Vergara - 1976 - Physiological and morphological adaptability of ri

80 CLIMATE AND RICE
by increasing genetic heterogeneity, which would provide populational buffering
(Allard and Bradshaur, 1964). In the upland areas ofSamar, Philippines, farmers
mix the seeds of white and purple-colored rice xvhen they broadcast the seeds.
This is some type of heterogeneity which would provide buffering in ease of
adverse environmental conditions. However, this type of mixture may not work
for world ricc markets unless the ricc grains look similar when processed.
In many areas such as Australia and India. adaptability of wheat is actually
dependent mainly on its drought resistance. Variations in drought resistance
among rice varieties are also known. The exact nature of the resistance is difficult
to specify, since different combinations of plant characters may give the
same degree of resistance. Drought resistance is a combination of the ability of
the plant to obtain and retain water. and also its metabolism during the period
of lowered tissue water potential.
According to Henekel (1970) the most important physiological characteristics
of the plant resistant to drought are high mitochondrial activity and preservation
of mitochondrial ultrastructure under drought. The higher energy level of the
drought-resistant plants enables them to maintain a higher capacity for protein
renewal during drought periods. while the higher resistance of the mitochondrial
structure to breakdown permits the plants to resume reparativc processes after
dehydration and overheating
It is hery difficult to determine the ability ofplants to obtain and retain water.
under laboratory conditions, since a field environment is difficult to simulate.
Ilowever, changes in the metabolism as a result of ixtater stress. such as the
proline content (Singh et al., 1972). or reaction to aetinomy-‘cin-D (Das and
Jain, 1972). may be easier to measure.
In rice, varietal differences have been noted in the proline accumulation as a
result of moisture stress (Yoshida et al., 1974). Many upland varieties had high
proline content. Whether the degree of accumulation of proline is related to
adaptability’ or stability index as calculated by the Finlay and Wilkinson method
has yet to be tested. Singh ct al. (1972) found a positive correlation in barley.
Morphological characters possibly related to drought resistance, such as thicker
and longer roots and fewer tillers. have been measured in rice, and a wide
range of values has been obtained for these characters. We have yet to determine
whether these characters are related to adaptability‘ of rice to xerie conditions.
Das and .Iain (I972) studied adaptability of wheat varieties by treating them
with 350 rig/ml aqueous solution of antimetabolite Actinomycin-D at seedling
stage. They found that the best adapted varieties (using Finlay and Wilkinsonis
fonnula) were relatively‘ less affected by Aetinomyicin-D. based on minimum
percentage of grovnh inhibition. The same authors also reported that the highly
adapted varieties showed a remarkable stability‘ in DNA synthesis at different
temperatures (20° to 38°C). lt appears that the presence ofa stable template may
be a characteristic feature ofa well-adapted gentityrpe. They also found (Jain and
Das, 1970) that the highly adapted wheat varieties based on multi-loeation yield
had constancy in DNA and plasticity in RNA synthesis under different environ-