Leaders of Germany's environmental party Die Gruenen (The Greens) Cem Oezdemir (R) and Claudia Roth (2nd L) and lead candidates for chancellor Juergen Trittin and Katrin Goering-Eckart acknowledge applause after addressing a party congress in Berlin, April 28, 2013. REUTERS/Tobias Schwarz (GERMANY - Tags: POLITICS) 

Sigmar Gabriel (C), chairman of the German Social Democratic Party (SPD), chats with lead candidates for chancellor Juergen Trittin and Katrin Goering-Eckart of Germany's environmental party Die Gruenen (The Greens) at a Greens party congress in Berlin April 27, 2013. Germany's Greens, itching to get back into power after eight years in opposition, are embroiled in a row over plans to raise taxes on high earners that looks set to spoil a party congress starting on Friday. REUTERS/Tobias Schwarz (GERMANY - Tags: POLITICS ENVIRONMENT) 

Sigmar Gabriel (C), chairman of the German Social Democratic Party (SPD), drinks a glass of tea as he arrives next to lead candidates for chancellor Juergen Trittin and Katrin Goering-Eckart of Germany's environmental party Die Gruenen (The Greens) at a Greens party congress in Berlin April 27, 2013. Germany's Greens, itching to get back into power after eight years in opposition, are embroiled in a row over plans to raise taxes on high earners that looks set to spoil a party congress. REUTERS/Tobias Schwarz (GERMANY - Tags: POLITICS ENVIRONMENT) 

Sigmar Gabriel (C), chairman of the German Social Democratic Party (SPD), drinks a glass of tea as he arrives, next to lead candidates for chancellor Juergen Trittin and Katrin Goering-Eckart of Germany's environmental party Die Gruenen (The Greens) at a Greens party congress in Berlin April 27, 2013. Germany's Greens, itching to get back into power after eight years in opposition, are embroiled in a row over plans to raise taxes on high earners that looks set to spoil a party congress starting on Friday. REUTERS/Tobias Schwarz (GERMANY - Tags: POLITICS ENVIRONMENT) 

Party leaders Claudia Roth (L) and Cem Oezdemir (R) of Germany's environmental party Die Gruenen (The Greens) chat with candidate for chancellor Juergen Trittin during a party congress in Berlin, April 26, 2013. Germany's Greens, itching to get back into power after eight years in opposition, are embroiled in a row over plans to raise taxes on high earners that looks set to spoil a party congress starting on Friday. REUTERS/Tobias Schwarz (GERMANY - Tags: POLITICS) 

Lead candidates for chancellor Juergen Trittin and Katrin Goering-Eckart of Germany's environmental party Die Gruenen (The Greens) acknowledge applause after addressing a party congress in Berlin, April 26, 2013. Germany's Greens, itching to get back into power after eight years in opposition, are embroiled in a row over plans to raise taxes on high earners that looks set to spoil a party congress starting on Friday. REUTERS/Tobias Schwarz (GERMANY - Tags: POLITICS) 

Lead candidates for chancellor Juergen Trittin and Katrin Goering-Eckart of Germany's environmental party Die Gruenen (The Greens) acknowledge applause after addressing a party congress in Berlin, April 26, 2013. Germany's Greens, itching to get back into power after eight years in opposition, are embroiled in a row over plans to raise taxes on high earners that looks set to spoil a party congress starting on Friday. REUTERS/Tobias Schwarz (GERMANY - Tags: POLITICS) 

Lead candidates for chancellor Juergen Trittin and Katrin Goering-Eckart of Germany's environmental party Die Gruenen (The Greens) address a party congress in Berlin, April 26, 2013. Germany's Greens, itching to get back into power after eight years in opposition, are embroiled in a row over plans to raise taxes on high earners that looks set to spoil a party congress starting on Friday. REUTERS/Tobias Schwarz (GERMANY - Tags: POLITICS) 

