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Zamioculcas zamiifolia (Araceae), a terrestrial East African aroid, with two defining attributes of crassulacean acid metabolism (CAM) (net C[O.sub.2] uptake in the dark and diel fluctuations of titratable acidity) is the only CAM plant described within the Araceae, a mainly tropical taxon that contains the second largest number of epiphytes of any vascular plant family. Within the Alismatales, the order to which the Araceae belong, Z. zamiifolia is the only documented nonaquatic CAM species. Zamioculcas zamiifolia has weak CAM that is upregulated in response to water stress. In well-watered plants, day-night fluctuations in titratable acidity were 2.5 [micro]mol [H.sup.+]*[(g fresh mass).sup.-1], and net C[O.sub.2], uptake in the dark contributed less than 1% to daily carbon gain. Following 10 d of water stress, net C[O.sub.2] uptake in the light fell 94% and net C[O.sub.2] uptake in the dark increased 7.5-fold, such that its contribution increased to 19% of daily carbon gain. Following rewatering, dark C[O.sub.2] uptake returned to within 5% of prestressed levels. We postulate that CAM assists survival of Z. zamiifolia by reducing water loss and maintaining carbon gain during seasonal droughts characteristic of its natural habitat. Key words: Araceae; C[O.sub.2] exchange; crassulacean acid metabolism; drought stress; photosynthesis; Zamioculcas.

Vertical stratification of epiphytes generally has not been reported for dry forests. For two epiphytic Crassulacean acid metabolism bromeliads that segregate vertically, it was hypothesized that different potentials for photoprotection or shade tolerance rather than drought tolerance is responsible for the observed stratification. The light environment, capacity for photoprotection, germination response to light quality, and responses to light and drought were thus examined for Tillandsia brachycaulos and T. elongata. Vertical and light-environment distributions differed for the two species but photoprotection and photodamage did not where they occurred at similar field locations; T. brachycaulos had a higher pigment acclimation to light. Tillandsia brachycaulos had higher acid accumulation under low light as opposed to T. elongata, which responded similarly to all but the highest light treatment. Tillandsia brachycaulos maintained positive total daily net C[O.sub.2] uptake through 30 d of drought; T elongata had a total daily net C[O.sub.2] loss after 7 d of drought. The vertical stratification was most likely the result of the sensitivity to drought of T. elongata rather than differences in photoprotection or shade tolerance between the two species. Tillandsia elongata occurs in more exposed locations, which may be advantageous for rainfall interception and dew formation. Key words: Crassulacean acid metabolism; dew; drought; photoprotection; threatened species; Tillandsia; vertical distribution; Yucatan, Mexico.

Loss of axial hydraulic conductance as a result of xylem cavitation was examined for roots of the Crassulacean acid metabolism (CAM) succulents Agave deserti and Opuntia ficus-indica. Vulnerability to cavitation was not correlated with either root size or vessel diameter. Agave deserti had a mean cavitation pressure of -0.93 +/- 0.08 MPa by both an air-injection and a centrifugal method compared to -0.70 +/- 0.02 MPa by the centrifugal method for O. ficus-indica, reflecting the greater tolerance of the former species to low water potentials in its native habitat. Substantial xylem cavitation would occur at a soil water potential of -0.25 MPa, resulting in a predicted 22% loss of conductance for A. deserti and 32% for O. ficus-indica. For an extended drought of 3 mo, further cavitation could cause a 69% loss of conductance for A. deserti and 62% for O. ficus-indica. A model of axial hydraulic flow based upon the cavitation response of these species predicted that water uptake rates are far below the maximum possible, owing to the high root water potentials of these desert succulents. Despite various shoot adaptations to aridity, roots of A. deserti and O. ficus-indica are highly vulnerable to cavitation, which partially limits water uptake in a wet soil but helps reduce water loss to a drying soil. Key words: Agave deserti; embolism; hydraulic conductance; Opuntia ficus-indica; vessel anatomy; water relations; xylem physiology.

