Craig S. Cook

Seasonal carbon isotope discrimination in a grassland community

SummaryGrassland communities of arid western North America are often characterized by a seasonal increase in ambient temperature and evaporative demand and a corresponding decline in soil moisture availability. As the environment changes, particular species could respond differently, which should be reflected in a number of physiological processes. Carbon isotope discrimination varies during photosynthetic activity as a function of both stomatal aperture and the biochemistry of the fixation process, and provides an integrated measure of plant response to seasonal changes in the environment. We measured the seasonal course of carbon isotope discrimination in 42 grassland species to evaluate changes in gas exchange processes in response to these varying environmental factors. The seasonal courses were then used to identify community-wide patterns associated with life form, with phenology and with differences between grasses and forbs. Significant differences were detected in the following comparisons: (1) Carbon isotope discrimination decreased throughout the growing season; (2) perennial species discriminated less than annual species; (3) grasses discriminated less than forbs; and (4) early flowering species discriminated more than the later flowering ones. These comparisons suggested that (1) species active only during the initial, less stressful months of the growing season used water less efficiently, and (2) that physiological responses increasing the ratio of carbon fixed to water lost were common in these grassland species, and were correlated with the increase in evaporative demand and the decrease in soil moisture.

Geo-location of heroin and cocaine by stable isotope ratios

Analyses of the carbon and nitrogen stable isotope ratios in heroin and cocaine samples obtained from different geographic regions indicated stable isotope ratio combinations that were strongly correlated with geographic location. Further analyses of the isotope ratios of morphine derived from the deacetylation of heroin exhibited more pronounced isotopic differences among regions, increasing its potential as a tool for geo-location and for sample-to-sample comparison.

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

SummaryThe 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.

Photosynthetic characteristics of Sonoran Desert winter annuals

SummaryPhotosynthesis in Sonoran Desert winter annuals appeared to be similar to those observed in other C3 photosynthetic pathway herbs, although photosynthetic capacities ranged from 18 to 65 μmol CO1 m-2 s-1 under natural conditions. The higher photosynthetic capacities were associated with high leaf conductances to water vapor (up to 39 mm s-1). Leaf Kjeldahl nitrogen contents were high, ranging up to 44.9 mg g-1. We suggest that the high photosynthetic capacities in several species may be related to resource availability and enable successful exploitation of the short, unpredictable growth periods experienced by these annuals. Although photosynthetic rates in desert winter annuals spanned a wide range, the relationship between leaf conductance and maximum photosynthesis appeared simiar to that of other C3 vascular plants. It is possible that the resulting constant intercellular, CO2 concentrations were related to minimizing excessive water loss, while not severely imposing limitations to photosynthetic gains.

FIELD WATER RELATIONS OF SONORAN DESERT ANNUALS

Field water relations were examined in winter and summer annuals of the Sonoran Desert. Both groups were characterized by low belowground biomass allocation. Winter annuals had large interspecific variation in water relations parameters, but their leaves generally had high con- ductances of water vapor and high water potentials during early growth periods. These high values were very short-lived and were reduced by peak flowering periods. Summer annuals were measured midway through the growing season; their leaves showed less interspecific variability in water vapor conductances and had values similar to the winter annuals for this stage of the growing season. Midday and diurnal courses of photosynthesis in the winter annuals exhibited lower rates than values previously reported for plants grown under unlimited water and nutrient regimes in glasshouses. This was probably due to the decreased leaf water potentials and increased vapor pressure deficits present in field situ- ations. Both the winter and the summer annuals showed a dichotomy among species in leaf water potentials, associated with either variations in soil water availability or the ability of leaves to adjust osmotically to decreased soil water availability. Although winter and summer annuals are ephemeral, some species are capable of tolerating low leaf water potentials and therefore are not drought evading in the traditional sense.

