Patricia L. Sáez

The dehydration tolerance of photosynthetic tissues varies greatly along land plant's phylogeny. We investigated the photochemical response to severe dehydration and subsequent rehydration in 318 species representing the major terrestrial biomes and photosynthetic lineages: lichens, bryophytes, pteridophytes and spermatophytes. Using a standardized portable dehydration test with the maximum photochemical efficiency of PSII (F _v /F _m ) as indicator, data meeting two key criteria were analyzed: (1) optimal physiological conditions before dehydration and (2) achieving relative water contents (RWC) below 25% at the dehydration stage (with approximately half dropping further to 5%–15%). Lichens and mosses displayed the highest overall Fv/Fm recovery after dehydration, whereas lower tolerance was observed in the other plant groups. Four distinct photochemical responses to dehydration emerged: dehydration‐sensitive (61.2%), rehydration‐sensitive (18.6%), dehydration‐tolerant (13.6%), and dehydration‐resistant (6.5%). Pteridophytes and spermatophytes predominantly exhibited dehydration‐ or rehydration‐sensitive responses, whereas dehydration‐tolerant dynamics were mainly observed in mosses and resurrection plants. Dehydration‐resistant patterns were rare and occurred particularly in polar and high‐altitude angiosperms such as Colobanthus quitensis , Deschampsia antarctica and Pozoa coriacea . These findings highlight the evolutionary divergence of dehydration tolerance strategies across the land plant phylogeny and provide insights into the underlying photochemical mechanisms.

Leaf hydraulics and their coordination with photosynthesis are central to maintaining plant physiological function, especially in plants inhabiting extreme environments. We investigated the hydraulic responses of the only two vascular plant species native to Antarctica, Deschampsia antarctica and Colobanthus quitensis , to long‐term in situ warming imposed by open‐top chambers (OTCs), and assessed how these responses covary with photosynthesis ( A _N ). After 7 years under OTC, D. antarctica reduced both leaf hydraulic conductivity ( K _leaf ) and A _N when compared with plants under natural conditions (OA), whereas C. quitensis increased both. A tight coordination between hydraulics and photosynthesis emerged in both species, supported by structural and ultrastructural leaf adjustments. In D. antarctica , reductions in K _leaf and A _N could be linked to outside‐xylem modulation and likely to senescence triggered by more frequent freeze–thaw events inside OTCs, where time below 0°C increased nearly tenfold compared with OA (from 32 to 309 h). Conversely, C. quitensis displayed increased cell wall elasticity, enhanced water transport and CO ₂ diffusion, triggered by changes in vascular anatomy, and the growth form that favors heat conservation. These results highlight that the two Antarctic vascular species represent complementary adaptive pathways to environmental change, illustrating two sides of a common adaptive continuum, balancing hydraulic safety and efficiency, stability and plasticity, critical for survival in one of the most stressful environments on Earth.

The dehydration tolerance of leaf photochemistry varies greatly along the land plant phylogeny. This study investigated the photochemical response to severe dehydration and subsequent rehydration in 444 species representing the major terrestrial biomes and photosynthetic lineages: lichens, bryophytes, pteridophytes and spermatophytes. Using a standardized, simple, and portable dehydration test, 318 species (84.3% of the total studied) met two key criteria: 1) optimal physiological conditions at the start of the test, and 2) achieving relative water contents (RWC) below 25% at the dehydration stage (with approximately half dropping even further to 5-15%). This ensured all species began under similar conditions and experienced severe tissue dehydration, allowing for subsequent fair comparisons. The maximal photochemical efficiency of PSII (F _v /F _m ) was employed as a physiological indicator at three stages: hydrated (initial), dehydrated and rehydrated. Altogether, lichens and mosses displayed the highest overall F _v /F _m recovery after dehydration, whereas lower tolerance was observed in liverworts, pteridophytes, gymnosperms, and angiosperms. Four distinct photochemical responses emerged: (i) ”dehydration-sensitive” (low F _v /F _m during dehydration and after rehydration) (LL) (61.2% of the total species), (ii) ”rehydration-sensitive” (high F _v /F _m during dehydration, but decreasing after rehydration) (HL) (18.6%), (iii) ”dehydration-tolerant” (low F _v /F _m during dehydration, high afterwards) (LH) (13.6%) (iv) and ”dehydration-resistant” (high F _v /F _m throughout) (HH) (6.5%). Pteridophytes and spermatophytes primarily exhibited LL or HL responses. LH species were predominantly found among mosses, lichens, and resurrection ferns, which displayed dehydration-induced reversible photochemical quenching as a major photoprotection strategy. Instead, the HH response showed a balanced distribution across phylogenetic groups but being the less frequent response type and included some angiosperms from polar regions and high-altitude mountain ranges as Colobanthus quitensis and Deschampsia antarctica from Antarctica and Pozoa coriacea from the Andean range. These findings highlight the evolutionary divergence of dehydration tolerance strategies across the land plant phylogeny and provide insights into the underlying photochemical mechanisms. Moreover, the methodology employed in this study offers a field-based tool to gain preliminary insights into the dehydration tolerance of almost any plant.

