiformis, V. sylvatica, V. onobrychioides, V. cappadocica, V. cretica, V. articulata, V. tetrasperma, V. ervilia, V. hirsuta and V. loiseleurii. Polymorphism with heterozygous and homozygous isozyme genotypes was found for V. cracca, V. tenuifolia, V. ochroleuca, V. villosa, V. sylvatica, V. cassubica, V. sparsiflora, V. megalotropis, V. altissima, V. onobrychioides, V. cassia, V. cretica and L. heterophyllus, reflecting outcrossing in these species. By contrast, V. benghalensis, V. palaestina, V. disperma, V. dumetorum, V. pisiformis, V. orobus, V. pauciflora, V. tetrasperma and V. loiseleurii had only homozygous isozyme genotypes at polymorphic loci. Isozyme-based phylogenetic trees are presented. CONCLUSIONS: Sections Cracca, Ervum, Pedunculatae and Lenticula of traditional taxonomy are monophyletic groups, whereas sections Oroboideae (= Vicilla) and Panduratae appear polyphyletic and section Cassubicae is split into two species-couples linked at a low level of support. Treatment of ervoid species in a separate subgenus Ervum is not supported because of its polyphyly.     

Allozyme and shell variation in two marine snails (Littorina, Prosobranchia) with different dispersal abilities

As adults Littorina littorea and L. saxatilis have overlapping distributions, both on the shore and on a geographical scale, this implies that they are affected by similar selective forces to a large extent. The two species have, however, different reproductive strategies. Littorina littorea has a planktonic larval stage lasting 4–6 weeks, whereas L. saxatilis has direct development. This influences dispersal rates of the two species, and gene flow and effective population size are both assumed to be much smaller in L. saxatilis than in L. littorea. Intraspeciflc variation in morphological and allozyme characters differs between the two species. Littorina littorea is homogeneous for both types of characters, both within and between populations, while populations of L. saxatilis may show pronounced differences in shell characters with micro-environment changes, and allozyme variation is markedly higher in this species too. The polymorphism in shell characters of L. saxatilis and the monomorphism of L. littorea, may be expected from theory as a consequence of the different dispersal strategies. Also the lack of divergence between geographically separate populations of L. littorea, but the high degree of between population differentiation in L. saxatilis, could be attributed to the high and low dispersal rates, respectively. However, the larger allozyme variation within subpopulations of L. saxatilis compared to that of L. littorea, is surprising as the former species has a smaller effective population size than the latter. Furthermore, there is no correlation between dispersal ability and overall allozyme heterozygosity, when 10 additional species of Littorina are included in the comparison. It is suggested that different evolutionary backgrounds of different species will possibly explain this, as neither selectionist nor neutralist models alone account for the observation. That is, an earlier passage through a population bottleneck may, for example, influence the genetic variation for a long period of time.     

Photosynthetic pathways in a midwestern rock outcrop succulent,Sedum nuttallianum Raf. (Crassulaceae)

Shoots of Sedum nuttallianum exhibited CAM^* acid fluctuations in the field. These nocturnal acid accumulations persisted in the laboratory under well-watered and water-stressed conditions. Simultaneous measurements of transpiration, however, indicated daytime stomatal opening and nocturnal stomatal closure. Measurements of CO₂ and H₂O vapor exchange continuously for six days after watering substantiated these results in part: the majority of CO₂ uptake occurred during the day early in the experiment; however, after several days without water, nighttime CO₂ uptake was stimulated and eventually was greater than the drastically reduced daytime CO₂ uptake. This nighttime uptake was never quite sufficient to account for all estimated increases in tissue acidity. Thus, a combination of CAM and CAM-cycling occurred early in the desiccation experiment. Evidence for CAM and a form of CAM-idling was found later in the experiment. Though nighttime CO₂ uptake occurred and persisted after only one day without water, rates were too low to alter the tissue ¹³C/¹²C value from a C₃-like number (–30). Thus, although CAM and CAM-idling may have survival value during extended droughts, shoots of S. nuttallianum apparently utilize the C₃ pathway to obtain most of their carbon.Abbreviations C₃ pathway - CO₂ fixation pathway in which an intermediate containing 3 carbon atoms is formed - CAM Crassulacean acid metabolism - Chl Chlorophyll - c_i internal CO₂ concentration - DW Dry weight - g_c mean conductance to CO₂ - FW Fresh weight - PAR Photosynthetically active radiation - SD Standard deviation - vpd Vapor pressure deficit - WUE Water use efficiency