G. Georgiev*, P. Peevska, E. Penchev
Dobrudzha Agricultural Institute, 9520 General Toshevo, Bulgaria

Abstract. Four new Bulgarian sunflower hybrids were investigated: Veleka and Vokil, which were officially registered in 2013 in Romania; Gaby, which will be registered in Romania in 2014, and Divna, which is now under official testing in Ukraine and Russia. The investigation was carried out at Dobrudzha Agricultural Institute, General Toshevo, Bulgaria during 2011 – 2012 with the aim to follow the phenological specificity of these hybrids under the conditions of Northeast Bulgaria. The hybrids were sown at three crop densities: 40000, 50000 and 60000 plants/ha on two sowing dates. The first one was normal for the conditions of this region – when soil temperature remained permanently above 8°C at depth 10 cm, and the second one was a month later. The investigated traits were duration of flowering (number of days), plant height (cm), diameter of inflorescence (cm), and vegetation period (days).The investigated factors sowing date and crop density in combination with the meteorological conditions over years, had significant effect on the phenological development of the investigated genotypes. With the higher crop densities the mean value of the trait plant height increased at both sowing dates. There was no significant difference in the plant height of the four investigated hybrids, most probably due to the fact that they have the same mother line (217A). The later sowing date was related to higher values of the trait diameter of the inflorescence. This investigation, however, similar to many previous studies, confirmed the rule that with the higher crop densitiestheinflorescencediameterwassmallerregardlessofthesowingdate. Attheoptimalsowingdate,floweringwasthelongestatthehighestcrop density. There was no difference between the individual genotypes involved in the testing. At late sowing the flowering period was shorter for all growing crop densities. The vegetation period depended on the sowing date. At later sowing it decreased with up to two weeks or more.