Vergara - 1976 - Physiological and morphological adaptability of ri

84 CLIMATE AND RICE
DISCUSSION
STANSEI. (Chairman): As Dr. Vergara pointed out. we can do little to change climate.
Therefore. we must tailor-make the physiological response of our cultivars to take
advantage of the existing climatic conditions. For example. in the southern rice area of
the United States. yields were increased by the development of non-photoperiod sensitive.
very early maturing. IOO-day’ upright-leaved varieties. This yield increase was attributed
to better utilization of existing climatic conditions. For example, the new varieties
are now planted 30 days earlier than were the older mid-season maturing varieties.
This earlier planting has increased yields by taking advantage of existing temperature
and sunlight conditions. The basic vegetative phase occurs during the cooler temperatures
of spring. thereby producing a shorter plant and preventing excessive vegetative
growth at higher nitrogen levels. This helps utilization of available sunlight by reducing
mutual shading. Earlier plantings also increased utilization of solar energy. since the
critical light-requiring stages of growth and development occurred during the periods of
maximum accumulative sunlight. The critical light-requiring period is defined as a
period of approximately‘ 6 weeks beginning with the panicle differentiation stage of
development and ending just prior to maturity. Heading occurs at about the middle of
this period. By adjusting planting dates. this critical sunlight-requiring period is
matched with anticipated maximum-sunlight conditions. with heading occurring close
to the longest day of the year. which. in Texas, is the time of maximum accumulative
sunlight conditions. This is an example of physiological and morphological adaptability
to existing climatic conditions for maximum yield.
There seems to be no limit to the variability available in plants. enabling them to
adapt to climatic conditions. However. the problem is much more complex than simply
cataloguing genetic material for its physiological and morphological response to specific
climatic factors. This is the first step. but it must he realized that the true key to these
responses may well be the interaction of two or more envirorunental factors. For example.
many of the earlier studies on photosynthetic rates of rice varieties showed little
response to temperature (within limits). When photosynthesis was measured under
light intensities approaching natural sunlight. hovrever. a response curve to temperature
effects on net photosynthesis rate was observed.
The identification of physiological and morphological response to components of
climate are a great step forward. It has been pointed out that Lhe response to climatic
factors changes with the growth and dexielopment stages of the plant. This is one form
of plant-climate interaction. One must-also be cautioned that climate is an integration
of its components. so their interactions with the plant at each stage of growth and development
become the etfects to consider in mans effort to feed himself. The challenge to
characterize physiological response to climate is there. We know little about the physiological
interaction with climate. but this information is direly needed to develop
climatic-adaptable cultivars.
ltlANUEL Other components of the environment. apart from temperature. can vary
with altitude, such as pressure and the spectral composition of light. l wonder if tolerance
to low temperature would be a SUfliCiBHl requirement for a plant to adapt to highaltitude
planting.
l ergara: We do not know the effect of low pressure on the growth of the rice plant.
Hon/ever. we know definitely that at high-altitude areas. low temperature is the main
limiting factor. Our experiments in the phytotron and at high-altitude areas showed
similar results. The effect of low pressure or spectral composition of the light was not
apparent in the parameters we measured.