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 (Fv/Fm) 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.

BACKGROUND AND AIMS:Coping with both UV-B radiation and desiccation was essential for early land plants. In bryophytes, UV-B screening and desiccation tolerance partially depend on the physicochemical features of cell walls, which may also constrain photosynthesis. This study tested the relationship between the accumulation of UV-absorbing compounds (UVACs) and photosynthetic capacity and desiccation tolerance. METHODS:Gas exchange, chlorophyll fluorescence, desiccation tolerance and UVACs were analysed for Colombian, Antarctic, Spanish and Australian species collected from locations of contrasting UV exposure. Apparent photosynthesis, quantum efficiency and heat dissipation were measured using PAM-coupled systems. Desiccation tolerance was evaluated by recovery of Fv/Fm after long-term dry storage (14 d). Cell wall-bound and soluble UVACs were quantified through spectrophotometry after methanolic and alkali extraction. Trait relationships were assessed by correlation tests and quantile regression. KEY RESULTS:A boundary line trade-off was found between apparent photosynthesis and cell wall-bound UVACs: no species simultaneously exhibited both high photosynthetic rates and large amounts of UVACs in its cell walls. The three phyla of bryophytes were segregated across this gradient, with mosses showing the highest photosynthetic capacity but lowest UVACs, and liverworts and hornworts presenting the highest UVACs together with low photosynthetic rates. No trend was observed with chlorophyll concentration or quantum efficiency of photosystem II. Long-term desiccation tolerance was not correlated with either cell wall-bound or soluble UVACs. On the contrary, it was associated with non-photochemical quenching, which is possibly linked to a minimum ROS scavenging capacity indispensable for recovering functionality after a long-term desiccation/rehydration cycle. CONCLUSIONS:Bryophytes exhibit a boundary-line trade-off between UV protection and photosynthetic capacity, possibly driven by limitations to CO2 diffusion associated with the accumulation of cell wall-bound UVAC. In contrast, desiccation tolerance appears to rely on different physiological mechanisms, related to heat dissipation.

Photosynthesis is inherently limited by abiotic stresses like extreme temperatures, water deficit, and nutrient deficiency. However, the combined impacts of these multiple stresses on photosynthetic capacity remain largely underexplored. This study investigates the effects of low temperature, drought, and nutrient scarcity on the photosynthetic responses of three distinct grass species: the polar Deschampsia antarctica (DA) from Antarctica, the cosmopolitan Poa pratensis (PP) from the Arctic, and the crop species Triticum aestivum (TA). Under optimal conditions, both DA and PP exhibited lower photosynthetic rates compared to TA. Crucially, when subjected to a combination of multiple abiotic stresses, DA demonstrated a significantly less pronounced photosynthetic decline than TA. This remarkable ability to maintain higher photosynthetic efficiency under suboptimal conditions not only boosts net carbon assimilation but also enhances overall plant performance in harsh environments. Our results reveal that DA's superior performance under multiple stress conditions is attributed to a unique combination of structural and biochemical traits. These include constitutively higher leaf mass per area (LMA) and cell wall hemicellulose content. Furthermore, DA showed a balanced strategy of reduced investment in photosynthetic machinery coupled with enhanced photoprotection and antioxidant status (indicated by an increased xanthophylls + carotene/chlorophylls ratio), a combination not observed in PP or TA. These findings underscore that DA possesses specific physiological adaptations, enabling it to successfully cope with simultaneous multiple abiotic stresses more effectively than PP and TA.

In the 19th century it was proposed that ecophysiology was best studied in regions with extreme climatic conditions. In the present perspective, we argue that perhaps this is more timely than ever. The main reason is the need to improve crops to be simultaneously more productive-due to the increased population-and more stress tolerant-due to climate change. Climate change induces plants to face not just harsh but also 'unexpected' (unpredictable) climatic conditions. In this sense, we hypothesize that 'sherplants', namely plants living in the extremes of plant life (e.g. hot deserts, Arctic and Antarctica, or high elevations) can provide cues on how to break the trade-off between productivity and stress tolerance, as they need to be produced quickly due to the very short growing period while being stress tolerant due to the harsh and unpredictable climate endured during most of the year. We present glimpses of results from three consecutive projects developed over the last 10 years, in which hundreds of species from different regions of the world have been studied. In particular, we propose a pathway for developing 'shercrops' learning from 'sherplants', debate whether some of the already studied species may have really broken the aforementioned trade-off, and present a number of interesting unforeseen discoveries made when studying plants from extreme climates.

