In areas with a Mediterranean climate, frost can occur in the field in winter or early spring. The production of ornamental plants in the nursery may be affected by freezing temperatures, which can produce severe injuries in the plants. Although methods are generally available to define and measure freezing temperature stress, quantifying the effect of stress on the plant is more difficult, because it depends on two closely related factors: 1) the time scale of exposure; and 2) the developmental strategy of the species in question (Huner et al., 1998).
The primary photochemical reactions of photosystem II (PSII) and PSI occur on a much faster time scale than electron transport and metabolism, and the exposure of plants to light energy in excess of that required for photosynthesis results in an energy imbalance that generally leads to photoinhibition (Osmond, 1994). Chilling can disrupt all major components of photosynthesis, including thylakoid electron transport, the carbon reduction cycle, and the control of stomatal conductance (gS) (Allen and Ort, 2001). To prevent damage on a time scale of minutes, organisms can acclimate in an attempt to compensate for exposure to high PSII excitation pressure. They do this by reducing energy transfer efficiency to PSII, either by diverting energy from PSII to PSI through state transitions or by dissipating excess energy as heat through non-photochemical quenching. On a longer time scale, photosynthetic acclimation to high PSII excitation pressure may occur as a consequence of a reduction in PSII antenna size (Huner et al., 1998). These mechanisms avoid damage to photosystems but result in a decrease in photosynthesis (D'Ambrosio et al., 2006). Hence, chlorophyll fluorescence is a useful tool for detecting the sensitivity of plants to stress, because the technique estimates the relative changes in PSII excitation pressure of organisms exposed to changing environmental conditions (Adams et al., 1995; Smillie and Hetherington, 1983). It is also useful for quantifying the extent to which the stress has damaged the photosynthetic apparatus (Percival and Fraser, 2001).
Chlorophyll fluorescence technique has been used in several species as an indicator of plant sensitivity to environmental stresses such as salinity (Percival and Fraser, 2001; Percival et al., 2003; Smillie and Nott, 1979), drought (Percival and Sheriffs, 2002), heat (Hetherington et al., 1983; Percival, 2005; Yamada et al., 1996), chilling (Hakam et al., 2000; Lurie et al., 1994; Percival and Henderson, 2003; Wilson and Greaves, 1990), and freezing (Forney et al., 2000; Zulini et al., 2010).
Several studies have indicated that reductions in chlorophyll fluorescence occur in plants at temperatures near their chilling threshold. Smillie and Nott (1979) reported that the decrease in chlorophyll fluorescence of leaves during rapid chilling at 0 °C can be used as an index of the chilling sensitivity of plant species. Khanizadeh et al. (2000) and Sthapit et al. (1995) suggested that chlorophyll fluorescence can be used for estimating the stress tolerance of strawberry and rice, respectively, which is useful in selecting resistant spring frost selections in breeding programs.
This investigation focuses on oleander, an evergreen shrub from the Mediterranean regions of southern Europe, which is of great importance in the commercial production of potted plants and is much appreciated for its ornamental value. The objective of this study was to characterize freezing-induced changes in chlorophyll fluorescence parameters of two cultivars of oleander with different degrees of sensitivity to freezing temperatures and relate these indicators with visual symptoms of freezing injury in leaves.
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