Vergara - 1976 - Physiological and morphological adaptability of ri

RESPONSE TO DEEP WATER STRESS 309
no explanation is given for this basic requirement before elongation of internodes
can oecur Age is more important than height of seedlings. according to
Borthakur (1971). The ability‘ to elongate apparently’ increases with age so that
early sowing has a better effect on the yield as it provides better seedling resistance
to flood.
The emergence of paniele marks the end of internode elongation and that of
flood resistance. At this stage the rice plant can no longer elongate. and heavyi
flood will mean total damage (Chou/dhury’ and Zaman, I970).
Stage
of leal‘ development
Sato (1952) observed that the internode which is supposed to elongate as a result
of submergence did so only after the leaf attached to that plant unit had ceased to
elongate. This is also the sequence ofdevclopment under ordinary conditions. A
rice plant unit consists of the lamina which is attached to the leaf sheath which
in turn is attached on the upper end of the intcmode. The unit also includes the
lateral bud located on the basal part of the internode, opposite the leaf sheath.
If the energy" for elongation of a particular internode comes mainly from the
leaf composing its unit. then the photosynthetic activity of that leaf is important
in the elongation of the intemode—and in the survival of the plant. What then
would be the effect of leaf size and leaf pruning on internode elongation‘? For a
floating variety’. is large leaf necessary?
Temperature of water
Under laboratory conditions. Kondo and Okamura (1932) reported that elongation
is greatest at 25° to 30°C. At 35° to 40°C. it is retarded and the plant dies if
submcrgencc is prolonged. 'l'hey noted that branching 0f the rice plant occurs
more frequently} the higher the water temperature.
Since the water temperature in the floating-rice areas was not reported, one
cannot even estimate the natural conditions.
Light and turbidity
Although darkness results in internode elongation in rice (Takahashi and
Wada, 1972), the elongation when the plant is submerged is apparently limited
by a factor greater than light. Elongation is more extensive in clear water than
in muddy water: damage is more and sun-‘ival is less in muddy water. According
to Kondo and Okamura (l934a). plants in muddy water for l0 days died. while
those in elear water regained their vitality‘ when water was removed.
Possibly the light received by the plant for photosynthesis is the limiting
factor. This reernphasizcs the importance ot‘ the photosynthetic activity" of the
leaf in the elongation of the intcmode.
Duration of submergence
Elongation is greater the longer the flooding time (Kondo and Okamura. 1932).
Hoxvever, elongation per day is less the longer the plant remains under water.