Vergara - 1976 - Physiological and morphological adaptability of ri

306 (tux-tats AND RICE
Fertilizer is not used in deep water areas even as a basal application. Apparently
the annual silt deposit is sufficient to maintain soil fertility‘ at a constant
level. and/or the varieties shoiv no positive response to added fertilizer.
Yield of floating rice is around 0.4 to 1.0 tfha in Africa (Oka and Chang.
n.d.)'. 1.5 to 2.0 tfha in Thailand which is higher or comparable to the national
average for the non-floating rice area (Morishima, 1964; Yantasast et al., 19110);
3.6 t/ha in Southern Manipur, India (Singh, 1962); 0.9 t/ha in Madras, India
(Srinivasan, 1966); around 2.4 t/ha for the heavy yielders in Bangladesh (Ah
and Sen, I955; Alim et at. 1962) or 3 t/ha under optimum conditions (Chowdhury
and Zaman, 1970). A maximum yield of 9.9 t/ha was obtained in India
(Singh. 1962).
In many deep water rice areas, floating rice is the only crop grown, so that
prosperity depends to a great extent on its success.
FACTORS AFFECTING
THE SURVIVAL OF SUBMIERGED RICE PLANTS
The ability to survive under complete submergenee is different from the
ability to elongate. In floating nee, the ability to sun~‘ive ctsmplete submergence
is necessary where the rise in water level is too rapid for the internodcs to eateh
up by elongation. Similar factors such as light, temperature, and carbohydrate
content affect the elongation and survival of submerged rice plants. HOWCVCI".
more studies have been done on the latter than on ivater level-internode elongation.
Submergenee inhibits the formation of leaf pigments although the plants may
not be suffering from light deficiency (Varga and Varga, 1967). Leaves under
water die and only those above the water level are alive.
Age of the plant
Resistance to submergenee in terms of survival increases with the age of the
seedlings (Kondo and Okamura, 1932; Alim et al., 1962; Palada. 1970;
Chowdhuiy’ and Zainan. 1970). Richharia and Parasuram (1963) noted that the
vegetatiitely’ propagated tillers had higher resistance to submergenee. Tillers in
this case are older and therefore more resistant. The use of older seedlings
(TO-day-old) as in the practice of “double transplanting“ also increases the
resistance to submergence.
In terms of grain yield. rather than survival. Konclo and Okarnura (1932)
studied the effect of submergence with ‘varying ages of the plant from 25 to 97
day's after sowing. varying temperatures of 20° to 40°C and different duration of
submergenee from 2 to lO days. Their findings showed that the harmful effect
of submergence on grain yield is least at seedling stage and worst at flovrering.
When tiowering is over. damage to grain yiield increases as harvest approaches.
The direct cause of this decrease in grain yield differed according to each growth
period-either inhibition of tillering. anthesis, and grain development, or germination
of grains on the panicle.