Vergara - 1976 - Physiological and morphological adaptability of ri

MlL“R()(_‘.Lll\-1_*-\'l'E or ms RICE CROP 139
studies for reasons presented in all modern tesrtbooks of plant physiology. The calorie and the
Langley have also been superseded by the Joule and the Joule per square meter but the relevant
international recommendation seems to be taking many years to reach the remoter parts of the
southem hemisphere!
Second, I was most interested to see that despite the large difference in leaf angle distribution
between Vlanryo and 1R8; the extinction coefficient for total radiation in the two canopies is
indistinguishable (Fig. 2A). This evidence confirms my suspicion that the high yields achieved
by very erect varieties of rice are not a direct consequence ot‘ light distribution.
Third, the range of wind speeds corresponding to Fig. 10A is extreme. about 0.15 to 5 mfs
accordmg to figures Dr. Uchijima has given me. ln a real canopy, only the top two curves of
Fig. 10A are likely‘ to be relevant (l to 5 mfs) and the effects of buoyancy would often reduce
even the small ditterence between them. The insensitivity of canopy photosynthesis to wind
speed was demonstrated in a paper by myself with Szeicz and Yabuki which Dr. Yoshida
quotes (sec p. 215).
FISCHER: “lurking on a crop where it is still a matter of considerable dispute whether erect
canopies give greater productivity than non-erect ones. can l assume that there is now complete
agreement amongst rice workers as to the advantages of erect canopies, that is effects on
albedo. on turbulence. on extinction coeflicient. and finally on crop photosynthesis?
Llchijima: From theoretical calculations. it is known that an erect-leaved canopy gives greater
productivity than non-erect-leaved ones. provided the leaf area index exceeds 4 and sun elemtion
is high. However, it is also mentioned that the possible maximum difference in photosynthetic
productivity between them is lower than 30%. Such an advantage of erect-leaved
canopies is due to even distribution of radiation in canopy, improvement of air mixing, and
prevention of excess rising of leaf temperature. Although the first advantage is confirmed by
both theoretical and experimental studies, the second and third have not been tested yct.
MANUEL! Did you determine the spectral characteristics of diffused and scattered light?
L7chyima: I did not measure the spectral distribution of radiant energy in diffuse radiation
and scattered radiation. However. with the data of radiant energy of coming radiation and of
spectra] characteristics in reflectivity and transinissibility’ of leaves. we can rather easily estimate
the spectral characteristics of these radiation fluxes at each level within a plant canopy.
A-lAbIIJEL: What is the evtent of total photosynthesis and transpiration accounted for by
diffused and scattered light?
Uchijimo: Because transpiration is determined by the total amount of radiation flux impinging
on leaf surface, spectral characteristics of radiation do not influence transpiration. On the
other hand. photosynthesis is closely correlated with the amount of photosynthetically active
radiation (0.4—(l.7t1), so the spectral characteristics of radiant energy should significantly‘
influence the amount of photosynthesis.
\’OS"ID1\.: How much difference in leaf temperature do you find as that is affected by leaf
inclination?
Ufhijlmfl.‘ Llnfortunately. l did not measure leaf temperature within rice canopies in relation
to the canopy‘ structure. However. if we use data similar to those shown in Fig. 4 of my
paper, it is very easy to make clear the difference in leaf temperature between erect and horizonta]
leaves, by using a heat-balance approach.
EVANS: Given the substantial reduction in CO, level within the canopy on bright still days.
the supply" of additional CO- could increase photosynthesis and yield substantially. In what
ways could this test be done. from the micrometeorological point of vie\v'?
Llchoimu: Although results obtained from computer simulation show that the additional
supply of CO; from underlying surface below plant canopy can increase the net photosynthesis
of plant canopy; somewhat in proportion to the amount of supplied C0,, a high percentage of
supplied C0,. is transferred and lost by turbulent transfer into the surface air layer. From this.
it seems to me that the supply of additional CO; is not very reliable under field conditions.
Whether the supply of additional CO; is a reliable way to increase photosynthesis under field
conditions can be tested by conducting field experiments similar to that made by Cooper el al.
(1973?) Agron. l. in a cotton field.
GH|LD‘i'.»\L: How does the estimate of water use by a rice crop by the energy-balance method
compare with pan evaporation‘?
Uchgiima: 1n Japan. ahnost all agricultural experiment stations carry out daily observation
of evaporation using an evaporirneter with a diameter of 7U cm. The evaporation of a rice field