Abschlussbericht Projekt: „Verminderung der ... - BLE

Discussion
Distribution of tubers with different specific gravities within each cultivar. In general,
the specific gravity of potato tubers is assumed to be special attributed to cultivars
(Laboski and Kelling 2007). However, presented investigations indicated that the specific
gravity of tubers ranged between 1.095 kg L -1 independent of given cultivars.
This is due to the fact that growing and storage conditions might overrule some conditions
of cultivar dependent specific gravity formations as mentioned by Laboski and Kelling
(2007). Tubers harvested from the same field differed in their specific gravity up to 0.008
to 0.010 units only. Investigated cultivars where grown in single hills in the same field and
performed an individual set of specific gravities, whereby the sets of specific gravities of
cultivars differed form each other about 0.010 units. The observed range of specific gravities
about 0.039 units within a single cultivar and hill is quite possible and followed the
hierarchy of tubers to accumulate dry matter and water as observed by Bussieres (1993),
Kolbe and Stephan-Beckmann (1997) and Struik et al. (1991). This sink-strengh hierarchy
is greatly affected by competitory relationships between plants and plant individual distributed
stems and are partially open to influence as described by Struik et al. (1990) in
detail.
Blackspot index and tuber components attributed to specific gravity. The BSI of tubers
of various specific gravities was not correlated with the concentrations of dry cell wall
material, pectin or non-pectin components (celluloses, hemicelluloses). Both parameters
affect cell wall thickness and cell wall firmness in potato tubers (Bush et al. 2001). Therefore
present results indicated that neither cell wall thickness nor cell wall firmness was
attributed to tuber blackspot bruise susceptibility. Other types of bruise like crushing, shatter
and cracking lead to visible cell wall rupture (Baritelle and Hyde 2003) but blackspot
bruise is invisible until the tuber is peeled because the tuber skin is not damaged or broken
after impact (Wright et al. 2005). Nevertheless, blackspot bruise formation depends on a
linkage between PPO and substrates normally stored in different cell compartments divided
by cell membranes. Therefore, the consistence of the cell membranes characterise
blackspot susceptibility of the tuber. Many reports assumed a decompartmentation of PPO
and substrates, while the cell membranes were ruptured during impact (Story 2007; van
Eck 2007; van Loon and van den Berg 2003/2004). In the present investigations structural
damages, which lead to necrosis or cell death were not observed. Moreover, tubers were
vital and turgid after harvest as well as after five and eight months of storage (Wulkow et
al. in preparation) independent of their blackspot bruise susceptibility. Presented findings
indicated an intact cell metabolism of potato tubers after impact and blackspot bruise formation.
According to the presented results some authors studied the appearance of cell
membrane using different microscope techniques and they detected no cell membrane rupture
after impact (Barkhausen 1978; Edgell et al. 1998; Reeve 1968). We assume that the
impact leading to blackspot bruise is a kind of mechanical stress, which might be focused
on mechanosensitive channels as described by Larcher (1994) and Yahraus (1995) in de-
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