Vergara - 1976 - Physiological and morphological adaptability of ri

PHYSIOLOGICAL AND NIORPIIOLOGICAL ADAPTABILITH’ 69
The plant characters necessary for wide adaptability of the rice plant as cited
here deal mainly vrith irrigated lowland rice where water is usually not a limiting
factor. The list does not include the other hydro-emrironmental conditions under
WhlCll the ricc plants are also grown, such as floating-ricc areas (rnorc than I50
cm deep), deep-ivater rice areas (SO-ISO cm deep), and "upland" rice areas. In
these places. specific adaptability is needed.
Drought resistance
Resistance to drought or ability to recover from moisture stress is an important
plant character for specific adaptability to rainfed lowland or upland rice areas.
Different types of specific adaptability are needed by the rice plant if it has to
survive in the special habitat where it is being grown, such as under coconut
trees or 5 meters of water,
An adaptive mechanism necessary for the floating-rice varieties is the ability
to elongate. While insensitivity to photoperitid is necessary‘ under loo/land and
upland conditions for wide adaptability, sensitivity to photopcriod, on the other
hand. is required for specific adaptability in the floating-rice areas. The long
growth duration (180 to 200 days) necessary for the floating-rice areas requires
sensitivity to photopcriod. since there are no known photopcriod-insensitive
varieties with groivth duration longer than 150 day's.
The varieties capable of producing very high grain yields under conditions of
widely: fluctuating seasonal environments have many differences in growth form
and phyisitiltigy. Although one can define certain characteristics of plants
adapted to specific environments, it is difficult if not impossible to define all the
possible combinations of characteristics necessary to provide good general
adaptability to a utidely fluctuating set of seasons (Finlay and Wilkinson, 1963).
Hence. the climatic factors have been divided into day length, wind, solar radiation,
precipitation, and temperature; and no attempt is made in this paper to
define all the possible combinations to provide general adaptability‘.
Most of the discussion will involve specific rather than general adaptability.
DAY LENG TI I
Like the wild rice, the early cultivated-rice varieties were grown in lovv-ly-‘ing
areas where water is available; and in many cases it is the only crop that can be
grown. because of too much water To avoid the cumbersome method of harvesting
rice while the water is still deep or the weather is too wet for drying of grain,
varieties have been selected that mature when the water has receded or when the
soil is starting to dry out.
In photoperiod-sensitivc varieties. delay in transplanting. resulting from late
rains, will not affect grain yields under lowland conditions. In this sense the
photopcriodscnsitive or the long growtlrduration varieties have a more flexible
seedling stage.
Before the introduction of the high-yielding ricc varieties in the tropics. the