Vergara - 1976 - Physiological and morphological adaptability of ri

PHYSIOLOGICAL non) MORPHOLOGICAI. ADAPTABILITE’ 79
performance, as evidenced in 1R8 and subsequent liaes released by the International
Rice Research Institute. Of course, pests and diseases should also be
considered. and these would make an entirely different concept of physiological
yield stability
In developing varieties better adapted to different environmental conditions,
the plant breeder is faced with the choicc of breeding either for specific ecological
conditions (ie. high- or levy-temperature areas) or for more varied conditions,
which include a considerable range of environment (ie. from upland rice to
lowland rice and deep-tvater rice). The latter approach requires varieties possess~
ing genera] adaptability. In rice, De Datta and Abilay (1974) have shown the
general adaptability of certain varieties to different water regimes. Making rice
plants adapted to temperature extremes (l2° to 17°C vs. 40° to 45°C) might be
harder than increasing adaptability to different levels of water. According to the
hypothesis ofAlexandrov (1967). the adaptation of a species to low temperature
will require an increase in the flexibility of the macrostrueture of the protein
molecules, which in turn will make them less stable to intense heat denaturation.
Increasing tolerance to low temperatures would decrease resistance to high
temperatures.
General adaptability even in a uniform edaphic environment is important
since there is a marked fluctuation of climatic conditions from season to season
(Finlay and Wilkinson. 1963). It is dificult to define all the possible combinations
of characteristics necessary to provide good general adaptability to a “ridely
fluctuating set tifseastins. Finlay and Wilkinson (1963) have devised a formula
for computing but not for characterizing an adaptable variety‘, based on 1nultilocation
planting. Some workers, however, have found this method to be not
completely workable for rice (Okuno et al.. 19H).
Using the same formula, Das and Jain (1971) tested wheat varieties which are
known to differ greatly’ in their adaptability‘ through multi-location yield tests.
They examined the phenotypie stability of several morphological characters
when wheat was grown under different agronomic conditions at a single location.
'l'he highly adapted wheat varieties showed a high degree of phenotypie stability’
for a number of its morphological characters. This is essentially the practice
being used by the plant breeders and agronomists at the International Rice
Research Institute.
Increasing adaptability to moisture stress by pro-sowing hardening of plants
to increase their ability to endure dehydration and overheating is a method being
used in many agricultural crops (Yarwood, 1967'. Henckel, 1970). The hardened
plants can use the metabolic water better than the non-hardened plants for
hydrating protoplasmic polymers during drought. Pre-sott-"ing hardening, according
to Henckel (1970). leads to fundamental physiological reorganization
and adaption of the plant without leading to a sharp reduction in plant growth
and productivity. Whether pre-sovtring hardening will tvork in rice is not known.
Henckel (1970) stressed that not all plants are able to ivithstarid the process.
Another ptissible method of increasing the general adaptability of the crop is