Incorporating pressure-volume traits into the leaf economics spectrum.

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.

Nutrient availability regulates Deschampsia antarctica photosynthetic and stress tolerance performance in Antarctica.

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.

Disentangling leaf structural and material properties in relationship to their anatomical and chemical compositional traits in oaks (Quercus L.).

BACKGROUND AND AIMS: The existence of sclerophyllous plants has been considered an adaptive strategy against different environmental stresses. Given that it literally means 'hard-leaved', it is essential to quantify the leaf mechanical properties to understand sclerophylly. However, the relative importance of each leaf trait for mechanical properties is not yet well established. METHODS: Genus Quercus is an excellent system to shed light on this because it minimizes phylogenetic variation while having a wide variation in sclerophylly. We measured leaf anatomical traits and cell wall composition, analysing their relationship with leaf mass per area and leaf mechanical properties in a set of 25 oak species. KEY RESULTS: The upper epidermis outer wall makes a strong and direct contribution to the leaf mechanical strength. Moreover, cellulose plays a crucial role in increasing leaf strength and toughness. The principal component analysis plot based on leaf trait values clearly separates Quercus species into two groups corresponding to evergreen and deciduous species. CONCLUSIONS: Sclerophyllous Quercus species are tougher and stronger owing to their thicker epidermis outer wall and/or higher cellulose concentration. Furthermore, section Ilex species share common traits, although they occupy different climates. In addition, evergreen species living in mediterranean-type climates share common leaf traits irrespective of their different phylogenetic origin.

Leaf structure and water relations of an allotetraploid Mediterranean fern and its diploid parents.

Allopolyploidy is a common speciation mechanism in plants; however, its physiological and ecological consequences in niche partitioning have been scarcely studied. In this sense, leaf traits are good proxies to study the adaptive capacity of allopolyploids and diploid parents to their respective environmental conditions. In the present work, leaf water relations (assessed through pressure-volume curves) and structural and anatomical traits of the allotetraploid fern Oeosporangium tinaei and its diploid parents, Oeosporangium hispanicum and Oeosporangium pteridioides, were studied under controlled conditions in response to a water stress (WS) cycle. O. hispanicum showed the lowest osmotic potential at turgor loss point (πtlp ) and leaf capacitance, together with higher leaf mass per area (LMA), leaf thickness (LT), leaf density (LD), and leaf dry matter content (LDMC), whereas O. pteridioides presented the opposite set of traits (high πtlp and capacitance, and low LMA, LT, LD, and LDMC). O. tinaei showed an intermediate position for most of the studied traits. The responsiveness (osmotic and elastic adjustments) to WS was low, although most of the traits explained the segregation of the three species across a range of drought tolerance according to the rank: O. hispanicum > O. tinaei > O. pteridioides. These trait differences may underlie the niche segregation among coexisting populations of the three species in the Mediterranean basin.

Photosynthesis, leaf hydraulic conductance and embolism dynamics in the resurrection plant Barbacenia purpurea.

The main parameters determining photosynthesis are stomatal and mesophyll conductance and electron transport rate, and for hydraulic dynamics they are leaf hydraulic conductance and the spread of embolism. These parameters have scarcely been studied in desiccation-tolerant (resurrection) plants exposed to drought. Here, we characterized photosynthesis and hydraulics during desiccation and rehydration in a poikilochlorophyllous resurrection plant, Barbacenia purpurea (Velloziaceae). Gas exchange, chlorophyll fluorescence, and leaf water status were monitored along the whole dehydration-rehydration cycle. Simultaneously, embolism formation and hydraulic functioning recovery were measured at leaf level using micro-computed tomography imaging. Photosynthesis and leaf hydraulic conductance ceased at relatively high water potential (-1.28 and -1.54 MPa, respectively), whereas the onset of leaf embolism occurred after stomatal closure and photosynthesis cessation (<-1.61 MPa). This sequence of physiological processes during water stress may be associated with the need to delay dehydration, to prepare the molecular changes required in the desiccated state. Complete rehydration occurred rapidly in the mesophyll, whereas partial xylem refilling, and subsequent recovery of photosynthesis, occurred at later stages after rewatering. These results highlight the importance of stomata as safety valves to protect the vascular system from embolism, even in a plant able to fully recover after complete embolism.

Linking water relations and hydraulics with photosynthesis.

