Vergara - 1976 - Physiological and morphological adaptability of ri

432 CLIMATE mo) RICE
higher the temperature: or the shorter the day length. the shorter the vegetative
grotvth. Even after the panicle-primtirdia initiation stage. new tillers come out
when the nitrogen content of the plant is kept high. As no panicle-primtirdiurn
is initiated in these new tillers when they come out. it can be considered that
so long as tillering continues, vegetative growth continues.
The reproductive phase is from the paniele-primtirdia initiation stage t0 the
flowering stage. The duration of this phase is fairly constant at about 25 day-‘s.
regardless of variety. although it is somewhat shorter when temperature is
higher. During this phase the panicle-primordia develop, three internodes at
the top of each stem elongate, and the top three leaves on each stem which were
differentiated in the previous phase. develop. Differentiation and development
of spikelets are influenced by environmental conditions. An ample supply of
nitrogen cattses an increase in the number of spikelets per panicle; a limited
supply of photosynthates due to a limited amount of solar radiation or mutual
shading among plants causes a decrease in the number of spikelets; temperatures
below a critical point cause incversible damage to floral organs, ete. A high
temperature or an abundant supply of nitrogen promotes the elongation of
inter-nodes and the expansion of upper leaves which may result in an inferior
plant type.
The ripening phase is from the flowering stage to maturity. The duration of
this phase is almost constant among varieties. In the tropics it is about 25-30
days. whereas in temperate areas the temperature. especially during the latter
part of this phase. is frequently quite low, and the duration can be 6t) days or
more. An ample supply of solar radiation promotes ripening and results in a
higher grain yield (De Datta and Zarate. 1970). Relatively low temperatures
above a critical level make ripening slow but continuous. and result in an
increased grain yield, whereas critically low temperatures stop ripening (Murata,
196-4).
Phasal development is a sequence of the development of the three phases;
the condition of the plant during the vegetative phase determines the tiller
number which is the potential number of panicles. a11d also the condition of
leaves which function during the reproductive phase. The condition during the
reproductive phase determines the number and size of spikelets. and also the
status of the leaves which contribute to ripening. In this way‘, each phase is the
result of the previous phase and the cause of the following phase. It is also
important to mention that some phenomena occurring within the plant at a
certain growth stage can be compensated for during later growth stages, but
those occurring at other stages cannot. For example, an attack by stem borers
on the main stem during the vegetative phase can be compensated for by
tillering and may not adversely affect grain yield. but such an attack at flowering
results in death of the panicle and a definite decrease in grain yield. The compensations
take place in various xvays in association with phasal development,
and complicate interpretation of the effect on yrield of a phenomenon occurring
at a certain growth stage.