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IX. Programming in Heterotic Hybrids

IX. Programming in Heterotic Hybrids

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HETEROSIS



169



The most fortunate and interesting aspect of the present analysis is that

almost all characters associated with better development, growth, and yield

of the plant are either dominant or partially dominant in F, hybrids. This

provides every chance to a plant breeder to play with his genetic stock

if he is aware of the genetics of these characters. Inevitably most of these

characters would be possibly multiallelic for the same locus to account

for the tremendous variation in each component character.



X.



Future Outlook



The analysis provided in this review, we believe, is the exposition of the

enigma of “heterosis,” which turns out to be a mirage. The phenomenon

of hybrid vigor, homeostasis, inbreeding depression, and general and specific combining ability can be explained on the basis of known principles

of genetics, biochemistry, and physiology.

The “directed heterosis” and its fixation appear to be distinct possibilities. However, we will have to remember that the yield heterosis based

on characters of low heritability, such as ears per plant in wheat, rice,

oats, barley, etc., or the fruits on tertiary and quarternary branches as in

mustard and various pulses will be difficult to obtain in the field. In these

crops, emphasis will have to be on increasing the productivity of individual

ears or increasing the potential of fruits on primary and secondary branches

along with the productivity of individual fruits. Any attempt to obtain

“heterosis” in any chemical constituent on per unit weight basis may also

be only an adventure.

This review provides a blueprint €or planned development of high-yielding genotypes in various crop plants. We hope that this analysis will induce

animal scientists to come forward with a similar physiological and biochemical analysis of their systems to enable understanding of the genetic

basis of heterosis in animals.

ACKNOWLEDGMENT

We thank Drs. Y. P. Abrol, S. Bhaskaran, V. L. Chopra, H. K. Jain, and K. R.

Sarkar for their discussions and constructive criticism. We are Grateful to Dr.

N. G. P. Rao for providing us seed material and encouragement.

The senior author records his thanks to his students and colleagues, Mrs. Renuka

Sanghi (nee Ghose), Dr. N. N. Singh, Mr. M. C. Ghildiyal, Dr. V. Balasubramanian,

Dr. V. Rajagopal, and Miss P. Shanthakumari for their valuable contributions.

We appreciate the cooperation of the Crop Science Society of America, the Annals

of Botany, the Iowa Agriculture Research Station, the Annals of Applied Biology,

the Indian Journal of Genetics and Plant Breedings, and several authors who very

kindly granted permission to reproduce the published data.



170



SURESH K. SINHA AND RENU KHANNA



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FERTILIZERS FOR USE UNDER TROPICAL CONDITIONS

0. P. Engelstad and D. A. Russel

Division of Agricultural Development, National Fertilizer Development Center,

Tennessee Valley Authority, Muscle Shoals, Alabama



Introduction ..........................................................

Brief Description of Tropics.. ...........................................

A. Climate and Soils.. ................................................

B. Types of Agriculture.. .............................................

C. Fertilizer-Related Infrastructure. ....................................

Ill. History of Fertilizer Use in the Tropics.. .................................

A. Time Trends in Consumption and Production.. .......................

B. Shifts in Production and Consumption of Nutrient Sources.. ...........

IV. Agronomic Considerations.. ............................................

A. Some Causes of Low Fertilizer Effectiveness and Possible Solutions.. ....

B. Research Needs in Fertilizer Use and Soil Fertility.. ...................

V. Fertilizer Technology Developments.. ....................................

References.. ..........................................................

1.



II.



175

176

176

179

181

182

182

185

186

186

197

202

204



I. Introduction



Accelerated population growth is still the leading problem facing the

human race. Continued growth places a heavy strain on traditional foodproducing areas of the world, particularly during periods of adverse

weather. It is now imperative that agricultural production be enhanced in

tropical regions. One of the important inputs in this effort will be fertilizers,

particularly with heavier population pressure forcing the abandonment of

shifting cultivation.

The objective of this chapter is to relate fertilizers to conditions found

in the tropics. It should not be inferred that problems in fertilizer use in

the tropics are different in principle from those in the temperate regions.

Rather the differences are more in degree of severity; i.e., problems in

fertilizer use and effectiveness in the tropics are often more serious and

restricting than their counterparts in temperate regions. Adaptive research

is needed therefore to determine how best to use fertilizers effectively under

local conditions.

Climatic conditions vary widely in the tropics. This, coupled with a great

variation in age and character of parent material, leads to wide diversity

175



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in soils. Likewise, a great variety of crops is grown in the tropics under

different cropping systems. Several texts have already been published that

provide rather specific guides to fertilization of tropical crops (De Geus,

1967, 1970; FAO, 1958; Jacob and von Uexkull, 1963). It is not intended

to duplicate these contributions here, but rather to relate known fertilizer

characteristics and behavior to problems associated with soils and climates

found in tropical regions. Some emphasis also is placed on efficiency of

fertilizer use under these tropical conditions; this is especially critical during periods of shortages and high prices. Hopefully, this emphasis brings

out principles involving fertilizers and their use without need for great

detail.



II.



Brief Description of Tropics



The region between the Tropic of Cancer and the Tropic of Capricorn

is regarded simply as “the Tropics.” These boundary lines are parallel to

and 23O 28‘ north and south of the equator. This region accounts for about

40% of the earth’s surface and receives over half of the world’s total rainfall; three-quarters of the area is covered with water (Wrigley, 1969).

A.



CLIMATE

AND SOILS



Climate is the most unique aspect of the tropics and is more variable

than commonly supposed. The main types of tropical weather patterns are

described in some detail by Webster and Wilson (1966) and in the President’s Science Advisory Committee Report ( 1967). Table I was adapted

from the latter publication, showing the major types of tropical climates

and their importance in terms of land area over the tropics. This variability

in rainfall patterns over the tropics contributes to widely differing soils,

vegetation, and agricultural patterns. Table 11, also adapted from the above

committee report, shows the area of principal kinds of soils by the major

tropical climatic regions.

The emphasis in this review is on the humid tropics, since they have

more agricultural potential than the desert regions. Soils found in the humid

tropics range from the slightly weathered Entisols to the highly weathered

Oxisols. Weathering of intermediate intensity is represented by the Inceptisols, Alfisols, and Ultisols (Aubert and Tavenier, 1972). It should be stressed

that the most highly weathered soils, while important, are by no means

dominant; this requires that more specific terminology be used than merely

“tropical soils.’’ Buol (1973) stated that the only parameter common to

all tropical soils is a relatively uniform temperature regime; the annual



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IX. Programming in Heterotic Hybrids

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