Lead candidates for chancellor Juergen Trittin and Katrin Goering-Eckart of Germany's environmental party Die Gruenen (The Greens) address a party congress in Berlin, April 26, 2013. Germany's Greens, itching to get back into power after eight years in opposition, are embroiled in a row over plans to raise taxes on high earners that looks set to spoil a party congress starting on Friday. REUTERS/Tobias Schwarz (GERMANY - Tags: POLITICS) 

Lead candidates for chancellor Juergen Trittin and Katrin Goering-Eckart of Germany's environmental party Die Gruenen (The Greens) address a party congress in Berlin, April 26, 2013. Germany's Greens, itching to get back into power after eight years in opposition, are embroiled in a row over plans to raise taxes on high earners that looks set to spoil a party congress starting on Friday. The slogan in the background reads: Germany is renewable. REUTERS/Tobias Schwarz (GERMANY - Tags: POLITICS) 

Green Party (Die Gruenen) co-leader Claudia Roth (3rd L), and top candidates in this year's German general election Katrin Goering-Eckardt (L) and Juergen Trittin (R) congratulate the top-candidates in yesterday's Lower Saxony state election Anja Piel and Stefan Wenzel at the party headquarters in Berlin, January 21, 2013. REUTERS/Thomas Peter (GERMANY - Tags: POLITICS ELECTIONS) 

Green Party (Die Gruenen) co-leader Claudia Roth (L), and top candidate in this year's German general election Juergen Trittin (R) congratulate the top-candidates in yesterday's Lower Saxony state election Anja Piel (2nd L) and Stefan Wenzel at the party headquarters in Berlin, January 21, 2013. REUTERS/Thomas Peter (GERMANY - Tags: POLITICS ELECTIONS) 

Juergen Trittin (L) and Claudia Roth (R), leader of Germany's Green party celebrate with supporters of Lower Saxony's Greens party (Die Gruenen) after first exit polls for the Lower Saxony federal state election in Hanover January 20, 2013. Angela Merkel's coalition and the centre-left opposition were neck-and-neck in exit polls in a state election in Lower Saxony on Sunday, which could deprive the German chancellor of a boost for her bid to win a third term later this year. REUTERS/Fabian Bimmer (GERMANY - Tags: POLITICS ELECTIONS) 

Top candidates for the 2013 German general elections Peer Steinbrueck (R) of the German Social Democratic Party (SPD) and German environmental Green Party (Die Gruenen) Juergen Trittin address a news conference in Berlin December 12, 2012. REUTERS/Tobias Schwarz (GERMANY - Tags: POLITICS) 

Top candidates for the 2013 German general elections Peer Steinbrueck (R) of the German Social Democratic Party (SPD) and German environmental Green Party (Die Gruenen) Juergen Trittin address a news conference in Berlin December 12, 2012. REUTERS/Tobias Schwarz (GERMANY - Tags: POLITICS) 

Top candidates for the 2013 German general elections Peer Steinbrueck (R) of the German Social Democratic Party (SPD) and German environmental Green Party (Die Gruenen) Juergen Trittin address a news conference in Berlin December 12, 2012. REUTERS/Tobias Schwarz (GERMANY - Tags: POLITICS) 

Top candidates for the 2013 German general elections Peer Steinbrueck (R) of the German Social Democratic Party (SPD) and German environmental Green Party (Die Gruenen) Juergen Trittin shake hands a news conference in Berlin December 12, 2012. REUTERS/Tobias Schwarz (GERMANY - Tags: POLITICS) 

Top candidates for the 2013 German general elections Peer Steinbrueck (R) of the German Social Democratic Party (SPD) and German environmental Green Party (Die Gruenen) Juergen Trittin leave after a news conference in Berlin December 12, 2012. REUTERS/Tobias Schwarz (GERMANY - Tags: POLITICS) 

Top candidates for the 2013 German general elections Peer Steinbrueck (R) of the German Social Democratic Party (SPD), and German environmental Green Party (Die Gruenen) Juergen Trittin address the media in Berlin December 12, 2012. REUTERS/Tobias Schwarz (GERMANY - Tags: POLITICS) 