Temperature significantly influences plant morphology and photosynthesis at high altitudes, shaping distinct altitudinal patterns. Lower temperatures at higher elevations typically increase leaf mass per unit area (LMA), potentially reducing mesophyll CO ₂ diffusion (gm) and chloroplast CO ₂ availability, which can limit photosynthetic rates (A _N ). However, most evidence comes from field studies where elevation-related factors, like gas pressure, water, and light, covary. Habitat type further affects plant responses, as wetland (azonal) plants may be more sensitive to thermal changes due to water’s buffering effect on temperature fluctuations. To isolate temperature effects, we conducted a controlled experiment with plants from two elevations (2600 and 3550 m a.s.l.), exposing them to two daytime temperatures (15 and 20 °C). We measured LMA, gm, and A _N . Growth temperature significantly influenced photosynthesis and gm but not LMA. Plant origin and habitat type (zonal vs. azonal) also shaped responses. Our findings reveal that temperature impacts photosynthesis by altering mesophyll diffusion independently of LMA. Furthermore, adaptations to specific elevations and environments, such as wetlands, contribute to the variability in responses. This study highlights the nuanced role of temperature in shaping photosynthetic traits in mountain plants. Highlight: Different growth temperatures affected photosynthesis independently from LMA, the response also varied from wetland species compared to non-wetland species and from plants coming from low or high elevation.

Climate change projections predict warming and increased weather variability, mainly in polar regions, altering freeze–thaw patterns. However, the effects of rising temperatures and more frequent freeze–thaw events on the water and CO₂ management of Antarctic plants remain unclear. To address this, we conducted a laboratory experiment to investigate how growth temperature (5°C and 15°C) and successive freeze–thaw cycles influence the hydraulic and photosynthetic performance of Deschampsia antarctica (D. antarctica) and Colobanthus quitensis (C. quitensis). Our results showed that warmer conditions improved hydraulic and photosynthetic performance in both species, driven by anatomical adjustments in leaf xylem vessels. Additionally, plants exposed to successive freeze–thaw cycles exhibited a coordinated decline in whole‐plant hydraulic conductivity and leaf gas exchange, regardless of growth temperature. The magnitude of changes (%) in photosynthetic traits after freeze–thaw cycles varied between species, with D. antarctica showing similar responses at both growth temperatures, while C. quitensis experienced more pronounced changes at the lower temperature. Overall, these findings suggest that while Antarctic plants benefit from warmer temperatures, repeated freeze–thaw events could disrupt their hydraulic balance and limit photosynthesis, particularly under natural environmental conditions.

Deschampsia antarctica and Colobanthus quitensis are extremophile species thriving from the Andes to the Subantarctic regions. Although they share this gradient with other species, notably, they are the only flowering plants growing naturally in Antarctica. These species exhibit traits typical of xeromorphy, such as high leaf mass area (LMA) and leaf density (LD), which influence mesophyll conductance and photosynthesis. However, under warmer conditions, Antarctic plants reduce their LMA and LD, optimizing photosynthesis and hydraulic conductivity, reflecting their ability to adjust these traits in response to temperature variations. While considerable research has focused on their physiological responses in Antarctica, it remains unclear whether these traits are exclusive to Antarctic provenances or are also found in populations outside Antarctica or in related species from similar environments. We hypothesize that the traits related to carbon assimilation and hydraulics in Antarctic species are specific adaptations to extreme Antarctic conditions and may not be present in Subantarctic provenances or in phylogenetically related species in other stressful environments. We grew D. antarctica and C. quitensis from Antarctic and Subantarctic provenances, along with related species: Deschampsia caespitosa, Avenella flexuosa , and Colobanthus apetalus , at 5°C, 15°C, and 23°C. Both Antarctic and Subantarctic D. antarctica showed consistent coordination of photosynthesis and hydraulic traits across temperatures. C. quitensis showed provenance‐specific responses, possibly due to ecotypic differentiation, with C. apetalus situated closely to Subantarctic C. quitensis in comparison with the Antarctic one. This research highlights some physiological responses of Antarctic plants and their adaptation to extreme climates.

