Vergara - 1976 - Physiological and morphological adaptability of ri

246 (fLllt-lATl-i AND RICE
agrees. indispensable for growth. i.e. for dry matter production. Without complicated experiments.
we can produce evidence to show that an increase in temperature up to a certain limit
promotes respiration and groudh. or that an insufficient supply of oxygen retards respiration
and growth.
The rice plant respires not only in organs which are photosynthesizing (the leaves]. but also
in growing organs. such as ripening grains. and also in the roots. By our data the percentages
of respiration in the leaf blades, the leaf sheath-and-culm (including the growing young shoot).
and the roots in the total respiration of a whole plant during vegetative growth are 20. 40. and
40%. respectively. Respiration in the panicles is about three times that in the leaf blades at
early stage of ripening. Active respiration of the ripening grains is indispensable for a high
yield. and that of the roots for an ample absorption of nutrient elements which are indispensable
for growth. Thus. more emphasis should be given to the eflieiency of respiration in the growth
and the dry matter production in relation to various environmental factors and varietal charac-
IBIS.
Dr. Tanaka reviews our present knowledge. but w'e have to admit that knotvledge is still
very limited. We still have enormous problems to be solved in relation to the subject of this
paper. I hope that the phytotrtm of IRRI. along with other similar setups. will function well
in solving such problems.
1. Tanaka: i’ would like to answer several points of question presented by Dr. A. Tanaka.
l. In general, high correlation is found between leaf nitrogen content and photosynthesis at
a given growth stage. However. when the data at different growth stages are pooled. the correlation
becomes lower. because enzyme activity in chloroplast, dittusion resistance of C0.
through stomata and leaf moisture content vary with growth stage. In this case. we need
multi-dimensional analysis.
2. There are many evidences that photorespiration increases considerably‘ with increasing
temperature. Thus. one of the reasons why C_. species shows wide temperature optimum for
net photosynthesis seems to be the accelerated photorespiration at high temperature. On the
other hand, we should draw our attention to the fact that the temperature-photosynthesis curves
are influenced by such factors as stomatal movement. leaf nitrogen content. light intensity and
CO; concentration. The detached leaves of rice plant were often used for measuring photosynthesis.
although their stomata are susceptible to closing from water shortage. The wide temperature
optimum for photosynthesis in the detached rice leaves may" partially be attributable
to the decrease in stomatal opening. especially at high temperature.
3. The hysteresis in the light-net photosynthesis ctm-‘e of rice population which indicated
higher photosynthetic rate in the morning than in the atternoon could be explained by the
accelerated dark respiration caused by’ an increase in temperature. This suggested that the
pliotorespiration might be parallel with the photosynthesis. though the photorespiration
rate was not calculated.
4. The ettect of low oxygen in an atmosphere on dry matter production may differ with
treated period, oxygen concentration. growth stage and plant species. because of complicated
interrelations betvreen oxygen. photorespiration and dark respiration. The low ovygen treatment
may bring about positive eITect on dry matter production through the depressed photorespiration.
while it may retard differentiation and elongation of cells. nutrient uptake. and carbohydrate
translocation due to a decrease in dark respiration. Besides, we should pay attention
to the change in honnonal metabolism as affected by low oxygen. We observed that the space
between tillers of rice was evtremelyt expanded 20 hours alter low oq/gen treatment. This
phenomenon resembled the response to 2.4-D.
l\=ILIR.aT.A: Concerning the Qlti of respiration of leaves and roots, the status of substrate
supply is important. I think. At what conditions were the data obtained?
I. Tanako: The respiration in Fig. 2 was measured by the Warburg method. In this experiment.
the decline in respiration of roots at high temperature was not recovered by the addition
of glucose as substrate. Thus. the temperature response of respiration may’ difler with plant
organs.
EVANS: Your experiment with low oxygen levels. which increased photosynthesis but reduced
yield, emphasizes the point that there may be little relation between photosynthesis and yield.
But the most interesting aspect was that lcttility’ of (TZIJPQFILI microiria, a C. plant. appeared to
be unaffected at low O, Is that true of other C, plants also‘?