Abstract
Swelling of epidermal cell walls decreases cell-to-cell adhesion and increases cracking susceptibility in sweet cherry. Ca is suggested to decrease cracking susceptibility by crosslinking of ...cell wall components and, possibly, by decreasing swelling. The objective is to test this hypothesis. The effect of Ca on swelling of anticlinal epidermal cell walls was quantified microscopically in vivo using excised skin sections and in vitro using extracted cell walls. After removal of turgor, cell wall thickness increased. Incubation in CaCl
2
decreased cell wall thickness up to 3 mM CaCl
2
. At higher concentrations thickness remained constant. Decreased cell wall swelling in vivo also occurred with other salts of divalent and trivalent cations, but not with those of monovalent cations. Decreased swelling was due to the Ca cation, the anions had no effect. Ca also decreased swelling of cell walls that were already swollen. CaCl
2
also decreased swelling of extracted cell walls in vitro. There was no effect on swelling pressure. The effect on swelling increased as the CaCl
2
concentration increased. Chlorides of divalent and trivalent cations, but not those of monovalent cations decreased swelling in vitro. The decrease in swelling among the divalent cations was linearly related to the radius of the cation. The results indicate that Ca decreases cracking susceptibility by decreasing swelling.
Rain cracking severely affects the commercial production of many fleshy-fruit species, including of sweet cherries. The objectives were to investigate how the gaping macroscopic cracks (macrocracks) ...of a rain-cracked fruit can develop from microscopic cracks in the cuticle (microcracks). Incubating fruit in deionized water is well known to cause significant macrocracking. We found that after a lag phase of 2 h, the numbers and lengths of macrocracks increased. Macrocrack number approached an asymptote at 12 h, whereas macrocrack length continued to increase. The rate of macrocrack propagation (extension at the crack tip) was initially 10.8 mm h-1 but then decreased to a near-constant 0.5 mm h-1. Light microscopy revealed three characteristic zones along a developing macrocrack. In zone I (ahead of the crack), the cuticle was intact, the epidermal cells were unbroken and their cell walls were thin. In zone II, the cuticle was fractured, the first epidermal cells died and their cell walls began to thicken (swell). In zone III, most epidermal cells had died, their cell walls were swollen and cell:cell separation began along the middle lamellae. The thickness of the anticlinal epidermal cell walls and the percentage of intact living cells along a crack were closely and negatively related. Cracks were stained by calcofluor white, but there was no binding of monoclonal antibodies (mAbs) specific for hemicelluloses (LM11, LM21, LM25). Strong binding was obtained with the anti-homogalacturonan mAb (LM19), indicating the presence of unesterified homogalacturonans on the crack surface. We conclude that macrocrack propagation is related to cell death and to cell wall swelling. Cell wall swelling weakens the cell:cell adhesion between neighbouring epidermal cells, which separate along their middle lamellae. The skin macrocrack propagates like a 'run' in a fine, knitted fabric.
Main conclusion
Swelling of sweet cherry cell walls is a physical process counterbalanced by turgor. Cell turgor prevents swelling in intact cells, whereas loss of turgor allows cell walls to swell.
...Swelling of epidermal cell walls precedes skin failure in sweet cherry (
Prunus avium
) cracking. Swollen cell walls lead to diminished cell:cell adhesions. We identify the mechanism of cell wall swelling. Swelling was quantified microscopically on epidermal sections following freeze/thaw treatment or by determining swelling pressure or swelling capacity of cell wall extracts. Releasing turgor by a freeze/thaw treatment increased cell wall thickness 1.6-fold within 2 h. Pressurizing cell wall extracts at > 12 kPa prevented swelling in water, while releasing the pressure increased swelling. The effect was fully reversible. Across cultivars, cell wall thickness before and after turgor release in two subsequent seasons was significantly correlated (before release of turgor:
r
= 0.71**,
n
= 14; after release of turgor:
r
= 0.73**,
n
= 14) as was the swelling of cell walls upon turgor release (
r
= 0.71**,
n
= 14). Close relationships were also identified for cell wall thickness of fruit of the same cultivars grown in the greenhouse and the field (before release of turgor:
r
= 0.60,
n
= 10; after release of turgor:
r
= 0.78**,
n
= 10). Release of turgor by heating, plasmolysis, incubation in solvents or surfactants resulted in similar swelling (range 2.0–3.1 µm). Cell wall swelling increased from 1.4 to 3.0 µm as pH increased from pH 2.0 to 5.0 but remained nearly constant between pH 5.0 and 8.0. Increasing ethanol concentration decreased swelling. Swelling of sweet cherry cell walls is a physical process counterbalanced by turgor.
Main conclusion
During fruit development, cell wall deposition rate decreases and cell wall swelling increases. The cell wall swelling pressure is very low relative to the fruit’s highly negative ...osmotic potential.
