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Epidermal hydathodes were found on leaves of 46 of 48 species of Crassula collected from the Namib Desert in southern Africa. The possibility that these structures might allow the absorption of surface water was investigated in 27 species (including subspecies). The presence of hydathodes on leaf epidermi correlated, in most cases, with increases in leaf thickness and enhanced rates of nocturnal, and sometimes diurnal, CO2 uptake following wetting of the leaves during the night. The precise nature of these responses varied depending on the species. In addition, wetting only the older leaves on the lower portion of the shoot of C. tetragona ssp. acutifolia not only resulted in increased thickness of these leaves, but also effected an increase in leaf thickness and stimulation of CO2 uptake rates in the distal, younger portion of the shoot that was not wetted. Overall, foliar hydathodes were implicated in the absorption of surface water in many species of Crassula such that the ecophysiology of these desert succulents was positively affected. Although rainfall in the Namib Desert is infrequent, surface wetting of the leaves is a more common occurrence as a result of nighttime dew or fog deposition. Presumably, species with hydathodes benefit directly from this source of moisture. These findings have important implications in understanding a relatively unexplored adaptation of some xerophytes to an extremely arid environment.

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Past studies have reported two unusual aspects of the water relations of the atmospheric CAM epiphyte Tillandsia usneoides L. (Bromeliaceae): a drought stimulation of nocturnal C0 2 uptake, and nocturnal absorption of water vapor. Contrary to past reports, a 10-d drought did not stimulate nocturnal C0 2 uptake in this species. On the other hand, previous reports of nocturnal water vapor absorption were confirmed in situ throughout a year, although tissue hydration from this source was insufficient to offset daytime water loss. Deposition of dew on the plants was never observed in the field. It is hypothesized that the unusual nature of the water relations of T. usneoides is attributable to the interactions between two "pools" of water and the external atmosphere. The dense indumentum of trichomes obscuring the surface of this epiphyte comprises one pool and is most likely responsible for rapid hydration early in the night and dehydration early in the day. In addition, stomata control water loss from the living mesophyll cells, the second pool, for the remainder of the night. The high rates of water loss observed throughout the day when stomata are closed probably result from leakage through the trichomes.

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 C 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 δC values reflected these variations in photosynthetic pathways. While all values of water-use efficiency (WUE) were greater than those reported for most C and C 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 C mode. Thus, relative water-use efficiencies of these species of Sedum were not predictable based on photosynthetic pathways alone.

Although physiological responses to drought have been examined in several species of epiphytic bromeliads, few have included a comprehensive methodological approach to the study of the carbon and water relations of a single species undergoing drought stress. Thus, physiological and anatomical responses to an imposed drought treatment were examined in the atmospheric Crassulacean acid metabolism (CAM) epiphyte Tillandsia ionantha. From 0 through 20 d without water, nocturnal malic acid accumulation and C0 2 uptake rates did not change despite a 17% reduction in relative water content. In addition, water potentials averaged -0.40 MPa and, unlike leaf water content, did not decline. The avoidance of further declines in leaf water content was attributed to the restriction of stomatal opening to the night (a characteristic feature of CAM), to low stomatal densities and small stomatal pores, and to a thick boundary layer resulting from a dense foliar trichome cover. The maintenance of high physiological activity during the first 20 d of the drought treatment was most likely a result of the high water potentials in the chlorenchyma, which were attributed, in part, to water movement from the water-storage parenchyma (= "hydrenchyma") to the chlorenchyma. Nocturnal malic acid accumulation and the rate of net C0 2 exchange declined in a linear fashion from 30 to 60 d without water, as did leaf water potential and osmotic potential. During this time, C0 2 recycling increased from ca. 20% to nearly 75%. Though declining throughout this later stage of the drought treatment, metabolic activity remained relatively high, possibly as a result of the observed osmotic adjustment as well as a potentially high cell wall elasticity.

The potential for Crassulacean acid metabolism (CAM) was investigated in the sandstone outcrop succulent Talinum calycinum in central Kansas. Field studies revealed CAM-like diurnal acid fluctuations in these plants. These fluctuations persisted under all moisture and temperature regimes in the laboratory. Despite this CAM-like acid metabolism, simultaneous gravimetric determinations of day-and nighttime transpiration rates indicated the presence of a C3 gas exchange pattern. Subsequent analyses of diurnal CO2 and H20 exchange patterns under wellwatered conditions and after 3, 5, and 7 days of drought confirmed these findings, though low rates of nocturnal CO2 uptake were observed on the fifth night after continuous drought. Finally, the 5'3C/'2C value of this succulent, -27.8%o, emphasizes the insignificance of any nocturnal C02 uptake in the lifelong accumulation of carbon in this species. Thus, it is proposed that T. calycinum is a C3 plant with some CAM charcteristics, including the ability to re-fix respiratory CO2 at night under all moisture regimes, potentially resulting in a conservation of carbon, and occasionally to fix atmospheric CO2 at night. These findings may prove to be common among rock outcrop succulents.

The heterophyllous epiphyte Tillandsia deppeana exhibits an atmospheric habit as a juvenile and a tank form as an adult. Both juveniles and adults utilize C photosynthesis. This is the first report of an atmospheric form of Tillandsia which does not exhibit CAM. Photosynthetic saturation occurred at approximately 10% of full sunlight in both forms, but the adults exhibited greater rates of photosynthesis at all levels of irradiance. The adults also had a higher and broader photosynthetic temperature optimum than did the juveniles. The adults transpired at greater rates than the juveniles; however, the water use efficiencies of both forms were similar and were high for C plants. In both forms the photosynthetic rate decreased in response to a decrease in humidity. After 8 days without water the juveniles were able to fix CO throughout the day. The adults, however, exhibited a net loss of CO on the second day without water and thereafter. These results indicate that the water-conservative atmospheric juvenile of T. deppeana is well adapted to establishment in the epiphytic habitat.