240
L. Kappen et al.
m-Zs- 1 PAR) but not in darkness (Fig. 11.7). Even though the leaves were
subjected to a reduced air humidity that would normally cause significant
stomatal closing, opening of the stomata was observed instead, most likely
because photosynthesis created a COz sink, and COz deficiency in the
stomatal cavity resulted. It was remarkable to observe a correlation between
stomatal aperture and period of COz deficiency. As soon as the COz concentration had returned to normal, the stomata closed but to a little lesser
extent than at the beginning of the experiment. The curve in Fig. 11.7 shows
a time sequence of only one stomatal apparatus, but other stomata in the
same experiment responded identically (Kappen et al. 1987).
COz exchange mirrors well the metabolic response to varying COz concentration in the air. Net photosynthesis decreased significantly with deTradescantia albiflora
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60 120 180 240 300 360
Time [min]
Fig.U.S. COz exchange and response of the stomatal apparatus of a green leaf in light
(390llmol photons m- z S-1 PAR, temperature 20°C) to changes in VPD, but air containing 177 ppm COz. (After Kappen and Haeger 1991)