To investigate root distribution with depth, which can affect competition for water, surface areas of young and old roots were determined in 4-cm-thick soil layers for the [C.sub.3] subshrub Encelia farinosa Torrey and A. Gray, the [C.sub.4] bunchgrass Pleuraphis rigida Thurber, and the CAM (crassulacean acid metabolism) leaf succulent Agave deserti Engelm. At a site in the northwestern Sonoran Desert these codominant perennials had mean rooting depths of only 9-10 cm for isolated plants. Young roots had mean depths of 5-6 cm after a winter wet period, but 11-13 cm after a summer wet period. Young roots were most profuse in the winter for E. farinosa, which has the lowest optimum temperature for root growth, and in the summer for P. rigida, which has the highest optimum temperature. Roots for interspecific pairs in close proximity averaged 2-3 cm shallower for A. deserti and a similar distance deeper for the other two species compared with isolated plants, suggesting partial spatial separation of their root niches when the plants are in a competitive situation. For plants with a similar root surface area, the twofold greater leaf area and twofold higher maximal transpiration rate of E. farinosa were consistent with its higher root hydraulic conductivity, leading to a fourfold higher estimated maximal water uptake rate than for P. rigida. Continuous water uptake accounted for the shoot water loss by A. deserti, which has a high shoot water-storage capacity. A lower minimum leaf water potential for P. rigida than for A. deserti indicates greater ability to extract water from a drying soil, suggesting that temporal niche separation for water uptake also occurs. Key words: Agave deserti; crassulacean acid metabolism; Encelia farinosa; Pleuraphis rigida; root growth; Sonoran Desert; water potential.

We used carbon stable-isotope techniques to test the hypothesis that the pallid bat, Antrozous pallidus (Chiroptera: Vespertilionidae), obtains substantial amounts of carbon from plants of the Cactaceae and Agavaceae, which employ the Crassulacean Acid Metabolism (CAM) photosynthetic pathway, in areas where there are bat-adapted flowers of cacti and agaves. Our analysis revealed significant geographic variation in carbon composition of the pallid bat during the blooming periods of bat-visited cacti and agaves. Although the pallid bat probably visits flowers to eat insects, it is a legitimate pollinator of bat-adapted CAM plants and may represent an early stage in the evolution of nectarivory and frugivory in the Vespertilionidae.

Plants (N = 102) from 23 populations of Talinum, representing five morphologically similar species, were examined in a test of the hypothesis that ploidal level may be correlated with levels of Crassulacean acid metabolism. When grouped by species and ploidal level, two groups of tetraploid plants, T. calcaricum and tetraploid individuals of T. calycinum, had significantly greater malic acid accumulations than any other group, including diploid individuals of T. calycinum. Another tetraploid, T. teretifolium, did not show greater malic acid fluctuations than the diploids, including two putative ancestors (T. mengesii and T. parviflorum). Similarly, malic acid fluctuations in tetraploid individuals of T. parviflorum were not different from diploid individuals of the same species. Nonetheless, when all plants were compared, nocturnal accumulations of malic acid in tetraploid plants were significantly greater than those of diploid plants.

In full sunlight (photosynthetic photon flux density, 0.4-0.7 micrometer, PPFD), stomatal conductance to water vapor (g) and transpiration rates (E) are usually higher, and water use efficiency lower, in C3 than C4 species. In the C3 grass Bromus inermis, the difference in conductance between full sun (photosynthetic photon flux (PPFD) > 1500 microns-mol center dot m to the -2 center dot s to the -1) and shade (PPFD approximately equal to 400 microns-mol center dot m to the -2 center dot s to the -1) was twice as large as in the C4 species Sorghastrum nutans (226.7 vs. 97.8 mmol center dot m to the -2 center dot s to the -1, respectively). Similar differences were measured in four additional C3 and C4 grasses. Variability in PPFD incident on leaves is very common in grasslands, and stomata are known to respond relatively rapidly to fluctuations in this resource. This study evaluated the consequences of different responses in stomatal conductance between C3 and C4 grasses during periods of sunlight variability. Photosynthesis (A, measured as CO2 uptake) and g varied substantially in both B. inermis and S. nutans as PPFD fluctuated between sun and shade levels (4-10 min intervals). Time constants derived from first-order equations describing responses in g after abrupt changes in PPFD indicated that g approached steady-state levels more rapidly in S. nutans than in B. inermis. However, because of the greater magnitude of change in g between sun and shade PPFD in B. inermis, the absolute rate of change in g in this C3 grass was twice as rapid compared to S. nutans (25.7 vs. 12.4 mmol center dot m to the -2 center dot s to the -1 center dot min to the -1 for reductions in g, respectively). New steady-state levels in g were achieved in 12-13 min after a change in PPFD in S. nutans and 15-19 min in B. inermis. A statistically based two-state (sun/shade) simulation model was used to quantify costs in A and E of the nonsteady-state responses measured for both species as PPFD varied. Costs were calculated as the difference between model output of A or E based on field responses and model output based on hypothetical gas exchange responses in which A and g changed instantaneously. Results suggested that even though the absolute rate of change in g was greater in the C3 grass, costs in E during periods of short-term (2-6 min) variations in PPFD were as much as sixfold higher than in the C4 species. Lower cost in E in C4 species during periods of variable PPFD (due to inherently smaller alterations in g) is a previously unrecognized consequence of the C4 photosynthetic pathway. C4 species, and grasses in particular, often are most successful in high-PPFD, water-limited environments in which leaf-level variability in PPFD occurs daily from cloud cover and within-canopy shading. In these environments, stomatal responses that conserve water under nonsteady-state conditions may contribute significantly to the success of C4 species.