Top candidates for the 2013 German general elections Peer Steinbrueck (R) of the German Social Democratic Party (SPD), and German environmental Green Party (Die Gruenen) Juergen Trittin address the media in Berlin December 12, 2012. REUTERS/Tobias Schwarz (GERMANY - Tags: POLITICS) 

Top candidates for the 2013 German general elections Peer Steinbrueck (R) of the German Social Democratic Party (SPD), and German environmental Green Party (Die Gruenen) Juergen Trittin address the media in Berlin December 12, 2012. REUTERS/Tobias Schwarz (GERMANY - Tags: POLITICS) 

Top candidates for the 2013 German general elections Peer Steinbrueck (R) of the German Social Democratic Party (SPD), and German environmental Green Party (Die Gruenen) Juergen Trittin address the media in Berlin December 12, 2012. REUTERS/Tobias Schwarz (GERMANY - Tags: POLITICS) 

German Green Party co-leader Claudia Roth (2ND L) receives congratulations by party members Volker Beck, Juergen Trittin and Katrin Goering-Eckardt (L-R) after her speech at the party convention of the Green Party in Hanover, November 17, 2012. Roth was re-elected by the delegates. REUTERS/Fabian Bimmer (GERMANY - Tags: POLITICS) 

German Green Party member and candidate Juergen Trittin delivers his speech at the party convention of the Green Party in Hanover, November 16, 2012. REUTERS/Fabian Bimmer (GERMANY - Tags: POLITICS) 

German Green Party member and candidate Juergen Trittin delivers his speech at the party convention of the Green Party in Hanover, November 16, 2012. REUTERS/Fabian Bimmer (GERMANY - Tags: POLITICS) 

German Green Party member and candidate Juergen Trittin delivers his speech at the party convention of the Green Party in Hanover, November 16, 2012. REUTERS/Fabian Bimmer (GERMANY - Tags: POLITICS) 

German Green Party members and candidates Juergen Trittin (L) and Katrin Goering-Eckardt wave scarfs with a slogan reading, "Green Will Win" after their speeches at the party convention of the Green Party in Hanover, November 16, 2012. REUTERS/Fabian Bimmer (GERMANY - Tags: POLITICS) 

German Green Party members and candidates Juergen Trittin (L) and Katrin Goering-Eckardt wave to delegates after their speeches at the party convention of the Green Party in Hanover, November 16, 2012. REUTERS/Fabian Bimmer (GERMANY - Tags: POLITICS) 

German Green Party members and candidates Juergen Trittin (R) and Katrin Goering Eckardt (R) wave to delegates at the party convention of the Green Party in Hanover, November 16, 2012. REUTERS/Fabian Bimmer (GERMANY) 

German Green Party members and candidates Juergen Trittin (R) and Katrin Goering Eckardt (R) wave to delegates at the party convention of the Green Party in Hanover, November 16, 2012. REUTERS/Fabian Bimmer (GERMANY - Tags: POLITICS) 

German Green Party members Juergen Trittin (L) and Katrin Goering Eckardt (R) are seen at the party convention in Hanover, November 16, 2012. REUTERS/Fabian Bimmer (GERMANY - Tags: POLITICS) 

Leaders of Germany's environmental Greens Party (Die Gruenen) Claudia Roth (R) and Cem Oezdemir (L) pose with their party members Katrin Goering-Eckardt and Juergen Trittin before a party board meeting in Berlin November 12, 2012. Last weekend members of the Green party elected by direct vote Goering-Eckardt and Trittin as top candidates for next year's general election. REUTERS/Tobias Schwarz (GERMANY - Tags: POLITICS) 

