Yoshida - 1981 - Fundamentals of Rice Crop Science

250 FUNDAMENTALS OF RICE CROP SCIENCE
Table 7.8. Effects of carbon dioxide enrichment before and after flowering on grain
yield and yield components of IR8, IRRI, 1972 dry season. a
Time of CO 2 enrichment
In relation Develop- Filled Grain
to flowering
mental Yield c Grains c spikelets c wt c,d
(days)
stage b (t/ha) (10 3 /m 2 ) (%) (mg)
Control e – 10.2 c 40.4 c 85.4 b 23.7 d
–33 to –24 I 11.3 b 45.0ab 84.2 b 23.8 d
–24 to –14 II 11.4 b 43.4 bc 86.2 b 24.7 c
–14 to 0 III 10.2 c 38.9 c 85.8 b 25.0 bc
–33 to 0 I–III 13.3a 48.2a 87.6 b 25.7ab
0 to 30 IV 11.5 b 38.9 c 92.4a 26.0a
a Yoshida (1976). b l = neck-node differentiation to differentiation of secondary rachisbranch,
II = differentiation of spikelets, III = differentiation of pollen mother cell, reduction
division stage to flowering, IV = flowering to harvest. c Any means followed by the
same letter are not significantly different at the 5% level. d Panicles have been dried at
75°C for 48 h, threshed, then grains were placed at 50°C for 12 h and weighed. e None.
plants after flowering, the results may imply that solar radiation during ripening
did not limit grain filling and, hence, the additional spikelets produced by the CO 2
enrichment were well filled in both the dry and wet seasons.
Further CO 2 -enrichment experiments in the dry season demonstrated that
preflowering enrichment increased grain yield from 10.2 to 13.3 t/ha, i.e., 30%
increase over the control, whereas the postflowering enrichment increased grain
yield by 10% (Table 7.8). The same experiment also revealed that the effective
stages for CO 2 enrichment before flowering is the time from the neck-node
differentiation to the spikelet differentiation stages. In the preflowering enrichment,
the yield increase is attributed to increased spikelet number, and, to a lesser
extent, also to increased grain weight. In the postflowering enrichment, the yield
increase resulted from increased filled-spikelet percentage and increased grain
weight.
These two experiments suggest that the sink size, largely determined by spikelet
number per unit of land area, is not completely limiting. Postflowering enrichment
increased the grain yield by increasing filled-spikelet percentage and weight per
grain. However, the magnitude of such grain yield increases seem distinctly
limited because both filled-spikelet percentage and grain size cannot be increased
greatly.
The total spikelet number per unit of land area is greatly increased by the CO 2
enrichment or possibly by increased photosynthesis before flowering. If the total
spikelet number can be increased, neither photosynthetic capacity nor light nor
CO 2 concentration after flowering is likely to limit grain-filling at Los Baños,
Philippines. Thus, to increase yield further, some way of increasing the spikelet
number or yield capacity (spikelet number × potential weight per grain) must be
found.