Assessments of photosynthesis is a good indicator for predicting plant stress. Because photosynthesis is sensitive to changes of environmental conditions, especially low temperature (Perks, Osborne & Mitchell 2004a). ChlF as a non-destructive, rapid and effective method has been used as a most reliable indicator of the performance in plants and crops products. Until now, it has been widely used to detect differential stress responses. Such as water stress, heat stress, salt stress, chilling stress etc. (Sayed 2003b). Low temperature injury can be divided into freezing and chilling:
At freezing temperature, leaf metabolism is greatly inhibited and recovers slowly, which is caused by the photo damage of PSII (Baker & Rosenqvist 2004). ChlF
37 parameter Fv / Fm ratios can be used to measure the status of the Photosystem II (PSII)
reaction center for detecting freezing tolerance (Sayed 2003b). In freezing studies, Fv /
Fm was linearly related to visual needle damage and short-term survival after freezing
stress. The parameter is available to accurately predict cold hardiness (Perks, Osborne & Mitchell 2004b). Moreover, it can be used to estimate the critical temperature for 50% needle damage after freezing (Lindgren & Hällgren 1993). Analysis of freezing injury of Palm showed that there is a strong positive correlation (r2 = 0.94) between injury index Fv / Fm values and extent of injury (Equiza & Francko 2010). In a potted Oleander
study, they found that after freezing treatment, non-photochemical quenching (NPQ) and the maximum photochemical efficiency of photosystem II (Fv / Fm) were decreased
(Julián et al. 2011).
Chilling injury influence the growth of the plant and development of cereals. The range of inducing temperatures is from 0 to 15 °C. According to the extent of the injury, it may be classified as primary or secondary chilling injury. The former indicates metabolic dysfunctions which can reverse when the temperature returns to the normal range, the latter happens irreversibly when metabolic dysfunctions occur (Sayed 2003b). Crops in temperate zones are generally much more tolerant of falling temperatures than subtropical and tropical species. Moreover, chilling inhibited metabolism can more rapidly recovery when temperatures rise in these plants. However, many temperate regions crops are also very sensitive to low temperature if the plants have not experienced effective low temperature acclimation. The growth and physiology of crops of subtropical and tropical districts can be influenced by exposure to low temperatures as high as 15 °C (Baker & Rosenqvist 2004). In general, compared with mature plants, the seedling growth stage is more sensitive to chilling stress and chilling injury leads to plant loss of viability. Moreover, flower organs will be more seriously influenced by chilling stress at the reproductive growth stage (Sayed 2003b). Chilling injury will lead to down-regulation and photo damage to PSII. This inhibition includes reduced stomatal aperture, membrane lipid changes and reduced CO2 fixation and
electron transport of PSII (Agati et al. 1996). Chilling stress leads to a decrease of Fv /
Fm, which has been widely used to screen differential chilling tolerance of crops.
In many districts of the world, drought from climate change is a major abiotic stress factor that severely affects food production. It leads to high temperatures and reduced water supply for crops (Mishra & Singh 2010). Improving drought tolerance with agronomic and physiological phenotype trait selection is important to reduce the
38 impact of water deficit on crop yield in a breeding program (Tuberosa & Salvi 2006). In many species, ChlF has been used as a probe for detecting and predicting the degree of drought response (Ögren 1990; Oraguzie et al. 2007; Razavi et al. 2008). In a barley study, they found that the values of chlorophyll content, F0, Fv / F0 and Fv / Fm in drought
tolerant genotypes were significantly higher than drought sensitive genotypes under drought stress (Li et al. 2006). ChlF parameters as indicators of membrane injury can be evaluated. The Fv / Fm ratio is employed to assess changes in the primary
photochemical reactions of the photosynthetic apparatus after dehydration (Kocheva et al. 2004). In addition, ChlF values can be used as phenotype information combined with population genotype information for detecting crop ChlF related QTLs at specific drought stages (Guo et al. 2008).
Salinization of soils is an extensive problem in crop production systems. The initial effects of increasing soil salinity are similar to drought effects when plants are exposed to drought (Mckersie & Ya’acov 1994). Fv / Fm values were found to be unaffected by
salinity treatments, whereas the overall or actual efficiency of photochemical energy conversion (actual quantum yield ΔF/ Fm) reduced with increasing salinity. Non-
photochemical quenching increased significantly with increasing salinity level in salt- sensitive cultivars (Dionisio-Sese & Tobita 2000). Fv / Fm was also demonstrated to
have a lack of change when maize was grown in high concentrations of NaCl (Shabala et al. 1998). Salt stress-induced effects in cereal crops include reduction of water uptake by roots due to perturbation of osmotic equilibrium, and inhibited cell expansion, reduced respiration and reduced cytokinesis (Blum & Johnson 1992; Sayed 2003a). Heat stress, may also limit the growth of crops and their yield. The most dominant heat stress traits are inactivation of PSII and thylakoid disorganization. The value of F0
has a sharp rise indicating the critical temperature which leads to PSII inactivation. Heat stress also enhances respiration and causes plants to consume saccharide reserves faster than photosynthesis replaces them (Baker & Rosenqvist 2004). In tomato studies, it was found Fv / Fm and Fv’/Fm’ decreased in the heat-sensitive genotypes. Fv’/Fm’ was better
that Fv / Fm for predicting crop heat-shock sensitivity (Hameed et al. 2015). CO2
assimilation and electron transport are both inhibited with heat stress, measurements of Fq’/Fm’ and Fv’/Fm’ also have potential for use in screening to identify tolerance to high
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