| Abstract |
Soil salinity existed long before humans and agriculture but the problem has been aggravated by agricultural practices such as irrigation. Today, ~20% of the worlds cultivated land and nearly half of all irrigated lands are affected by salinity (Zhu, 2001). High concentrations of salts cause ion imbalance and hyperosmotic stress in plants. As a consequence of these primary effects, secondary stresses such as oxidative damage often occur that lead to reduced plant growth. The nature of the damage that high salt concentrations inflict on plants is not entirely clear. The integrity of cellular membranes, the activities of various enzymes, nutrient acquisition and function of photosynthetic apparatus are all known to be prone to the toxic effects of high salt stress. Very little is known about changes in PSII photochemistry during the initial stages of the responses to salinity stress. These early responses may be crucial since they may determine whether the organism will be able to survive the fast transition and then undergo a somewhat longer metabolic stage of acclimation to elevated salinity (Lu & Vonshak, 2002). Still the adaptation of PSII photochemistry to salinity stress is a complex process (Vonshak et al., 1996; Lu et al., 1999). Other studies have revealed the participation of the LHCIIs size regulation in plant photosynthetic responses to various environmental stressors, such as high ozone concentration (Navakoudi et al., 2003), UVB radiation (Sfichi et al., 2004), high CO2 concentration (Logothetis et al., 2004). In all of the above cases externally manipulated polyamine (Putresine, Spermidine, Spermine) concentrations were effective in reversing the sensitivity of the photosynthetic apparatus to the stressor, since they were found to be attached to photosynthetic subcomplexes (Kotzabasis et al., 1993; Del Duca et al., 1994). Thus, polyamines play a major role in regulating the LHCIIs size status and therefore the photosynthetic efficiency, and consequently increasing plant production under unfavorable environmental conditions. In light of all this information we used the unicellular green alga Scenedesmus obliquus to study the effects of salinity stress on the photosynthetic apparatus, and the role of exogenous treated polyamine levels (mainly putresine) in salt stressed cells, with the purpose of creating salt tolerant caltures. The results showed that high concentrations of NaCl drastically reduced photosynthetic efficiency (Fv/Fm), by increasing the LHCII antenna size and deactivating a considerable percentage of photosynthetic reactive centers (RC/CSo), resulting in increased dissipation energy per reaction center (DIo/RC) that damages the photosynthetic apparatus and causes the maximal photosynthetic oxygen evolution activity to reduce. The exogenously increased quantity of putresine (or increase in Put/Spm ratio) led to protection of the photosynthetic apparatus from the harmful effects of salt stress, by reducing the LHCII antenna size, and consequently reducing the excess dissipation energy and increasing the maximal photosynthetic oxygen evolution activity. The role of light in differentiating the effect of salt stress was also discussed.
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Μηχανισμοί ανθεκτικότητας των φυτών στην αλατότητα μέσω διαφοροποιήσεων της μοριακής δομής και λειτουργίας του φωτοσυνθετικού μηχανισμού
