Extreme drought can deactivate ABA biosynthesis in embolism-resistant species.

The phytohormone abscisic acid (ABA) is synthesised by plants during drought to close stomata and regulate desiccation tolerance pathways. Conifers and some angiosperms with embolism-resistant xylem show a peaking-type (p-type) response in ABA levels, in which ABA levels increase early in drought then decrease as drought progresses, declining to pre-stressed levels. The mechanism behind this dynamic remains unknown. Here, we sought to characterise the mechanism driving p-type ABA dynamics in the conifer Callitris rhomboidea and the highly drought-resistant angiosperm Umbellularia californica. We measured leaf water potentials (Ψl ), stomatal conductance, ABA, conjugates and phaseic acid (PA) levels in potted plants during a prolonged but non-fatal drought. Both species displayed a p-type ABA dynamic during prolonged drought. In branches collected before and after the peak in endogenous ABA levels in planta, that were rehydrated overnight and then bench dried, ABA biosynthesis was deactivated beyond leaf turgor loss point. Considerable conversion of ABA to conjugates was found to occur during drought, but not catabolism to PA. The mechanism driving the decline in ABA levels in p-type species may be conserved across embolism-resistant seed plants and is mediated by sustained conjugation of ABA and the deactivation of ABA accumulation as Ψl becomes more negative than turgor loss.

Passive stomatal closure under extreme drought in an angiosperm species.

The phytohormone abscisic acid (ABA) plays a major role in closing the stomata of angiosperms. However, recent reports of some angiosperm species having a peaking-type ABA dynamic, in which under extreme drought ABA levels decline to pre-stressed levels, raises the possibility that passive stomatal closure by leaf water status alone can occur in species from this lineage. To test this hypothesis, we conducted instantaneous rehydration experiments in the peaking-type species Umbellularia californica through a long-term drought, in which ABA levels declined to pre-stress levels, yet stomata remain closed. We found that when ABA levels were lowest during extreme drought, stomata of U. californica were passively closed by leaf water status alone, with stomata reopening rapidly to maximum rates of gas exchange on instantaneous rehydration. This contrasts with leaves early in drought, in which ABA levels were highest and stomata did not reopen on instantaneous rehydration. The transition from ABA-driven stomatal closure to passively driven stomatal closure as drought progresses in this species occurs at very low water potentials facilitated by highly embolism-resistant xylem. These results have important implications for understanding stomatal control during drought in angiosperms.

Femoral central venous catheters are not associated with higher rates of infection in the pediatric critical care population.

BACKGROUND: Adult data show a difference in central venous catheter (CVC) infection rates between 3 major sites: subclavian (SC), internal jugular (IJ), and femoral veins. We hypothesized that in patients in pediatric intensive care units (PICUs), there is no difference in rates of CVC infection among these three sites, but specifically the femoral compared to all other sites. METHODS: In this retrospective cohort study, data from January 1999 to January 2008 were collected prospectively for internal review and quality assurance. All PICU patients with a CVC were enrolled. The rate of CVC infection was determined using Cox regression survival analysis to account for various durations of CVC placement at the various sites, then adjusted for severity of illness, number of lumens, and patient age. Mortality was compared in patients with a CVC infection versus those without. RESULTS: A total of 4,512 patients with a CVC were enrolled. No site was associated with an increased risk of infection compared with the other sites, with hazard ratios of 0.951 (95% confidence interval [CI], 0.612-1.478) for the SC site, 0.956 (95% CI, 0.593-1.541) for the IJ site, and 1.120 (95% CI, 0.753-1.665) for the femoral site. No significant association between mortality and presence of CVC infection was found when adjusted for age, severity of illness, and duration of CVC placement. An association was found between the presence of a CVC infection and prolonged PICU length of stay (3.98 days longer; P < .001). CONCLUSION: Femoral CVCs are not associated with higher rates of infection in the PICU. In addition, the presence of CVC infection does not affect mortality, but is associated with longer PICU admission.