Yong-Jiang Zhang

Size-dependent mortality in a Neotropical savanna tree: the role of height-related adjustments in hydraulic architecture and carbon allocation

Size-related changes in hydraulic architecture, carbon allocation and gas exchange of Sclerolobium paniculatum (Leguminosae), a dominant tree species in Neotropical savannas of central Brazil (Cerrado), were investigated to assess their potential role in the dieback of tall individuals. Trees greater than approximately 6-m-tall exhibited more branch damage, larger numbers of dead individuals, higher wood density, greater leaf mass per area, lower leaf area to sapwood area ratio (LA/SA), lower stomatal conductance and lower net CO(2) assimilation than small trees. Stem-specific hydraulic conductivity decreased, while leaf-specific hydraulic conductivity remained nearly constant, with increasing tree size because of lower LA/SA in larger trees. Leaves were substantially more vulnerable to embolism than stems. Large trees had lower maximum leaf hydraulic conductance (K(leaf)) than small trees and all tree sizes exhibited lower K(leaf) at midday than at dawn. These size-related adjustments in hydraulic architecture and carbon allocation apparently incurred a large physiological cost: large trees received a lower return in carbon gain from their investment in stem and leaf biomass compared with small trees. Additionally, large trees may experience more severe water deficits in dry years due to lower capacity for buffering the effects of hydraulic path-length and soil water deficits.

Midday stomatal conductance is more related to stem rather than leaf water status in subtropical deciduous and evergreen broadleaf trees

Midday depressions in stomatal conductance (g(s) ) and photosynthesis are common in plants. The aim of this study was to understand the hydraulic determinants of midday g(s) , the coordination between leaf and stem hydraulics and whether regulation of midday g(s) differed between deciduous and evergreen broadleaf tree species in a subtropical cloud forest of Southwest (SW) China. We investigated leaf and stem hydraulics, midday leaf and stem water potentials, as well as midday g(s) of co-occurring deciduous and evergreen tree species. Midday g(s) was correlated positively with midday stem water potential across both groups of species, but not with midday leaf water potential. Species with higher stem hydraulic conductivity and greater daily reliance on stem hydraulic capacitance were able to maintain higher stem water potential and higher g(s) at midday. Deciduous species exhibited significantly higher stem hydraulic conductivity, greater reliance on stem capacitance, higher stem water potential and g(s) at midday than evergreen species. Our results suggest that midday g(s) is more associated with midday stem than with leaf water status, and that the functional significance of stomatal regulation in these broadleaf tree species is probably for preventing stem xylem dysfunction.

Water relations and hydraulic architecture in Cerrado trees: adjustments to seasonal changes in water availability and evaporative demand

O objetivo deste estudo foi determinar os ajustamentos na morfologia e fisiologia que permitem arvores das savanas neotropicais do Brasil Central (Cerrado) de evitar deficits hidricos e de manter um balanco hidrico interno praticamente constante apesar das variacoes sazonais da precipitacao e no deficit de saturacao do ar (D). A precipitacao na area de estudo e fortemente sazonal, com cerca de cinco meses praticamente sem chuva durante os quais D e duas vezes maior aos valores medidos na epoca chuvosa. Como consequencia da flutuacao sazonal das chuvas e de D, o potencial hidrico do solo muda substancialmente, nos primeiros 100 cm do solo, mas permanece quase constante abaixo de 2 m de profundidade. A arquitetura hidraulica e os parâmetros relacionados a relacoes hidricas das arvores do Cerrado se ajustaram durante a estacao seca para evitar o deficit hidrico crescente e assegurar a homeostase nos valores minimos de potencial hidrico foliar ψL e na perda total diaria de agua pela planta (iso-hidria). O comportamento iso-hidrico das arvores do Cerrado foi o resultado de uma diminuicao da superficie foliar total por arvore, um forte controle estomatico das perdas por evaporacao, um aumento na condutividade hidraulica especifica da folha e na condutância hidraulica foliar e um aumento da quantidade de aguas retirada dos reservatorios internos do caule, durante a estacao seca. A eficiencia no transporte de agua aumentou, nas mesmas proporcoes, nas folhas e nos ramos terminais durante a estacao seca. Todos estes ajustamentos sazonais foram importantes para a manutencao de ψL acima de limiares criticos, com isto contribuindo para uma reducao na formacao de embolismos nos ramos e ajudando a evitar a perda de turgor em tecidos foliares durante a epoca seca. Esses ajustes permitem que os ramos das especies lenhosas do Cerrado operem bem distanciados do ponto de disfuncao catastrofica para a cavitacao, enquanto as folhas operam proximas e sofrem embolismos em uma base diaria, especialmente durante a estacao seca.

