Influence of Plum Rootstocks on the Content of Reducing Sugars in the Annual Shoots of Cultivar ‘Kubanskaya Kometa’

The lack of suitable plum rootstocks for Baltic conditions has become a problem during recent years due to changing climatic conditions. Rapid temperature fluctuations between freezing and thawing are occurring more frequently. The winter-hardiness of rootstocks is essential for overwintering of trees in such conditions. The content of accumulated reducing sugars is an important physiological factor influencing wintering ability of trees. The dynamics of reducing sugars was investigated during two winter seasons (2010/2011 and 2011/2012) in one-year-old ‘Kubanskaya Kometa’ (Prunus x rossica Erem.) hybrid plum shoots from two orchards planted in 2001 at Pūre Horticultural Research Centre (Latvia) and Polli Horticultural Research Centre (Estonia). Cultivar ‘Kubanskaya Kometa’ was grafted on eight clonal rootstocks: ‘St. Julien A’, ‘Brompton’ cuttings, ‘Ackermann’, ‘Pixy’, GF8/1, G5/22, GF655/2, ‘Hamyra’ and eight seedling rootstocks: ‘St. Julien INRA 2’, ‘St. Julien d’Orleans’, ‘St. Julien Noir’, ‘Brompton’ seedlings, ‘Wangenheims Zwetsche’, ‘St. Julien Wädenswill’, ‘Myrobаlan’ and Prunus cerasifera var. divaricata. Trees were planted at 5×3 m spacing in four replications per rootstock with three trees per plot. Shoot samples were harvested five times during the winter period. The concentration of reducing sugars (mg g^-1 dry weight) was determined with Bertran’s method. Significant differences in concentration of reducing sugar were found between samples coming from different locations and in two seasons. The maximum concentration of reducing sugar was found in December or January depending on growing location and meteorological conditions