Search for: All records

Abstract Background and AimsLeaf abscission is the process whereby plants actively shed leaves through physical detachment at the abscission zone (AZ). Leaf abscission is generally preceded by senescence, during which there is an active reclamation of leaf nutrients. The physiological regulation of leaf abscission remains poorly studied in trees, with a suite of environmental and endogenous signals believed to regulate the process. Here, we sought to characterize the role of water status, leaf gas exchange, senescence and the phytohormone abscisic acid (ABA) in regulating leaf abscission in temperate trees. MethodsWe developed a novel method to quantify AZ competency (AZC) and simultaneously measured leaf gas exchange, chlorophyll content, water potential, AZC and ABA levels from late summer until leaf death in four temperate tree species representing deciduous, brevi-deciduous, and marcescent leaf habits. We tested for associations between changes in key physiological traits and AZC in all species. Key ResultsThe two deciduous species showed contrasting physiological patterns leading to leaf abscission: one species degraded chlorophyll and ceased photosynthesis before complete AZC, while the other retained chlorophyll and continued photosynthesis until complete AZC. The brevi-deciduous species degraded most chlorophyll but developed AZC gradually over a longer period. The marcescent species’ leaves fully senesced but did not develop AZC. ConclusionsThese findings demonstrate that leaf senescence and abscission are distinct and variably timed processes across temperate tree species. These results have implications for predicting future leaf lifespan as the climate changes, with the characterization of physiological diversity in the regulation of leaf abscission profoundly understudied. 

Abstract The limiting factors of tree recovery from drought, particularly the coordination between carbon sources and sinks, remain poorly understood. In this study, juvenile Douglas fir (Pseudotsuga menziesii) were exposed to 28 d of mild or severe drought, followed by 35 d of recovery. We continuously monitored CO₂ and H₂O fluxes in shoots and roots to derive gas exchange and carbon accumulation, while measuring basal area to estimate stem growth and sapwood development. To identify underlying mechanisms of drought recovery, we periodically measured nonstructural carbohydrates (NSC), midday water potential (Ψmd), and foliar abscisic acid (ABA). We found no evidence that ABA or Ψmd limited gas exchange recovery, with stomatal conductance recovery instead related to drought-induced reductions in sapwood development. While carbon accumulation ultimately recovered to control levels following mild stress, severe stress caused persistent impairments, ultimately reducing carbon accumulation by 51%, with stem growth similarly affected. We found no evidence of growth being limited by NSC, which remained abundant. However, we suggest that drought-induced limitations to stem development govern this pattern. This became clear when considering the diurnal growth cycle, where daytime growth was largely absent in trees after exposure to severe drought despite accounting for up to 30% of total growth in control trees. Daytime growth appeared to depend on sufficient sapwood area, which likely buffered xylem tension to support growth conditions. Our findings suggest drought-induced reductions of stem hydraulic development constrain the recovery of gas exchange and growth. Further, altered diurnal growth patterns may explain prolonged productivity declines in forests following drought. 

Abstract Using the optical vulnerability method, we evaluated leaf and stem embolism resistance across three Populus species (P. trichocarpa, P. deltoides, and P. grandidentata) grown under field and glasshouse conditions to explore the mechanisms of vulnerability segmentation between organs. Classical vulnerability segmentation occurs when leaf xylem is more vulnerable to embolism than stem xylem, with leaves serving as hydraulic ‘fuses’ that protect the hydraulic integrity of the stem during drought. Recent evidence suggests that reverse vulnerability segmentation—when leaves have higher embolism resistance than stems—may occur in Populus species. We observed reverse segmentation exclusively in field-grown older stems, while no segmentation was found in glasshouse-grown or newly formed stems. X-ray micro-computed tomography and hydraulic measurements confirmed that the more vulnerable field-grown older stems had significant native embolism (>25% loss of conductivity). These findings support the hypothesis that reverse segmentation arises not from inherent xylem properties, but from the accumulation of native embolism, probably induced by winter freeze–thaw cycles or stem age. Our results provide a mechanistic explanation for reverse segmentation and suggest that its occurrence in Populus may be a byproduct of environment and life history rather than an adaptive trait of xylem architecture. 