Rain cracking of sweet cherry fruit is preceded by the swelling of the cell walls. Cell wall swelling decreases both the cell: cell adhesion and the cell wall fracture force. Rain cracking susceptibility increases during fruit development. The objectives were to relate developmental changes in cell wall swelling to compositional changes taking place in the cell wall. During fruit development, total mass of cell wall, of pectins and of hemicelluloses increases, but total mass of cellulose remains constant. The mass of these cell wall fractions increases at a lower rate than the fruit fresh mass—particularly during stage II and early stage III. During stage III, on a whole-fruit basis, the HCl-soluble pectin fraction, followed by the water-soluble pectin fraction, the NaOH-soluble pectin fraction and the oxalate-soluble pectin fraction all increase. At maturity, just the HCl-soluble pectin decreases. Cell wall swelling increases during stages I and II of fruit development, with little change thereafter. This was indexed by light microscopy of skin sections following turgor release, and by determinations of the swelling capacity, water holding capacity and water retention capacity. The increase in cell wall swelling during development was due primarily to increases in NaOH-soluble pectins. The in vitro swelling of cell wall extracts depends on the applied pressure. The swelling pressure of the alcohol-insoluble residue is low throughout development and surprisingly similar across different cell wall fractions. Thus, swelling pressure does not contribute significantly to fruit water potential.
Display omitted
•In fjords benthos utilized a wide range of organic matter sources.•Arctic fjord and shelf benthos displayed a high degree of omnivory.•Feeding on higher trophic levels dominated in ...deep-sea benthos.•Macrofauna mainly relied on fresher organic matter arriving to the sea floor.•Meiofauna mainly relied on the reworked organic matter from the sediments.
The knowledge on benthic trophic relations is particularly important for understanding the functioning of still pristine and less studied Arctic Ocean ecosystems. This study examines the benthic food-web structure in the European sector of the Arctic Ocean and assesses if and how it differs along depth gradients in the marginal sea-ice zone during spring. Samples of the sediment organic matter were collected in May/June 2015 and May 2016 at stations representing different sedimentary habitats (fjord, shelf, slope/basin), and stable isotopes of δ13C and δ15N were used to determine macro- and meiofauna food-web structure. Our results show that the food-web structure differed both among the three studied habitat types and between macro- and meiofauna components of benthic communities, and that these differences were related to the quality and quantity of organic matter. Meiofauna in fjords and on the shelf mainly relied on the reworked sediment organic matter while macrofauna utilized more fresh organic matter, sedimenting to the sea floor. In fjords and on the shelf, benthos displayed a high degree of omnivory and non-selective feeding while on the slope and in deep basins feeding on higher trophic levels dominated. In the latter, benthos seemed also to have utilized highly reworked organic matter. As the organic matter quantity and quality were major drivers of trophic relations in all studied areas, the benthic food webs will likely face cascading effects following the modification of pelagic food webs due to climatic changes.
The Arctic Ocean north of Svalbard has recently experienced large sea ice losses and the increasing prominence of Atlantic water (AW) advection. To investigate the impact of these ongoing changes on ...annual particle export, two moorings with sequential sediment traps were deployed in ice‐free and seasonally ice‐covered waters on the shelf north (NSv) and east (ESv) of Svalbard, collecting sinking particles nearly continuously from October 2017 to October 2018. Vertical export of particulate organic carbon (POC), total particulate matter (TPM), planktonic protists, chlorophyll a, and zooplankton fecal pellets were measured, and swimmers were quantified and identified. Combined with sensor data from the moorings, these time‐series measurements provided a first assessment of the factors influencing particle export in this region of the Arctic Ocean. Higher annual TPM and POC fluxes at the ice‐free NSv site were primarily driven by the advection of AW, higher grazing by large copepods, and a wind‐induced mixing event during winter. Higher diatom fluxes were observed during spring in the presence of sea ice at the ESv site. Along with sea ice cover, regional differences in AW advection and the seasonal presence of grazers played a prominent role in the biological carbon pump along the continental shelf off Svalbard.
Plain Language Summary
Recently, the area north of Svalbard has experienced the largest reduction of winter sea ice extent in the Arctic Ocean and an increasing influence of warm Atlantic water (AW) in surface waters. The consequences for the marine ecosystem remain unclear. In the present study, we investigated the fate of algal production and organic matter by measuring the amount and composition of the material sinking toward the seafloor. Using sediment traps with automatically rotating bottles, the seasonal variability in the quality and quantity of the organic matter sinking to ∼100 m was investigated at two sites with and without winter sea ice on the shelf north and east of Svalbard from 2017 to 2018. Our results suggest that the AW inflow along the shelf break induces a gradient with more zooplankton in the west and more ice in the east. More zooplankton in the west rework the organic matter through grazing, resulting in higher flux of carbon than in the east. Less zooplankton in the east potentially leaves more algal cells to sink ungrazed. As algal fluxes were higher in the presence of sea ice, a future with less sea ice may result in more reworked material sinking to the seafloor.
Key Points
Sea ice melt led to higher springtime and annual diatom fluxes at the seasonally ice‐covered site
Aside from algal fluxes, annual particle fluxes were higher at the ice‐free site
Atlantic water advection, zooplankton grazing, wind‐induced mixing, and resuspension contributed to the higher fluxes at the ice‐free site
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