Abstract
Aiming to mimic a more realistic field condition and to determine convergent and divergent responses of individual stresses in relation to their combination, we explored physiological, biochemical, and metabolomic alterations after drought and heat stress imposition (alone and combined) and recovery, using a drought-tolerant Eucalyptus globulus clone. When plants were exposed to drought alone, the main responses included reduced pre-dawn water potential (Ψpd) and gas exchange. This was accompanied by increases in malondialdehyde (MDA) and total glutathione, indicative of oxidative stress. Abscisic acid (ABA) levels increased while the content of jasmonic acid (JA) fell. Metabolic alterations included reductions in the levels of sugar phosphates accompanied by increases in starch and non-structural carbohydrates. Levels of α-glycerophosphate and shikimate were also reduced while free amino acids increased. On the other hand, heat alone triggered an increase in relative water content (RWC) and Ψpd. Photosynthetic rate and pigments were reduced accompanied by a reduction in water use efficiency. Heat-induced a reduction of salicylic acid (SA) and JA content. Sugar alcohols and several amino acids were enhanced by the heat treatment while starch, fructose-6-phosphate, glucose-6-phosphate, and α-glycerophosphate were reduced. Contrary to what was observed under drought, heat stress activated the shikimic acid pathway. Drought-stressed plants subject to a heat shock exhibited a sharp decrease in gas exchange, Ψpd and JA, no alterations in electrolyte leakage, MDA, starch, and pigments and increased glutathione pool in relation to control. Comparing this with drought stress alone, subjecting drought stressed plants to an additional heat stress alleviated Ψpd and MDA, maintained an increased glutathione pool and reduced starch content and non-structural carbohydrates. A novel response triggered by the combined stress was the accumulation of cinnamate. Regarding recovery, most of the parameters affected by each stress condition reversed after re-establishment of control growing conditions. These results highlight that the combination of drought and heat provides significant protection from more detrimental effects of drought-stressed eucalypts, confirming that combined stress alter plant metabolism in a novel manner that cannot be extrapolated by the sum of the different stresses applied individually.
Highlights
Forest trees, as all sessile plants, have evolved many mechanisms that enable them to thrive in variable environmental conditions, ranging from circadian regulation (Dodd et al, 2005) to recovery from overpowering stress (Brodribb and Cochard, 2009)
Drought and combined stress induced a significant reduction in pd, with the extent of reduction being higher in the drought stress alone than in the combined stress (Figure 1A)
Plants growing in the field encounter a number of different co-occurring abiotic stresses that most probably cannot be extrapolated by the sum of the different stresses applied individually, altering plant metabolism in a novel manner (Rizhsky et al, 2002; Zandalinas et al, 2016)
Summary
As all sessile plants, have evolved many mechanisms that enable them to thrive in variable environmental conditions, ranging from circadian regulation (Dodd et al, 2005) to recovery from overpowering stress (Brodribb and Cochard, 2009). Despite these physiological adaptations, the long life-span of trees does not allow for rapid genetic adaptation to environmental changes, rendering forests susceptible to climate change (Lindner et al, 2010). Relatively little attention has been given to the combined effects of abiotic stresses, for example, in the field water deficit does not occur alone but associated with high temperature or high light (Chaves et al, 2002)
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