BERLIN (Reuters) - Germany s Social Democrat (SPD) leader Martin Schulz told a television debate with Chancellor Angela Merkel that he would stop Turkey s bid to join the European Union if he was elected leader of Europe s most populous country.  When I m chancellor, I ll cancel ... the accession talks of Turkey with the European Union,  Schulz said in the debate.  No German citizen can travel safely to Turkey anymore,  Schulz said, referring to the detention of 12 German citizens on political charges in the past months.  Whoever is the next chancellor has the task to tell Turkey in order to protect the Federal Republic of Germany: all red lines have now been crossed, and therefore this country can no longer become a member of the EU,  Schulz said. Merkel repeated her standard position that she was in favour of stopping further discussions with Ankara about its participation in a EU customs union. But she cautioned against pulling the plug on Turkey s accession talks right now, adding such a step should be considered cautiously. 