Drought stress: primary and secondary responses
Limited water availability is a key factor affecting productivity across world ecosystems, and therefore, the question of how plants cope with curbed water availability continues to be an area of vigorous investigation. Immediate drought responses, extent of acclimation and drought tolerance strongly differ among plant species (Ogaya and Peñuelas 2003, 2006, Blackman et al. 2009, Gallé et al. 2011, Barigah et al. 2013) and populations within species (Marron et al. 2003, Costa e Silva et al. 2004, Monclus et al. 2009, Correia et al. 2014, Granda et al. 2014), but underlying mechanisms are still not wholly resolved even for classical drought-triggered phenomena such as reductions in stomatal conductance and photosynthetic rate.
Apart from changes in photosynthetic metabolism, drought induces a plethora of additional cellular responses that are manifested in changes in whole plant transcriptome and metabolome (Ramakrishna and Ravishankar 2011, Arbona et al. 2013, Xu et al. 2013, Granda et al. 2014, Zhang et al. 2014), ultimately resulting in major changes in plant chemical composition. Accumulation of osmotica, including ubiquitous chemicals such as sugars and salt ions (e.g., Epron and Dreyer 1996, Clifford et al. 1998, Morales et al. 2013), and species-specific osmotica such as betaine (e.g., Grieve and Maas 1984), glycinebetaine (e.g., Sakamoto and Murata 2000), proline (e.g., Somal and Yapa 1998, Morales et al. 2013) and quercitol (e.g., Arndt et al. 2008) constitutes the most conspicuous chemical modification that importantly enhances plant drought resistance. In addition to osmotic modifications, reprogramming of plant metabolism in droughted plants results in multiple other changes in plant secondary metabolism, the role of which in immediate plant drought resistance is less clear, making these changes often hard to explain and predict, but as the study of McKiernan et al. (2016) in this issue of Tree Physiology demonstrates, diverse responses in secondary chemistry might be the only modifications standing out from routine patterns.
Why is understanding ‘secondary’ responses difficult?
Given that secondary metabolites such as isoprenoids and phenolics are typically highly reduced and contain more carbon than the products of primary metabolism, ‘carbon-based secondary compounds’ and the source-sink ‘carbon-nutrient balance’ hypotheses have been proposed to predict changes in concentrations in secondary metabolites in stressed plants (Bryant et al. 1983, Herms and Mattson 1992, Peñuelas and Estiarte 1998). According to these hypotheses, any stress that suppresses growth more than photosynthesis leads to accumulation of secondary chemicals. Drought particularly strongly affects growth that is directly dependent on plant turgor pressure (e.g., Tardieu et al. 2000), suggesting that carbon-rich secondary chemicals should accumulate in drought-stressed plants. There is some evidence of enhanced concentrations of terpenoids (Gershenzon 1984, Munné-Bosch et al. 1999, Blanch et al. 2009, Marchese et al. 2010) and phenolics (Llusià and Peñuelas 1998, Pääkkönen et al. 1998, Ayaz et al. 2000, McKiernan et al. 2014), but also lack of drought effects on terpenoid concentration (Llusià and Peñuelas 1998, Dunford and Vazquez 2005, Marchese et al. 2010), and even reductions in terpenoid (Yani et al. 1993, McKiernan et al. 2014) and phenolic (Thomas and Schafellner 1999, McKiernan et al. 2014) concentrations. Apparently, drought effects can be associated with drought severity (Marchese et al. 2010), and vary for different compound classes, but the information is limited.
In eucalypts studied by McKiernan et al. (2016), drought effects were different for secondary metabolites synthesized via shikimate and isoprenoid synthesis pathways. Isoprenoids, mono- and sesquiterpenes, were unaffected by drought, but the concentration of condensed tannins was enhanced, and concentrations of macrocarpals, the condensation products of shikimic acid and isoprenoid pathway intermediates specific to eucalypts, decreased upon drought (McKiernan et al. 2016). Thus, this study underscores that the drought responses in secondary chemistry are more complex than simple carbon balance theories suggest. It is even unclear what are the immediate physiological implications of the observed changes in secondary chemistry for acclimation to drought stress per se. However, from an ecological perspective, it is relevant to consider that stress factors more often than not co-occur or occur in sequence (Niinemets 2010). Drought is typically accompanied by high temperature and photoinhibition stresses, and some modifications in secondary chemistry, such as elevated tocopherol and carotenoid contents in drought-stressed plants, have been linked to improved photoinhibition tolerance (Munné-Bosch et al. 1999, Munné-Bosch and Alegre 2000, Peñuelas and Munné-Bosch 2005). Furthermore, plants in their natural habitats are typically exposed to chronic biotic stresses, in particular, herbivory and pathogen attacks, that can be especially devastating for drought-stressed plants due to reduced carbon input and depleted carbon reserves (Mattson and Haack 1987, Robison and Raffa 1998, Kurz et al. 2008, Niinemets 2010). Thus, drought-induced phenotypic modifications as the result of correlative reprogramming of metabolic pathways clearly could fulfill important ecological tasks, some of which we might not even be aware of.
