Inter-tracheid pitting and the hydraulic efficiency of conifer wood: the role of tracheid allometry and cavitation protection

Journal ArticlePlant xylem must balance efficient delivery of water to the canopy against protection from air entry into the conduits via air-seeding. We investigated the relationship between tracheid allometry, end wall pitting, safety from air-seeding, and the hydraulic efficiency of conifer wood in order to better understand the trade-offs between effective transport and protection against air entry

Weak tradeoff between xylem safety and xylem-specific hydraulic efficiency across the world\u27s woody plant species.

The evolution of lignified xylem allowed for the efficient transport of water under tension, but also exposed the vascular network to the risk of gas emboli and the spread of gas between xylem conduits, thus impeding sap transport to the leaves. A well-known hypothesis proposes that the safety of xylem (its ability to resist embolism formation and spread) should trade off against xylem efficiency (its capacity to transport water). We tested this safety-efficiency hypothesis in branch xylem across 335 angiosperm and 89 gymnosperm species. Safety was considered at three levels: the xylem water potentials where 12%, 50% and 88% of maximal conductivity are lost. Although correlations between safety and efficiency were weak (r(2) \u3c 0.086), no species had high efficiency and high safety, supporting the idea for a safety-efficiency tradeoff. However, many species had low efficiency and low safety. Species with low efficiency and low safety were weakly associated (r(2) \u3c 0.02 in most cases) with higher wood density, lower leaf- to sapwood-area and shorter stature. There appears to be no persuasive explanation for the considerable number of species with both low efficiency and low safety. These species represent a real challenge for understanding the evolution of xylem

Horizontal transfer of an adaptive chimeric photoreceptor from bryophytes to ferns

Ferns are well known for their shade-dwelling habits. Their ability to thrive under low-light conditions has been linked to the evolution of a novel chimeric photoreceptor-neochrome-that fuses red-sensing phytochrome and blue-sensing phototropin modules into a single gene, thereby optimizing phototropic responses. Despite being implicated in facilitating the diversification of modern ferns, the origin of neochrome has remained a mystery. We present evidence for neochrome in hornworts (a bryophyte lineage) and demonstrate that ferns acquired neochrome from hornworts via horizontal gene transfer (HGT). Fern neochromes are nested within hornwort neochromes in our large-scale phylogenetic reconstructions of phototropin and phytochrome gene families. Divergence date estimates further support the HGT hypothesis, with fern and hornwort neochromes diverging 179 Mya, long after the split between the two plant lineages (at least 400 Mya). By analyzing the draft genome of the hornwort Anthoceros punctatus, we also discovered a previously unidentified phototropin gene that likely represents the ancestral lineage of the neochrome phototropin module. Thus, a neochrome originating in hornworts was transferred horizontally to ferns, where it may have played a significant role in the diversification of modern ferns

Control of C¦4 photosynthesis at low temperatures in high-elevation grasses

grantor: University of TorontoCurrent surveys and paleodistributions indicate that C\sb4 species favour warm, subtropical regions. Some factors may account for the low-temperature sensitivity of C\sb4 plants: Pyruvate P\sb{\rm i}-Dikinase is inactivated below 12\sp\circC and damage to Photosystem II may also occur. The gas-exchange and biochemical results in this study demonstrated that in the C\sb4 montane grasses Bouteloua gracilis and Muhlenbergia montanum, the activity of Rubisco correlated with photosynthesis below 20\sp\circC during both short- and long-term chilling. Neither Phosphoenolpyruvate Carboxylase nor Pyruvate P\sb{\rm i}-Dikinase activities indicated that these enzymes limit CO\sb2 assimilation below 20\sp\circC. During long-term exposure to chilling conditions, the photosynthesis rate above 25\sp\circC improved in M. montanum indicating that cold-acclimation in some C\sb4 species can occur. Additionally, prolonged cold treatment had little effect on Photosystem II in this specie. Rubisco activity appears to limit C\sb4 photosynthesis at cooler temperatures, and thus may have prevented the expansion of C\sb4 flora to cooler habitats.M.Sc

