Vergara - 1976 - Physiological and morphological adaptability of ri

308 CLINIATE AND RICE
of the plant to submergence. Yamada (1959) reported that nitrogen topdressing
before submergence caused a large decrease in resistance to submergence
because of the decrease in starch content of the plant. Plants grown on 10W
nitrogen had higher carbohydrate content and higher percentage of survival
when submerged (Palada and Vergara, 1972).
Longer duration of submergence. higher water temperature, greater turbidity.
lOW6T light intensity’, higher soil nitrogen. and deeper submergence can decrease
the carbohydrate content of the plant and thus decrease the percentage of
suntival under submerged conditions.
PHYSIOLOGICAL BASIS OF RESISTANCE TO SUBMERGENCE
The mechanism of reaction involved in elongation of the intemode when xvater
level rises is different from the reaction regarding resistance to submergence
(with or without intcrnodc elongation)
When the plant is submerged. lhe internal oxygen content is decreased so that
anaerobic respiration is promoted, resulting in an increased consumption of
respiratory substrates until the available carbohydrates are exhausted. Starch
contained in the plant was used up first, then the reducing and non-reducing
sugars, follottved by some portions of the acid-hydrolyvtable polyjsaccharides
(Yamada et a1, 1954; Yaniada. 1959). Based on these findings. Yamada (1959)
proposed that the resistance of the rice plant to complete subrnergence is determined
by the quantity of respiratory substrates contained in the plant immediately
before submergence. the respiratory rate which determines the rate of
consumption of the substrates, and the extent of protein hydrolysis. Protein
hjrdrtilvsis tends to decrease with deficiency’ of (my gen.
In examining several varieties. Yamada and Ota (1956) found that resistance
to submergence is determined by the carbohydrate content and the rate of
respiration of the varieties. However. they did not find an_v significant correlation
between resistance and the amount of soluble or insoluble nitrogen-fraction
content. According to Kuo (1965), the adaptation of the cytochrome oxidase to
limited oxygen supply probably plays an important role in the tolerance of rice
to submergence. Panja and Pillay (1944) were able to induce flood resistance by
keeping the germinated seeds in a continuous current of oxygen-free air for 3
davs.
FACTORS AFFECTING THE ELONGATION
OF INTERNODES OF RICE PLANTS
Age of the plant
Several papers indicate that the plants must be at least 3 to 6 uteeks» old before
the advent of flood. otherwise elongation of internodes will not be effective and
sitfficient for the survival of the plants (Richharia and Misro. 1960; Alim et al..
19621 Singh, 1962; Chowdhury' and Zaman. 1970'. Borthakur, 1971). However,