Yoshida - 1981 - Fundamentals of Rice Crop Science

94 FUNDAMENTALS OF RICE CROP SCIENCE
period. When the rice crop was given carbon dioxide enrichment before flowering
to increase spikelet number per square meter but was kept under normal atmospheric
conditions after flowering, it yielded 7.7 t/ha (Yoshida 1976). Thus, the
recorded maximum yields are close to the estimated potential yields.
2.5 RAINFALL AND FIELD WATER RELATIONSHIPS
Under rainfed rice culture and where temperatures are within the critical low and
high ranges, rainfall is perhaps the most limiting factor in rice cultivation. When
irrigation is provided, however, growth and yield are determined largely by
temperature and solar radiation. Because of the unpredictable variation in frequency
and amount of rainfall for a given location over time, it is extremely
difficult to find a simple relationship between annual rainfall and grain yield. At
best, only the probable occurrence of drought can be predicted based on long-term
records. Furthermore, it is difficult to generalize on the water requirement of rice
because of variations in topography, soil characteristics, and length of the cropgrowing
period among different locations, and because local practices affect the
amount of water necessary for land preparation. Currently, however, rainfed rice
cultivation is limited to areas where the annual rainfall exceeds 1,000 mm.
2.5.1. Transpiration and growth
Transpiration is the loss of water in the form of vapor from plant surfaces. The
plant will wilt or die unless water is supplied to compensate for the water lost by
transpiration. Transpiration occurs mainly through stomates and to a much lesser
extent through the cuticle. Thus, the transpiration is controlled primarily by the
opening and closing of stomates.
Transpiration may increase the rate of upward movement of salt in the plant or it
tends to cool the leaves because 590 calories is removed per 1 gram of water. Yet,
these may not be absolutely essential for plant life. Thus, transpiration is usually
considered an unavoidable evil, unavoidable because of the structure of leaves and
evil because it often results in water deficits and injury by desiccation (Kramer
1969).
The amount of water lost through transpiration is directly related to amount of
growth. The transpiration ratio is the number of grams of water transpired per gram
of dry matter produced. It varies with soil moisture, climate, variety, growth
stage, and growth duration of the plant. It ranges from 171 to 766 g/g but it is
generally around 250-350 g/g (Matsushima 1962, Yoshida 1975c). The transpiration
ratio implies that dry matter production is proportional to the amount of
water transpired by the plant. When calculated for grain production, the transpiration
ratio of an early-maturing variety is only one-third that of a late-maturing
variety (Matsushima 1962). Thus, the former uses water for grain production more
efficiently than the latter.
Transpiration is, however, only part of total water loss by a field-grown rice
crop. Water is lost through evaporation from soil or water surfaces and through
percolation.