Vergara - 1976 - Physiological and morphological adaptability of ri

NITROGEN RESPONSE or RICE [N TROPICAL CONDITIONS 527
Since upland rice is grown during the cloudy" monsoon season, yield is restricted.
The potential of upland rice is at best similar to the potential of
rainfed lowland rice (De Datta and Beachell, I972). Very few data are available
on sunlight intensity for most rice-growing areas and even less for upland rice.
Most of the principles established. therefore. are based on information gathered
in the Philippines.
Sinee 1969. a number of field experiments have been conducted in the Philippines
to study the effects of solar energy and rainfall distribution on the nitrogen
response and grain yield of upland rice. Since solar radiation effects are closely
associated with moisture supply‘. both factors were studied together.
The varieties and lines used in each trial varied from year to year. In all trials.
one recommended upland variety from the Philippines was included. Generally.
the recommended varieties were tall_ low tillering, and low yielding. [Iowever.
being grown under less favorable soil and moisture conditions. they
produced low but stable yields.
In the I967 wet season, the importance of improved plant type for obtaining
high yields under upland conditions was demonstrated for the first time (IRRI,
I967; .Iana and De Datta. 197]). as had been previously’ done for lowland conditions
(Beachell and Jennings. 1944). The varieties used were semidwarfs 1R8
and IR400—28. Milfor-6(2). which is a medium-statured Philippine variety
commonly grown under flooded and upland conditions, and Palawan, a tall
Philippine upland variety.
The grain yields of 1R8 and lR400—28 responded positively to the increased
solar radiation during the reproductive period, but those of Milfor-6(2) and
Palawan did not. In spite of the limited rainfall, the grain yields of the improved
varieties were higher. The extremely low yield of the September-seeded crop was
due primarily to the soil-moisture stress which occurred during the reproductive
stage of groivth (Jana and De Datta, 1971).
Results from the 1969 wet season showed that the grain yields and nitrogen
response (average of six varieties) gradually decreased as the planting date was
delayed from July 6 to August 22. because of higher soil-moisture tension recorded
fer later seeded crops. Except for IRS. the crops seeded on July 6 headed
between September 22 and 28 when soil-moisture tensions were very; low. The
moisture tension was below 33 centibar (ch) for the entire G-“teek period before
harvest. which explains the high yields obtained (Jana and De Datta.
I97]; De Datta and Beachell. I972).
To evaluate our findings at IRRI at other locations. additional upland rice
field experiments involving four dates of planting during the 1970 Vtffil season
were started at IRRI and at the Maligayia Rice Research and Training Center
in the Philippines. Soil analyses data for IRRI and the experimental site at
Maligayza are given in Table l.
At the IRRI fann, 1R5 and an experimental line, IR442—2—58. consistently
outyielded the upland variety Ml-48. The superior perfonnance of the IR442—
2-58 line under upland conditions is not surprising since it is derived from a
cross involving the deep-water rice variety Leb Mue Nahng III from Thailand.