266
w. Larcher
functional, and distributional type (Tables 13.1 and 13.2). These temperatures are not constant but depend on the ontogenetic stage and the state of
activity (growth period, dormancy) of the plant. Furthermore, the temperature thresholds vary under the influence of changing environmental conditions within the specific range of the reaction norm.
13.3 Use of Photosynthetic Responses for Determining Heat Tolerance
Heat damage to photosynthesis arises from inactivation of the highly sensitive water-splitting reaction, disconnection of PS II centers from the bulk
pigments, thermal uncoupling of photophosphorylation, and biomembrane
lesions (Berry and Bjorkman 1980; Santarius and Weis 1988; McCain et al.
1989; Santarius et al. 1991).
The pronounced sensitivity of photosynthesis to heat can be used to
detect early disturbances and injuries caused in green plant tissues. Particularly suitable for this purpose is the use of the in vivo chlorophyll fluorescence as an intrinsic indicator of thylakoid organization and changes in
membrane fluidity. Measurements of CO 2 exchange are less reliable since
they can be influenced by stomatal closure not induced primarily by heat.
When a leaf is heated slowly (1 Kmin- 1 ), the heat-induced curve of the
basic fluorescence Fo shows a sharp discontinuity (Schreiber and Berry
1977). At the threshold temperature at which Fo begins to increase, the
quantum yield for CO2 fixation suddenly drops. Seeman et al. (1984) found
in desert plants, and Larcher et al. (1991a) in Persea species, close agreement between the temperature for the heat limit for apparent CO2 uptake
and the critical breakpoint, To in the heat-induced Fo-curve (Fig. 13.2).
From a comparison of the Fo-curves as a function of temperature of 26
herbaceous and woody plants of different origin, Bilger et al. (1984) derived
a clear relationship between the critical breakpoint temperature and the
temperature that causes necrotic damage (L T 50) in the leaves after 30 min
heating.
Under natural conditions, few studies have been devoted to the temperature thresholds for the heat inactivation of photosynthesis and for heat
damage. In the field, overheating of the leaves results from strong insolation.
The midday depression of photosynthesis on clear, hot days is due, among
other factors, to heat-related reversible photoinhibition (Demmig-Adams et
al. 1989). On the other hand, weak light (30-50f.lmol photons m- 2 s- 1 ) can
alleviate a heat-induced inhibition of photosynthesis (Havaux et al. 1991).
A number of different species are able to raise their heat threshold for
photosynthetic functioning (Larcher 1980) and for viability by 2-4 K from
morning to afternoon (Alexandrov 1977; Kappen 1981). A rise in heat
tolerance can be achieved within a few hours by synthesis of polypeptides
and heat shock proteins, which also protect the chloroplasts (Sachs and Ho
1986; Yordanov et al. 1989).
w. Larcher
functional, and distributional type (Tables 13.1 and 13.2). These temperatures are not constant but depend on the ontogenetic stage and the state of
activity (growth period, dormancy) of the plant. Furthermore, the temperature thresholds vary under the influence of changing environmental conditions within the specific range of the reaction norm.
13.3 Use of Photosynthetic Responses for Determining Heat Tolerance
Heat damage to photosynthesis arises from inactivation of the highly sensitive water-splitting reaction, disconnection of PS II centers from the bulk
pigments, thermal uncoupling of photophosphorylation, and biomembrane
lesions (Berry and Bjorkman 1980; Santarius and Weis 1988; McCain et al.
1989; Santarius et al. 1991).
The pronounced sensitivity of photosynthesis to heat can be used to
detect early disturbances and injuries caused in green plant tissues. Particularly suitable for this purpose is the use of the in vivo chlorophyll fluorescence as an intrinsic indicator of thylakoid organization and changes in
membrane fluidity. Measurements of CO 2 exchange are less reliable since
they can be influenced by stomatal closure not induced primarily by heat.
When a leaf is heated slowly (1 Kmin- 1 ), the heat-induced curve of the
basic fluorescence Fo shows a sharp discontinuity (Schreiber and Berry
1977). At the threshold temperature at which Fo begins to increase, the
quantum yield for CO2 fixation suddenly drops. Seeman et al. (1984) found
in desert plants, and Larcher et al. (1991a) in Persea species, close agreement between the temperature for the heat limit for apparent CO2 uptake
and the critical breakpoint, To in the heat-induced Fo-curve (Fig. 13.2).
From a comparison of the Fo-curves as a function of temperature of 26
herbaceous and woody plants of different origin, Bilger et al. (1984) derived
a clear relationship between the critical breakpoint temperature and the
temperature that causes necrotic damage (L T 50) in the leaves after 30 min
heating.
Under natural conditions, few studies have been devoted to the temperature thresholds for the heat inactivation of photosynthesis and for heat
damage. In the field, overheating of the leaves results from strong insolation.
The midday depression of photosynthesis on clear, hot days is due, among
other factors, to heat-related reversible photoinhibition (Demmig-Adams et
al. 1989). On the other hand, weak light (30-50f.lmol photons m- 2 s- 1 ) can
alleviate a heat-induced inhibition of photosynthesis (Havaux et al. 1991).
A number of different species are able to raise their heat threshold for
photosynthetic functioning (Larcher 1980) and for viability by 2-4 K from
morning to afternoon (Alexandrov 1977; Kappen 1981). A rise in heat
tolerance can be achieved within a few hours by synthesis of polypeptides
and heat shock proteins, which also protect the chloroplasts (Sachs and Ho
1986; Yordanov et al. 1989).