Vergara - 1976 - Physiological and morphological adaptability of ri

NITROGEN RESPONSE or RICE IN TROPICAL CONDITIONS 515
since at 14°N latitude, day length varies only slightly throughout the year (Moomaw
and Vergara. 1964).
At a given harvest date, solar radiation is subject to yearly variability‘. For
example. during the 2-year period of analyses made by Montafio and Barker
(1974). the lowest solar energy reading recorded in the January harvest was l6
kealfsq em per 45 days in 1969. a year of drought. 43 % above 11.2 kcal/sq em
per 45 day's in 1970. These year-to—_\-'car variations within the same date of harvest
have profound effects on the nitrogen response of rice varieties (De Datta and
Zarate. 1970; De Datta. 1973).
Effect of air temperature and relative humidity
Between l5°N and l5°S latitudes. average air temperature is high and constant.
These high temperatures in the tropics are not critical for rice production. There
is no study on the effect of air temperature on nitrogen response in tropical rice.
Furthermore. in some areas in the tropics such as Los Bafios (14°N), Philippines,
air temperature is confounded with solar energy‘. At Los Bafios. higher air
temperatures during the ripening period for the May harvest are confounded
with high solar energy for that harvest month. Contrarily’, relative humidity for
the ripening period of the May harvest is negatively correlated with the solar
energy total for the ripening period of the same harvest month.
Multiple regression analyses revealed that when nitrogen levels and some
environmental factors were correlated, the variations in grain yield of four varieties
(IRS. 1R5. Milfor-6(2)._ and H-4) were betvrcen 71 and 88%. Of these
factors affecting the nitrogen response and grain yields of the four varieties. the
amount of solar energy received during the 45-day" period before harvest was the
most important (De Datta and Zarate. 1970).
Effect 0|‘ moisture supply
At optimum levels of management (insect. disease. and weed control), nitrogen
response within a season will vary with the moisture level. The differences in
nitrogen response between irrigated and rainfed rice will depend on moisture
sufficiency‘ from rain. If rainwater provides enough moisture. nitrogen response
between irrigated and rainfed rice should be identical. provided no other factors
such as insects. diseases. and weeds are limiting grain yiield. Such was the case
with IRZO during the 1973 wet season at the IRRI farm when nitrogen response
under rainfed and under irrigated conditions were similar (lRRl, 1974).
Even in an irrigated area. water supply seldom matches experiment station
conditions. At the farm level. the seasonal effects on nitrogen response are less
pronounced. For example. in experiments conducted by the IRRI Agricultural
Economies Department under irrigation in Nueva Eeija. the nitrogen responses
of 11220 during the 1974 dry season and the 1973 wet season were identical (Fig. 3).
In the dry season. water supply from irrigation is more limited than in the wet
season. Under less favorable irrigation water supply; the advantage of higher