Vergara - 1976 - Physiological and morphological adaptability of ri

340 Cl.l.\t.-'\'l'E AND RICE
DIS CLTSSION
ISHIZLIKA (Chairman): While listening to Dr. Buyer's paper, I noticed one feature of his work
which is quite different from those of Dr. Satake and Dr. Vergara. When we are concerned with
temperature. for which there are definite measurable units. the reproducing of experimental
conditions is comparatively easy with present techniques. and anybody can repeat the experiment.
In the case of deep-water stress, it is also definable and though it is not as easy as for
temperature. it is possible to reproduce the same condition aniticiallyn It is therefore possible
to compare the results of similar experiments conducted by difierent scientists.
In the case of “droughtf however. the situation is very different.
It is of course possible to explain the relationship between the physiological functioning and
the water content of a leaf. but it is difficult to define clearly the relation between the water
content of the leaf and the environmental moisture conditions surrounding the plant. including
xvater in the soil. Therefore it is very difficult to reproduce experimental conditions with precision.
lvloreovcr. under drought conditions. there exists a kind of hysteresis phenomenon. When air
temperatures change suddenly, the crop will be subject to the influence of the cold temperature
directly and simply. In the case of deep-water stress the problem arose when water depth increased
suddenly within a short time. and this is a simple. definable tactor.
On the other hand, a crop does not suffer from drought suddenly and immediately after rain
ceases. except the fairly rare case of the effect of a
vrater gradually as
The problem will occur around this stage. and morphological characteristics of roots also
play an important role at this stage.
The author has already mentioned in his conclusion that breeding for increased leaf growth
and decreased senescence could have a positive effect on agricultural production if it reduces
the effect of mild or moderate drought.
I would suggest that we are mainly concerned with moderate rather than very severe drought
and that the discussion should therefore be limited to moderate droughts.
lvIONTEITHI How do you explain that in your first slide the marked peak in both water
potential and elongation rate occurred at about 2 am. and it was ftillovred by a sharp decrease
in both quantities before sunrise?
Boyer‘: This is a deceptive graph because of the problem of "smoothing" between points. It
should be redrawn to indicate the problem area with a dashed line. However. this particular
figure will not be appearing in the present manuscript.
FISCHER: You stated in your paper that osmotic compensation in face of drying does not
occur in leaves. lvly general impression of the research in this area is a growing number of
reports showing such compensation with slow field drying. I would even suggest this is likely a
mechanism by which crop species and perhaps varieties differ in drought resistance.
Buyer: Although we have not seen osmotic compensation in sunflower leaves. it could xvell be
present in maize or other species that we have not tested. Ifso. it would be valuable to explore
its role in drought resistance. particularly‘ since there could well be differences in this character
among genotypes.
lvlULsTA: l. In your Fig. 1 in the text you compared net photosynthesis and elongation.
l-Vhat is the respiration rate which is more closely‘ connected with growth processes‘?
2. Concerning Fig. 5 in the text, what is the water-holding ability of going and old leaves‘?
According to our experience young leaves have higher resistance than old ones.
Borer: l. Dark respiration is one of the most stable metabolic processes during drought. ln
Fig. l. it would have decreased about 50% by the time photosynthesis had decreased 9O to
100°». Typically, the disappearance of dark respiration signals the death of the tissue.
2. l am not completely certain of the water-holding capacity that Dr. Murata has in mind.
but if we define it as the turgid water content per unit dry weight. young leaves world have more
capacity than older leaves. largely because the older leaves have increased in dry weight as they
matured.
If on the other hand. we define WZtlCT-htlldltlg capacity as the tvater content of the tissue as
desiccation occurs. one would probably conclude the opposite. Old tissues. because of rigid cell
walls, tend to lose turgor when only a small amount of water is lost. Young tissues must lose a