For land plants, water is the principal governor of growth. Photosynthetic performance is highly dependent on the stable and suitable water status of leaves, which is balanced by the water transport capacity, the water loss rate as well as the water capacitance of the plant. This review discusses the links between leaf water status and photosynthesis, specifically focussing on the coordination of CO2 and water transport within leaves, and the potential role of leaf capacitance and elasticity on CO2 and water transport.

Cell wall composition strongly influences mesophyll conductance in gymnosperms.

Cell wall thickness is widely recognized as one of the main determinants of mesophyll conductance to CO2 (gm ). However, little is known about the components that regulate effective CO2 diffusivity in the cell wall (i.e. the ratio between actual porosity and tortuosity, the other two biophysical diffusion properties of cell walls). The aim of this study was to assess, at the interspecific level, potential relationships between cell wall composition, cell wall thickness (Tcw ) and gm . Gymnosperms constitute an ideal group to deepen these relationships, as they present, on average, the thickest cell walls within spermatophytes. We characterized the foliar gas exchange, the morphoanatomical traits related with gm , the leaf fraction constituted by cell walls and three main components of primary cell walls (hemicelluloses, cellulose and pectins) in seven gymnosperm species. We found that, although the relatively low gm of gymnosperms was mainly determined by their elevated Tcw , gm was also strongly correlated with cell wall composition, which presumably sets the final effective CO2 diffusivity. The data presented here suggest that (i) differences in gm are strongly correlated to the pectins to hemicelluloses and cellulose ratio in gymnosperms, and (ii) variations in cell wall composition may modify effective CO2 diffusivity in the cell wall to compensate the negative impact of thickened walls. We speculate that higher relative pectin content allows higher gm because pectins increase cell wall hydrophilicity and CO2 molecules cross the wall dissolved in water.

Differences in biochemical, gas exchange and hydraulic response to water stress in desiccation tolerant and sensitive fronds of the fern Anemia caffrorum.

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 optimize photosynthesis despite very thick cell walls by means of chloroplast distribution.

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 (Tcw) 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.

Cell wall thickness and composition are involved in photosynthetic limitation.

The key role of cell walls in setting mesophyll conductance to CO2 (gm) and, consequently, photosynthesis is reviewed. First, the theoretical properties of cell walls that can affect gm are presented. Then, we focus on cell wall thickness (Tcw) reviewing empirical evidence showing that Tcw varies strongly among species and phylogenetic groups in a way that correlates with gm and photosynthesis; that is, the thicker the mesophyll cell walls, the lower the gm and photosynthesis. Potential interplays of gm, Tcw, dehydration tolerance, and hydraulic properties of leaves are also discussed. Dynamic variations of Tcw in response to the environment and their implications in the regulation of photosynthesis are discussed, and recent evidence suggesting an influence of cell wall composition on gm is presented. We then propose a hypothetical mechanism for the influence of cell walls on photosynthesis, combining the effects of thickness and composition, particularly pectins. Finally, we discuss the prospects for using biotechnology for enhancing photosynthesis by altering cell wall-related genes.

Acclimation of mesophyll conductance and anatomy to light during leaf aging in Arabidopsis thaliana.

Mesophyll conductance (gm ), a key limitation to photosynthesis, is strongly driven by leaf anatomy, which is in turn influenced by environmental growth conditions and ontogeny. However, studies examining the combined environment × age effect on both leaf anatomy and photosynthesis are scarce, and none have been carried out in short-lived plants. Here, we studied the variation of photosynthesis and leaf anatomy in the model species Arabidopsis thaliana (Col-0) grown under three different light intensities at two different leaf ages. We found that light × age interaction was significant for photosynthesis but not for anatomical characteristics. Increasing growth light intensities resulted in increases in leaf mass per area, thickness, number of palisade cell layers, and chloroplast area lining to intercellular airspace. Low and moderate-but not high-light intensity had a significant effect on all photosynthetic characteristics. Leaf aging was associated with increases in cell wall thickness (Tcw ) in all light treatments and in increases in leaf thickness in plants grown under low and moderate light intensities. However, gm did not vary with leaf aging, and photosynthesis only decreased with leaf age under moderate and high light, suggesting a compensatory effect between increased Tcw and decreased chloroplast thickness on the total CO2 diffusion resistance.

Decreased Levels of Thioredoxin o1 Influences Stomatal Development and Aperture but Not Photosynthesis under Non-Stress and Saline Conditions.

