Vergara - 1976 - Physiological and morphological adaptability of ri

RICE oaowrn m DIFFERENT ENVIRONMENTS 433
The grains are the twerall results of the whole cycle of ontogenesis. llo\\=e\-'er,
as the major constituents of grains are the photosynthetic products during the
ripening phase, there is no doubt that (lfjf-mflltfil‘ production during this phase is
most important for grain production. It is controlled by two factors: (a) the
potential ability of the population to photosynthesize (the source). and (b) the
capacity of spikelets to accept the photosy/nthates (the sink) (Tanaka. 1972). The
potential of the source is a function of morphological and phjvsioltigical status
of the population. such as leaf area, arrangement of leaves. and their nutrient
status. and also of the climatic condition. The status ofplants can be modified by
varietal improvements or cultural methods. but it is impossible to improve solar
radiation or temperature. Thus. the climatic environment during the ripening
phase imposes the ceiling of potential of the source. The sink is composed of (a)
the panicle number per unit field area, (b) the spikelct number per panic-le. (c)
the field grain percentage, and (d) the siZe of individual spikelets. Among these,
(a) is predominantly determined during the vegetative phase. (b) and (d) during
the reproductive phase. and (c) during the reproductive phase and flowering.
There are often negative correlations between paniele number and spikelet
number per panicle. or between spikelet number per unit field area and filled
grain percentage or IOOO-grain weight. due to compensation effects. Thus. the
optimization of the balance among these yield components within the limit of
the ceiling of potential source through manipulations of the growth pattern is
difficult. but indispensable in accomplishing a breakthrough in the ceiling of
grain yield in a given environmental condition.
GROWTH
PATTERNS
The growth patteni is an imprecise term. but it is here taken to mean the relative
strength and the sequence of the three growth phases.
Types of growth patterns vary according to the combinations of varieties,
environmental conditions. and cultural practices.
The general pattern of the growth curve can be interpreted as follows: the
leaf-area index (lAl) of a population of rice increases slowly at early grotvtlt
stages, more rapidly in later stages. continues to increase until flou-‘ering, and
then decreases due to the death of lower leaves. The crop growth rate (CGR)
follows a pattern almost similar to that [)f].1AI until flowering, it is kept high
even after the flowering stage because of increasing grain weight. and then it
decreases. The relation between LAI and CGR is asymptotic. The CGR increases
with an increase of LAI; the rate of increase of CGR decreases when
LAT becomes large dtie to a decrease of the net assimilation rate (NAR) caused
by the mutual shading among leaves; there is an optimum LA]. where CGR is
at the maximum and above which it decreases (Tanaka et al.. 1966). An twerly
vigorous increase in dry matter, consequently too fast an increase of LAI. at
early growth stages is frequently’ associated with a low CGR during later growth
stages, whereas a rather slow growth at early stages is associated with a high