Recovery from stress: under- or overcompensation?
In addition to variation in how plant species respond to lack of water, there is also a large species variability in the dynamics and degree of recovery upon rewatering that can play at least as important a role in determining plant survival and fitness as survival of the drought period. Furthermore, different traits recover with different time kinetics and to a different extent. In an ideal fully elastic (homeostatic) recovery, the trait value reaches the prestressed value. However, sometimes the trait value fails to reach the previous level (undercompensation) or recovery leads to a stronger, overcompensatory, response (Figure 1). In the case of stomatal conductance and photosynthesis, the way recovery occurs often depends on past stress level. In mildly or moderately droughted plants, the recovery is rapid and typically elastic (Liang and Zhang 1999, Vomáčka and Pospíšilová 2003, Gallé et al. 2011), or the recovery may sometimes even be somewhat overcompensatory (Ruiz-Sánchez et al. 1997, Morales et al. 2013, Correia et al. 2014). In contrast, a more severe stress is associated with slow recovery and undercompensation (Liang and Zhang 1999, Xiloyannis et al. 1999, Vomáčka and Pospíšilová 2003, Gallé et al. 2007, Gallé et al. 2009, 2011), although not always (Ruiz-Sánchez et al. 1997, Correia et al. 2014).
Schematic representation of plant stress response and recovery after stress. Upon stress, the rate of process changes as the result of the immediate stress response and acclimation that enhances plant stress resistance. Upon stress relief, the trait can fully return to the prestressed state (elastic or homeostatic response) or it can recover less (undercompensation) or more (overcompensation) compared with the stress-induced change in the trait. In both latter cases, plant phenotype has changed after the stress compared with the prestress phenotype and this can have important implications for the reaction to any subsequent stress. The figure illustrates stress- and recovery-dependent changes in a trait that is positively affected by stress. The stress and recovery kinetics depicted in the figure are mirrored for a trait that is negatively affected by stress (overcompensation would mean stronger upregulation and undercompensation lower upregulation compared with a homeostatic response). As characteristic examples, the content of leaf osmotica represents a trait that is positively affected by water stress, while leaf photosynthesis a trait that is negatively affected by water stress.
As with drought responses, changes in secondary metabolite response upon recovery are less straightforward to link to past stress history (Xu et al. 2010, Ramakrishna and Ravishankar 2011). In the study of McKiernan et al. (2016), photosynthetic traits fully recovered upon rewatering, while overcompensation was observed for several groups of shikimate pathway products. Leaves formed after the stress were less robust with lower leaf dry mass per area and higher water content than the corresponding control leaves (McKiernan et al. 2016). Both less robust leaves and lower investment in secondary defenses are associated with faster rates of carbon gain (Wright et al. 2004) that could potentially compensate for the carbon ‘lost’ due to drought. Yet, such an overcompensation might increase the vulnerability to possible biotic attacks. At any rate, the study of McKiernan et al. (2016) demonstrates that after-stress effects can lead to significantly changed plant phenotype that can differently respond to any subsequent stress events. Future studies are called for to understand such after-stress effects on biotic interactions.
Generality and specificity in stress responses among species and populations
Apart from trait variation across species, within any given species, there is a significant variation in any trait due to genetic and ecotypic components of variance such that it is justified to speak about a species trait spectrum within the global trait spectrum (Niinemets 2015). While trait variations within species have been studied over species range in several cases (James and Bell 1995, Anderson et al. 1996, Luoma 1997, Tognetti et al. 1997), much less information is available for within-species variations in stress and after-stress responses (Costa e Silva et al. 2004, Correia et al. 2014, Granda et al. 2014). Such variations are important to resolve to predict responses of species and populations of species to global change, in particular, to potential changes in soil water availability.
In McKiernan et al. (2016) study, a common garden experiment with two Eucalyptus species each represented by four provenances of different geographic origin was conducted to gain insight into the genetic differentiation in drought and recovery responses. Species and provenances importantly differed in constitutive levels of multiple traits, but surprisingly, interspecific and intraspecific variations in drought effects on foliar photosynthesis, structure and chemistry and on whole plant traits were minor. This result raises an important question of what is the role of variation in constitutive levels of traits in drought tolerance and overall fitness? Relatively mild drought stress employed in McKiernan et al. (2016) might be responsible for the lack of significant fitness benefits of provenances from drier areas. However, it might also be that some of the fitness benefits provided by constitutive variations in secondary chemistry traits can only be detected in a field setting where biological interactions are present. It is recommended that future studies investigating the genetic variations in drought resistance consider treatments with different severity of drought to gain insight into the variations in drought resistance as driven by constitutive and plastic components of trait variation.
