Syngenta AG, the Swiss agricultural chemicals maker, was seen opening up more than 7 percent after a source said Monsanto Co. had sweetened its takeover offer, according to premarket indicators in Zurich. U.S.-based Monsanto increased its offer to buy Syngenta to around $47 billion, or 470 Swiss francs ($503.70) per share from 449 francs per share previously, a person familiar with the matter said on Monday. Syngenta officials did not return a request for comment on Tuesday morning about whether a sweetened deal from Monsanto would convince them to agree to discuss a transaction.