Vergara - 1976 - Physiological and morphological adaptability of ri

YIELD AND YIELD (.‘.(IJl\rfP()NEl\"l"S OF LC)\\"'L.‘—\I\'I) RItTE IN THE TROPICS 493
grain yield potential in rice varieties should come from direct seeding and not lrom transplanting.
Of course varieties must be lodging-resistant. Would Dr. Yoshida comment on this?
S. lbshida: Spikelet number per square meter cannot be increased by simply increasing LAI
at the plateau level (about LAI of 6 and above). I would agree that direct seeding would
offer a better chance for a further increase in grain yield because it could improve light environment
in the community by resulting in smaller plants with more uniform distribution. In practice
however the direct-seeded crop is susceptible t0 lodging. l believe that the next major breakthrough
in grain yield would come from a substantial increase in spikelet number per square
meter by genetic manipulation and not from improved arrangements of individual plants in
the community, i.e. direct seeding.
NluRATAt: In our experiment we observed very high r between the number of ctfectivc tillers
and solar radiation in the 3-week period alter transplanting. I think this will have some relationship
to the number of spiltelets, with climatic factors. What do you think about this?
S Yoshrda: I think that the early growth is more important in detennining final grain yield in
the temperate region than in the tropics. Under Los Bafios conditions. in the 3-week period
after transplanting. solar radiation is not too important. as we have shown in Figure 6.
NIOSITEFTI! : The elegant shading experiment described in this paper and the analysis of yield
components provide conclusive evidence thr a phenomenon which several contributors have
drawn attention to/the invariance of grain weight in a rice varietya Day before yesterday I
made a comment in jest that biologists seem to be fond of variability. so I am glad to see Dr.
Yoshida emphasizing the constancy of this characteristic. Zt-ly colleagues at Nottingham have
recently shown that many varieties of barley" grown in Britain show a similar stability of grain
weight which changes lrom year to year by less than r 5% when yield and total dry matter
production change by a factor of 2 or 3. By analyzing changes of dry weight alter anthesis. we
have found that constant grain weight is achieved by translocation from the stem. by an amount
which compensates almost exactly for any deficit in current photosynthesis. In the papers read
here I have seen very few" reference to the possibility of this phenomenon in rice. ls it important.
or is there always a surplus of assimilate allowing both grain and stem to get heavier in the
ripening stage?
S. Yoshrda: In our previous experiment using “C. we have shown that the accumulated
carbohydrate in the leaf sheath plus stem before anthesis accounts for 24 to 27% of grain carbohydrate
or grain yicld equivalent to 2 tsha in the dry season. The accumulated carbohydrate
would insure good ripening when weather conditions become unfavorable. I would think.
however. that constant grain weight for a given variety comes from the size of the rigid hull in
rice. Thus_ when the assimilate supply is adequate most grains ripen, and a surplus remains in
the straw. When there is shortage of carbohydrate. some grains ripen at the expense of other
grains (i.c. untillcd grains) thereby attaining almost constant weight.
NIANIEEL: The duration of the various growth stages of the rice plant varies. Were the reductions
(shading treatrnents) in amount of sunlight in Table 2 applied for the same durations
during these stages?
S. lbshida: The variety we used in our experiment matures in 96 days. Estimated duration
for panicle development is 25 days. i.e. 7 or 8 days shorter than for the medium-maturing variety
in which duration is normally about 32 to 33 days.
MITCHELL: At exactly’ what stages in crop development did you change your shading to full
light or change from full light to shading?
S. Foshlda: They were transplanting neck-node ditYerentiation stage and flowering time.
PACARDOZ With reference to your Figure 6, have you studied the relationship between
spikclct number and vegetative grouih? If the relationship is positive. then solar radiation may
also influence yield. at least indirectly.
S. Yoshida." Dry matter production at the end of shading treatment during the vegetative
stage varied from 256 gfml under 100% sunlight to 101 g.='m-' under 25 “n sunlight. This ditfercncc
did not have a noticeable ctfect on spikclct number and hence yicld. when these crops
were exposed to normal sunlight.
FISCHER.’ I wonder if Dr. Yoshida should resist the temptation to apply his conclusions and
model from IRRI to the data presented from other countries. I suspect it would give a very
good fit for the latter data as well.