Plants in extreme environments face pronounced seasonal variations in abiotic conditions, influencing their growth and carbon gain. However, our understanding of how plants in cold-arid mountains sustain carbon assimilation during short growing seasons remains limited. Here, we investigate seasonal dynamics and interspecific variability in photochemical performance of 310 individuals, comprising 10 different dicotyledon plant species across 3100-5300 m in the NW Himalayas, spanning semi-deserts to subnival zones. From early June to late September, we measured Fv/Fm and ΦPSII, assessing ΦPSII relationships with leaf traits (N, P, C, C:N ratio, LMA, and LDMC) and environmental factors (temperature, soil moisture content, etc.). Our findings revealed that high-Himalayan plants maintained relatively stable photosynthetic performance (Fv/Fm = 0.7-0.85), indicating optimal function even under potential stress. Contrary to our hypothesis that ΦPSII peaks mid-season in alpine and subnival zones and early season in steppes and semi-deserts, it declined by 33% across species and habitats throughout the season. This decline was closely associated with nutrient depletion, leaf senescence, and energy-water limitations. Species exhibited distinct strategies, with some prioritising structural resilience over photosynthesis, while others optimised photochemical performance despite environmental constraints. Alpine and subnival plant performance was constrained more by soil moisture deficits and high temperatures than cold temperatures, while deep-rooted steppe and semi-desert plants were primarily constrained by high temperatures and evaporative forcing rather than soil moisture deficit. These results provide new insights into how Himalayan plants adapt to extreme environmental conditions, highlighting the crucial interplay between moisture and temperature in shaping their performance within cold-arid mountains.

We investigated the impact of open-top chamber (OTC) passive warming systems at molecular and ecophysiological levels on Deschampsia antarctica (DA) and Colobanthus quitensis (CQ) in Antarctica. In this field campaign, OTC led to more benign conditions early in the growing season but ultimately intensified drought stress and increased extreme heat events, affecting photosynthetic capacity, metabolism and dehydration tolerance in DA; however, CQ remained relatively unaffected. DA exhibited significant reductions in photosynthesis primarily due to stomatal and mesophyll limitations. Furthermore, DA plants grown under OTC conditions showed a notable 17% decrease in leaf mass per area (LMA), a crucial trait associated with stress tolerance. Metabolic profiling revealed an increased accumulation of osmoprotectants and protein stabilisers (soluble sugars, trehalose, myo-inositol and galactinol), secondary metabolite precursors (tryptophan and nicotinate) and cell wall constituents (xylose) in OTC-grown DA, suggesting a robust metabolic response to stress. However, these metabolic adjustments were insufficient to counteract the decline in LMA and maintain dehydration tolerance. This study thereby provides new insights into the physiological and metabolic limitations of Antarctic vascular plants under future warming and drying scenarios.

I performed my PhD in grapevine physiology under the supervision of Dr. H. Medrano, standing in the vineyards from pre-dawn to sunrise during many hot, wet and sunny days with my colleagues J.M.E. and J.B. I also spent many days and nights facing ticks year-round working in Mediterranean macchias with J.Gu. and M.M. Later I was able to supervise PhD students on grapevines – like A.P. and M.T. – and on Mediterranean vegetation – like J.Gal. With the incorporation to the group of M.R.-C. 'the puzzle' was completed and, combining the aforementioned studies, we could conclude (more than 20 years ago) things like: (1) stomatal conductance is the best proxy for 'water stress' in studies on photosynthesis; (2) steady-state chlorophyll fluorescence retrieves photosynthesis under saturating light; (3) photoinhibition is not a major photosynthetic limitation under water stress; (4) mesophyll conductance instead is; and (5) mesophyll conductance is a major driver of leaf water use efficiency. In view of these achievements which arose from the study of grapevines extended to other species, we realized that most current knowledge on plant ecophysiology is build on studies in temperate and Mediterranean species. Impelled especially by R.E.C. and in company of J.Gu., J.Gag., M-J.C-M., M.C., F.C and others we started studying plants in remote sites like Antarctica, Himalaya, Andes, etc … What we found there and why it may imply the 'and back' to agriculture is the object of this talk, which is intended as a homage to my supervisor and friend Hipólito Medrano on his retirement.