NPI-031G (puerarin) reduces anxiogenic effects of alcohol withdrawal or benzodiazepine inverse or 5-HT2C agonists

Because extracts of kudzu have been used as a hangover remedy in China for many centuries, we tested the ability of NPI-031G (puerarin), an isoflavone isolated from kudzu, to counteract anxiogenic effects associated with withdrawal from chronic alcohol exposure. NPI-O31G (50 and 150 mg/kg ip) significantly increased the social interaction and locomotor activity reduced by withdrawal from 17 days of alcohol (7%) diet. The effects of NPI-031G resembled those of the benzodiazepine antagonist, flumazenil (5 mg/kg), and the 5-HT(2C) antagonist, SB 242084 (1 mg/kg). In a separate study, control rats were pretreated with NPI-031G (30 min) and then given the anxiogenic compounds DMCM, a benzodiazepine inverse agonist, or Ro 600175, a 5-HT(2C) agonist. NPI-031G significantly counteracted the reduction in social interaction induced by either compound. To identify a potential mechanism of action of NPI-031G, synaptoneurosomes were isolated from the cerebral cortex of untreated rats and chloride uptake assays were carried out. NPI-031G did not have any effect on the stimulation of chloride uptake by muscimol, a GABA(A) agonist. However, it reduced the potentiation of muscimol-stimulated chloride uptake by flunitrazepam, a benzodiazepine agonist, at a concentration of 100 microM. A reduction in [3H]flunitrazepam binding was also seen at this concentration. These findings are consistent with NPI-031G being a weak benzodiazepine site antagonist.

Recovery of diurnal depression of leaf hydraulic conductance in a subtropical woody bamboo species: embolism refilling by nocturnal root pressure.

Despite considerable investigations of diurnal water use characteristics in different plant functional groups, the research on daily water use strategies of woody bamboo grasses remains lacking. We studied the daily water use and gas exchange of Sinarundinaria nitida (Mitford) Nakai, an abundant subtropical bamboo species in Southwest China. We found that the stem relative water content (RWC) and stem hydraulic conductivity (K(s)) of this bamboo species did not decrease significantly during the day, whereas the leaf RWC and leaf hydraulic conductance (K(leaf)) showed a distinct decrease at midday, compared with the predawn values. Diurnal loss of K(leaf) was coupled with a midday decline in stomatal conductance (g(s)) and CO(2) assimilation. The positive root pressures in the different habitats were of sufficient magnitude to refill the embolisms in leaves. We concluded that (i) the studied bamboo species does not use stem water storage for daily transpiration; (ii) diurnal down-regulation in K(leaf) and gs has the function to slow down potential water loss in stems and protect the stem hydraulic pathway from cavitation; (iii) since K(leaf) did not recover during late afternoon, refilling of embolism in bamboo leaves probably fully depends on nocturnal root pressure. The embolism refilling mechanism by root pressure could be helpful for the growth and persistence of this woody monocot species.

Hydraulic redistribution in dwarf Rhizophora mangle trees driven by interstitial soil water salinity gradients: impacts on hydraulic architecture and gas exchange

Rhizophora mangle L. trees of Biscayne National Park (Florida, USA) have two distinct growth forms: tall trees (5-10 m) growing along the coast and dwarf trees (1 m or less) growing in the adjacent inland zone. Sharp decreases in salinity and thus increases in soil water potential from surface soil to about a depth of 1 m were found at the dwarf mangrove site but not at the tall mangrove site. Consistent with our prediction, hydraulic redistribution detected by reverse sap flow in shallow prop roots was observed during nighttime, early morning and late afternoon in dwarf trees, but not in tall trees. In addition, hydraulic redistribution was observed throughout the 24-h period during a low temperature spell. Dwarf trees had significantly lower sapwood-specific hydraulic conductivity, smaller stem vessel diameter, lower leaf area to sapwood area ratio (LA/SA), smaller leaf size and higher leaf mass per area. Leaves of dwarf trees had lower CO(2) assimilation rate and lower stomatal conductance compared to tall trees. Leaf water potentials at midday were more negative in tall trees that are consistent with their substantially higher stomatal conductance and LA/SA. The substantially lower water transport efficiency and the more conservative water use of dwarf trees may be due to a combination of factors such as high salinity in the surface soil, particularly during dry periods, and substantial reverse sap flow in shallow roots that make upper soil layers with high salinity a competing sink of water to the transpiring leaves. There may also be a benefit for the dwarf trees in having hydraulic redistribution because the reverse flow and the release of water to upper soil layers should lead to dilution of the high salinity in the rhizosphere and thus relieve its potential harm to dwarf R. mangle trees.

