Vergara - 1976 - Physiological and morphological adaptability of ri

RICE PR()[)t.t(’l‘I\"l'I‘Y IN ("LIMATIC REGIONS or" JAPAN 455
influence on leaf area development through the supply of material for growth.
i.e. photosynthates. Concerning this point. Mtirata (l96l) shovred by a shading
experiment using young Iiee plants that the relative growth rate in leaf area was
practically’ free from the influence of solar radiation so long as the level of radiation
was higher than about one third of the full incident radiation. The following
result may be mentioned to explain the most probable cause of this phenomcnon.
A marked increase in specific leaf area took place in young rice plants kept
for 1 week under reduced light intensities. while the relative leaf growth rate on
a weight basis was sharply reduced in response to the reduced light intensities.
as compared with the relative growth rate expressed on a leaf area basis.
In relation to this problem. Kumura (1975) mentioned three factors. namely.
NAR, the partition ratio 0f dry matter to leaves. and specific leaf area. as
components of the relative growth rate of leaf area. He analyzed the influence of
climatic factors on each of them using the IBP seedling experiment data. He
found that solar radiation had a high positive partial correlation of 0.552. significant
at the 0.1 % level. for NAR. excluding the influence of temperature. while it
had negative correlations. also significant at the 0.1 % level. both for partition
ratio (-0.301) and for specific leaf area (-0.359). It has been reasoned out from
these results that the overall influence of solar radiation was so small as a net
consequence of cancellation of the positive and negative influences.
Leaf area development and growth stage
As described earlier. the developmental stage of a plant exerts a great influence
on leaf. Leaf growth takes place only before heading stage and leaf area decreases
after heading. Miyasaka et a1. (1975) have very recently shown from
analysis of the IBP field experiment that the effect of growth stage of rice plants
can be very well represented by the percentage total nitrogen content of leaf
blades. The leaf area growth rates, LRGR. sampled at various growth stages are
plotted in Figure 2 against the corresponding values of N. using the pooled
data for 5 years. Close relationships are apparent between the two factors,
LRGR and N
Three \t'ery interesting points are apparent from Figure 2. First. the close
relationship is observed not only during the period when leaf area is increasing
but also when leaf area is decreasing; second. a considerable part of the _vearl_v
deviation in LRGR is explained by’ the LRGR-N relationship; and third. the
relationship shows no great change due to tiariety or cultural season, so long as
cultural practices are not greatly altered.
The first point implies that the nitrogen content has a close relationship not
only with the processes of leaf area development but also with those of leaf
area decrease due to withering of lotvcr leaves. Results obtained at seven stations
show that leaf area decrease occurred when the nitrogen content of the leaves
decreased to 2-3 percent.
As to the second and third points. it has been recognized that another factor