Monsanto Co., the world’s largest seed company, turned down Bayer AG’s $62 billion acquisition bid as “incomplete and financially inadequate” on Tuesday, but said it was open to engage further in negotiations.

Monsanto’s decision, first reported earlier on Tuesday by Reuters, puts pressure on Bayer to decide whether to raise its bid, even as the company faces criticism from some shareholders that its $122-per-share cash offer is already too high. The other options are to walk away, or mount a hostile bid.

Bayer offered no immediate comment.

Monsanto shares rose 2.5 percent to $108.70 in afternoon trading in New York, but remained far below Bayer’s bid price, underscoring some investor skepticism that a deal can be done. Bayer shares rose 3.23 percent at 87.15 euros in Frankfurt.

“We believe in the substantial benefits an integrated strategy could provide to growers and broader society, and we have long respected Bayer’s business,” Monsanto Chief Executive Hugh Grant said in a statement.

“However, the current proposal significantly undervalues our company and also does not adequately address or provide reassurance for some of the potential financing and regulatory execution risks related to the acquisition,” he added.

It was not clear what price Monsanto would be willing to sell for. Several analysts have suggested Bayer would have to pay much more than it is currently offering to clinch a deal.

“We believe it is unlikely that the deal gets done at $122 and still believe $135 is a more likely price,” JPMorgan analysts wrote in a research note last week.

Global agrochemicals companies are racing to consolidate, partly in response to a drop in commodity prices that has hit farm incomes. Seeds and pesticides markets are also increasingly converging. This has driven Monsanto to consider a tie-up to build strength.

Monsanto approached Bayer in March to express interest in its crop science unit, Reuters reported at the time. Among the possibilities discussed were an outright acquisition of the crop science unit and a joint venture, or other type of partnership between the two companies.

ChemChina plans to buy Switzerland’s Syngenta for $43 billion, after Syngenta rejected a bid from Monsanto. Dow Chemical Co. and DuPont are forging a $130 billion business.

With German rival BASF SE having previously considered a tie-up with Monsanto, Bayer has moved to avoid being left behind.

Leverkusen-based Bayer’s unsolicited bid for Monsanto is the largest all-cash takeover on record, according to Thomson Reuters data, just ahead of InBev SA’s $60.4 billion offer for Anheuser-Busch in June 2008.

Bayer said on Monday it would finance its cash bid with a combination of debt and equity.