The accumulation of crystal calcium oxalate (CaOx) in plants is linked to a type of stress-induced photosynthesis termed ‘alarm photosynthesis’, serving as a carbon reservoir when carbon dioxide (CO₂) exchange is constrained. Colobanthus quitensis is an extremophyte found from southern Mexico to Antarctica, which thrives in high-altitude Andean regions. Growing under common garden conditions, C. quitensis from different latitudinal provenances display significant variations in CaOx crystal accumulation. This raises the following questions: are these differences maintained under natural conditions? And is the CaOx accumulation related to mesophyll conductance (g_m) and net photosynthesis (A_N) performed in situ? It is hypothesized that in provenances with lower g_m, C. quitensis will exhibit an increase in the use of CaOx crystals, resulting in reduced crystal leaf abundance. Plants from Central Chile (33°), Patagonia (51°), and Antarctica (62°) were measured in situ and sampled to determine gas exchange and CaOx crystal accumulation, respectively. Both A_N and g_m decrease towards higher latitudes, correlating with increases in leaf mass area and leaf density. The crystal accumulation decreases at higher latitudes, correlating positively with A_N and g_m. Thus, in provenances where environmental conditions induce more xeric traits, the CO₂ availability for photosynthesis decreases, making the activation of alarm photosynthesis feasible as an internal source of CO₂.

Antarctic flowering plants have become enigmatic because of their unique capability to colonize Antarctica. It has been shown that there is not a single trait that makes Colobanthus quitensis and Deschampsia antarctica so special, but rather a set of morphophysiological traits that coordinately confer resistance to one of the harshest environments on the Earth. However, both their capacity to inhabit Antarctica and their uniqueness remain not fully explained from a biological point of view. These aspects have become more relevant due to the climatic changes already impacting Antarctica. This review aims to compile and update the recent advances in the ecophysiology of Antarctic vascular plants, deepen understanding of the mechanisms behind their notable resistance to abiotic stresses, and contribute to understanding their potential responses to environmental changes. The uniqueness of Antarctic plants has prompted research that emphasizes the role of leaf anatomical traits and cell wall properties in controlling water loss and CO₂ exchange, the role of Rubisco kinetics traits in facilitating efficient carbon assimilation, and the relevance of metabolomic pathways in elucidating key processes such as gas exchange, nutrient uptake, and photoprotection. Climate change is anticipated to have significant and contrasting effects on the morphophysiological processes of Antarctic species. However, more studies in different locations outside Antarctica and using the latitudinal gradient as a natural laboratory to predict the effects of climate change are needed. Finally, we raise several questions that should be addressed, both to unravel the uniqueness of Antarctic vascular species and to understand their potential responses to climate change.

One of the well-documented effects of regional warming in Antarctica is the impact on flora. Warmer conditions modify several leaf anatomical traits of Antarctic vascular plants, increasing photosynthesis and growth. Given that CO₂ and water vapor partially share their diffusion pathways through the leaf, changes in leaf anatomy could also affect the hydraulic traits of Antarctic plants. We evaluated the effects of growth temperature on several anatomical and hydraulic parameters of Antarctic plants and assessed the trait co-variation between these parameters and photosynthetic performance. Warmer conditions promoted an increase in leaf and whole plant hydraulic conductivity, correlating with adjustments in carbon assimilation. These adjustments were consistent with changes in leaf vasculature, where Antarctic species displayed different strategies. At higher temperature, Colobanthus quitensis decreased the number of leaf xylem vessels, but increased their diameter. In contrast, in Deschampsia antarctica the diameter did not change, but the number of vessels increased. Despite this contrasting behavior, some traits such as a small leaf diameter of vessels and a high cell wall rigidity were maintained in both species, suggesting a water-conservation response associated with the ability of Antarctic plants to cope with harsh environments.