In recent years, attempts have been made in linking pressure-volume parameters and the leaf economics spectrum to expand our knowledge of the interrelationships among leaf traits. We provide theoretical and empirical evidence for the coordination of the turgor loss point and associated traits with net CO2 assimilation (An ) and leaf mass per area (LMA). We measured gas exchange, pressure-volume curves and leaf structure in 45 ferns and angiosperms, and explored the anatomical and chemical basis of the key traits. We propose that the coordination observed between mass-based An , capacitance and the turgor loss point (πtlp ) emerges from their shared link with leaf density (one of the components of LMA) and, specially, leaf saturated water content (LSWC), which in turn relates to cell size and nitrogen and carbon content. Thus, considering the components of LMA and LSWC in ecophysiological studies can provide a broader perspective on leaf structure and function.

Deschampsia antarctica is one of the only two native vascular plants in Antarctica, mostly located in the ice-free areas of the Peninsula's coast and adjacent islands. This region is characterized by a short growing season, frequent extreme climatic events, and soils with reduced nutrient availability. However, it is unknown whether its photosynthetic and stress tolerance mechanisms are affected by the availability of nutrients to deal with this particular environment. We studied the photosynthetic, primary metabolic, and stress tolerance performance of D. antarctica plants growing on three close sites (<500 m) with contrasting soil nutrient conditions. Plants from all sites showed similar photosynthetic rates, but mesophyll conductance and photobiochemistry were more limiting (~25%) in plants growing on low-nutrient availability soils. Additionally, these plants showed higher stress levels and larger investments in photoprotection and carbon pools, most probably driven by the need to stabilize proteins and membranes, and remodel cell walls. In contrast, when nutrients were readily available, plants shifted their carbon investment towards amino acids related to osmoprotection, growth, antioxidants, and polyamines, leading to vigorous plants without appreciable levels of stress. Taken together, these findings demonstrate that D. antarctica displays differential physiological performances to cope with adverse conditions depending on resource availability, allowing it to maximize stress tolerance without jeopardizing photosynthetic capacity.

Extreme environments, such as deserts and high-elevation ecosystems, are very important from biodiversity and ecological perspectives. However, plant physiology at those sites has been scarcely studied, likely due to logistic difficulties. In the present study, leaf physiological traits in native plants were analyzed from arid zones across an elevational transect in Western China, from Turpan Basin to the Qinghai-Tibet Plateau (QTP) at Delingha. The aim of this study was to use leaf physiological traits to help identifying potentially threatened species and true extremophiles. Physiological measurements in the field, and particularly in situ measurements of gas exchange and chlorophyll fluorescence, have been determined to be useful to determine the current state of plants at a given environment. Using this approach plus a combination of leaf traits, several species performing particularly well at the QTP were identified, e.g. Hedysarum multijugum, as well as at Manas drylands, e.g. Peganum harmala and Setaria viridis. On the other hand, several species showed marked signs of severe stress, in particular a very low photosynthetic rate over its potential maximum, as well as other negative traits, like low water and/or nitrogen-use-efficiency, which should be considered in conservation plans. Interestingly, all C-4 species studied except Setaria viridis were among the most stressed species. Despite their higher water use efficiency and drought tolerance reputation, they presented a much larger photosynthesis depression than most C-3 species. This is an intriguing and interesting observation that deserves further studies.

Global warming and changes in land use are some of the main threats to high mountain species. Both can interact in ways not yet assessed. In this study, we evaluated the photosynthetic responses of six common páramo species within a warming experiment using open-top chambers (OTC) in conserved páramo areas with different land use histories. We did not find significant differences in the photochemical performance of the species as measured through Fv/Fm, ETR, and NPQ in response to passive warming, indicating that warmed plants are not stressed. However, NPQ values were higher in recovering areas, especially in the driest and warmest months. Leaf transpiration, stomatal conductance, and Ci were not affected by the OTC or the land use history. The photosynthetic capacity, maximum photosynthetic capacity, and carboxylation rate of RuBisCO increased in response to warming but only in the area with no anthropogenic intervention. These results suggest that species will respond differently to warming depending on the history of páramo use, and therefore not all páramo communities will respond equally to climate change. In disturbed sites with altered soil conditions, plants could have a lower breadth of physiological response to warming.

