chapter 8 
Page 
Perspective on Plant Breeding 137 
The Plant Breeder’s Approach to 
Commercialization of New Varieties 138 
Major Constraints on Crop Improvement 139 
Genetic Technologies as Breeding Tools 140 
New Genetic Technologies for Plant Breeding . 141 
Phase I: Tissue Culture to Clone Plants 141 
Phase II: Engineering Changes to Alter 
Genetic Makeup; Selecting Desired Traits. . 144 
Phase III: Regenerating Whole Plants From 
Cells in Tissue Culture 146 
Constraints on the New Genetic Technologies . 149 
Technical Constraints 149 
Institutional Constraints 150 
Impacts on Generating New V'arieties 151 
Examples of New Genetic Approaches 152 
Selection of Plants for Metabolic Efficiency . . . 152 
Nitrogen Fixation 152 
Genetic Variability, Crop Vulnerability, and 
Storage of Germplasm 154 
The Amount of Genetic Erosion That Has 
Taken Place 154 
The Amount of Germplasm Needed 154 
The National Germplasm System 155 
The Basis for Genetic Uniformity 157 
Six Factors Affecting Adequate Management 
of Genetic Resources 158 
Page 
Summary 160 
Issues and Options— Plants 161 
Technical Notes 162 
Tables 
Table No. Page 
24. Average Yield per Acre of Major Crops in 
1930 and 1975 137 
25. Some Plants Propagated Through Tissue 
Culture for Production or Breeding 143 
26. Representative List of Tissue Culture 
Programs of Commercial Significance in 
the United States 143 
27. Gene Resource Responsibilities of Federal 
Agencies 155 
28. Estimated Economic Rates of Return From 
Germplasm Accessions 156 
29. Acreage and Farm Value of Major U.S. Crops 
and Extent to Which Small Numbers of 
Varieties Dominate Crop Average 157 
Figures 
Figure No. Page 
28. The Process of Plant Regeneration From 
Single Cells in Culture 147 
29. A Model for Genetic Engineering of Forest 
Trees 149 
