Photosynthesis as a Tool for Indicating Temperature Stress Events
273
question the relevance and applicability of such results for plants in the
field. The use of portable systems for measuring photosynthesis and data
acquisition now make it possible to follow the freezing process in situ
(Schroeter et al. 1991). Using the noninvasive chlorophyll fluorescence technique, Bodner and Beck (1987) recorded at an altitude of 4200 m in the
upper Teleki valley on Mt. Kenya, the progress of supercooling and freezing
of intact leaves of giant Afroalpine rosette plants during several nights.
Hourly recorded fluorescence transients during a day-night cycle are presented in Fig. 13.5. In the night of the 24th to 25th February 1985 the leaves
of Lobelia telekii remained supercooled, and on rewarming the following
morning the immediate onset of photosynthetic activity was indicated by
normal induction transients. In the more severe frost in the night of the 28th
February to March 1st, with temperatures below -lOoC, the leaves of
Dendrosenecio brassica remained supercooled down to -8.8°C, after which
they froze stiff. The following morning photosynthesis was also completely
reactivated. In an environment in which night frosts can occur at any time of
the year, including the growing and flowering seasons, the plants have
evolved peculiar mechanisms of protection against the effects of apoplastic
freezing (Beck et al. 1987). These mechanisms could only be identified and
investigated in the field, since these plants did not attain the frost-hardy
state when cultivated in a greenhouse.
13.6 Conclusions
The knowledge gained by research on photosynthesis and the development
of new methods employing easily used apparatus for the nondestructive
measurement of photosynthetic parameters mark a great step forward not
only in physiological ecology, but also in botanical stress physiology and
pathology. Since photosynthesis is especially easily disturbed, it is recognized
as being a sensitive, early warning indicator for many types of stress events,
and especially for temperature stress.
The purpose of this chapter is to show the way in which deviations from
normal photosynthesis or its component processes can provide us with information about the nature and degree of stress undergone by a plant. Nevertheless, measurement and analysis of photosynthetic perturbations, although
they provide valuable insight into stress events, can only give a valid picture
of the state of stress in combination with additional diagnostic data.
In the case of heat stress, procedures involving gas exchange analysis and
in vivo chlorophyll fluorometry give clear and reliable information about the
limits of metabolic performance and of viability. The onset of low temperature stress in chilling-susceptible plants can also be detected conveniently
by measuring photosynthesis, whereas for determining severe impairment
273
question the relevance and applicability of such results for plants in the
field. The use of portable systems for measuring photosynthesis and data
acquisition now make it possible to follow the freezing process in situ
(Schroeter et al. 1991). Using the noninvasive chlorophyll fluorescence technique, Bodner and Beck (1987) recorded at an altitude of 4200 m in the
upper Teleki valley on Mt. Kenya, the progress of supercooling and freezing
of intact leaves of giant Afroalpine rosette plants during several nights.
Hourly recorded fluorescence transients during a day-night cycle are presented in Fig. 13.5. In the night of the 24th to 25th February 1985 the leaves
of Lobelia telekii remained supercooled, and on rewarming the following
morning the immediate onset of photosynthetic activity was indicated by
normal induction transients. In the more severe frost in the night of the 28th
February to March 1st, with temperatures below -lOoC, the leaves of
Dendrosenecio brassica remained supercooled down to -8.8°C, after which
they froze stiff. The following morning photosynthesis was also completely
reactivated. In an environment in which night frosts can occur at any time of
the year, including the growing and flowering seasons, the plants have
evolved peculiar mechanisms of protection against the effects of apoplastic
freezing (Beck et al. 1987). These mechanisms could only be identified and
investigated in the field, since these plants did not attain the frost-hardy
state when cultivated in a greenhouse.
13.6 Conclusions
The knowledge gained by research on photosynthesis and the development
of new methods employing easily used apparatus for the nondestructive
measurement of photosynthetic parameters mark a great step forward not
only in physiological ecology, but also in botanical stress physiology and
pathology. Since photosynthesis is especially easily disturbed, it is recognized
as being a sensitive, early warning indicator for many types of stress events,
and especially for temperature stress.
The purpose of this chapter is to show the way in which deviations from
normal photosynthesis or its component processes can provide us with information about the nature and degree of stress undergone by a plant. Nevertheless, measurement and analysis of photosynthetic perturbations, although
they provide valuable insight into stress events, can only give a valid picture
of the state of stress in combination with additional diagnostic data.
In the case of heat stress, procedures involving gas exchange analysis and
in vivo chlorophyll fluorometry give clear and reliable information about the
limits of metabolic performance and of viability. The onset of low temperature stress in chilling-susceptible plants can also be detected conveniently
by measuring photosynthesis, whereas for determining severe impairment