Salinity has a negative impact on plant growth, with photosynthesis being downregulated partially due to osmotic effect and enhanced cellular oxidation. Redox signaling contributes to the plant response playing thioredoxins (TRXs) a central role. In this work we explore the potential contribution of Arabidopsis TRXo1 to the photosynthetic response under salinity analyzing Arabidopsis wild-type (WT) and two Attrxo1 mutant lines in their growth under short photoperiod and higher light intensity than previous reported works. Stomatal development and apertures and the antioxidant, hormonal and metabolic acclimation are also analyzed. In control conditions mutant plants displayed less and larger developed stomata and higher pore size which could underlie their higher stomatal conductance, without being affected in other photosynthetic parameters. Under salinity, all genotypes displayed a general decrease in photosynthesis and the oxidative status in the Attrxo1 mutant lines was altered, with higher levels of H2O2 and NO but also higher ascorbate/glutathione (ASC/GSH) redox states than WT plants. Finally, sugar changes and increases in abscisic acid (ABA) and NO may be involved in the observed higher stomatal response of the TRXo1-altered plants. Therefore, the lack of AtTRXo1 affected stomata development and opening and the mutants modulate their antioxidant, metabolic and hormonal responses to optimize their adaptation to salinity.

Mesophyll conductance: the leaf corridors for photosynthesis.

Besides stomata, the photosynthetic CO2 pathway also involves the transport of CO2 from the sub-stomatal air spaces inside to the carboxylation sites in the chloroplast stroma, where Rubisco is located. This pathway is far to be a simple and direct way, formed by series of consecutive barriers that the CO2 should cross to be finally assimilated in photosynthesis, known as the mesophyll conductance (gm). Therefore, the gm reflects the pathway through different air, water and biophysical barriers within the leaf tissues and cell structures. Currently, it is known that gm can impose the same level of limitation (or even higher depending of the conditions) to photosynthesis than the wider known stomata or biochemistry. In this mini-review, we are focused on each of the gm determinants to summarize the current knowledge on the mechanisms driving gm from anatomical to metabolic and biochemical perspectives. Special attention deserve the latest studies demonstrating the importance of the molecular mechanisms driving anatomical traits as cell wall and the chloroplast surface exposed to the mesophyll airspaces (Sc/S) that significantly constrain gm. However, even considering these recent discoveries, still is poorly understood the mechanisms about signaling pathways linking the environment a/biotic stressors with gm responses. Thus, considering the main role of gm as a major driver of the CO2 availability at the carboxylation sites, future studies into these aspects will help us to understand photosynthesis responses in a global change framework.

Born to revive: molecular and physiological mechanisms of double tolerance in a paleotropical and resurrection plant.

Resurrection plants recover physiological functions after complete desiccation. Almost all of them are native to tropical warm environments. However, the Gesneriaceae include four genera, remnant of the past palaeotropical flora, which inhabit temperate mountains. One of these species is additionally freezing-tolerant: Ramonda myconi. We hypothesise that this species has been able to persist in a colder climate thanks to some resurrection-linked traits. To disentangle the physiological mechanisms underpinning multistress tolerance to desiccation and freezing, we conducted an exhaustive seasonal assessment of photosynthesis (gas exchange, limitations to partitioning, photochemistry and galactolipids) and primary metabolism (through metabolomics) in two natural populations at different elevations. R. myconi displayed low rates of photosynthesis, largely due to mesophyll limitation. However, plants were photosynthetically active throughout the year, excluding a reversible desiccation period. Common responses to desiccation and low temperature involved chloroplast protection: enhanced thermal energy dissipation, higher carotenoid to Chl ratio and de-epoxidation of the xanthophyll cycle. As specific responses, antioxidants and secondary metabolic routes rose upon desiccation, while putrescine, proline and a variety of sugars rose in winter. The data suggest conserved mechanisms to cope with photo-oxidation during desiccation and cold events, while additional metabolic mechanisms may have evolved as specific adaptations to cold during recent glaciations.

What drives photosynthesis during desiccation? Mosses and other outliers from the photosynthesis-elasticity trade-off.

In vascular plants, more rigid leaves have been linked to lower photosynthetic capacity, associated with low CO2 diffusion across the mesophyll, indirectly resulting in a trade-off between photosynthetic capacity (An) and bulk modulus of elasticity (ε). However, we evaluated mosses, liverworts, and Chara sp., plus some lycophytes and ferns, and found that they behaved as clear outliers of the An-ε relationship. Despite this finding, when vascular and non-vascular plants were plotted together, ε still linearly determined the cessation of net photosynthesis during desiccation both in species with stomata (either actively or hydro-passively regulated) and in species lacking stomata, and regardless of their leaf structure. The latter result challenges our current view of photosynthetic responses to desiccation and/or water stress. Structural features and hydric strategy are discussed as possible explanations for the deviation of these species from the An-ε trade-off, as well as for the general linear dependency between ε and the full cessation of An during desiccation.