Seasonal pools undergo marked diurnal changes in pH, free carbon dioxide, and oxygen levels. Previous studies showed that Isoetes howellii utilized crassulacean acid metabolism (CAM) photosynthesis as a means of assimilating carbon at night when ambient carbon dioxide levels are high. However, much of the pool flora is not CAM. We hypothesized that coexistence under extreme carbon-limiting conditions would select for other photosynthetic characteristics in these non-CAM species. Quantitative carbon uptake measurements were made in the field on the CAM Isoetes howellii and the non-CAM Eleocharis acicularis, Downingia bella, and Plagiobothrys undulatus. Despite wide phylogenetic separation, these species are structurally convergent in that they all produce an aquatic stage with an isoetid growth form. They are functionally similar in that none appear to utilize bicarbonate, and all are carbon limited through much of the day, as indicated by a marked midday depression in carbon uptake that lasts through the afternoon. All four species were capable of carbon fixation at night, although only in Isoetes howellii was this accompanied by an overnight accumulation of acid. It was estimated that dark fixation in I. howellii contributed up to 40% of its total carbon gain under submerged conditions, but was [is less than]1% under aerial conditions. The role of dark fixation in the other three, non-CAM, species is unknown. Short-term (1 s) steady-state [sup.14.C] tracer studies in the laboratory revealed that, under submerged conditions in the light, all species assimilated carbon into the [C.sub.3] photosynthetic product phosphoglycerate (PGA) and the [C.sub.4] photosynthetic products of malate, citrate, and aspartate. The proportion of label fixed into [C.sub.4] organic acids was greatest in E. acicularis. The ratio of RuBP carboxylase/PEP carboxylase was broadly similar in all species, ranging from 8.8 to 11.5. When the pools dried down and the plants became aerial, photosynthetic rates increased and a midday depression such as occurred under submergence was not observed. Carbon fixation in the dark became negligible, and the RuBP carboxylase/PEP carboxylase ratio increased. Seasonal changes in biomass showed I. howellii and E. acicularis dominated the pools early in the season, and the biomass of the other two species increased later when the pools dried. The latter species, D. bella and P. undulatus, also had higher rates of photosynthesis under aerial conditions.

Leaf water becomes enriched in the heavy isotopes of oxygen and hydrogen during evapotranspiration. The magnitude of the enrichment has been shown to be influenced by temperature and humidity, but the effects of species-specific factors on leaf water enrichment of D and [sup.18.O] have not been studied for different plants growing together. Accordingly, to learn whether leaf water enrichment patterns and processes for D and [sup.18.O] are different for individual species growing under the same environmental conditions we tested the proposal that leaf waters in plants with crassulacean acid metabolism (CAM) show higher slopes (m in the leaf water equation [delta]D = m [[delta].sup.18.O] + b) than in [C.sub.3] plants. We determined the relationships between the stable hydrogen ([delta]D) and oxygen ([[delta].sup.18.O])isotope ratios of leaf waters collected during the diurnal cycle of evapotranspiration for Yucca schidigera, Ephedra aspera, Agave deserti, Prunus ilicifolia, Yucca whipplei, Heteromeles arbutifolia, Dyckia fosteriana, Simmondsia chinensis, and Encelia farinosa growing at two sites in southern California. Slopes (m in the above leaf water equation) ranged from 1.50 to 3.21, compared to [is nearly equal]8 for meteoric water, but differences in slope could not be attributed to carboxylation pathway (CAM vs. [C.sub.3]) nor climate (coastal California vs. Sonoran Desert). Higher slopes were correlated with greater overall ranges of leaf water enrichment of D and [sup.18.O]. Water in plants with higher slopes also differed most from unaltered meteoric water. Leaf water isotope ratios in plants with lower slopes were better correlated with temperature and humidity. The findings indicate that m in the aforementioned equation is related to the overall residence time for water in the leaf and proportions of water subjected to repeated evapotranspiration enrichments of heavy isotopes.

C4 plants may dominate high altitude grasslands due to their resistance to droughts rather than due to their tolerance to low CO2 levels. C4 photosynthetic mechanism enables these plants to trap CO2 and, thereby, to control water balance. The population of C4 plants is predominant at low latitudes and is low at the poles.

Crassulacean acid metabolism (CAM) is an adaptation of certain plants, such as cacti, to arid and water-stressed environments. CAM is characterized by nocturnal uptake of CO.sub.2 through open stomata, carboxylation [...]