Co-leader of Germany's environmental Greens Party (Die Gruenen) Claudia Roth (R) hugs her party member Katrin Goering-Eckardt next to co-leader Cem Oezdemir (2nd L) and Juergen Trittin before a party board meeting in Berlin November 12, 2012. Last weekend members of the Green party elected by direct vote Goering-Eckardt and Trittin as top candidates for next year's general election. REUTERS/Tobias Schwarz (GERMANY - Tags: POLITICS) 

Leaders of Germany's environmental Greens Party (Die Gruenen) Claudia Roth (R) and Cem Oezdemir (L) pose with their party members Katrin Goering-Eckardt and Juergen Trittin before a party board meeting in Berlin November 12, 2012. Last weekend members of the Green party elected by direct vote Goering-Eckardt and Trittin as top candidates for next year's general election. REUTERS/Tobias Schwarz (GERMANY - Tags: POLITICS) 

Green Party's Katrin Goering-Eckardt speaks during a news conference in Berlin after she was announced one of the party's top-candidates in next year's general election, November 10, 2012. Members of the Green party elected by direct vote Katrin Goering-Eckardt and Juergen Trittin as top candidates for next year's general election. REUTERS/Thomas Peter (GERMANY - Tags: POLITICS ELECTIONS HEADSHOT) 

Green Party's Juergen Trittin speaks during a news conference in Berlin after he was announced one of the party's top-candidates in next year's general election, November 10, 2012. Members of the Green party elected by direct vote Katrin Goering-Eckardt and Juergen Trittin as top candidates for next year's general election. REUTERS/Thomas Peter (GERMANY - Tags: POLITICS ELECTIONS HEADSHOT) 

Green Party's Katrin Goering-Eckardt (L) and Juergen Trittin leave a news conference in Berlin after they were announced as their party's top-candidates in next year's general election, November 10, 2012. Members of the Green party elected by direct vote Katrin Goering-Eckardt and Juergen Trittin as top candidates for next year's general election. REUTERS/Thomas Peter (GERMANY - Tags: POLITICS ELECTIONS) 

Green Party's Katrin Goering-Eckardt (L) and Juergen Trittin arrive for a news conference in Berlin after they were announced as their party's top-candidates in next year's general election, November 10, 2012. Members of the Green party elected by direct vote Katrin Goering-Eckardt and Juergen Trittin as top candidates for next year's general election. REUTERS/Thomas Peter (GERMANY - Tags: POLITICS ELECTIONS) 

A worker changes the stage design after the Green party announced its top-candidates in next year's general election in Berlin, November 10, 2012. Members of the Green party elected by direct vote Katrin Goering-Eckardt and Juergen Trittin as top candidates for next year's general election. The poster reads: "Green Party election by direct vote 2012'" REUTERS/Thomas Peter (GERMANY - Tags: POLITICS ELECTIONS) 

Green Party's Katrin Goering-Eckardt (L) and Juergen Trittin leave a news conference in Berlin after they were announced as their party's top-candidates in next year's general election, November 10, 2012. Members of the Green party elected by direct vote Katrin Goering-Eckardt and Juergen Trittin as top candidates for next year's general election. REUTERS/Thomas Peter (GERMANY - Tags: POLITICS ELECTIONS) 

Journalists wait for Green Party's Katrin Goering-Eckardt to leave a news conference in Berlin after she was announced one of the party's top-candidates in next year's general election, November 10, 2012. Members of the Green party elected by direct vote Katrin Goering-Eckardt and Juergen Trittin as top candidates for next year's general election. REUTERS/Thomas Peter (GERMANY - Tags: POLITICS ELECTIONS) 

Green Party's Katrin Goering-Eckardt (R) and Juergen Trittin attend a news conference in Berlin after they were announced as their party's top-candidates in next year's general election, November 10, 2012. Members of the Green party elected by direct vote Katrin Goering-Eckardt and Juergen Trittin as top candidates for next year's general election. REUTERS/Thomas Peter (GERMANY - Tags: POLITICS ELECTIONS) 

