Showing papers in "Advances in Agronomy in 1983"

Mycorrhizas and their Significance in Nodulating Nitrogen-Fixing Plants

Abstract: Publisher Summary This chapter discusses the role of mycorrhizas in the growth and nutrition of nitrogen-fixing nodulated plants. It reviews some general, well-established principles on mycorrhizal types, morphology, physiology, and function. The interactions between nodular and mycorrhizal endophytes related to the formation and effects of these dual symbioses that greatly enhance the development of the common host plant are discussed. It presents the ecological significance of plants bearing the two types of symbioses, with emphasis on the possibilities of harnessing them to increase crop yield. Mycorrhizal associations play an important role in the growth and nutrition of higher plants. It is recognized that this symbiosis can be harnessed to improve nutrient cycling and crop productivity by reducing industrial fertilizer inputs, thereby conserving and reducing environmental costs. Mycorrhizal infection can help plants to become reestablished in eroded or degraded habitats, to thrive in arid conditions, to deter pathogens, and to cope with various stress situations. The common mycorrhizal types, the vesicular-arbuscular mycorrhizas (VAM), are nearly omnipresent and are studied intensively throughout the world. VAM enhance growth, nodulation, and nitrogen fixation in grain and forage legumes, crops of the greatest interest for food production in the biosphere.

Current Status and Future Prospects for Breeding Hybrid Rice and Wheat

Abstract: Publisher Summary This chapter reviews the present status and future prospects for breeding hybrid rice and wheat. Rice and wheat together constitute the world's two most important food crops. Wheat is the staple food of many developed countries and constitutes the leading cereal in terms of total world production. Because of the importance of rice in many developing countries, especially in Asia, considerable effort has been expended during the past decades to develop varieties capable of responding to improved management and to reduce production hazards through the incorporation of genes resistant to major biological, chemical, and physical stresses. The successful development of hybrid maize in the 1930s provided an important impetus for breeders of other crops, including self-pollinating cereals such as wheat, rice, barley, and cross-pollinating sorghum, to utilize the principles of hybrid production. Heterosis was first reported in wheat when it was found that F1 plants were generally taller than the tall parent. The major yield components in rice and wheat are number of panicles per square meter, spikelet number per panicle, spikelet fertility percentage, and 1,000-grain weight. Significant positive mid-parent, high-parent, or standard heterosis has been observed for one or more of these components in a number of rice crosses.

The Convergent Evolution of Annual Seed Crops in Agriculture

Abstract: Publisher Summary This chapter considers that the yield potential of annual crop species increase at a faster rate than with empirical selection for yield if suitable ideotypes are identified. A considerable list of common features and practices that influence yield in all annual seed crops is identified, and it is possible to design a basic ideotype for all these crops, involving principles of crop physiology and associated agronomic practices equally applicable to any annual seed crop. A sharp distinction is drawn between the ecology of annual field crops grown for their seed and that of most horticultural crops. Horticultural plants are cultivated for their fruits, unripe seeds, roots, stems, or leaves. The chapter recognizes several categories within Darwin's general processes of selection by man and natural selection. These are discussed in relation to annual seed crops. It is evident that all crops have been subject to many similar selective and evolutionary processes during domestication. They have become adapted to both the natural environment of the region and the manmade environment of local cultural methods. Crop yield is a man-made concept. It does not necessarily relate to natural selection or to crop evolution, and it is expressed by the nonbiological criterion of weight of product per unit area. The harvest in some crops is a vegetative part whereas in others it is a reproductive organ.

Herbicide Antidotes: Development, Chemistry, and Mode of Action

Abstract: Publisher Summary This chapter discusses the development, chemistry, and mode of action of herbicide antidotes. Organic herbicides represent the most effective weapon available to farmers worldwide in their war against weeds. A fundamental reason for the widespread use of these chemicals in modern agriculture is their ability to control selectively a wide spectrum of weeds in a variety of crops. The concept of using herbicide antidotes offers a potential alternative for increasing the selectivity of available herbicides. A desirable herbicide antidote is a chemical agent that selectively protects crops from herbicide injury without protecting weeds. This selectivity is the result of either a very specific crop–herbicide–antidote interaction or a selective treatment such as the dressing of crop seeds with the antidote. Herbicide antidotes are developed primarily through random screening techniques that involve most combinations of important herbicides, major crops, and candidate antidotes. The timing of herbicide and antidote applications to the crop as well as differential intraspecific tolerance of crop cultivars to combinations of herbicides plus antidotes needs to be established for optimum effectiveness of herbicide antidotes in the field.

Nitrogen Availability Indexes for Submerged Rice Soils

Abstract: Publisher Summary This chapter reviews the methods proposed for assaying the nitrogen-supplying capacity of wetland rice soils and recommends those methods that have potential for predicting soil nitrogen (N) availability, thus making possible the judicious and efficient use of fertilizer nitrogen for rice production. It discusses the mineralization process that is basic to soil nitrogen availability to wetland rice. The mineralizable N pool in soils plays a dominant role in N nutrition of wetland rice. Studies using 15 N-labeled fertilizers show that approximately one-half to two-thirds of the total N utilized by a rice crop, even in well-fertilized rice paddies, comes from the soil-mineralizable N pool. Numerous biological and chemical laboratory methods have been proposed for predicting soil N availability to various crops, including rice. Mineralization of organic nitrogen, which does not proceed past ammonium production in wetland rice soils, is the most important biological process that is involved in the availability of soil N to rice grown under submerged conditions. The chapter discusses simple equations describing the relationships between the amounts of NH 4 + –N released under waterlogged conditions and environmental factors such as temperature and other soil characteristics. These relationships have been formulated from studies with diverse soils from a particular region and are limited in that they have not been widely tested.