Decreased mortality from necrotizing pancreatitis

BACKGROUND Necrotizing pancreatitis has been associated with mortality rates of 25% to 80%. We reviewed our experience to determine whether aggressive debridement and comprehensive critical care improves survival. METHODS The records of 989 patients with the diagnosis of pancreatitis admitted between January 1990 and September 1997 were retrospectively reviewed. Twenty-six patients required surgery for necrotizing pancreatitis and are the subjects of this review. RESULTS Five of twenty-six patients (19%) died. For all patients, mean Ransons score was 4.3 of 11, mean admission APACHE II score was 17.2, and mean Multiple Organ Dysfunction (MOD) score was 9.1. Poor outcome was associated with infected pancreatic necrosis (P = 0.03), elevated APACHE II score on admission (P = 0.04), and progression of MOD during the week after admission (P = 0.02). CONCLUSIONS This review demonstrates improved survival in seriously ill patients with necrotizing pancreatitis as a result of comprehensive surgical and critical care.

Characteristics of Encelia species differing in leaf reflectance and transpiration rate under common garden conditions

SummaryThe performance of coastal and desert species of Encelia (Asteraceae) were evaluated through common garden growth observations. The obectives of the study were to evaluate the roles of leaf features, thought to be of adaptive value (increased leaf reflectance and/or transpirational cooling), on plant growth in the hot, arid, desert garden versus their impact on growth under cooler, relatively more moist coastal garden conditions. E. californica native to the coast of southern California and E. farinosa, and E. frutescens, interior desert species, were grown in common gardens at coastal (Irvine, California) and interior (Phoenix, Arizona) sites under both irrigated and natural conditions. Although all species survived in both gardens during the two and a half year study period, there were large differences in their sizes. In the desert garden, leaf conductance and leaf water potential were both lower than at the coastal site. E. californica shrubs were leafless much of the time under natural conditions in the desert garden and had the smallest size there as well. Under natural conditions, E. farinosa, with its reflective leaf surface, was able to maintain lower leaf temperatures and attained a large size than the other two species in the desert garden. The green-leaved species (E. californica and E. frutescens) were not able to maintain leaves into the drought periods in the desert garden, with the exception of the irrigated E. frutescens which did maintain its leaf area if provided with supplemental watering to maintain transpirational leaf cooling. In the coastal garden, all species survived and there were few clear differences in the physiological characteristics among the three species. E. californica, the coastal native, attained a larger size in the coastal garden when compared with either of the two desert species.

Unusually low carbon isotope ratios in plants from hanging gardens in southern Utah

Abstract Leaf carbon isotope ratios (δ13C) and photosynthetic gas exchange were measured on plants growing in hanging garden communities in southern Utah, USA. Hanging gardens are unusual, mesic cliff communities occurring where water seeps from the sandstone bedrock in an otherwise extremely arid region; there is very limited overlap in species distributions inside and outside these gardens. Solar exposure in hanging gardens varied with orientation and one of the gardens (Ribbon Garden) was shaded throughout the day. The leaf δ13C values of plants in hanging gardens were significantly more negative than for plants from either nearby ephemeral wash or riparian communities. In Ribbon Garden, the observed δ13C values were as low as −34.8‰, placing them among the most negative values reported for any terrestrial plant species growing in a natural environment. Hanging garden plants were exposed to normal atmospheric CO2 with an average δ13C value of −7.9‰ and so the low leaf δ13C values could not be attributed to exposure to a CO2 source with low 13C content. There was a seasonal change toward more negative leaf δ13C values at the end of the growing season. The observed leaf δ13C values were consistent with photosynthetic gas exchange measurements that indicated unusually high leaf intercellular CO2 concentrations associated with the relatively low light levels in hanging gardens. Thus, extremely negative leaf δ13C values would be expected if significant amounts of the seasonal carbon gain occur at light levels low enough to be near the light compensation point. Maximum observed photosynthetic rates varied with light levels at each of the gardens, with maximum rates averaging 20.3, 14.6, and 3.1 μmol m−2 s−1 at Double Garden, Lost Garden, and Ribbon Garden, respectively. Leaf nitrogen contents averaged 18.5 mg g−1 in species from the more shaded hanging gardens (Lost and Ribbon). When expressed on a leaf area basis, nitrogen contents averaged 117 mmol N m−2 at Lost Garden and 65 mmol N m−2 at Ribbon Garden (shadiest of the two gardens). Leaf nitrogen isotope ratios averaged −2.3‰ (range of −0.7 to −6.1‰), suggesting that most of the nitrogen was derived from a biological fixation source which is most likely the Nostoc growing on the sandstone walls at the seep. These values contrast with leaf nitrogen isotope ratios of 5–9‰ which have been previously reported for arid zone plants in nearby ecosystems.