Vergara - 1976 - Physiological and morphological adaptability of ri

RESPONSE TO DEEP \\~'.ATER STRESS 313
water conditions than in shallow water. However, there is a distinct physiological
difference between the ability to elongate and the ability to withstand
submergence; both characters are needed by the floating rice varieties.
Improvement 0n the following physio-morphological characters is needed
to overcome the deep water stress in floating rice:
l. Increase ability to Withstand abrupt rise in water level or total submergencc
for at least 1 to 3 weeks without losing physiological stability‘. This
ability is less important in Thailand since the rise of water is generally very slow.
2. Increase ability to grow quickly above the water level. This ability to elongate
will depend upon the depth and rate of increase in water level. Varieties for
Bangladesh must be able to elongate at a faster rate (60 to 9U crn/day WBICI’
rise) than the Thailand varieties (3 to 8 em/dayx water rise).
3. Better tillering ability" both before and after the flood. However. Berthakur
(1971) reported that tillers formed after the flood do not contribute to
grain yield so that vigorous tillering capacity after flooding is not necessary’.
Also. it has been reported that plants which had more tillers suffered greater loss
when submerged than those which had a lesser number of tillers (Chowdliury
and Zarnan, 1970).
A preliminary survey was made in the floating-rice area of Thailand. The
crop of the floating-rice variety has a LAI of 0.81) to 1.74 at flowering; the
short variety has a LAI of 6-8 under normal \vater conditions. The large difference
in LAI results from fewer tillers per unit area. The leaf area per tiller
is essentially’ the same for a floating-rice crop and a non-floating rice crop.
Although the tiller or panicle number per unit area is small, the weights per
paniele of floating-rice varieties are much greater. Ilence. the average national
yield is relatively high compared to the national average for the non-floating
area planted to traditional varieties.
4. Increase ability to withstand uprooting. In some areas. as in Bangladesh.
severe winds result in enonnous waves carrying the rice plants to great distances.
It was reported, hourever, that early uprooting did not make a significant difference
in grain yield from the normal plants. When the uprooting was delayied,
the yield was reduced (Alim and Zaman. 1958).
To withstand uprooting, the plant must have a deep root system and high
tensile strength to avoid snapping of the stern (‘Richharia and Misro, 1960:
Richhana and (iovindastvami. 1.966). Howexter. it is not clear from the literature
where the break in the culm usually occurs.
5. Varieties for deep water areas must have a long growth duration (150 to
270 days) so that panicle emergence does not occur when the water level is
still increasing. This would necessitate the use of photoperiod-sensitive varieties
since there are no known photopcriod-insensitive varieties with a growth
duration longer than 5 months. Oil-site testing would be needed even for growth
duration. Since the variety must have a photoperiod response that fits exactly
the latitude where it will be sown (so that it will flower at the right time), the