Yoshida - 1981 - Fundamentals of Rice Crop Science

234 FUNDAMENTALS OF RICE CROP SCIENCE
Table 7.1. Photosynthetic activity of leaves, sheaths, and panicles at a light intensity
of 60 klx at milky stage. a
Leaf blade 1 c
2
3
4
All leaves
Leaf sheath plus culm
Panicle
Total
Surface
area
(cm 2 )
24
54
51
37
Assimilation b
Rate
(mg CO 2 /h) (mg CO 2 /dm 2 per h)
P n R d P g P n R d
8.3 (30) 0.44 (9) 8.7 (27) 34 1.8
10.1 (36)0.52 (11) 10.6 (33) 19 1.0
5.6 (20)0.46 (11) 6.1 (19) 11 0.9
2.2 (8) 0.34 (7) 2.5 (8) 6 0.9
26.2 (94) 1.76 (38) 27.9 (87)
1.2 (4) 0.60 (14) 1.8 (5) – –
0.6 (2) 2.05 (48) 2.7 (8) – –
28.0(100) 4.46(100) 32.4(100) – –
a Yoshida and Cock (1971); variety IR22 grown in the field. b P n = net photosynthesis, R d =
dark respiration, P g = gross photosynthesis. Figure in parentheses indicate percentage
of the total. c Counted from the top.
Table 7.1). The potential net photosynthesis of the leaves was 94% of the total.
The fourth leaf had a very low net photosynthesis both on a per-leaf basis and on a
per-unit-area basis. The top leaf had the highest net photosynthesis per unit area
but the second leaf was larger in size and had a greater potential net photosynthesis
on a per-leaf basis. In rice, all the leaves from the flag leaf down to the third leaf
from the top export assimilates to the panicle. Lower leaves send their assimilates
to the roots (Tanaka 1958). The top 3 leaves of plants in an IR8 crop made up
74% of the total leaf area when LAI was 5.5 at heading. Flag leaves were only 19%
of the total leaf area, and the second and third leaves made up 55% (Yoshida and
Cock 1971).
The net photosynthesis of the leaf sheath and panicle is extremely small. Even in
terms of gross photosynthesis ( P g ), the panicle (8%) and the leaf sheath (5%) have
extremely low photosynthetic capacity. Hence, both can be considered nonphotosynthetic
tissues (Tsuno et al 1975).
The longevity of the green tissue must be considered when assessing the
photosynthetic contribution of different plant parts during ripening. The panicle
becomes yellow relatively early in ripening but the leaves remain green much
longer. The flag leaf remains green and maintains active photosynthesis until
maturity (Takeda and Maruta 1956). At later stages of ripening, however, photosynthesis
may not make much of a net contribution to grain production because
rapid grain growth occurs earlier and slows down toward maturity.
The light environment of different plant parts in a crop canopy is extremely
important for determining the real photosynthetic activity of a given part. Panicles
of improved indica varieties tend to bend and become positioned below the flag
leaf where they are heavily shaded by the leaf canopy, particularly when the LAI is
large. Panicles of these varieties are unable to make a significant contribution to
grain filling because of their low potential photosynthetic activity and low light