Yoshida - 1981 - Fundamentals of Rice Crop Science

CLIMATIC ENVIRONMENT AND ITS INFLUENCE 109
Table 2.17. Yield of variety Jaya under 3 levels of submergence
(25, 50, 75% of crop ht) at each of the 3 growth phases during
aman (Jul–Nov) and boro (Jan–Apr). 1973–74. a
Relative grain
yield (%)
Plant growth stage Ht of plant 1973
submerged (%) (aman)
Control (continuous
submergence 5 ± 2 cm) 100
Seedling establishment 25 82
to maximum tillering 50 75
75 68
Maximum tillering to 25 81
flowering 50 71
75 72
Flowering to maturity 25 79
50 76
75 70
a Adapted from Pande (1976).
1974
(boro)
100
75
62
58
74
64
56
71
66
50
c. Submerged areas. The water level and duration are variable but the plants are
usually completely submerged. In some areas submergence occurs almost every
year. In many lowland rice fields submergence occurs only during typhoons or
unusually heavy, continuous rains.
Attention is currently focused on improving rice varieties adapted to flood
areas by incorporating the elongation genes of floating rice into improved semidwarf
high-yielding varieties. These new varieties, called deepwater rices, can
remain short when the water is shallow and grow tall in response to increased water
depths (Jackson et al 1972).
A large portion of the rainfed lowland rice area falls into the category of
submerged areas, where partial submergence is the common problem. When a rice
crop is submerged to different degrees at different growth stages, the grain yield
decreases in different proportions. For example, with 25% plant height submergence
at tillering, yield decreases by 18% in aman and 25% in boro; with 75%
submergence at ripening, yield decreases by 30% in aman and 50% in boro (Table
2.17). The yield-decreasing effects of partial submergence could be attributed to
impaired tillering and decreased area of photosynthetic leaf surface. To overcome
the adverse effects of partial submergence on grain yield, varieties with intermediate
plant height (about 110–130 cm, taller than semidwarfs and shorter than
traditional tall varieties) are replacing semidwarf varieties in shallow water areas
that are subject to occasional floods.
2.6. ANNUAL PRODUCTIVITY IN TEMPERATE REGIONS AND
THE TROPICS
In temperate regions, one or two rice crops can be grown annually. In the tropics,
three to four crops a year are possible, provided irrigation is available. Thus, rice