The maximum height of grasses is determined by roots

Grasses such as bamboos can produce upright stems more than 30 m tall, yet the processes that constrain plant height in this important group have never been investigated. Air embolisms form commonly in the water transport system of grasses and we hypothesised that root pressure-dependent refilling these embolisms should limit the maximum height of grass species to the magnitude of their root pressure. Confirming this hypothesis, we show that in 59 species of bamboo grown in two common gardens, the maximum heights of culms of 67 clones are closely predicted by the maximum measured root pressure overnight. Furthermore, we demonstrate that water transport in these bamboo species is dependent on root pressure to repair hydraulic dysfunction sustained during normal diurnal gas exchange. Our results established the critical importance of root pressure in the tallest grass species and provide a new basis for understanding the limits for plant growth.

Reversible Deformation of Transfusion Tracheids in Taxus baccata Is Associated with a Reversible Decrease in Leaf Hydraulic Conductance

The reversible collapse of leaf transfusion tracheids of Taxus baccata under desiccation is related to reversible declines in leaf hydraulic conductance, suggesting a circuit breaker-like function that protects the xylem from excessive tensions. Declines in leaf hydraulic conductance (Kleaf) with increasing water stress have been attributed to cavitation of the leaf xylem. However, in the leaves of conifers, the reversible collapse of transfusion tracheids may provide an alternative explanation. Using Taxus baccata, a conifer species without resin, we developed a modified rehydration technique that allows the separation of declines in Kleaf into two components: one reversible and one irreversible upon relaxation of water potential to −1 MPa. We surveyed leaves at a range of water potentials for evidence of cavitation using cryo-scanning electron microscopy and quantified dehydration-induced structural changes in transfusion tracheids by cryo-fluorescence microscopy. Irreversible declines in Kleaf did not occur until leaf water potentials were more negative than −3 MPa. Declines in Kleaf between −2 and −3 MPa were reversible and accompanied by the collapse of transfusion tracheids, as evidenced by cryo-fluorescence microscopy. Based on cryo-scanning electron microscopy, cavitation of either transfusion or xylem tracheids did not contribute to declines in Kleaf in the reversible range. Moreover, the deformation of transfusion tracheids was quickly reversible, thus acting as a circuit breaker regulating the flux of water through the leaf vasculature. As transfusion tissue is present in all gymnosperms, the reversible collapse of transfusion tracheids may be a general mechanism in this group for the protection of leaf xylem from excessive loads generated in the living leaf tissue.

Strong leaf morphological, anatomical, and physiological responses of a subtropical woody bamboo (Sinarundinaria nitida) to contrasting light environments

AbstractDwarf bamboos are an important understory component of the lowland and montane forests in the subtropical regions of Asia and South America, yet little is known about their physiology and phenotypic plasticity in response to changing light environments. To understand how bamboo species adapt to different light intensities, we examined leaf morphological, anatomical, and physiological differentiation of Sinarundinaria nitida (Mitford) Nakai, a subtropical woody dwarf bamboo, growing in open and shaded natural habitats in the Ailao Mountains, SW China. Compared with leaves in open areas, leaves in shaded areas had higher values in leaf size, specific leaf area, leaf nitrogen, and chlorophyll concentrations per unit area but lower values in leaf thickness, vein density, stomatal density, leaf carbon concentration, and total soluble sugar concentration. However, stomatal size and leaf phosphorus concentration per unit mass remained relatively constant regardless of light regime. Leaves in the open habitat exhibited a higher light-saturated net photosynthetic rate, dark respiration rate, non-photochemical quenching, and electron transport rate than those in the shaded habitat. The results of this study revealed that the bamboo species exhibited a high plasticity of its leaf structural and functional traits in response to different irradiances. The combination of high plasticity in leaf morphological, anatomical, and physiological traits allows this bamboo species to grow in heterogeneous habitats.

Stomatal Closure, Basal Leaf Embolism, and Shedding Protect the Hydraulic Integrity of Grape Stems

Grape stomata are regulated to close before xylem cavitation. If the plant continues to dehydrate, basal leaf embolism and shedding protect the hydraulic integrity of younger leaves and the stem. The time scale of stomatal closure and xylem cavitation during plant dehydration, as well as the fate of embolized organs, are under debate, largely due to methodological limitations in the evaluation of embolism. While some argue that complete stomatal closure precedes the occurrence of embolism, others believe that the two are contemporaneous processes that are accompanied by daily xylem refilling. Here, we utilize an optical light transmission method to continuously monitor xylem cavitation in leaves of dehydrating grapevine (Vitis vinifera) in concert with stomatal conductance and stem and petiole hydraulic measurements. Magnetic resonance imaging was used to continuously monitor xylem cavitation and flow rates in the stem of an intact vine during 10 d of dehydration. The results showed that complete stomatal closure preceded the appearance of embolism in the leaves and the stem by several days. Basal leaves were more vulnerable to xylem embolism than apical leaves and, once embolized, were shed, thereby preventing further water loss and protecting the hydraulic integrity of younger leaves and the stem. As a result, embolism in the stem was minimal even when drought led to complete leaf shedding. These findings suggest that grapevine avoids xylem embolism rather than tolerates it.