**Acknowledgements**

caused by drought and salt stress applied individually. Additionally, subsequent multivariate statistical analysis allowed identifying those metabolites that specifically responded to the combined stress, namely two TCA cycle intermediates (citrate and fumarate) and four amino acids (the branched chain amino acids—valine, leucine and isoleucine, and the aro-

Up to date, studies on the combined effects of salt and heat stress in plants have revealed both positive and negative interactions on plant growth, yield and physiological traits (**Figure 2**). In wheat, the combination of salt and heat stress enhanced the transpiration rate, which in turn, was already induced by heat stress itself. On the other hand, this stress combination also

 ions by the plant [109, 110]. The effects of the combination of salt and heat stress were evaluated in tomato plants (*Solanum lycopersicum* cv. Optima) [111]. This stress combination was observed to induce a specific response by the plants through the accumulation in the levels of glycine betaine and trehalose, both well-known for their osmoprotectant roles. The accumulation of glycine betaine

a better performance of the cell water status and photosynthesis when compared to the salt

To the best of our knowledge, metabolomics studies aiming at dissecting metabolite responses induced by salt and heat stress are scarce, highlighting the need for further research in this

Climate change disturbs a number of variables that determine how much plants can grow and

and changes to soil conditions will essentially make it more challenging for plants to thrive. Overall, climate change is expected to decline the growth and development of plants, particularly with reference to agricultural systems. Declining plant growth also dramatically changes the habitats that are necessary for many species to survive. Undoubtedly, under the current threat of climate change, it is urgent to address the molecular and biochemical mechanisms that underlie plant responses to several abiotic stresses and combinations thereof. However, a complete understanding of plant responses to climate change is best obtained if data is integrated at several levels, including morpho-physiological and developmental studies as well as molecular studies that comprise the so-called *omics* technologies. Up to now, metabolomics studies have already provided a promising basis for facilitating our understanding of the plant's flexibility to reconfigure central metabolic pathways (i.e., carbon, nitrogen and energy metabolism) as well as the degree by which plants tolerate and/or are susceptible to a climate change scenario. Nevertheless, more research efforts are crucial for a more comprehensive

:K<sup>+</sup>

together with a decrease in water availability

ratio, thereby leading to

matic amino acid—phenylalanine) [107].

122 Plant, Abiotic Stress and Responses to Climate Change

promoted a higher uptake of Na<sup>+</sup>

stress alone [111].

**5. Concluding remarks**

develop. Extreme temperatures, elevated CO<sup>2</sup>

area.

**4.3. Metabolite responses to combined salt and heat stress**

and trehalose was associated to the maintenance of a lower Na<sup>+</sup>

C. António gratefully acknowledges support from Fundação para a Ciência e a Tecnologia (FCT) through the FCT Investigator Program (IF/00376/2012/CP0165/CT0003) and from the ITQB NOVA research unit Green-IT "Bioresources for sustainability" (UID/Multi/04551/2013). T.F. Jorge acknowledges FCT for the PhD grant (PD/BD/113475/2015) from the ITQB NOVA International PhD program "Plants for Life" (PD/00035/2013).