Green Party's Katrin Goering-Eckardt (R) and Juergen Trittin attend a news conference in Berlin after they were announced as their party's top-candidates in next year's general election, November 10, 2012. Members of the Green party elected by direct vote Katrin Goering-Eckardt and Juergen Trittin as top candidates for next year's general election. REUTERS/Thomas Peter (GERMANY - Tags: POLITICS) 

Green Party's Katrin Goering-Eckardt (R) and Juergen Trittin attend a news conference in Berlin after they were announced as their party's top-candidates in next year's general election, November 10, 2012. Members of the Green party elected by direct vote Katrin Goering-Eckardt and Juergen Trittin as top candidates in next year's general election. REUTERS/Thomas Peter (GERMANY - Tags: POLITICS ELECTIONS) 

Green Party's Katrin Goering-Eckardt (R) and Juergen Trittin attend a news conference in Berlin after they were announced as their party's top-candidates in next year's general election, November 10, 2012. Members of the Green party elected by direct vote Katrin Goering-Eckardt and Juergen Trittin as top candidates in next year's general election. REUTERS/Thomas Peter (GERMANY - Tags: POLITICS ELECTIONS) 

Green Party Secretary General Steffi Lemke speaks during a news conference in Berlin to announce the outcome of an election selecting the party's candidates for next year's general election, November 10, 2012. Members of the Green party elected by direct vote Katrin Goering-Eckardt and Juergen Trittin as top candidates in next year's general election. REUTERS/Thomas Peter (GERMANY - Tags: POLITICS ELECTIONS) 

Green Party Secretary General Steffi Lemke speaks during a news conference in Berlin to announce the outcome of an election selecting the party's candidates for next year's general election, November 10, 2012. Members of the Green party elected by direct vote Katrin Goering-Eckardt and Juergen Trittin as top candidates in next year's general election. REUTERS/Thomas Peter (GERMANY - Tags: POLITICS ELECTIONS) 

Green Party Secretary General Steffi Lemke arrives at a news conference in Berlin to announce the outcome of an election selecting the party's candidates for next year's general election, November 10, 2012. Members of the Green party elected by direct vote Katrin Goering-Eckardt and Juergen Trittin as top candidates in next year's general election. REUTERS/Thomas Peter (GERMANY - Tags: POLITICS ELECTIONS) 

Cem Oezdemir (R) and Juergen Trittin of the Green Party (Die Gruenen) leave the Chancellery after talks between the government and opposition leaders in Berlin June 21, 2012. Angela Merkel's government and the opposition reached an agreement on Thursday on economic growth measures for Europe that will allow Germany's parliament to approve the permanent bailout scheme for the euro zone and the fiscal pact next week.
REUTERS/Thomas Peter (GERMANY - Tags: POLITICS BUSINESS) 

Cem Oezdemir (R) and Juergen Trittin (2nd R) of the Green Party (Die Gruenen) talk to the media after talks between the government and opposition leaders in Berlin June 21, 2012. Angela Merkel's government and the opposition reached an agreement on Thursday on economic growth measures for Europe that will allow Germany's parliament to approve the permanent bailout scheme for the euro zone and the fiscal pact next week.
REUTERS/Thomas Peter (GERMANY - Tags: POLITICS BUSINESS) 

Cem Oezdemir (L) and Juergen Trittin (3rd L) of the Green Party (Die Gruenen) listen to Frank-Walter Steinmeier (foreground R) of the Social Democratic Party (SPD) as he talks to the media after talks between the government and opposition leaders in Berlin, June 21, 2012. Angela Merkel's government and the opposition reached an agreement on Thursday on economic growth measures for Europe that will allow Germany's parliament to approve the permanent bailout scheme for the euro zone and the fiscal pact next week. REUTERS/Thomas Peter (GERMANY - Tags: POLITICS) 

