A comprehensive understanding of the physiological responses of plants to extreme temperatures is essential for future strategies for plant improvement. Obvious advantages can result from the study of highly adapted plant species, such as the model tree Populus euphratica Olivier that naturally thrives under extreme temperatures, saline soils, and drought. The present paper addresses the issue of P. euphratica thermotolerance using a cell suspension model system. P. euphratica suspended cells were subjected to a range of temperatures (from 5 up to 75°C) for 20 min, and cultures were evaluated for cell viability and biomass content at specific time points. The results have shown that cell viability was only affected after a temperature stress higher than 40°C, although in these conditions it was observed that a cell growth increases after the recovery period. In contrast, a total decline in cell viability was observed in suspended cells treated at 50°C or higher temperatures, which did not show growth recovery capacity. Therefore, the known natural tolerance of P. euphratica to thermal stress was not observable at the cellular level. The greater susceptibility to high temperatures in suspended cells as compared to field plants suggests that high thermotolerance can only be achieved when cells are integrated into a tissue. 1. Introduction Plants often grow under unfavorable conditions that extensively alter their development and productivity. One such environmental challenge is exposure to adverse temperatures, which can significantly affect many essential metabolic processes and disrupt an extensive range of cellular components. Heat stress can vary in severity, depending upon the intensity and extent of the stress as well as the rate of temperature variation. As sessile organisms, plants have developed several metabolic responses that minimize injuries caused by the constant exposure of plants to daily temperature fluctuations and their association with other abiotic factors [1]. The deeper knowledge on plant abiotic stress resistance has been fundamental in the development of effective engineering strategies leading to enhanced stress tolerance. Plant transformation with genes conferring thermal tolerance has been successfully achieved [2, 3]. However, many molecular mechanisms involved in thermotolerance are likely still unknown. A successful strategy for the assignment of gene function has been the study of species that are naturally adapted to survive in extreme environments. The advantages of using members of the poplar genus (Populus) as genomic
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