INFLUENCE OF PRECIPITATION AND THERMAL CONDITIONS ON STARCH CONTENT IN POTATO TUBERS FROM MEDIUM-EARLY CULTIVARS GROUP

The basis of this elaboration constituted data concerning mean starch content in nine medium-early cultivars of potato grown in six experimental stations belonging to COBORU (Research Centre for Cultivar Testing) in years 2010–2013. Using stepwise regression analysis the influence of thermal and precipitation conditions on starch content in potato cultivars was examined. Such analysis was performed for locations, years and particular cultivars. The analysis showed that starch content in tubers of medium-early potato cultivars depends more on precipitation than on thermal conditions. The relationship between starch content in tubers and precipitation and thermal conditions varied in different stations. In four stations (Karżniczka, Sulejów, Uhnin, Węgrzce) it depended on precipitation noted in August, in two (Uhnin and Słupia) – in July and in other two (Naroczyce and Słupia) – in June. Positive influence of temperature on starch content was noted only in August in Karżniczka. Starch content in tubers of Ametyst and Gawin cultivars depended on precipitation sum in August, Satina cultivar – in July, Oberon cultivar – in June and July, and Cekin, Finezja, Jurek, Stasia and Tajfun cultivars – on precipitation in July and August.


INTRODUCTION
Chemical composition and value of potato tubers are mainly determined by cultivars features which are modified under the influence of agrotechnology and climate and soil conditions.According to Sawicka et.al. [2011] starch content results most of all from cultivars features.The contrary opinion presents Bombik et.al. [2007] as they claim that starch content to the greatest extent depends on vegetation conditions in particular years.These conditions are formed mainly by temperature and precipitation.Excessive amount of precipitation during the growing season of potato has a negative influence on starch accumulating in tubers [Puła and Sokwera 2004], while drought results in increase of dry matter content as well as decrease in starch content in some cultivars [Boguszewska 2007].However Zarzecka et. al. [2004] claim that content of this polysaccharide is determined by cultivars and years of study and there is a significant relationship between these two factors.
The aim of this study was to define the influence of precipitation and thermal conditions on starch content formation in medium-early cultivars of potato grown in six different locations in a period of four years.

MATERIALS AND METHODS
Material constituted the results of long-term filed trials (2010-2013) as well as meteorological observations derived from six COBORU stations (Karżniczka, Naroczyce, Słupia, Sulejów, Uhnin and Węgrzce).The experiments were performed in incomplete blocks system.Potatoes were planted in the third decade of April with a spacing of 75 cm and distance between furrows in a row of 33 cm.They were collected in a period of full maturity.Fertilization was used according to principles of COBORU methodology.During the experiments full plants protection against potato blight, Colorado potato beetle and against weeds was used.Mean starch content in Ametyst, Cekin, Finezja, Gawin, Jurek, Oberon, Satina, Stasia and Tajfun cultivars were obtained as a result of cumulative trial (total from repetitions) collected in each location in subsequent years.
In order to define the influence of precipitation and thermal conditions on starch content in potato tubers grown in the analyzed years, the stepwise regression analysis was calculated.The dependent variable was respectively: locations, years and cultivars, while the independent variables were precipitation and mean air temperature in particular months: X 1 -June temperature, X 2 -July temperature, X 3 -August temperature, X 4 -June precipitation, X 5 -July precipitation, X 6 -August precipitation.As a measurement of model fit coefficient of determination was taken.All calculations were performed using STATIS-TICA 10.0 where p≤0,05.

RESULTS AND DISCUSSION
Table 1 presents mean monthly air temperature and monthly precipitation sum in each of the locations in years 2010-2013.In the analyzed years mean monthly air temperature in all stations was similar to mean long-term values .Whereas monthly precipitation sums, especially in June and July, in all stations analyzed in years 2010-2013 were significantly higher than sums from multi-year .In all the years in each of the stations, there were noted months with precipitation sum over 100 mm higher than the multi-year sum.It can be said that second part of the growing seasons in the analyzed years was characterized by excessive moisture.Monthly precipitation sums were to a large extent higher than precipitation demands of medium-early cultivars of potato.
On the basis of multiple regression analysis it was stated that in the analyzed stations starch content in tubers of medium-early cultivars of potato depended on different elements according to the station location (Table 2).In Sulejów

Table 1 .
Mean monthly air temperature and monthly atmospheric precipitation sum at the analyzed stations in 2010-2013