Photosynthesis as a Tool for Indicating Temperature Stress Events
269
ment and of recovery rates. The overriding effect of photoinhibition can
lead to different assessments of the susceptibility of cultivars (Smillie et al.
1988; Greer and Hardacre 1989). On the other hand, weak light has a
protective effect on photosynthetic function and recovery (Wise et al. 1990;
Neuner and Larcher 1991). It is therefore important to differentiate between
effects of chilling per se and combined effects of low temperature and light.
13.5 Assay and Analysis of Freezing Events by Monitoring Photosynthesis
Freezing represents a much more serious constraint to cell organization than
the mere effect of the low thermodynamic state accompanying cold. Formation of ice crystals in plant tissues sets off contraction and concentration
effects which, via toxic accumulation of ions and by membrane lesions,
eventually lead to cell death (Heber and Santarius 1973; Steponkus and
Webb 1992).
The CO2 uptake by assimilatory organs is interrupted as soon as ice forms
in the tissue, which is between - 2 and - 10 °C according to plant species and
season (Pisek et al. 1967). If the leaves manage to survive freezing without
suffering injury, in many plants photosynthetic activity remains low even
after thawing, and this is more pronounced with greater degree and duration
of the frost. These post-freezing effects are primarily due to inhibition of
chloroplast functions at the ultrastructural and biochemical levels following
freeze-dehydration (Senser and Beck 1977; bquist and Martin 1986). The
inhibition of photosynthesis seems to be based on diminished activation
of Calvin cycle enzymes; more severe freezing stress impairs the wateroxidation system (Krause et al. 1988).
The onset of tissue freezing is indicated by characteristic changes in the
fluorescence induction kinetics. Even before ice nucleation occurs, the fluorescence rise to the peak becomes delayed as the temperature is lowered,
and the decrease in the variable chlorophyll fluorescence following the peak
disappears. As soon as ice forms in the tissue, the chlorophyll fluorescence
attains the height of maximal fluorescence (Melcarek anp Brown 1979). If
the mesophyll cells have survived the freezing stress, the typical induction
transient reappears when the ice thaws. Partial damage to photosynthetic
function results in a lower fluorescence yield, whereas in totally frozen
tissues the variable fluorescence disappears after thawing (see Fig. 13.4).
By monitoring the course of freezing and thawing via chlorophyll fluorescence important cryophysiological characteristics can be determined:
1. Discrimination between freezing avoidance and freezing tolerance.
Plants are able to survive subfreezing temperatures by two basically different
mechanisms, i.e., by delay or prevention of ice nucleation in the tissues
("freezing avoidance" according to Levitt 1980), and by tolerance to extra-
269
ment and of recovery rates. The overriding effect of photoinhibition can
lead to different assessments of the susceptibility of cultivars (Smillie et al.
1988; Greer and Hardacre 1989). On the other hand, weak light has a
protective effect on photosynthetic function and recovery (Wise et al. 1990;
Neuner and Larcher 1991). It is therefore important to differentiate between
effects of chilling per se and combined effects of low temperature and light.
13.5 Assay and Analysis of Freezing Events by Monitoring Photosynthesis
Freezing represents a much more serious constraint to cell organization than
the mere effect of the low thermodynamic state accompanying cold. Formation of ice crystals in plant tissues sets off contraction and concentration
effects which, via toxic accumulation of ions and by membrane lesions,
eventually lead to cell death (Heber and Santarius 1973; Steponkus and
Webb 1992).
The CO2 uptake by assimilatory organs is interrupted as soon as ice forms
in the tissue, which is between - 2 and - 10 °C according to plant species and
season (Pisek et al. 1967). If the leaves manage to survive freezing without
suffering injury, in many plants photosynthetic activity remains low even
after thawing, and this is more pronounced with greater degree and duration
of the frost. These post-freezing effects are primarily due to inhibition of
chloroplast functions at the ultrastructural and biochemical levels following
freeze-dehydration (Senser and Beck 1977; bquist and Martin 1986). The
inhibition of photosynthesis seems to be based on diminished activation
of Calvin cycle enzymes; more severe freezing stress impairs the wateroxidation system (Krause et al. 1988).
The onset of tissue freezing is indicated by characteristic changes in the
fluorescence induction kinetics. Even before ice nucleation occurs, the fluorescence rise to the peak becomes delayed as the temperature is lowered,
and the decrease in the variable chlorophyll fluorescence following the peak
disappears. As soon as ice forms in the tissue, the chlorophyll fluorescence
attains the height of maximal fluorescence (Melcarek anp Brown 1979). If
the mesophyll cells have survived the freezing stress, the typical induction
transient reappears when the ice thaws. Partial damage to photosynthetic
function results in a lower fluorescence yield, whereas in totally frozen
tissues the variable fluorescence disappears after thawing (see Fig. 13.4).
By monitoring the course of freezing and thawing via chlorophyll fluorescence important cryophysiological characteristics can be determined:
1. Discrimination between freezing avoidance and freezing tolerance.
Plants are able to survive subfreezing temperatures by two basically different
mechanisms, i.e., by delay or prevention of ice nucleation in the tissues
("freezing avoidance" according to Levitt 1980), and by tolerance to extra-