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L. Kappen et al.
(see also Willmer 1983) - that is equal to the air CO2 concentration during
the Ice Ages (Peel 1983) - even though air humidity was high. Stomata
obviously react differently if they are subjected to COz-free air or pure
nitrogen (Louguet 1972; Laffray et al. 1984). The question remains, however, whether other response mechanisms exist. If we assume that stomata
open because they react to a CO2 gradient that is caused by an assimilatory
sink, as is quite obvious by stomatal response to low CO 2 concentrations
in light, we have little reason to postulate stomatal opening in darkness.
The CO2 concentration in the tissue region is increased by respiration and
consequently a gradient to a lower ambient CO2 concentration may instead
cause a tendency to close. If pure nitrogen is supplied, the lack of oxygen
would principally change the system (Louguet 1972).
During evaluation of stomatal response to increasing temperature, it was
interesting that leaf conductance as calculated by the moisture relations in
the gas exchange cuvette differed from that calculated from the observed
changes in stomatal aperture dimensions. Unless these differences can be
explained by other experiments, they may indicate that the. calculation
of conductance using the chosen parameters (VPD, temperature) is insufficient, and that other parameters like convection have to be considered as
well.
11.5 Conclusions
It is important to state that in all our experiments with herbaceous plants
(and in general also with woody plants) the stomata respond in the same
way to influences on the whole leaf. The stomatal response that we investigated was also in general accordance with transpiration and CO2 uptake.
Consequently, it is hard to believe that we have seen only an exceptional
area of the leaf that accidentally corresponded with the gas exchange signals. The observed stomatal aperture cannot, however, be taken as an
absolute measure because steady-state apertures vary to a large degree. This
variation is apparently greater in leaves of woody plants than in those
of herbaceous plant species (Kappen et al. in preparation). An unequal
distribution of stomatal opening (patchiness) was demonstrated in mediterranean sclerophyllous leaves (Beyschlag and Pfanz 1990) and we suggest
that it may be due to different steady-state apertures of stomata rather than
to a pattern of patches with divergently reacting stomata. However, for the
investigation of sclerophyllous plants, we must respect the technical limitations of our method because size and anatomy of these stomata do not allow
direct observation.
Acknowledgment. The authors wish to thank Dr. R.J. Palmer Jr. for editing the English
version of the manuscript.
L. Kappen et al.
(see also Willmer 1983) - that is equal to the air CO2 concentration during
the Ice Ages (Peel 1983) - even though air humidity was high. Stomata
obviously react differently if they are subjected to COz-free air or pure
nitrogen (Louguet 1972; Laffray et al. 1984). The question remains, however, whether other response mechanisms exist. If we assume that stomata
open because they react to a CO2 gradient that is caused by an assimilatory
sink, as is quite obvious by stomatal response to low CO 2 concentrations
in light, we have little reason to postulate stomatal opening in darkness.
The CO2 concentration in the tissue region is increased by respiration and
consequently a gradient to a lower ambient CO2 concentration may instead
cause a tendency to close. If pure nitrogen is supplied, the lack of oxygen
would principally change the system (Louguet 1972).
During evaluation of stomatal response to increasing temperature, it was
interesting that leaf conductance as calculated by the moisture relations in
the gas exchange cuvette differed from that calculated from the observed
changes in stomatal aperture dimensions. Unless these differences can be
explained by other experiments, they may indicate that the. calculation
of conductance using the chosen parameters (VPD, temperature) is insufficient, and that other parameters like convection have to be considered as
well.
11.5 Conclusions
It is important to state that in all our experiments with herbaceous plants
(and in general also with woody plants) the stomata respond in the same
way to influences on the whole leaf. The stomatal response that we investigated was also in general accordance with transpiration and CO2 uptake.
Consequently, it is hard to believe that we have seen only an exceptional
area of the leaf that accidentally corresponded with the gas exchange signals. The observed stomatal aperture cannot, however, be taken as an
absolute measure because steady-state apertures vary to a large degree. This
variation is apparently greater in leaves of woody plants than in those
of herbaceous plant species (Kappen et al. in preparation). An unequal
distribution of stomatal opening (patchiness) was demonstrated in mediterranean sclerophyllous leaves (Beyschlag and Pfanz 1990) and we suggest
that it may be due to different steady-state apertures of stomata rather than
to a pattern of patches with divergently reacting stomata. However, for the
investigation of sclerophyllous plants, we must respect the technical limitations of our method because size and anatomy of these stomata do not allow
direct observation.
Acknowledgment. The authors wish to thank Dr. R.J. Palmer Jr. for editing the English
version of the manuscript.