Parliamentary faction co-leader of the opposition Green party (Die Gruenen) Juergen Trittin (C) talks to media outside the chancellery in Berlin before talks between government and opposition leaders about the EU fiscal pact June 21, 2012. Germany's small Left Party said it would lodge a complaint with the constitutional court in an attempt to prevent ratification of the European Union's fiscal pact, which the Berlin government has championed, and the ESM permanent bailout scheme.
REUTERS/Thomas Peter (GERMANY - Tags: POLITICS BUSINESS MEDIA) 

Juergen Trittin (4th L) and Cem Oezdemir (front R) of the opposition Green Party address the media outside the Chancellery in Berlin after talks with German Chancellor Angela Merkel and her coalition partners about the EU fiscal pact, June 13, 2012. Germany's government and opposition parties failed on Wednesday to resolve a row holding up parliamentary ratification of the EU's new fiscal treaty and will resume talks next week, a government source said. REUTERS/Thomas Peter (GERMANY - Tags: BUSINESS POLITICS) 

Juergen Trittin (L) and Cem Oezdemir of the opposition Green Party enter the Chancellery in Berlin for talks with German Chancellor Angela Merkel and her coalition partners about the EU fiscal pact, June 13, 2012. Germany's opposition Social Democrats (SPD) could soon resolve a row with Chancellor Angela Merkel's coalition over a European financial transaction tax and vote for the EU fiscal pact before the summer break, one party leader said on Tuesday. REUTERS/Thomas Peter (GERMANY - Tags: POLITICS BUSINESS) 

A combination picture shows the heads of the leaders of Germany's opposition parties enter the Chancellery in Berlin for talks with German Chancellor Angela Merkel and her coalition partners about the EU fiscal pact, June 13, 2012. Germany's opposition Social Democrats (SPD) could soon resolve a row with Chancellor Angela Merkel's coalition over a European financial transaction tax and vote for the EU fiscal pact before the summer break, one party leader said on Tuesday. Pictured are (L-R): the parliamentary faction leader of the Social Democratic Party (SPD), Frank-Walter Steinmeier, SPD head Sigmar Gabriel, Juergen Trittin and Cem Oezdemir of the Green party and Katja Kipping and Gregor Gysi of the left-wing Die Linke party. REUTERS/Thomas Peter (GERMANY - Tags: BUSINESS POLITICS) 

Cem Oezdemir (R) of Germany's Green Party and party fellow Juergen Trittin leave the Chancellery after talks in Berlin May 24, 2012. German Chancellor Angela Merkel meets the heads of Germany's parliamentary parties on Thursday in a bid to get them on board for the two votes. She fears a delay to approving the pact on budget discipline in Germany would send a disastrous signal to the euro zone. REUTERS/Tobias Schwarz (GERMANY - Tags: POLITICS) 

Cem Oezdemir (R) of Germany's Green Party and party fellow Juergen Trittin leave the Chancellery after talks in Berlin, May 24, 2012. German Chancellor Angela Merkel meets the heads of Germany's parliamentary parties on Thursday in a bid to get them on board for the two votes. She fears a delay to approving the pact on budget discipline in Germany would send a disastrous signal to the euro zone. REUTERS/Tobias Schwarz (GERMANY - Tags: POLITICS) 

Die Gruenen (Green party) faction leader Juergen Trittin (L) shakes hands with Lower Saxony's Prime Minister David McAllister as they arrive for a meeting on nuclear waste storage facilities at the German Environmental Ministry in Berlin April 24, 2012. REUTERS/Tobias Schwarz (GERMANY - Tags: POLITICS ENVIRONMENT) 

Die Gruenen (Green party) faction leader Juergen Trittin arrives for a meeting on nuclear waste storage facilities at the German Environmental Ministry in Berlin April 24, 2012. REUTERS/Tobias Schwarz (GERMANY - Tags: POLITICS ENVIRONMENT) 

Die Gruenen (Green party) faction leaders Juergen Trittin smiles during an interview with Reuters in Berlin March 27, 2012. REUTERS/Fabrizio Bensch (GERMANY - Tags: POLITICS HEADSHOT)