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

Enhanced Stomatal Conductance Supports Photosynthesis in Wheat to Improved NH4+ Tolerance

The impact of ammonium (NH4+) stress on plant growth varies across species and cultivars, necessitating an in-depth exploration of the underlying response mechanisms. This study delves into elucidating the photosynthetic responses and differences in tolerance to NH4+ stress by investigating the effects on two wheat (Triticum aestivum L.) cultivars, Xumai25 (NH4+-less sensitive) and Yangmai20 (NH4+-sensitive). The cultivars were grown under hydroponic conditions with either sole ammonium nitrogen (NH4+, AN) or nitrate nitrogen (NO3-, NN) as the nitrogen source. NH4+ stress exerted a profound inhibitory effect on seedling growth and photosynthesis in wheat. However, these effects were less pronounced in Xumai25 than in Yangmai20. Dynamic photosynthetic analysis revealed that the suppression in photosynthesis was primarily attributed to stomatal limitation associated with a decrease in leaf water status and osmotic potential. Compared to Yangmai20, Xumai25 exhibited a significantly higher leaf K+ concentration and TaAKT1 upregulation, leading to a stronger stomatal opening and, consequently, a better photosynthetic performance under NH4+ stress. In conclusion, our study suggested stomatal limitation as the primary factor restricting photosynthesis under NH4+ stress. Furthermore, we demonstrated that improved regulation of osmotic substances contributed to higher stomatal conductance and enhanced photosynthetic performance in Xumai25.

Cell wall polysaccharide and glycoprotein content tracks growth-form diversity and an aridity gradient in the leaf-succulent genus Crassula

Cell wall traits are believed to be a key component of the succulent syndrome, an adaptive syndrome to drought, yet the variability of such traits remains largely unknown. In this study, we surveyed the leaf polysaccharide and glycoprotein composition in a wide sampling of Crassula species that occur naturally along an aridity gradient in southern Africa, and we interpreted its adaptive significance in relation to growth form and arid adaptation. To study the glycomic diversity, we sampled leaf material from 56 Crassula taxa and performed comprehensive microarray polymer profiling to obtain the relative content of cell wall polysaccharides and glycoproteins. This analysis was complemented by the determination of monosaccharide composition and immunolocalization in leaf sections using glycan-targeting antibodies. We found that compact and non-compact Crassula species occupy distinct phenotypic spaces in terms of leaf glycomics, particularly in regard to rhamnogalacturonan I, its arabinan side chains, and arabinogalactan proteins (AGPs). Moreover, these cell wall components also correlated positively with increasing aridity, which suggests that they are likely advantageous in terms of arid adaptation. These differences point to compact Crassula species having more elastic cell walls with plasticizing properties, which can be interpreted as an adaptation toward increased drought resistance. Furthermore, we report an intracellular pool of AGPs associated with oil bodies and calcium oxalate crystals, which could be a peculiarity of Crassula and could be linked to increased drought resistance. Our results indicate that glycomics may be underlying arid adaptation and drought resistance in succulent plants.

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.

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.

Within-branch photosynthetic gradients are more related to the coordinated investments of nitrogen and water than leaf mass per area

Nitrogen (N) and water are key resources for leaf photosynthesis and the growth of whole plants. Within-branch leaves need different amounts of N and water to support their differing photosynthetic capacities according to light exposure. To test this scheme, we measured the within-branch investments of N and water and their effects on photosynthetic traits in two deciduous tree species Paulownia tomentosa and Broussonetia papyrifera. We found that leaf photosynthetic capacity gradually increased from branch bottom to top (i.e. from shade to sun leaves). Concomitantly, stomatal conductance (g_s) and leaf N content gradually increased, owing to the symport of water and inorganic mineral from root to leaf. Variation of leaf N content led to large gradients of mesophyll conductance, maximum velocity of Rubisco for carboxylation, maximum electron transport rate and leaf mass per area (LMA). Correlation analysis indicated that the within-branch difference in photosynthetic capacity was mainly related to g_s and leaf N content, with a relatively minor contribution of LMA. Furthermore, the simultaneous increases of g_s and leaf N content enhanced photosynthetic N use efficiency (PNUE) but hardly affected water use efficiency. Therefore, within-branch adjustment of N and water investments is an important strategy used by plants to optimize the overall photosynthetic carbon gain and PNUE.

Changes in endogenous abscisic acid and stomata of the resurrection fern, Pleopeltis polypodioides, in response to de- and rehydration

PREMISE

While angiosperms respond uniformly to abscisic acid (ABA) by stomatal closure, the response of ferns to ABA is ambiguous. We evaluated the effect of endogenous ABA, hydrogen peroxide (H₂ O₂ ), nitric oxide (NO), and Ca²⁺ , low and high light intensities, and blue light (BL) on stomatal opening of Pleopeltis polypodioides.

METHODS

Endogenous ABA was quantified using gas chromatography-mass spectrometry; microscopy results and stomatal responses to light and chemical treatments were analyzed with Image J.

RESULTS

The ABA content increases during initial dehydration, peaks at 15 h and then decreases to one fourth of the ABA content of hydrated fronds. Following rehydration, ABA content increases within 24 h to the level of hydrated tissue. The stomatal aperture opens under BL and remains open even in the presence of ABA. Closure was strongly affected by BL, NO, and Ca²⁺ , regardless of ABA, H₂ O₂ effect was weak.

