Yoshida - 1981 - Fundamentals of Rice Crop Science

GROWTH AND DEVELOPMENT OF THE RICE PLANT 57
When the filaments elongate further, the anthers are positioned just above or
droop over the lip of the lemma and palea, and pollen grains are released into the
air. These pollen grains may be blown by the wind to the stigmas of other spikelets.
Since self-pollination precedes cross-pollination, the percentage of the latter is
usually less than 1.
Pollen grains are viable for only 5 minutes after emerging from the anther,
whereas the stigma can be fertilized for 3–7 days. Pollen grains are more sensitive
to high temperatures than is the stigma. When treated with hot water at 43°C for 7
minutes, they lose viability. The stigma remains active under the same conditions.
This differential response to high temperature is used for emasculation in hybridization.
When a portion of the panicle has exserted, anthesis will occur, starting with the
spikelets at the tip of upper panicle branches. Hence, the date of anthesis is the
same as the date of heading. The date of anthesis of individual spikelets varies with
the positions of the spikelets within the same panicle. Spikelets on the upper
branches have anthesis earlier than those on the lower branches; within a branch, a
spikelet at the tip flowers first.
It takes 7–10 days for all the spikelets within the same panicle to complete
anthesis; most of the spikelets complete anthesis within 5 days. Within the same
field it takes 10–14 days to complete heading because panicle exsertion varies
within tillers of the same plant and between plants in the same field. Hence, it takes
about 15–20 days for all the spikelets of a crop to complete anthesis.
The time of day when anthesis occurs varies with weather conditions. Under
normal weather conditions in the tropics, most rice varieties ( O. sativa ) begin
anthesis at about 0800 and end at about 1300 hours. When temperatures are low,
anthesis may start late in the morning and continue into the late afternoon. On rainy
days, pollination may occur without the opening of the lemma and palea.
There is a marked difference in anthesis time between O. sativas and O.
glaberrimas (Fig. 1.47). For instance, at 0900 hours, about 60% of the spikelets in
1.47. Anthesis time of O. sativa
and O. glaberrima selections
(IRRI 1978).