New insights into bordered pit structure and cavitation resistance in angiosperms and conifers

The article discusses water transport in angiosperms and conifers, focusing on structural factors that influence a plant's resistance to xylem cavitation. Such cavitation increases resistance to water flow, places limits on gas exchange, and may lead to starvation or death of the plant. The paper describes the geometry, material components, and functions of the xylem. It notes that pit membranes facilitate water flow between cells, but restrict passage of gases and pathogens. The pit membranes also account for a large proportion of the hydraulic resistance exhibited by the xylem

New insights into bordered pit structure and cavitation resistance in angiosperms and conifers

The article discusses water transport in angiosperms and conifers, focusing on structural factors that influence a plant's resistance to xylem cavitation. Such cavitation increases resistance to water flow, places limits on gas exchange, and may lead to starvation or death of the plant. The paper describes the geometry, material components, and functions of the xylem. It notes that pit membranes facilitate water flow between cells, but restrict passage of gases and pathogens. The pit membranes also account for a large proportion of the hydraulic resistance exhibited by the xylem

Analysis of Freeze-Thaw Embolism in Conifers. The Interaction between Cavitation Pressure and Tracheid Size

Ice formation in the xylem sap produces air bubbles that under negative xylem pressures may expand and cause embolism in the xylem conduits. We used the centrifuge method to evaluate the relationship between freeze-thaw embolism and conduit diameter across a range of xylem pressures (P(x)) in the conifers Pinus contorta and Juniperus scopulorum. Vulnerability curves showing loss of conductivity (embolism) with P(x) down to −8 MPa were generated with versus without superimposing a freeze-thaw treatment. In both species, the freeze-thaw plus water-stress treatment caused more embolism than water stress alone. We estimated the critical conduit diameter (D(f)) above which a tracheid will embolize due to freezing and thawing and found that it decreased from 35 μm at a P(x) of −0.5 MPa to 6 μm at −8 MPa. Further analysis showed that the proportionality between diameter of the air bubble nucleating the cavitation and the diameter of the conduit (kL) declined with increasingly negative P(x). This suggests that the bubbles causing cavitation are smaller in proportion to tracheid diameter in narrow tracheids than in wider ones. A possible reason for this is that the rate of dissolving increases with bubble pressure, which is inversely proportional to bubble diameter (La Place's law). Hence, smaller bubbles shrink faster than bigger ones. Last, we used the empirical relationship between P(x) and D(f) to model the freeze-thaw response in conifer species

Leaf Mechanical Strength Corresponds to Tissue Water Relations in Twelve Species of California Ferns

The dominant vegetation types in southern California’s coastal foothills are chaparral and costal sage scrub. Chaparral shrubs have mechanically strong evergreen leaves whereas coastal sage scrubs bear mechanical weak, facultative deciduous leaves. What about the ferns that live in the understory of these vegetation types, especially considering their adaptations to a summer dry, Mediterranean-type climate? We tested the hypothesis that some fern leaves are stronger than others and mechanically strong leaves are associated with greater dehydration tolerance. Twelve fern species were examined. Tissue water relations were assessed via pressure volume curves using Scholander-Hammel pressure chambers. We estimated osmotic potential at saturation (Ψs,sat) and at the turgor loss point (Ψs,tlp). We examined pinna strength using an Instron Mechanical Testing Device to measure Young’s Modulus (YM) and tensile stress at break (TSB). We also measured vein density to determine if it was associated with mechanical strength. We found significant differences among our 12 fern species. Young’s Modulus was positively correlated with dehydration tolerance of leaf tissues, increasing with osmotic potentials at saturation (r2 = 0.514) and osmotic potentials at the turgor loss point (r2 = 0.536). Consistent with our initial hypothesis, we also found vein density to increase with mechanical strength and Young’s Modulus to increase with increasing tensile stress at break. We conclude that similar to species of chaparral shrubs and coastal sage scrubs in California the mechanical strength that increasing mechanical strength of leaves may be associated with increasing dehydration tolerance of their cellular tissues