Summary The mesophyll provides a critical signal for stomatal responses to red light (RL) and CO₂in angiosperms. By contrast, the stomatal response to blue light (BL) is largely guard cell‐specific. It is not known whether substomatal or mesophyll anatomy influences the effectiveness of the mesophyll signal driving stomatal responses to RL and CO₂.Here we utilize the diverse substomatal anatomy in Restionaceae to investigate whether mesophyll anatomy has an influence on stomatal responses to light and CO₂. Restionaceae from the subfamily Restionoideae have distinctive nonphotosynthetic, cuticle‐covered protective cells that line a large substomatal cavity, while most species from the subfamily Leptocarpoideae have a small substomatal cavity surrounded by mesophyll cells.We found that representative Restionoideae species do not have a stomatal response to RL or CO₂, with only BL driving stomatal responses to light. By contrast, representative Leptocarpoideae species have a stomatal response to RL, BL and CO₂.The absence of stomatal responses to RL and CO₂in species with protective cells lining the substomatal cavity demonstrates the importance of mesophyll anatomy in stomatal control and suggests that the mesophyll signal that drives stomatal responses to RL and CO₂requires close proximity of photosynthetic cells to the guard cells. 

Abstract Water moves through trees pulled under tension through a series of dead tubes called xylem. During drought, conduits can be invaded by air, causing an embolism, leading to tissue or whole-plant death. Fagus sylvatica is the most abundant European broadleaf forest tree, and is currently under threat due to an increasing frequency and severity of drought, resulting in xylem embolism, dieback, and death. Here, we investigated the variation in embolism resistance across globally distributed individuals of F. sylvatica f. purpurea receiving between 563 mm and 1362 mm of rainfall per year, as well as variation in embolism resistance across the canopy. We found no difference in the water potential when 50% of the stem xylem was embolized (P50) across locations in this clonal form of F. sylvatica but we did find variation in P50 across the canopy driven by light, with shade branches being significantly more vulnerable than part or full sun-adapted branches. The lack of variation in response to annual rainfall in a globally distributed clone has implications for predicting the risk of mortality driven by periodic droughts and long-term shifts in rainfall patterns due to climate change in F. sylvatica. Our work also highlights the importance of horticultural resources such as globally distributed clones as a model system for examining plant responses to the environment. 

Abstract PremiseHydraulic segmentation, caused by the difference in embolism resistance across plant organs, provides a sacrificial layer of cheaper plant organs, like leaves, to protect more costly organs, such as stems, during drought. Within‐leaf hydraulic segmentation has been observed in two compound‐leaved tree species, with leaflets being more vulnerable than the rachis or petiole. Many herbaceous species have compound leaves, and some species have leaflets that are associated with pulvini at the base of the lamina, which could provide an anatomical means of preventing embolism from spreading within a leaf because of the higher number of vessel endings in the pulvinus. MethodsWe used the optical vulnerability method to investigate whether differences in embolism resistance were observed across the leaf tissues of six herbaceous species and one deciduous tree species with compound leaves. Our species selection included both palmately and pinnately‐compound leaved species, one of each with a pulvinus at the base of the leaflets. ResultsWe found considerable variation in embolism resistance across the species measured, but no evidence of variation in embolism resistance within the leaf. In two species with pulvini, we observed major embolism events crossing the pulvinus, spreading from the rachis or petiole into the lamina, and embolizing both tissues at the same water potential. ConclusionsWe conclude that within‐leaf hydraulic segmentation, caused by variation in embolism resistance, is not a universal phenomenon to compound‐leaved species and that the presence of a pulvinus does not provide a barrier to embolism spread in compound leaves. 