Summary Desiccation tolerant plants can survive extreme water loss in their vegetative tissues. The fern Anemia caffrorum produces desiccation tolerant (DT) fronds in the dry season and desiccation sensitive (DS) fronds in the wet season, providing a unique opportunity to explore the physiological mechanisms associated with desiccation tolerance. Anemia caffrorum plants with either DT or DS fronds were acclimated in growth chambers. Photosynthesis, frond structure and anatomy, water relations and minimum conductance to water vapour were measured under well‐watered conditions. Photosynthesis, hydraulics, frond pigments, antioxidants and abscisic acid contents were monitored under water deficit. A comparison between DT and DS fronds under well‐watered conditions showed that the former presented higher leaf mass per area, minimum conductance, tissue elasticity and lower CO2 assimilation. Water deficit resulted in a similar induction of abscisic acid in both frond types, but DT fronds maintained higher stomatal conductance and upregulated more prominently lipophilic antioxidants. The seasonal alternation in production of DT and DS fronds in A. caffrorum represents a mechanism by which carbon gain can be maximized during the rainy season, and a greater investment in protective mechanisms occurs during the hot dry season, enabling the exploitation of episodic water availability.

Resurrection plants are vascular species able to sustain extreme desiccation in their vegetative tissues. Despite its potential interest, the role of leaf anatomy in CO2 diffusion and photosynthesis under non-stressed conditions has not been explored in these species. Net CO2 assimilation (An) and its underlying diffusive, biochemical, and anatomical determinants were assessed in 10 resurrection species from diverse locations, including ferns, and homoiochlorophyllous and poikilochlorophyllous angiosperms. Data obtained were compared with previously published results in desiccation-sensitive ferns and angiosperms. An in resurrection plants was mostly driven by mesophyll conductance to CO2 (gm) and limited by CO2 diffusion. Resurrection species had a greater cell wall thickness (T-cw) than desiccation-sensitive plants, a feature associated with limited CO2 diffusion in the mesophyll, but also greater chloroplast exposure to intercellular spaces (Sc), which usually leads to higher gm. This combination enabled a higher An per Tcw compared with desiccation-sensitive species. Resurrection species possess unusual anatomical features that could confer stress tolerance (thick cell walls) without compromising the photosynthetic capacity (high chloroplast exposure). This mechanism is particularly successful in resurrection ferns, which display higher photosynthesis than their desiccation-sensitive counterparts.

Biomass production in marginal lands represents one of the most challenging and promising alternatives to sustainably produce biofuels. Native species seem to be the most adequate option to obtain a profitable output when low-input techniques are applied, and biomass is grown in depleted soils and harsh climatic conditions. In this study, a 5-year field trial in the island of Majorca served to investigate different autochthonous and naturalized Mediterranean perennial grasses as novel candidate lignocellulosic bioenergy crops for the semi-arid Mediterranean area and compare them with commercial ones (both Mediterranean and non-Mediterranean). Species and growing season had a significant effect on biomass production, perennialism and biomass quality. Arundo donax (winter crops) and Piptatherum miliaceum (autumn crops) performed better than the commercial species tested (Panicum virgatum for winter crops and Festuca arundinacea for autumn crops) in biomass production and perennialism. In terms of biomass quality, Panicum virgatum was the best species, having high structural content (mainly cellulose and hemicellulose), low non-structural content and the lowest ash. However, Ampelodesmos mauritanicus and Arundo donax rendered similar results, with no significant difference in terms of cellulose production for this latter but with higher lignin content. For the autumn species, Festuca arundinacea was the species with the best biomass quality but with the highest ash production for all the species considered. Hence, both for winter or autumn regimes, native or naturalized plants seem to be better suited than the commercial commonly used for biomass production with energy-producing purposes. Further research must be conducted in terms of seed biology and physiology, seedbed preparation methods, sowing time, seedling density and weed control before they can firmly be proposed as adequate alternatives for energy purposes.