Cell wall composition and thickness affect mesophyll conductance to CO2 diffusion in Helianthus annuus under water deprivation.

Water deprivation affects photosynthesis, leaf anatomy, and cell wall composition. Although the former effects have been widely studied, little is known regarding those changes in cell wall major (cellulose, hemicelluloses, pectin, and lignin) and minor (cell wall-bound phenolics) compounds in plants acclimated to short- and long-term water deprivation and during recovery. In particular, how these cell wall changes impact anatomy and/or photosynthesis, specifically mesophyll conductance to CO2 diffusion (gm), has been scarcely studied. To induce changes in photosynthesis, cell wall composition and anatomy, Helianthus annuus plants were studied under five conditions: (i) control (i.e. without stress) (CL); (ii) long-term water deficit stress (LT); (iii) long-term water deficit stress with recovery (LT-Rec); (iv) short-term water deficit stress (ST); and (v) short-term water deficit stress with recovery (ST-Rec), resulting in a wide photosynthetic range (from 3.80 ± 1.05 µmol CO2 m-2 s-1 to 24.53 ± 0.42 µmol CO2 m-2 s-1). Short- and long-term water deprivation and recovery induced distinctive responses of the examined traits, evidencing a cell wall dynamic turnover during plants acclimation to each condition. In particular, we demonstrated for the first time how gm correlated negatively with lignin and cell wall-bound phenolics and how the (cellulose+hemicelloses)/pectin ratio was linked to cell wall thickness (Tcw) variations.

A field portable method for the semi-quantitative estimation of dehydration tolerance of photosynthetic tissues across distantly related land plants.

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.

Possible link between photosynthesis and leaf modulus of elasticity among vascular plants: a new player in leaf traits relationships?

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.

Physiological performance of transplastomic tobacco plants overexpressing aquaporin AQP1 in chloroplast membranes.

The leaf mesophyll CO2 conductance and the concentration of CO2 within the chloroplast are major factors affecting photosynthetic performance. Previous studies have shown that the aquaporin NtAQP1 (which localizes to the plasma membrane and chloroplast inner envelope membrane) is involved in CO2 permeability in the chloroplast. Levels of NtAQP1 in plants genetically engineered to overexpress the protein correlated positively with leaf mesophyll CO2 conductance and photosynthetic rate. In these studies, the nuclear transformation method used led to changes in NtAQP1 levels in the plasma membrane and the chloroplast inner envelope membrane. In the present work, NtAQP1 levels were increased up to 16-fold in the chloroplast membranes alone by the overexpression of NtAQP1 from the plastid genome. Despite the high NtAQP1 levels achieved, transplastomic plants showed lower photosynthetic rates than wild-type plants. This result was associated with lower Rubisco maximum carboxylation rate and ribulose 1,5-bisphosphate regeneration. Transplastomic plants showed reduced mesophyll CO2 conductance but no changes in chloroplast CO2 concentration. The absence of differences in chloroplast CO2 concentration was associated with the lower CO2 fixation activity of the transplastomic plants. These findings suggest that non-functional pores of recombinant NtAQP1 may be produced in the chloroplast inner envelope membrane.

The photosynthetic capacity in 35 ferns and fern allies: mesophyll CO2 diffusion as a key trait.

Ferns and fern allies have low photosynthetic rates compared with seed plants. Their photosynthesis is thought to be limited principally by physical CO2 diffusion from the atmosphere to chloroplasts. The aim of this study was to understand the reasons for low photosynthesis in species of ferns and fern allies (Lycopodiopsida and Polypodiopsida). We performed a comprehensive assessment of the foliar gas-exchange and mesophyll structural traits involved in photosynthetic function for 35 species of ferns and fern allies. Additionally, the leaf economics spectrum (the interrelationships between photosynthetic capacity and leaf/frond traits such as leaf dry mass per unit area or nitrogen content) was tested. Low mesophyll conductance to CO2 was the main cause for low photosynthesis in ferns and fern allies, which, in turn, was associated with thick cell walls and reduced chloroplast distribution towards intercellular mesophyll air spaces. Generally, the leaf economics spectrum in ferns follows a trend similar to that in seed plants. Nevertheless, ferns and allies had less nitrogen per unit DW than seed plants (i.e. the same slope but a different intercept) and lower photosynthesis rates per leaf mass area and per unit of nitrogen.