Abschlussbericht Projekt: „Verminderung der ... - BLE

tubers to some extent (Heinecke 2007). During blackspot formation malnutrition related to
Se, K, Mg and/or Fe deficiency may promote further spontaneous oxidation and polymerisation
of substrates catalysed by the enzyme PPO generating dopaquinone. Free tyrosine,
cystein, chlorogenic acid and caffeic acid are PPO specific substrates present in potato tubers
(Dean et al. 1993; Kirk et al. 2006). The amount of these substrates might be restricted
or decreased at an optimal supply of K (McNabnay et al. 1999), B (Mondy and Munschi
1993), Se (Munshi et al. 1990) and Fe (Bergmann 1993). Despite the fact that a well balanced
Fe concentration might mitigate blackspot susceptibility Fe also forms a complex
with phenolic substances like chlorogenic acid esters during the cooking process of potato
tubers causing after cooking darkening (Muneta and Kaisaki 1983; Griffiths and Bain
1997). Nevertheless, malnutrition associated with K, B, Se and/or Fe deficiency might increase
substrate concentrations and, therefore, stimulate the continuing non-enzymatic polymerization
reaction with nucleophilic amino acid residues of proteins and form the black
nitrogen containing pigment melanin (Dean et al. 1993; Kirk et al. 2006). Depending on
the compounds intermediates with varying colours occur (Dean et al. 1993; Kirk et al.
2006). With the present study we payed attention to the above described relationships between
nutrient concentrations and blackspot susceptibility of potato tubers and investigated
the effect of fertilizer on tuber quality when nutrients applied in amounts close to the requirement
for producing optimal yield.
Material and methods
Material. Eight cultivars of table potatoes (Adretta, Afra, Gala, Granola, Lolita, Marabel,
Nicola and Renate) were grown on sandy soils near Dethlingen, Germany, during two
vegetation periods in 2006 and 2007. Conventional farming methods were practiced, including
proper irrigation, adequate fertilization (urea and ammonium nitrate solution,
diammonium phosphate as well as fertilizer combinations of KCL + Mg and K 2 SO 4 + Mg
were applied) and an integrated pest management. To evaluate cultivar specific differences
tubers of similar size from 40 to 50 mm and corresponding specific gravities were considered.
Analyses were performed in tubers harvested two weeks before usual harvest time
(pre-harvest) and in tubers reaped during ideal time of harvest as well as in tubers stored
for five and eight months, respectively, at 4°C and 95 % relative humidity. Furthermore,
the samples were divided into two parts. Tubers from the first part were used to determine
their blackspot index (BSI). Those of the second part were sliced, shock-frozen in liquid
nitrogen, lyophilized (Epsilon 2-40, Christ) and ground to 50 mm using a micro hammer
mill (Culatti AG, Switzerland) to analyse the concentrations of Se, B, Fe, Ca, Mg, P and K,
respectively.
Analyses. Blackspot index (BSI) of whole potato tubers of different specific gravities was
determined according to the method proposed by the “Bundessortenamt” (BSA, Federal
Plant Variety Office, Germany). The tubers were cooled down to 4 to 5°C and mechanical
treated for 50 s in a vegetable washing machine with a rotating drum (Flott 18 K, Flottwerk
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