Vergara - 1976 - Physiological and morphological adaptability of ri

492 (‘Lib-LATE AND RICE
DISCUSSION
EVANS (Chairman): Since grain filling is largely dependent on currently assimilated carbohydrate.
there has been emphasis on the rate and duration of photosynthesis during ripening. and
on the relation between yield and radiation in the period afier anthesis. Several authors have
shown a close relation between rice yields and radiation during ripening, both in Japan (e.g.
Murata. Ivlurakzimi) and at IRRI (eg. De Datta and Zarate).
However. radiation following anthesis is often closely related to radiation in the 3 to 4 weeks
before anthesis. when spikelet numbers are being determined. making it ditticult to detennine
which stage is more critically influenced by radiation level. This is a problem of major importance
with all cereals. and Yoshida and Parao‘s experiments constitute the clearest resolution
so far achieved.
Yoshidzfs earlier field experiments with various levels of shading, or increase in (JO; supply
applied at the vegetative, reproductive (pre-anthesis), or ripening (post-anthesis) stages indicated
that photosynthesis during the reproductive stage could be even more important as a yield
detemtinant than photosynthesis during grain tilting.
However. temperature and radiation are often correlated in the field. and the results of serialsowfing
experiments like those of Yoshida and Parao can he difficult to interpret unless phytotron
experiments can unravel the effects of temperature and radiation. Access to controlled
environments allowed ‘foshida to show that spikclet numbers fall as temperature rises during
the reproductive phase. in the range relevant to Los Baftos. Anned with this relation. the authors
were then able to demonstrate the strong positive relation between radiation during the
reproductive phase and spikelet number. and therefore yield capacity. in crops of rice at Los
Bafios. Combining these relations. they constructed a climatic productivity index which was
highly‘ correlated with _vields there.
These results emphasize that. for the cultivar and environment used. storage capacity. as
determined positively by radiation and negatively by temperature in the pre-anthesis period.
was the major detenninant of yield. With other eultivars and environments this may not he so.
as Murata has found. and as the results given by Yoshida and Parao for crops in Thailand
suggest.
Nor should one extend this conclusion too readily to other cereals. Since weight per grain in
rice is relatively stable (grain size is tixed by the enclosing glumes). there is less possibility of
variation in rice yields due to conditions afier anthesis than there is in crops such as wheat.
where grain size can vary considerably".
Also. although there is some seasonal variation in conditions during crop growth at Los
Bafios. it is far less marked than at higher latitudes. such as in Japan. There. spikelet numbers
are dctcnnined under higher radiation and temperatures. and longer days. than occur during
grain tilting. Under such conditions it is quite likely that radiation during grain tilting may be
more limiting to yield than that during spikelet differentiation. and that Japanese cultivars will
have been selected for the appropriate balance between length of the reproductive stage and
rate of grain titling.
A. TANARAI To increase rice yield further at Los Bafios I agree that we should increase
spikelet number. My question is. does the importance of ripening period still remain relatively
low even alter achieving a significant increase in spikelet number‘?
S. Yoshida: If we could make a substantial increase in spikelet number. I would think the
ripening period might home limiting. It‘ we eliminate one thctor. the other will home limiting.
So. l agree with you that the ripening period could become the limiting tactor after we have
made a substantial increase in spikelet number. At the same time. such an increase might be
aceompannied by an increase in source activity‘. imyway, we have to substantiate this.
STANSEL: Is an attempt being made to determine a comparative difference of CO; level
between shaded and unshaded canopies in terms of wind movement?
S. Torhida: We have not attempted this. but our experimental setup allowed the free ventilation
of natural wind. Sn I don't think there is any problem as to the CO; concentration in the
shaded crop.
DE DATIAI Spikelet number per square meter can be increased by increasing LA] by broadcast
seeding. but tilled grain percentage decreases. Therefore l conclude that the next increase in