Abstract The rate of residual water loss is a major determinant of plant survival during drought, yet how the major paths of residual water flow develop as leaves expand is poorly understood. Here, we tracked the rate of residual water loss, the compositional development of cuticular wax, stomatal differentiation, pore formation, and xylem development as leaves expand in two co-occurring, deciduous tree species Tilia americana and Fagus grandifolia. As leaves expanded, residual conductance declined rapidly, primarily driven by decreases in cuticular conductance, which was the main pathway for residual water loss from branches with young leaves. Very little water was lost through stomatal pores as leaves expanded, because the outer cuticular ledge only formed above the majority of stomata once leaves approached complete expansion. Similar development of residual conductance was observed between the two species despite differences in cuticle composition and stomatal development timing as leaves expanded. Our work suggests that the cuticle is the primary pathway through which water is lost from the youngest expanding leaves, and residual conductance is minimized only when cuticular wax deposition is complete, stomata have formed, and leaves have fully expanded. 

Summary Tip‐to‐base conduit widening is critical for hydraulic efficiency; yet few studies have investigated the full developmental potential of xylem.We utilized the diverse growth habits of ferns to test whether a wide developmental potential of xylem is constrained by growth habit. We documented xylem hydraulically weighted conduit diameter (Dh) across the vertical stem of the tallEquisetum giganteumL.; in the soil surface rhizome that roots at each node ofPhlebodium pseudoaureum(Cav.) Lellinger and its vertical leaf. In addition, we determinedDhacross aquatic and aerially suspended rhizomes ofMarsilea hirsutaL.Considerable tip‐to‐base conduit widening was observed in the stems ofE. giganteum, aerially‐suspended rhizomes ofM. hirsuta, and the vertical leaves ofP. pseudoaureum, organs in which selection would favor the maintenance of hydraulic efficiency toward the apex. No change inDhwas observed along the soil‐surface rhizomes ofP. pseudoaureumor the aquatic rhizomes ofM. hirsuta.Our results indicate that there is considerable developmental potential for xylem conduit diameter in ferns apparent under conditions in which hydraulic efficiency is not required for survival; but that tip‐to‐base conduit widening is consistently observed in fern organs that require an efficient long‐distance transport of water to the apex. 

Abstract Stomatal closure in response to water deficit is crucial for maintaining plant water balance. While the mechanisms driving daytime stomatal closure under drought are well studied, the mechanism driving progressive declines in nighttime transpiration (Enight) during drought remains less understood. To investigate whether either abscisic acid (ABA) or declining leaf water status drives progressive declines in Enight during drought in vascular plants, we conducted experiments using representative fern, gymnosperm, and angiosperm species, including a severe ABA-deficient mutant and tree species. These species span a spectrum of stomatal control by ABA, ranging from insensitive to endogenous ABA in the fern to reliance on ABA for stomatal closure in the herbaceous angiosperm. We found that reductions in Enight during drought are driven by hydropassive stomatal closure in ferns and gymnosperms, transitioning to ABA regulation in gymnosperms under severe stress, and are triggered by ABA in herbaceous angiosperms. In all species, the proportion of total transpiration occurring at night increased as stomata closed during the drought. The reduction of Enight during drought appears to be a convergent stomatal response across vascular land plants but is driven by diverse regulatory mechanisms linked to evolutionary history and ecological strategy. 

Summary In the temperate Northern Hemisphere, herbaceous community composition undergoes major seasonal phenological shifts. Despite significant variation in water availability across growing seasons, few studies have associated physiological traits with seasonal shifts in community composition.Key ecophysiological traits, including leaf embolism resistance, were measured in 41 phylogenetically diverse herbaceous species native to the central forest‐grassland ecotone of North America. Traits were tracked monthly inSolidago canadensisL. to explore seasonal plasticity. Correlations between ecophysiological traits and flowering time, plant height, and ecological guild were examined. Further analyses were conducted to investigate if traits were constrained by phylogeny or seasonal differences in climatic variables.Taller and later flowering species had more embolism‐resistant leaf xylem and greater stomatal safety margins (SSM) than smaller, spring‐flowering species. We found no seasonal variation in traits inS. canadensis, suggesting minimal seasonal phenotypic plasticity in these traits. No phylogenetic signal was found for any trait suggesting that climate rather than phylogeny drives variation in ecophysiological traits in herbs.We conclude that leaf embolism resistance and SSM are adaptively relevant traits associated with the phenological differentiation of temporally disjunct herbaceous species. The association of leaf embolism resistance and phenology has implications for herbaceous community adaptation to changing climates.