Unfortunately, most of the major food crops including maize, wheat, rice, and potatoes cannot. On the contrary, many of the high-yielding hybrid varieties of these food crops bred during the Green Revolution of the 1960 s were selected specifically to give high yields in response to generous applications of nitrogen fertilizer. This poses an additional, formidable challenge to plant geneticists: they must work on enhancing fixation within the existing symbioses. Unless they succeed, the yield gains of the Green Revolution will be largely lost even if the genes in legumes that equip those plants to enter into a symbiosis with nitrogen fixers are identified and isolated, and even if the transfer of those gene complexes, once they are found, becomes possible. The overall task looks forbidding, but the stakes are too high not to undertake it.

　　20. The primary purpose of the passage is to

　　expose the fragile nature of the foundations on which the high yields of modern agriculture rest

　　argue that genetic engineering promises to lead to even higher yields than are achievable with synthetic fertilizers

　　argue that the capacity for nitrogen-fixing symbioses is transferable to nonleguminous plants

　　explain the reasons for and the objectives of current research on nitrogen-fixing symbioses

　　describe the nature of the genes that regulate the symbiosis between legumes and certain bacteria

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