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Predawn Water Potential (predawn + water_potential)

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Life Sciences	100%

Selected Abstracts

Chlorophyll fluorescence, predawn water potential and photosynthesis in precipitation pulse-driven ecosystems , implications for ecological studies

FUNCTIONAL ECOLOGY, Issue 3 2008
V. Resco
Summary 1A major research focus in population and community ecology is to establish a mechanistic understanding of plant interactions and demographic responses. The first step towards this mechanistic approach relies on understanding the differences in stress caused by different environmental conditions. Leaf-level photosynthetic rate (A) within and among plant populations provides important insight into population and community processes, but is difficult to acquire with sufficient replication under field conditions. Instead, chlorophyll fluorescence (Fv/Fm) and predawn water potential (,pd) are often used in arid and semi-arid ecosystems. 2Fv/Fm reflects the photoactivation status of photosystem II (PSII), whereas ,pd indicates water availability in the rhizosphere. Here we compare these indices with A in two perennial C4 grasses (native Heteropogon contortus and invasive Eragrostis lehmanniana) and in seedlings of the C3 shrub Prosopis velutina growing on highly contrasting sandy loam and loamy clay soils in experimental plots. Measurements were made the day prior to and up to 7 days following a 39-mm rainfall pulse after 2 months of drought. 3A was more sensitive across a broad range of environmental conditions, whereas Fv/Fm and ,pd only responded to periods of protracted drought. The use of these measures was further complicated because their values varied daily and we observed different time-lags in their response to precipitation pulses. 4We suggest sampling schemes and a priori measurements to capture the value that is representative for the question of interest, and that match the pulsed biological activity in these ecosystems. Finally, we suggest the use of these measures in combination with measurements providing integration over longer time periods, such as ,13C, ,18O and N concentration in bulk leaf tissue. [source]

Leaf-level resource use for evergreen and deciduous conifers along a resource availability gradient

FUNCTIONAL ECOLOGY, Issue 3 2000
B. D. Kloeppel
Abstract 1.,We compared leaf-level carbon, nitrogen and water use for a deciduous (Larix occidentalis Nutt.) and sympatric evergreen (Pseudotsuga menziesii, Beissn., Franco, or Pinus contorta Engelm.) conifer along a resource availability gradient spanning the natural range of L. occidentalis in western Montana, USA. 2.,We hypothesized that leaf photosynthesis (A), respiration (r), specific leaf area (SLA) and foliar nitrogen concentration (N) would be higher for deciduous than sympatric evergreen conifers in mixed stands, and that these interspecies differences would increase from high to low resource availability. We also hypothesized that leaf-level nitrogen and water-use efficiency would be higher for the co-occurring evergreen conifer than L. occidentalis. 3.,In general, mass-based photosynthesis (Am) was significantly higher for L. occidentalis than co-occurring evergreen conifers in the drier sites, but Am was similar for evergreen and deciduous conifers at the mesic site. 4.,Mass-based foliar nitrogen concentration (Nm) was positively correlated to SLA for all species combined across the gradient (R2 = 0·64), but the relationship was very weak (R2 = 0·08,0·34) for evergreen and deciduous species separately. Mass-based Am and rm were poorly correlated to Nm for all species combined across the gradient (R2 = 0·28 and 0·04, respectively). 5.,For each site-species combination, daily maximum Am was negatively correlated to vapour pressure deficit (VPD) (R2 = 0·36,0·59), but was poorly correlated to twig predawn water potential (R2 < 0·04). 6.,Instantaneous nitrogen-use efficiency (NUEi; Am divided by Nm) and water-use efficiency (,13C) increased significantly (P = 0·05) from high to low resource availability for both evergreen and deciduous conifers, except for NUEi in L. occidentalis. [source]

Shading delays bud break in Brachsyegia spiciformis

AFRICAN JOURNAL OF ECOLOGY, Issue 4 2008
R. A. Richer
Abstract Whole tree manipulation experiments were performed in the common southern African tree species, Brachystegia spiciformis to test a novel hypothesis that decreasing Total nonstructural carbohydrates (TNC) in the stem could cause bud break in Brachystegia spiciformis. The experimental treatments included fertilization, canopy defoliation, shading and stem heating to decrease stem carbohydrates. None of the treatments significantly decreased mean stem TNC. Likewise the heating, fertilization and defoliation treatments did not significantly affect the date of bud break. However, shading significantly delayed bud break. This delay in bud break could not be attributed to changes in leaf level photosynthetic traits, stem water content, leaf predawn water potential or delayed leaf fall. These results question widely accepted hypotheses about the mechanisms controlling bud break and suggest a carbohydrate homeostatic mechanism. Résumé Des expérimentations de manipulations d'arbres très complètes ont été réalisées sur l'espèce très commune en Afrique du Sud Brachystegia spiciformis pour tester une nouvelle hypothèse selon laquelle une diminution des TNC, les hydrates de carbone nonstructuraux, dans le tronc pourrait causer l'éclosion des bourgeons chez cette espèce. Les traitements expérimentaux comprenaient une fertilisation, le défeuillage de la canopée, la mise à l'ombre ou le chauffage des troncs pour diminuer les hydrates de carbone. Aucun de ces traitements n'a diminué significativement les TNC dans les troncs. Le chauffage, la fertilisation ou le défeuillage n'ont pas non plus affecté la date de l'éclatement des bourgeons. Le fait d'être à l'ombre a, par contre, significativement retardé l'éclosion des bourgeons. Ce retard ne pouvait pas être attribué aux changements du taux de photosynthèse des feuilles, à la teneur en eau des troncs, au potentiel aqueux des feuilles avant l'aube, ni au retard de la chute des feuilles. Ces résultats remettent en question les hypothèses largement acceptées au sujet des mécanismes qui contrôlent l'éclosion des bourgeons et suggèrent l'existence d'un mécanisme homéostatique lié aux hydrates de carbone. [source]