Vergara - 1976 - Physiological and morphological adaptability of ri

290 CLIh-IATE AND RICE
us to make a more precise study to determine the sensitive stage. In the earlier
studies the procedure of estimating the sensitive stage consisted of comparing
sterility ofpanicles with the same heading date. Even in the panieles of the same
heading dates there is a difference of l day in maturation between the earliest
and the latest heading; and within the same panieles there is a great maturation
variation (7 days) between the apical and basal portion of spikelets.
Satake and Ilayase (1970). instead of heading date. adopted auricle distance
at intervals of l cm (the auricle distance of our materials changed at the rate of
3 cm every day"). Instead of panicle unit. we used spikelets. divided by the location
of the uppermost primary branches. To obtain spikelets at the different
ages. we used panieles in the auricle distance ranging from — l6 cm to O em.
Because we used spikelets from different locations on the uppermost primary
branches of panieles. only one spikclet was sampled from a panielc. In this
method. about 1.000 uniform panieles were necessary to estimate the relation
between pollen developmental stage and fertility. In the limited areas of the
phytotron it is very difficult to grow 1.000 panieles synchronized as to growth
and development. However, this difficulty could be tivercome by ZO-plant
circular-dense culture ofplants in l/SDOO-a pots.
The most sensitive stage
Following these procedures, results of the third-from-the-top spikelets. as an
example. are shown in Fig. 4 (Satake and Ilayase. l97l0). Experiments using two
varieties over 2 years indicated the same results. As obviously indicated in Fig.
4. the stage most sensitive to coolness is that of the _voung microsporc, from
tetrad to the first contraction phase after the meiotic division. Our further
studies (Satake and Hayase. 1974) show that there is a slightly (secondary)
sensitive stage-just before and at leptotene stage-tvith a primary sensitive
stage to coolness. These results have changed the hypothesis believed to be true
for the past 30 years.
The interval between the meiotic division stage and the young microsporc
stage is only l or l I12 days. This time discrepancy is of great importance in studying
the mechanism of sterility". although perhaps of no practical agricultural
significance. Meiosis is marked by visible alterations such as chromosome movement.
formation of cell membrane. and so on. However. pollen mother cells
protected by callosc wall are physiologically‘ inactive and increase in size to a
slight extent. After the callosc wall of the tetrad dissolves and microspores are rcleascd
into anther loculi. the microspores grow rapidly with the greater metabolic
activity. The corresponding physiological change may occur in the surrounding
tapctal cells. Sakai (194921) pointed out that the tapctal hypertrophy at the
young microsporc stage occurred with the highest frequency’. Murakami et al.
(1958) reported that cooling treatment induced the histochcinical abnormalities
in the tapctal cells at the young microsporc stage. Moreover. Fujishita (1970),
employing several vegetable crops (tomato, eggplant. bean, pepper. melon. and
strawberijv). made cytological observations after low and high temperature