A deal would give Monsanto, which dominates the market for seeds and genetically-modified crops, access to lucrative crop protection chemicals and create an industry behemoth with combined sales of more than $31 billion. Syngenta rebuffed the cash-and-shares offer, saying it undervalued its prospects and did not fully take into account regulatory risks. Monsanto, which earlier confirmed it had made an offer, declined to comment on whether it would improve the bid. A spokesman for Syngenta declined to comment.