Two relatively recent independent developments stand behind the current major research effort on nitrogen fixation, the process by which bacteria symbiotically render leguminous plants independent of nitrogen fertilizer. The one development has been the rapid, sustained increase in the price of nitrogen fertilizer. The other development has been the rapid growth of knowledge of and technical sophistication in genetic engineering. Fertilizer prices, largely tied to the price of natural gas, huge amounts of which go into the manufacture of fertilizer, will continue to represent an enormous and escalating economic burden on modern agriculture, spurring the search for alternatives to synthetic fertilizers. And genetic engineering is just the sort of fundamental breakthrough that opens up prospects of wholly novel alternatives. One such novel idea is that of inserting into the chromosomes of plants discrete genes that are not a part of the plants' natural constitution: specifically, the idea of inserting into nonleguminous plants the genes if they can be identified and isolated, that fit the leguminous plants to be hosts for nitrogen-fixing bacteria. Hence, intensified research on legumes.
Nitrogen fixation is a process in which certain bacteria use atmospheric nitrogen gas, which green plants cannot directly utilize, to produce ammonia, a nitrogen compound plants can use. It is one of nature's great ironies that the availability of nitrogen in the soil frequently sets an upper limit on plant growth even though the plants' leave are bathed in a sea of nitrogen gas. The leguminous plants - among them crop plants such as soybeans, peas, alfalfa, and clover - have solved the nitrogen supply problem by entering into a symbiotic relationship with the bacterial genus Rhizobium; as a matter of fact, there is a specific strain of Rhizobium for each species of legume. The host plant supplies the bacteria with food and protected habitat and receives surplus ammonia in exchange. Hence, legumes can thrive in nitrogen-depleted soil.
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
(A) expose the fragile nature of the foundations on which the high yields of modern agriculture rest
(B) argue that genetic engineering promises to lead to even higher yields than are achievable with synthetic fertilizers
(C) argue that the capacity for nitrogen-fixing symbioses is transferable to nonleguminous plants
(D) explain the reasons for and the objectives of current research on nitrogen-fixing symbioses
(E) describe the nature of the genes that regulate the symbiosis between legumes and certain bacteria
21. According to the passage, there is currently no strain of Rhizobium that can enter into a symbiosis with
(A) alfalfa
(B) clover
(C) maize
(D) peas
(E) soybeans
22. The passage implies that which of the following is true of the bacterial genus Rhizobium?
(A) Rhizobium bacteria are found primarily in nitrogen-depleted soils.
(B) Some strains of Rhizobium are not capable of entering into a symbiosis with any plant.
(C) Newly bred varieties of legumes cannot be hosts to any strain of Rhizobium.
(D) Rhizobium bacteria cannot survive outside the protected habitat provided by host plants.
(E) Rhizobium bacteria produce some ammonia for their own purposes.
23. It can be inferred from the passage that which of the following was the most influential factor in bringing about intensified research on nitrogen fixation?
(A) The high yields of the Green Revolution
(B) The persistent upward surge in natural gas prices
(C) The variety of Rhizobium strains
(D) The mechanization of modern agriculture
(E) The environmental ill effects of synthetic fertilizers
24. Which of the following situations is most closely analogous to the
situation described by the author as one of nature's great ironies?
(A) That of a fanner whose crops have failed because the normal midseason rains did not materialize and no preparations for irrigation had been made
(B) That of a long-distance runner who loses a marathon race because of a wrong turn that cost him twenty seconds
(C) That of shipwrecked sailors at sea in a lifeboat, with one flask of drinking water to share among them
(D) That of a motorist who runs out of gas a mere five miles from the nearest gas station .
(E) That of travelers who want to reach their destination as fast and as cheaply as possible, but find that cost increases as travel speed increases
25. According to the passage, the ultimate goal of the current research on nitrogen fixation is to develop
(A) strains of Rhizobium that can enter into symbioses with existing varieties of wheat,rice, and other nonlegumes
(B) strains of Rhizobium that produce more ammonia for leguminous host plants than do any of the strains presently known
(C) varieties of wheat, rice, and other nonlegumes that yield as much as do existing varieties, but require less nitrogen
(D) varieties of wheat, rice, and other nonlegumes that maintain an adequate symbiotic relationship with nitrogen-fixing bacteria and produce high yields
(E) high-yielding varieties of wheat, rice, and other nonlegumes that are genetically equipped to fix nitrogen from the air without the aid of bacteria
26. The author regards the research program under discussion as
(A) original and extensive but ill-defined as to method
(B) necessary and ambitious but vulnerable to failure
(C) cogent and worthwhile but severely underfunded
(D) prohibitively expensive but conceptually elegant
(E) theoretically fascinating but practically useless
27. Most nearly parallel, in its fundamental approach, to the research program described in the passage would be a program designed to
(A) achieve greater frost resistance in frost-tender food plants by means of selective breeding, thereby expanding those plants' area of cultivation
(B) achieve greater yields from food plants by interplanting crop plants that are mutually beneficial
(C) find inexpensive and abundant natural substances that could, without reducing yields, be substituted for expensive synthetic fertilizers
(D) change the genetic makeup of food plants that cannot live in water with high salinity, using genes from plants adapted to saltwater
(E) develop, through genetic engineering, a genetic configuration for the major food plants that improve the storage characteristics of the edible portion of the plants
14 Feb 2020, 04:04
100% (03:40) correct
