Yoshida - 1981 - Fundamentals of Rice Crop Science

CLIMATIC ENVIRONMENT AND ITS INFLUENCE 87
earth’s surface ranges from about 40% of the solar constant in humid regions to
nearly 80% in arid regions where the cloud cover is sparse. The maximum solar
radiation recorded under cloudless skies at noontime is 1.57 cal/cm 2 per minute
(Monteith 1977).
The most commonly used unit of solar radiation in agriculture is the calorie per
square centimeter per day (cal/cm 2 per day). Physicists and plant scientists,
however, use other units, such as kilojoule per square meter per second and watt
per square meter for solar radiation and other radiant energy in artificially lighted
environments. The following lists the relationships between the different units.
1 cal/cm 2 per min = 1 langley/min
= 697 J/m 2 per s (J = Joule)
= 697 W/m 2 (W = Watt)
1 kJ/m 2 per s = 1 kW/m 2 = 1.43 cal/cm 2 per min (k = kilo = × 10 3 )
lGJ/m 2 = 23.9 kcal/cm 2 (G = giga = × 10 9 )
The annual income of solar radiation in the tropics ranges from about 8 GJ/m 2 on
the margins of the subtropical deserts to about 5 GJ/m 2 in wet equatorial regions.
The monthly mean solar radiation at 26 sites in 15 rice-growing countries
range from 50 cal/cm 2 per day in December at Milano, Italy, to 700 cal/cm 2 per day
or higher in June or July at Lisboa, Portugal, and Davis, California, USA, or from
November through January at Griffith, Australia (Fig. 2.14). Most places, however,
appear to receive about 300 cal/cm 2 per day or more during the rice-ripening
period.
2.4.2. Solar radiation requirement at different growth stages
The solar radiation requirements of a rice crop differ from one growth stage to
another (Table 2.11). Shading during the vegetative stage only slightly affects
yield and yield components. Shading during the reproductive stage, however, has
a pronounced effect on spikelet number. During ripening it reduces grain yield
considerably because of a decrease in the percentage of filled spikelets. A quantitative
relationship between yield and solar radiation at different growth stages is
shown in Fig. 2.15. Solar radiation at the reproductive stage has the greatest effect
on grain yield; that at the ripening stage, the next highest effect; and that at the
vegetative stage, an extremely small overall effect.
Solar radiation of 300 cal/cm 2 per day during the reproductive stage makes
yields of 5 t/ha possible. Less solar radiation during ripening is required to achieve
the same yield level. Thus, the effect of solar radiation is apparent only when grain
yield is higher than 5 t/ha. When grain yield is below that, sunlight per se may not
have any direct significance. Since that conclusion was obtained with a 95-day
variety, yield could be even higher with the same level of solar radiation and a
medium growth-duration variety. Thus, the right variety and good management
could achieve a yield of about 5 or 6 t/ha during the tropical wet season.