Abschlussbericht Projekt: „Verminderung der ... - BLE

of any blackspot development and therefore no blackspot susceptibility (Σ 1 ). Discolouration
of up to one quarter of length and less than 5 mm in depth denoted slight blackspot
susceptibility (Σ 2 ). Medium blackspot susceptibility corresponded to a discolouration of up
to one quarter of length and more that 5 mm in depth or one half of the length and a maximum
of 5 mm in depth (Σ 3 ). Serious blackspot susceptibility was indicated by any discolouration
stronger than that described before (Σ 4 ). The blackspot susceptibility (BSI), expressed
as the percentage of discoloured tubers, was calculated as (0.3Σ 2 +0.5Σ 3 +Σ 4 )/Σ tuber *
100.
Water relation parameters of tubers of the second batch were determined on tissue discs (6
mm diameter and 2 mm thick) from stem end and bud end of the tuber. With a cork borer
tissue cylinders were axially cut from the middle of the tubers. The analysed samples consisted
of both cortex and parenchyma tissue, which is important because blackspot occurs
in the vascular bundles and parenchyma tissue under the cortex.
Water potential of the tissue discs was determined psychrometrically (von Willert et al.,
1995) using 10 C-52 dew point hygrometer chambers connected to a HR-33T micro voltmeter
via a PS-10 switchbox (all Wescor Inc., Logan, USA). The samples were inserted
into the deep sample holders of the C-52 chambers and equilibrated for 60 min in the
closed chambers. Subsequently, the discs were removed from the chambers, their fresh
mass (FM) obtained (electronic balance BP 210 S, Satorius AG, Goettingen, Germany),
and the discs were allowed to transpire freely for 30 min. Afterwards, the water potential
and fresh mass were measured again.
Finally, samples were filled into pre-weighted 1,5 ml micro test tubes (Eppendorf AG,
Hamburg, Germany), frozen, thawed, centrifuged for 3 min (14000g, MiniSpin® plus, Eppendorf
AG, Hamburg, Germany) and the supernatant analysed for its molal osmotic concentration
(c osm ; mmol kg -1 ) with a vapour pressure osmometer (VAPRO 5520, Wescor
Inc., Logan, USA). The osmotic potential (Ψ π ) was calculated from the osmotic concentration
using the van’t Hoff’s relation (Ψ π = c osm *ρ solution *R*T; c.f. von Willert et al., 1995).
From the difference of water potential and osmotic potential, pressure potential (Ψ P ) was
obtained. Finally tissue dry mass (DM, g kg -1 FM) was obtained after oven-drying at 85°C
(24 hours) and the fresh mass based water concentration of the discs was calculated from
fresh and dry mass ((FM-DM)/FM*1000). From changes in pressure potential and fresh
mass during the two subsequent measurements multiplied by the initial fresh mass (FM i )
the volumetric modulus of elasticity (ε, c.f. von Willert et al., 1995) of the tissue samples
was calculated according to Landahl et al. (2004) as ε = (ΔΨ P )/ΔFM)*FM i .
Statistics. The obtained data were analysed with Sigma Stat statistical program (STAT32).
All data sets were tested for normal distribution and variance homogeneity (p=0.05). Following
analyses of variance were calculated for cultivar and specific gravity. Calculated
means were compared by Tukey's test (p