Glossary Plant Breeding

Physiological Homeostasis. A type of buffering reaction of the organism against
fluctuating environmental conditions by internal self-regulatory systems that resist
those changes either by varying phenotypically or by remaining constant in the face of
environmental changes. It is frequently associated with heterozygosity of genotypes.
Physiological Races. Pathogens of the same species with similar or identical
morphology but differing pathogenic capabilities.
Physiological Threshold. The limit beyond which physiological functioning are affected
so adversely that individuals can no longer survive and thus selection is powerless to
effect any change even in the presence of substantial genetic variation in the
population.
Phytoalexins. The biochemical substances (phenolic compounds) formed by the host
plant in response to injury, physiological stimuli, infectious agents or their products
that accumulate to levels which inhibit the growth of microorganisms and repel pests
and pathogens, e.g., DIMBOA in maize and onions, pisatin in peas, phaseolins in
beans, rishtin and phytotuberin in potatoes. These biochemical substances confers
resistance on the concerned hosts against respective pests or pathogens.
Phytoplankton. Minute floating plants, usually algae, distributed throughout the pond as
deep as light penetrates.
Phytotoxins. Chemical compounds that kill plants. These compounds are produced by
pathogens. The resistant hosts in such cases are insensitive to toxins. For example, the
causal organism of victoria blight in oat, Drecheslera victoriae, produces a toxin
called ‘victorin’. Resistant varieties of oats are resistant to this toxin.
Phytotron. A controlled environment facility to assist plant scientists in conducting crop
breeding and crop management studies without any constraint of seasons and
locations. This unique facility has been created at IARI, New Delhi in 1997. It has an
area of 2500 m 2 , 22 growth chambers and 10 green houses where temperature,
relative humidity, CO 2 levels, and light can be controlled to produce a wide range of
simulated conditions. Further, by manipulating these components, plants can be raised
up to maturity whenever required. It is particularly suited to rain-fed and other noncongenial
production regimes inhabited mostly by resource-poor farmers. It can
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