V. NEW APPROACH TO WATER RELATIONS OF DESERT PLANTS
273
limited to the Crassulaceae and therefore should not be called Crassulacean
acid metabolism.
Experiments for simultaneous measurement of C 0 2 absorption and
transpiration of Bryophyllum daigremontianum (grown in pots, with the
above-ground part of the plant in a cuvette, and in a 12-hour dark-light
cycle, with 7000-lux light intensity) showed a very close parallelism between C 0 2 absorption and transpiration in well-watered plants (Kluge and
Fischer, 1967). As earlier reported, changes in stomatal aperture corresponded with gas exchange. At the beginning of the light period, as
stomata open, a temporary increase in C 0 2 absorption and transpiration
occurred. One hour later, transpiration and CO L > absorption dropped sharply
with closure of the stomata, and there was even a slight release of C0 2 .
Whether the temporary increase of C 0 2 uptake and transpiration at the
beginning of the light period is caused by the sudden exposure to illumination, which does not occur in nature, was not checked.
Toward the end of the light period, transpiration and C 0 2 absorption
increased slightly and, after the beginning of the dark period, a transient,
sudden drop in C 0 2 absorption and transpiration was again shown. However, 30 minutes after the beginning of the dark period, this transient
reaction was overcome and transpiration and C 0 2 absorption increased constantly toward a maximum value, parallel with the opening of the stomata.
Bryophyllum plants having leaves without epidermis (the epidermis can
be easily stripped off without damaging the tissue underneath) showed with
the same light-dark cycle a similar trend of increase and decrease in
C0 2 absorption as did plants having intact leaves (Fig. 31). This indicates
that stomatal absorption of C 0 2 is controlled by the leaf mesophyll rather
than by stomatal aperture, which in turn is synchronized by the concentration of C 0 2 in the intercellular spaces. Transpiration, however, depends on
stomatal aperture, and shows no cyclic changes in plants with leaves without epidermis (Fig. 3IB). In this case transpiration becomes a passive process and approaches evaporation.
When irrigation of the Bryophyllum plant is discontinued and water
stress develops gradually, the curve for C 0 2 absorption during the light
period becomes lower, and finally goes below the zero line. The peak at
the beginning of the light period becomes lower, whereas the one at the
end disappears completely. Finally, C 0 2 uptake occurs only during the
dark period and to a lesser degree. At high water stress, C 0 2 uptake may
be limited by hydroactive stomatal closure. Transpiration parallels C 0 2
uptake indicating stomatal control of gas exchange. The original cycle is
reestablished almost immediately after rewatering (Fig. 32).
The change of C 0 2 absorption and transpiration in Bryophyllum under
water stress condition is a good example of a biologically controlled pro-
273
limited to the Crassulaceae and therefore should not be called Crassulacean
acid metabolism.
Experiments for simultaneous measurement of C 0 2 absorption and
transpiration of Bryophyllum daigremontianum (grown in pots, with the
above-ground part of the plant in a cuvette, and in a 12-hour dark-light
cycle, with 7000-lux light intensity) showed a very close parallelism between C 0 2 absorption and transpiration in well-watered plants (Kluge and
Fischer, 1967). As earlier reported, changes in stomatal aperture corresponded with gas exchange. At the beginning of the light period, as
stomata open, a temporary increase in C 0 2 absorption and transpiration
occurred. One hour later, transpiration and CO L > absorption dropped sharply
with closure of the stomata, and there was even a slight release of C0 2 .
Whether the temporary increase of C 0 2 uptake and transpiration at the
beginning of the light period is caused by the sudden exposure to illumination, which does not occur in nature, was not checked.
Toward the end of the light period, transpiration and C 0 2 absorption
increased slightly and, after the beginning of the dark period, a transient,
sudden drop in C 0 2 absorption and transpiration was again shown. However, 30 minutes after the beginning of the dark period, this transient
reaction was overcome and transpiration and C 0 2 absorption increased constantly toward a maximum value, parallel with the opening of the stomata.
Bryophyllum plants having leaves without epidermis (the epidermis can
be easily stripped off without damaging the tissue underneath) showed with
the same light-dark cycle a similar trend of increase and decrease in
C0 2 absorption as did plants having intact leaves (Fig. 31). This indicates
that stomatal absorption of C 0 2 is controlled by the leaf mesophyll rather
than by stomatal aperture, which in turn is synchronized by the concentration of C 0 2 in the intercellular spaces. Transpiration, however, depends on
stomatal aperture, and shows no cyclic changes in plants with leaves without epidermis (Fig. 3IB). In this case transpiration becomes a passive process and approaches evaporation.
When irrigation of the Bryophyllum plant is discontinued and water
stress develops gradually, the curve for C 0 2 absorption during the light
period becomes lower, and finally goes below the zero line. The peak at
the beginning of the light period becomes lower, whereas the one at the
end disappears completely. Finally, C 0 2 uptake occurs only during the
dark period and to a lesser degree. At high water stress, C 0 2 uptake may
be limited by hydroactive stomatal closure. Transpiration parallels C 0 2
uptake indicating stomatal control of gas exchange. The original cycle is
reestablished almost immediately after rewatering (Fig. 32).
The change of C 0 2 absorption and transpiration in Bryophyllum under
water stress condition is a good example of a biologically controlled pro-