Abschlussbericht Projekt: „Verminderung der ... - BLE

enzyme polyphenoloxidase (PPO) to form melanin. The pigment is responsible for the dark
discolouration found in bruised tissue. In non-bruised tissue PPO and its substrates are
separated by intracellular compartmentation. PPO is largely associated with the plastids
and the substrates are located in the vacuole. Impact on tissue causes a coincidence of PPO
and its substrates, leading to blackspot bruise formation in the presence of oxygen (Edgell
et al. 1998). This bruise threshold depends on the stage of crop growth and the biochemical
potential of the tuber to produce superoxide radicals (Story 2007). Bruise resistance refers
to the mechanical properties of the tissue, which are influenced by tuber specific gravity
(Baritelle and Hyde 2003; Scanlon et al. 1996). Specific gravity is generally assumed to be
a cultivar characteristic (Laboski and Kelling 2007) and reflects the dry matter concentration
of potato tubers (Haase 2003/2004). The term dry matter refers to all substances of the
potato tuber, except water (Putz 1998) and includes approximately 80 % starch, 10 % cytoplasmatic
components and 10 % cell wall material (Lærke 2001). Many studies mentioned
the starch concentration as the predominant factor, which influences the blackspot
susceptibility (e.g. Baritelle and Hyde 2003; Lærke 2001; Scanlon et al. 1996). However,
information are still rare if this relation also exists among different cultivars, environmental
factors and storage conditions. Also the cell wall cohesion and the firmness of the
cell wall might affect any impact on the mechanical properties of the tissue and internal
structural changes (Alvarez et al. 2000; Story 2007). The cell wall cohesion and the firmness
of the cell wall depend on its structure (Taylor et al. 2007) and its pectin concentration
(Keijbets and Pilnik 1974). Pectin is a component of the middle lamella of the primary cell
wall (Braun 1989) belonging to the group of polygalacturonic acids and varying in its
structure of side chains. The side chains consist of arabinose and galactose and a complex
ramified network of mono saccharides (Kohorn 2000). Depending on the complexity and
the degree of esterification of the side chains the physiological properties of pectin are
modified (Guillotin 2005). Non-esterified carboxyl groups of adjacent side chains are
linked by multivalent ions like Ca 2+ and Mg 2+ , which might predefine cell wall firmness
(Braun 1989). The cell wall structure (Taylor et al. 2007) and its pectin concentration
(Braun 1989; Moledina et al. 1981; van Eck 2007) determine the texture and quality of
processed tubers. However, from our knowledge, no results are available if the pectin concentration
of potato tubers influences their blackspot susceptibility.
The aim of this study was to investigate the components which are attributed to tubers specific
gravity and their influence on blackspot bruise susceptibility of table potato cultivars,
respectively.
Material and methods
Material. Eight cultivars of table potatoes as Adretta, Afra, Gala, Granola, Lolita, Marabel,
Nicola and Renate were grown near Dethlingen, Germany with conventional farming
methods during three vegetation periods in 2005, 2006 and 2007. From harvested plants
tubers with a similar size of 40 to 50 mm were selected to get comparable results. Analyses
195