In this work, we review the physiological and molecular mechanisms that allow vascular plants to perform photosynthesis in extreme environments, such as deserts, polar and alpine ecosystems. Specifically, we discuss the morpho/anatomical, photochemical and metabolic adaptive processes that enable a positive carbon balance in photosynthetic tissues under extreme temperatures and/or severe water-limiting conditions in C-3 species. Nevertheless, only a few studies have described the in situ functioning of photoprotection in plants from extreme environments, given the intrinsic difficulties of fieldwork in remote places. However, they cover a substantial geographical and functional range, which allowed us to describe some general trends. In general, photoprotection relies on the same mechanisms as those operating in the remaining plant species, ranging from enhanced morphological photoprotection to increased scavenging of oxidative products such as reactive oxygen species. Much less information is available about the main physiological and biochemical drivers of photosynthesis: stomatal conductance (g(s)), mesophyll conductance (g(m)) and carbon fixation, mostly driven by RuBisCO carboxylation. Extreme environments shape adaptations in structures, such as cell wall and membrane composition, the concentration and activation state of Calvin-Benson cycle enzymes, and RuBisCO evolution, optimizing kinetic traits to ensure functionality. Altogether, these species display a combination of rearrangements, from the whole-plant level to the molecular scale, to sustain a positive carbon balance in some of the most hostile environments on Earth.

The rapid growth of many Mediterranean cities in the past 50 years has led to a deficit of urban green space where water availability is the main limiting factor. The aim of this experiment was to evaluate the establishment performance of six Mediterranean species for use in extensive green roofs in Mediterranean and semi-arid areas. Asteriscus maritimus, Brachypodium phoenicoides, Crithmum maritimum, Limonium virgatum, Sedum sediforme and Sporobolus pungens plants were grown under well-watered conditions and two levels of water-deficit conditions to assess the effects of water-deficit on their growth capacity, water consumption, photosynthetic capacity, water relations and aesthetic value (relative appearance). After 88 days, the results showed that the water-deficit treatments affected each species differently and revealed differences in the potential ecosystem service benefits that different species could provide. Sedum sediforme appeared to be the best adapted species to water-deficit in terms of relative appearance; however, this species had the lowest capacity to grow (between 7.55 and 4.72 g) and consume water (between 1.58 and 0.82 mm day(-1)) under both well-watered and water-deficit conditions, limiting its usefulness for storm water management. Brachypodium phoenicoides and Limonium virgatum showed a satisfactory aesthetical performance under water-deficit conditions and had higher water consumption (between 7.66 and 3.48 mm day(-1)) and biomass accumulation (between 40.11 and 18.98 g) than Sedum sediforme. By contrast, Asteriscus maritimus showed variable relative appearance results than that species. Crithmum maritimum, a succulent species, showed similar biomass accumulation and water consumption to S. sediforme but lower relative appearance than that species. Sporobolus pungens, a C-4 species, showed the highest water use efficiency at the plant level (between 5.79 and 4.81 g L-1). In summary, B. phoenicoides and L. virgatum are suggested as interesting species to complement S. sediforme in Mediterranean and semi-arid green roofs.

Plants in the high Arctic are exposed to a 24-h photoperiod for several months. These conditions can be damaging for plants at lower latitudes. When common crops are artificially maintained under continuous light (CL), photosynthetic processes maintain endogenous circadian rhythms, but it is unclear whether plants naturally acclimated to CL also maintain such rhythmicity. Alternatively, Arctic plants have to utilise the favourable conditions during the short Arctic summers. In this study, we evaluate the existence of daily cycles in pigment composition in three Arctic plants in a tundra ecosystem at 78 °N that display examples of different growth forms: the bryophyte Polytrichum hyperboreum , the herb Bistorta vivipara and the dwarf shrub Salix polaris . Changes in pigment composition are excellent indicators of the restructuring of the light-harvesting apparatus. Most pigment parameters analysed did not show any consistent pattern of variation between subjective noon and midnight. P. hyperboreum had the highest level of rhythmicity, while S. polaris was the most stable. Despite these subtle changes, the primary effects observed were induced by light and its effect on the de-epoxidation state of the xanthophyll cycle pigments (AZ/VAZ). Both short- and long-term adjustments of the AZ/VAZ correlated with changes in photochemical efficiency ( Φ PSII ). When the plants were artificially darkened during the night, it became evident that the midnight sun prevents the complete relaxation of the xanthophyll cycle. These results indicate that light is the primary driver of photochemical efficiency in Arctic plants, and consequently, photosynthesis is not completely interrupted at night.