CONCLUSIONS

The decrease in the ABA content during extended dehydration and insensitivity of the stomata to ABA suggests that the drought tolerance mechanism of Pleopeltis polypodioides is independent of ABA.

Ecophysiological Differentiation among Two Resurrection Ferns and Their Allopolyploid Derivative

Theoretically, the coexistence of diploids and related polyploids is constrained by reproductive and competitive mechanisms. Although niche differentiation can explain the commonly observed co-occurrence of cytotypes, the underlying ecophysiological differentiation among cytotypes has hardly been studied. We compared the leaf functional traits of the allotetraploid resurrection fern Oeosporangium tinaei (HHPP) and its diploid parents, O. hispanicum (HH) and O. pteridioides (PP), coexisting in the same location. Our experimental results showed that all three species can recover physiological status after severe leaf dehydration, which confirms their 'resurrection' ability. However, compared with PP, HH had much higher investment per unit area of light-capturing surface, lower carbon assimilation rate per unit mass for the same midday water potential, higher non-enzymatic antioxidant capacity, higher carbon content, and lower contents of nitrogen, phosphorus, and other macronutrients. These traits allow HH to live in microhabitats with less availability of water and nutrients (rock crevices) and to have a greater capacity for resurrection. The higher assimilation capacity and lower antioxidant capacity of PP explain its more humid and nutrient-rich microhabitats (shallow soils). HHPP traits were mostly intermediate between those of HH and PP, and they allow the allotetraploid to occupy the free niche space left by the diploids.

An abrupt increase in foliage ABA levels on incipient leaf death occurs across vascular plants

Forest mortality during drought has been attributed to hydraulic failure, which can be challenging to measure. A limited number of alternative proxies for incipient leaf death exist. Here we investigate whether a terminal increase in abscisic acid (ABA) levels in leaves occurs across vascular land plants and is an indicator of imminent leaf death. For different species across vascular plants, we monitored ABA levels during lethal drought as well as leaf embolism resistance, across the canopy as leaves die following senescence, or when leaves are exposed to a heavy, lethal frost late in the growing season. We observed a considerable increase in foliage ABA levels once leaves showed signs of incipient leaf death. This increase in ABA levels upon incipient leaf death, could be induced by embolism during drought, by freezing or as leaves age naturally, and was observed in species spanning the phylogeny of vascular land plants as well as in an ABA biosynthetic mutant plant. A considerable increase in foliage ABA levels may act as an indicator of impending leaf death.

Desiccation tolerance in bryophytes relates to elasticity but is independent of cell wall thickness and photosynthesis

Mosses have been found outliers of the trade-off between photosynthesis and bulk elastic modulus described for vascular plants. Hence, potential trade-offs among physical features of cell walls and desiccation tolerance, water relations, and photosynthesis were assessed in bryophytes and other poikilohydric species. Long-term desiccation tolerance was quantified after variable periods of desiccation/rehydration cycles. Water relations were analyzed by pressure-volume curves. Mesophyll conductance was estimated using both CO₂ curve-fitting and anatomical methods. Cell wall elasticity was the parameter that better correlated with the desiccation tolerance index for desiccation tolerant species and was antagonistic to higher absolute values of osmotic potential. Although high values of cell wall effective porosity were estimated compared with the values assumed for vascular plants, the desiccation tolerance index negatively correlated with the porosity in desiccation tolerant bryophytes. Neither cell wall thickness nor photosynthetic capacity were correlated with the desiccation tolerance index of the studied species. The existence of a potential evolutionary trade-off between cell wall thickness and desiccation tolerance is rejected. The photosynthetic capacity reported for bryophytes is independent of elasticity and desiccation tolerance. Furthermore, the role of cell wall thickness in limiting CO₂ conductance would be overestimated under a scenario of high cell wall porosity for most bryophytes.

Novel insights into water-deficit-responsive mRNAs and lncRNAs during fiber development in Gossypium hirsutum

BACKGROUND

The fiber yield and quality of cotton are greatly and periodically affected by water deficit. However, the molecular mechanism of the water deficit response in cotton fiber cells has not been fully elucidated.

RESULTS

In this study, water deficit caused a significant reduction in fiber length, strength, and elongation rate but a dramatic increase in micronaire value. To explore genome-wide transcriptional changes, fibers from cotton plants subjected to water deficit (WD) and normal irrigation (NI) during fiber development were analyzed by transcriptome sequencing. Analysis showed that 3427 mRNAs and 1021 long noncoding RNAs (lncRNAs) from fibers were differentially expressed between WD and NI plants. The maximum number of differentially expressed genes (DEGs) and lncRNAs (DERs) was identified in fibers at the secondary cell wall biosynthesis stage, suggesting that this is a critical period in response to water deficit. Twelve genes in cotton fiber were differentially and persistently expressed at ≥ five time points, suggesting that these genes are involved in both fiber development and the water-deficit response and could potentially be used in breeding to improve cotton resistance to drought stress. A total of 540 DEGs were predicted to be potentially regulated by DERs by analysis of coexpression and genomic colocation, accounting for approximately 15.76% of all DEGs. Four DERs, potentially acting as target mimics for microRNAs (miRNAs), indirectly regulated their corresponding DEGs in response to water deficit.

CONCLUSIONS

This work provides a comprehensive transcriptome analysis of fiber cells and a set of protein-coding genes and lncRNAs implicated in the cotton response to water deficit, significantly affecting fiber quality during the fiber development stage.