[Experiments on male-sterility of sun-flowers, caused by genetical, physiological and applied chemical factors].

1. Male-sterile plants could be selected for several generations. These produce by sib mating (malesterile x fertile) 50% male-sterile and 50% fully fertile descendants. In two cases crossing resulted in 75% malesterile plants in F1. 2. Two new varieties, which had originated from a combination of breeding by self pollinations and by multiple crossings and which show earlier flowering under shorter photoperiods than do other varieties, were sawn in July 1966 and 1967. Flowering began at the end of September and in the beginning of October, there was found an 80-100% male-sterility (retrogressive antheres). Varieties which react less to short day treatment had complete fertile flowers. 3. Experiments which were started some years earlier to induce male-sterility of sun-flowers by active substances brought the following results in 1963-1967: a) Gibberellic acid treatment (0.5-0.25 mg/plant) applied at the beginning of bud formation, induced malesterility. In this way it is possible to produce small quantities of hybrid seed and to test topcrosses. b) Some combinations of gibberellic acid with other active substances, e.g. 0.25 mg gibb. ac. + 0.25 mg TJBS, 0.25 mg gibb. ac. + 0.1 mg NES, 0.25 mg gibb. ac. + 0.1 mg IES and 0.25 mg gibb. ac. + 0.5 mg CCC induced malesterility but permitted sufficient female fertility. c) At a male-sterility of 90-100% total average fertility of 10 plants was only 25-75%. In treating 87 plants of the inbred line 'HEBH' a production of kernels ranging from 0 to 86% was found, 39% on the average, that is 202 kernels/flower, as compared to 90% for untreated plants. d) Varieties and inbred lines show different reactions to the treatment. There are also marked differences in effects of active substances under different climatic conditions, which makes the practical application of treatments difficult. It therefore becomes necessary to test different varieties for reaction to active substances or to combinations of these substances under different climatic conditions.