Salinization is one of the major causes of agricultural soil degradation worldwide. In arid and semi-arid regions with calcareous soils, phosphorus (P) deficiency further worsens the quality of salinized soils. Nonetheless, nutrient poor soils could be suitable of producing second-generation energy crops. Due to its high biomass production, Arundo donax L. (giant reed) is one of the most promising species for energy and second-generation biofuel production. A. donax can be propagated by micropropagation, an in vitro technique that produces high number of homogeneous plantlets. However, crop establishment is often compromised due to poor plantlet acclimatization to the soil environment. Arbuscular mycorrhizal fungi (AM) are components of soil-plant systems able to increase root phosphorus uptake and to confer the plant an increase tolerance to salinity with a consequent enhancement effect of plant growth and yield. In the present study, the relative importance of the early symbiosis establishment between AM fungi and A. donax micropropagated plantlets in the response to salt stress under low phosphorus availability was determined. A commercial inoculum which contained two different AM fungi species: Rhizophagus intraradices and Funneliformis mosseae was used. AM-symbionts (AM) and non-symbionts plants were grown at two phosphorus [2.5 μM (C) and 0.5 mM (P)] and three NaCl (1, 75 and 150 mM) concentrations in a room chamber under controlled conditions. After 5 weeks, AM root colonization was 60, 26 and 15% in 1, 75 and 150 mM NaCl-treated plants, respectively. At 1 and 75 mM NaCl, AM plants showed increased growth. In all saline treatments, AM plants had decreased Na+ uptake, Na+ root-to-shoot translocation, Na+/K+ ratio and increased P and K use efficiencies with respect to C and P plants. AM improved the nutritional status of A. donax plants by enhancing nutrient use efficiency rather than nutrient uptake. Increased phosphorus use efficiency in AM plants could have benefited ion (Na+ and K+) uptake and/or allocation and ultimately ameliorate the plant's response to saline conditions.

Desiccation tolerant (DT) plants withstand complete cellular dehydration, reaching relative water contents (RWC) below 30% in their photosynthetic tissues. Desiccation sensitive (DS) plants exhibit different degrees of dehydration tolerance (DHT), never surviving water loss >70%. To date, no procedure for the quantitative evaluation of DHT extent exists that is able to discriminate DS species with differing degrees of DHT from truly DT plants. We developed a simple, feasible and portable protocol to differentiate between DT and different degrees of DHT in the photosynthetic tissues of seed plants and between fast desiccation (< 24 h) tolerant (FDT) and sensitive (FDS) bryophytes. The protocol is based on (1) controlled desiccation inside Falcon tubes equilibrated at three different relative humidities that, consequently, induce three different speeds and extents of dehydration and (2) an evaluation of the average percentage of maximal photochemical efficiency of PSII (Fv /fm) recovery after rehydration. Applying the method to 10 bryophytes and 28 tracheophytes from various locations, we found that (1) imbibition of absorbent material with concentrated salt-solutions inside the tubes provides stable relative humidity and avoids direct contact with samples; (2) for 50 ml capacity tubes, the optimal plant amount is 50-200 mg fresh weight; (3) the method is useful in remote locations due to minimal instrumental requirements; and (4) a threshold of 30% recovery of the initial Fv /fm upon reaching RWC ≤ 30% correctly categorises DT species, with three exceptions: two poikilochlorophyllous species and one gymnosperm. The protocol provides a semi-quantitative expression of DHT that facilitates comparisons of species with different morpho-physiological traits and/or ecological attributes.

A compromise between carbon assimilation and structure investment at the leaf level is broadly accepted, yet the relationship between net assimilation per area (An ) and leaf mass per area has been elusive. We propose bulk modulus of elasticity (ε) as a suitable parameter to reflect both leaf structure and function, and an inverse relationship between ε and An and mesophyll conductance (gm ) is postulated. Using data for An , gm and ε from previous studies and new measurements on a set of 20 species covering all major growth forms, a negative relationship between An or gm and ε was observed. High ε was also related to low leaf capacitance and higher diffusive limitations to photosynthesis. In conclusion, ε emerges as a key trait linked with photosynthetic capacity across vascular plants, and its relationship with gm suggests the existence of a common mechanistic basis, probably involving a key role of cell walls.