Influence of rootstocks on the dynamic of proline content in the annual shoots of 'Victoria' plum

Sharp temperature fluctuations, when thaws are changing with frost have been observed in the Baltic region with changing climate conditions more and more often. Therefore the question about rootstock influence on plum tree winter hardiness is essential for good overwintering of trees in such conditions. The content of proline is one of the biochemical factors influencing wintering ability of trees. Its content in one-year shoots increases with decreasing of air temperature. The dynamics of proline content in one-year shoots was investigated during three wintering periods (2010/2011, 2011/2012 and 2012/2013) in two locations. Orchards were planted in 2001 in Pūre Horticultural Research Centre (Latvia) and in Polli Horticultural Research Centre (Estonia). Well-known European plum cultivar ‘Victoria’ (P. domestica L.) was grafted on eight clonal rootstocks: ‘St. Julien A’, ‘Brompton’, ‘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’, ‘Wangenheims Zwetche’, ‘St. Julien Wädenswill’, ‘Myrobalan’ and Prunus cerasifera var. divaricata. Plants were planted in orchard with 5×3 m spacing, three trees plot^-1, in four replications. Annual shoots samples were harvested four times during the wintering period (at the end of October, December, January, and March). The content of proline in dry matter (mg g^-1) was determined by using ninhydrin method. Significant differences between proline content in shoot samples harvested from trees on different rootstocks were not stated, whereas significant differences were stated between years and locations. From this we conclude that proline content is changing under the influence of climatic conditions. The maximum content of proline was found in December and January depending on minimal temperatures, but during the wintering period of 2011/2012 also in March.