Shares in Monsanto gained, even as the seeds giant rejected a $62bn bid from Germany's Bayer as "financially inadequate", as the US group said that was open, nonetheless, to "constructive" talks on a deal.
Hugh Grant, the Monsanto chairman and chief executive, said that Bayer's $122-a-share offer, which was unveiled on Monday, "significantly undervalues our company".
The deal, which offers a premium of 37% to Monsanto shareholders, also falls short in details on financing, which Bayer has said it will raise through debt and a rights issue, and on winning approval from antitrust regulators, Mr Grant said.
The offer "does not adequately address or provide reassurance for some of the potential financing and regulatory risks related to the acquisition", he said.
However, Monsanto added that it was "open to continued and constructive conversations" over whether a deal in the best interests of its shareholders could be sealed.
"We believe in the substantial benefits an integrated strategy could provide to growers and broader society, and have long respected Bayer's business," Mr Grant said.
Monsanto shares stood 2.7% higher at $108.86 in late deals in New York.
In fact, Bayer, which is in agriculture strong in agrichemicals rather than seeds, would appear one of the best bets, on antitrust grounds, for a tie-up with Monsanto, which has a relatively small presence in sprays, so limiting the threat of a deal raising substantial monopoly concerns.
Monsanto has long espoused the benefits to mergers in the sector, given the growing costs of research and the more difficult market climate prompted by low crop prices, and indeed last year failed in an effort to buy Swiss rival Syngenta.
Bayer on Monday said it was "prepared to proceed immediately with due diligence and negotiations and to achieve an agreed transaction" with Monsanto.
The German group also said it was "highly confident in its ability to finance the transaction based on advanced discussions with and support from its financing banks".
And on antitrust, Bayer said it had "a successful track record of working with global authorities to secure the necessary regulatory approvals".
By Mike Verdin