Drought stress and carbon assimilation in a warming climate: Reversible and irreversible impacts

Abstract

Global change is characterized by increased CO2 concentration in the atmosphere, increasing average temperature and more frequent extreme events including drought periods, heat waves and flooding. Especially the impacts of drought and of elevated temperature on carbon assimilation are considered in this review. Effects of extreme events on the subcellular level as well as on the whole plant level may be reversible, partially reversible or irreversible. The photosynthetically active biomass depends on the number and the size of mature leaves and the photosynthetic activity in this biomass during stress and subsequent recovery phases. The total area of active leaves is determined by leaf expansion and senescence, while net photosynthesis per leaf area is primarily influenced by stomatal opening (stomatal conductance), mesophyll conductance, activity of the photosynthetic apparatus (light absorption and electron transport, activity of the Calvin cycle) and CO2 release by decarboxylation reactions (photorespiration, dark respiration). Water status, stomatal opening and leaf temperature represent a "magic triangle" of three strongly interacting parameters. The response of stomata to altered environmental conditions is important for stomatal limitations. Rubisco protein is quite thermotolerant, but the enzyme becomes at elevated temperature more rapidly inactivated (decarbamylation, reversible effect) and must be reactivated by Rubisco activase (carbamylation of a lysine residue). Rubisco activase is present under two forms (encoded by separate genes or products of alternative splicing of the premRNA from one gene) and is very thermosensitive. Rubisco activase was identified as a key protein for photosynthesis at elevated temperature (non-stomatal limitation). During a moderate heat stress Rubisco activase is reversibly inactivated, but during a more severe stress (higher temperature and/or longer exposure) the protein is irreversibly inactivated, insolubilized and finally degraded. On the level of the leaf, this loss of photosynthetic activity may still be reversible when new Rubisco activase is produced by protein synthesis. Rubisco activase as well as enzymes involved in the detoxification of reactive oxygen species or in osmoregulation are considered as important targets for breeding crop plants which are still productive under drought and/or at elevated leaf temperature in a changing climate.

A spring rainfall pulse causes greater in situ photosynthetic upregulation for Bromus tectorum compared to co-occurring native herbaceous species

“…Relative to L. argenteus, a lower range of values of ETR for the grasses suggests that they may have dissipated more energy through non-photochemical processes. The ratio ETR/A can infer the relative amount of electrons allocated to photorespiration compared to photochemistry; watering should reduce the ratio if photorespiration decreases relative to photochemistry (Feller, 2016). However, reductions in ETR/A were small and non-significant (except for L. argenteus), possibly due to the small sample sizes and decreasing values of g s and A over time as soil water was depleted.…”

Selection and Breeding of Suitable Crop Genotypes for Drought and Heat Periods in a Changing Climate: Which Morphological and Physiological Properties Should Be Considered?

“…Redistribution processes via the phloem are controlled by the source/sink network with leaves, roots and maturing fruits as major players. This network can be considerably disturbed by abiotic stresses [100][101][102][103]. Related to this transport functions is the capacity of stems and petioles to store solutes (especially carbohydrates and amino acids) [104,105].…”

“…Changes in these fluxes affect leaf morphology and functions in a complex manner and influence finally plant productivity and yield. Since heat and drought impacts on photosynthesis and leaf physiology were reviewed recently [6,7,100,103] these aspects are only briefly summarized here in Figure 3. Gas exchange between the atmosphere and the photosynthetically active mesophyll cells is a key aspect and depends on properties of the cuticle, stomatal conductance and mesophyll conductance [106].…”

“…A reduced stomatal density in halophytes exposed to salt stress (compared to unstressed control plants) was reported by several groups [107][108][109][110] and such adaptations were also considered for selecting/breeding crop varieties with improved abiotic stress tolerance by "learning from halophytes" [109]. Besides the morphological adaptations in drought-exposed leaves, other important mechanisms are based on changes in the protein pattern (e.g., accumulation of chaperonins or increased activities of enzymes involved in the detoxification of reactive oxygen species or the production of compatible solutes [18,103]. Two aspects of abiotic stress impacts relevant for genotype selection and breeding are emphasized here and discussed in more detail: intactness of organelles (especially chloroplasts and mitochondria) and reactive oxygen species (production and detoxification).…”

Relationships between climate of origin and photosynthetic responses to an episodic heatwave depend on growth CO2 concentration for Eucalyptus camaldulensis var. camaldulensis

“…Heatwaves can lead to high-temperature stress within plant cells that impairs electron flow within PSII, caused in part by production of reactive oxygen species that affects the repair processes of PSII (Allakhverdiev and Murata 2004;Allakhverdiev et al 2008). Photosynthetic CO 2 assimilation is sensitive to high-temperature events, largely through effects on Rubisco activation (Feller 2016). Whereas eCO 2 may result in enhanced tolerance of thermal stress for some species (Shanmugam et al 2013), improved heat stress tolerance in eCO 2 is not universal Bauweraerts et al 2014).…”