Vergara - 1976 - Physiological and morphological adaptability of ri

154 culture ANT) RICE
enviromnent adequately‘. and this is the balance of PAR (determining a photosynthesis rate)
and net radiation exchange (determining a transpiration rate). Despite a bit of special pleading
in Lloyd's paper. and despite liens assertion that it is possible to simulate drought in a phytotron
by managing the root environment properly. I don't see how we can hope to get the factors
detennining the atmospheric demand for water into the right balance with the tactors that
detennine growth. ln order to relate drought performance in the phytotron and in the field, it
will be essential to make careful and regular measurements of quantities such as stomatal resistance
and the water potential of soil and plant tissue. Transpiration rate by itself is not an
adequate index of water stress.
Two minor points. The spectrum of the blue light from the sky must be added to l\-'Ioon’s
sunlight spectrum to get the right spectral balance for daylight. As some of the light scattered
out of the direct beam is received as diffuse light at the surface. the spectrum of total radiation
changes less during the day than the direct-beam spectrum alone.
Finally. transpiration will not be proportional to the vapor-pressure deficit measured with
respect to air temperature (which is presumably‘ what is quoted in the paper) but with respect
to leaf temperature. Although the two temperatures may be close. the dilference will usually
be significant in this context.
Jl-fIICfTBi/J In terms of realism of simulation, it is practicable if we take trouble to simulate the
great majority of field conditions. The constant problem is whether for understanding operational
crop physiology it is worth the clfort. The more he are able to twffer awide range of simutation
the more we find that the field man is uncertain as to the accuracy ot‘ his descriptions
of particular environmental features which are important to simulate. Essentially. controlled
envirorunent conditions allow you to examine this field of uncertainty by sequential testing.
Hence. a greater part of the initial strategy is not to get too complicated but to keep your
simulation simple.
I think the chairman. who is a very‘ distinguished micromcteortiltigist, has erred slightly in
his assessment of the implications of simulating radiation transfer. We can essentially simulate
the radiation transfer levels as met in the field. That is particularly for the incoming radiation
within the visible. and also including the infrared component. It is a question of how far it is
worth attempting to simulate exactly‘ in terms of what it means to the plant.
NIQNTEITH: This means that you have a surface that is 20"C below ambient temperature. like
the clear blue sky?
ittirchell: No. I am talking of total incoming radiation load. and total net radiation. As we
read the physiology; of plant response, its implication is essentially in terms of the effects of
radiation load on the temperature of the leaf surface. The key here is not how accurately‘ you
have simulated sky‘ radiation in terms of its long infrared wavelengths, but what the temperature
is of the leaf surface under the operating conditions you have. As long as that is realized. then it
eliminates arguments about getting exact infrared simulation.
NIX: In the example presented of high levels of irradiance reducing growth and leaf area
increase. how much were tissue temperatures raised by the higher radiation levels‘? Over what
time period were thcsc differences established?
itfitcheil; Leaf temperatures were raised approximately 5"‘C by the higli-iittensity’ radiation.
Plants were adapted to the conditions from 10-14 days. "Then measurements ‘were made for
subsequent 7-10 day’ periods.
Om: The difference in day and night temperatures plays an important rule in the development
of plants. and the tolerance to constant temperature differs according to plant genotypes.
May I have your comment on this point‘?
Mitchell: I agree. It is a matter for investigation to ascertain the best temperatures at various
stages of growth for different genotypes.
SASTRYZ The duration of leaf wetness is an important factor in the development of forecasting
models for crop-disease development. Have any attempts been made to simulate these conditions
at different temperatures and light levels in the controlled environment‘?
iltltcheil." Yes. They appear to be working successfully with a study of fungal infection of
leaves of Pimts mdiata.
GHILDYAL: In characterizing the physics of plant environment we tend to forget about soil
physical environment. Our observations show that plant-water potential and plant temperature
are directly related to soil temperature, and we know that both of these parameters significantly