Vergara - 1976 - Physiological and morphological adaptability of ri

5 l2 CLIMATE AND RICE
tently flooded water regime. The maximum yield of 11.7 tfha obtained with 1R8
was at 480 kg N/ha. Within a specific plant type. the earlier-maturing cultivar
produced a higher response to N than its later-maturing counterpart. The data
from Peru suggest a significant interaction between plant type and growth duration
as factors affecting N response in a high solar radiation environment.
Sanchez et al. (1973) reported the weather pattern during the cropping season in
relation to growth stages of the eultivars tested in the experiment conducted in
Peru. temperatures were low at the seedling and ripening stages (particularly
the night temperature) but were around 20°C during tillering stages. This helped
in obtaining high tiller number in each variety The main reason for marked
response to up to 320 kg N/ha was primarily the high solar radiation during the
entire growing period. The average solar radiation value for the ricegrovviitg
season in Peru was 488 cal - sq curl - day“ which is 19% higher (Sanchez et al..
1973) than we reported as an average value for a dryt-season rice cropping in
Los Bafios. Philippines (De Datta and Zarate. 1970). High solar radiation and
low night temperature and intermittent flooding in Peruvian conditions (Sanchez
et al.. 1973) are in direct contrast to low solar radiation and high temperature
in the hot, tropical Asian conditions (De Datta et al.. 1966. 1968', De Datta
and Zarate. 1970).
Correlation between nitrogen response and solar radiation
Institute experiments have revealed strong positive correlations between grain
yield and solar radiation values during the last 30 days of crop growth. The
planting date should be adjusted so that the crop will receive maximum solar
energy’ during the reproductive stage, particularly during the ripening period
(Moomaw et al.. 1967). This is explained by Tanaka et al. (1966) who found that
the increase in dry matter following flowering of rice is primarily in the fonn of
grain. lt is also dependent on the amount of solar energy received by the crop
during that period. Later experiments at IRRI. though. showed that correlations
were recorded between solar energy total for the last 45 day's before harvest and
dry-matter production for that period (De Datta et al.. 1968). This period corresponds
to the particle initiation to manirity’ stage. Since photosynthesis for
grain production is also important for the period between panicle initiation and
heading (Murata. 1966). high correlation values for grain yield response to solar"
energy total. which include that period. are understandable.
Experiments conducted during 1968 and 196‘) showed that in l2 harvests
of the four varieties tested. the highest nitrogen response and grain yield o1‘
9.3 t/ha were obtained with 1R8 when harvested in May (Fig. 2). In each of 12
harvests, 1R8 produced at least 5 tfha indicating its consistent high yielding
ability’. On the other hand. the tall tropical variety Il—4 produced its highest
grain yield of6.3 tfha in the May harvest. but gave a yield of only 2 tfha in the
August harvest. 1n the dry season. as the solar radiation received during the
45-day period before harvest increased from ll kcal/sq cm for the January
harvest to about 20 kcal/sq em for the May harvest, the ititrogcn response of 1R8