Vergara - 1976 - Physiological and morphological adaptability of ri

292 curtrars AND RICE
postulated that abnormalities of meiotic division and hypertrophy of tapetal
tissues were the major cause of sterility llayase and Satake (1970) made cyjttilogical
observation on the pollen developmental stage of rice plants cooled at the
meiotic division stage. but they’ found that at tetrad the frequency of abnormal
microspores was much less than that found by Sakai. Nishiy=ama et al. (1970)
found that the frequency of tapetal hypertrophy was not so high compared with
the percentage of sterility, and doubted Sakafs hypothesis. Though the rate of
tapetal hypertrophy’. as Sakai reported, correlated with sterility’. its frequency
was much loyver when compared yyith the percentage of sterility’. Even in Sakai's
data. the rate of anther loculi having tapetal hypertrophy’ was less than l0 percent
in coiriparison with 50 percent sterility. Considering these facts. Nishiyama
ct al. (1970) considered that tapetal hypertrophy was not the main cause of
sterility However. the cooling time of the anthers they; observed was not the
young microspore stage. but the meiotic stage.
We have been making cyitological observations on the anthers cooled at the
young microsporc stage. In this treatment the balloon-type of tapetal hypertrophy
has been recognized at a fairly high frequency’. The tapetal hypertrophy’
became perceptible on the second day of cooling treatment, while the microspore
abnormalities were observed on the fourth day. These tapetal and microspore
abnormalities were observed with much higher frequency’ in the smaller anther
loculi than in the larger ones of the same anthers. Further microscopical observations
on the anthers cooled at the young microspore stage will reveal tlie significance
of the higher frequency’ of abnormalities in smaller anther loculi to
sterility Perhaps the elaborate observations may’ give an adequate explanation
of the significance of tapetal abnormalities to the mechanism of sterility outbreak.
Function of tapetum
As mentioned in the foregoing paragraph, further studies are required to determine
the significance of tapetal abnormality on sterility. Plowever. the belief
that tapetal abnormality’ is the main cause of sterility‘ remains predominant.
Abnormal development of microspores means that nutrient movement is suppressed
through tapetal cclls to microspores. Judging from the dense cytoplasm.
tapetal cells are not only the passage way’ for nutrients. but also they may ha\-’e
metabolic 3Cll\'ll_\~'. If some injury occurs in this metabolic system. the supply’ of
nutrients to microspores may be disturbed.
ln the anthers during the period from the first contraction phase to the second
contraction phase at the end of cooling treatment, Murakami et al. (I958)
found a remarkably high increase in reducing substances (presumably chiefly
reducing sugars) in the cooled tapetal and transitory’ tissues. They assumed that
the inhibited transport of sugars from the surrounding tissues to microspores
w'as the main cause of sterility. They found no detectable abnormality’ in the
microspore side at the end of cooling treatment. At present we have no evidence
to determine whether microspores or tapetal cells {or both) are the tissues sensi-