Protein Transformation in Soil

Abstract: Publisher Summary Protein degradation is carried out mostly by microorganisms in conjunction with mesofauna such as earthworms and insect larvae. The mineralization of protein has a considerable effect on soil fertility and soil scientists have been studying microbial proteolysis in situ and in vitro in order to understand the genesis of soil nitrogen. This chapter provides an evaluation of the work done in this area and to point out its significance in the study of soil dynamics. It also discusses the transformations proteins undergo in the presence of biotic and abiotic components, and how these reactions may influence protein decomposition and nitrogen availability. Most of the nitrogen present in unfertilized soils is organic in nature. This organic nitrogen represents an important nutrient reservoir and a large part of it appears to be derived from protein. Sulfur and nitrogen are added to soil through the decomposition of methionine and cysteine. Some research suggests that amino compounds contribute to the structure of soil by their incorporation into humic acid, from which they are slowly released by enzymes. After transformation, they are used as plant nutrients. The breakdown of protein is crucial for composting, the development of soils, the digestion of sewage sludge, and the biodegradation of solid waste.

Thermodynamics and Potassium Exchange in Soils and Clay Minerals

Abstract: Publisher Summary This chapter reviews stimulating research on potassium exchange in soil and clay minerals. The essential elements of a thermodynamic analysis of cation-exchange equilibria are presented and their application to potassium exchange in soil and clays are examined. The useful applications have been limited to characterizing the exchange properties of soils and establishing the selectivity of various soils and clays for potassium. No thermodynamic parameter has been found to predict crop yield or response to K + fertilizer from soil K + measurements. The use of calorimetrically measured enthalpies of exchange to detect small amounts of impurities in formerly pure clay minerals aids the clay mineralogist. It may also aid agriculturalists, because a more detailed picture of soil mineralogy helps to explain and predict the reaction of a soil when K + fertilizer is added. One of the most important areas of agricultural research is the modeling of soil-plant processes.

Applications of Induced and Spontaneous Mutation in Rice Breeding and Genetics

Abstract: Publisher Summary This chapter reviews some of the applications of induced and spontaneous mutation in rice improvement. Induced semidwarf mutants are being used widely in breeding programs in Japan. The most useful types of mutants in rice have been characters that were simply inherited and usually controlled by single recessive genes: semidwarfism, early maturity, waxy endosperm, genetic male sterility, and various phenotypic markers such as hull color. Such mutants have been used in three general modes: for direct release as improved cultivars, for donor gene sources in standard cross-breeding or hybridization programs, and for developing near-isogenic comparisons for testing agronomic and physiologic hypotheses. Mutants are very useful in situations where only one or two simple changes in well-adapted local cultivars are needed, especially when the local cultivar carries gene complexes adapted to modem agriculture. Such complexes include cold tolerance, grain quality, insect or pest resistance, and tolerance to environmental stress. An application of induced mutation is the induction of genetic male sterility for use in facilitating crossing in population improvement schemes.

Buffalo Gourd and Jojoba: Potential New Crops for Arid Lands

Abstract: Publisher Summary This chapter reviews the progress made in developing two potential crops for arid lands: buffalo gourd and jojoba. These plants have different attributes and require different cultural techniques, but they both appear to offer considerable potential for becoming important new crops for warm, arid regions of the world. These crops are profitable to produce on a very large scale and are well adapted to mechanization. The grain crops are not particularly perishable and can be stored and transported with relative ease. Each species is particularly adapted to large growing areas where they produce excellent yields. The wheat plant that dominates the semiarid croplands of the world fills the need in these areas for a cultivated crop with a lower demand for water and a greater tolerance for drought. Rice, a grain crop adapted to growing in fields flooded by water and requiring a long growing season to mature, is the staple grain of much of the world's tropical agriculture. These and other major staple world crops are grown in the most desirable agricultural areas as determined by soil and climate. The potential for any new crop to achieve an established position in the highly competitive agricultural and industrial economy of the United States requires outstanding attributes as well as a long, adequately funded period of research and development.

Submicroscopic Examination of Soils

Abstract: Publisher Summary This chapter discusses the submicroscopic examination of soils that makes possible the in situ investigation of soil materials in thin sections, soil peds, soil aggregates, minerals, and other entities. Electron microscopy, with or without equipment for in situ microchemical analysis, is used for the study of clays, weathered and newly formed minerals, organic matter, and soil structure and fabric. Ion microscopy and laser-microprobe mass-analysis techniques make it possible to study both trace and major chemical elements, which is not possible with electron microscopy. Submicroscopic studies can provide microscale in situ information, whereas X-ray diffraction and wet-chemical analyses are done on bulk and disturbed samples. Information from light microscopy that precedes submicroscopy during the in situ study of soil materials is a powerful determination technique when correlated with data from X-ray diffraction and wet chemistry. Submicroscopy with the scanning electron microscope, especially after the introduction of new detector systems, can be used to study the form of pores and minerals in a thin section if used in combination with equipment for image analysis. Thin sections are used to investigate chemical elements of soil particles.