Size-dependent variability of leaf and shoot hydraulic conductance in silver birch

Variation in leaf and shoot hydraulic conductance was examined on detached shoots of silver birch (Betula pendula Roth), cut from the lower third (shade leaves) and upper third of the crown (sun leaves) of large trees growing in a natural temperate forest stand. Hydraulic conductances of whole shoots (K _S), leaf blades (K _lb), petioles (K _P) and branches (i.e. leafless stem; K _B) were determined by water perfusion using a high-pressure flow meter in quasi-steady state mode. The shoots were exposed to irradiance of photosynthetic photon flux density of 200–250 μmol m⁻² s⁻¹, using different light sources. K _lb depended significantly (P < 0.001) on light quality, canopy position and leaf blade area (A _L). K _lb increased from crown base to tree top, in parallel with vertical patterns of A _L. However, the analysis of data on shade and sun leaves separately revealed an opposite trend: the bigger the A _L the higher K _lb. Leaf anatomical study of birch saplings revealed that this trend is attributable to enhanced vascular development with increasing leaf area. Hydraulic traits (K _S, K _B, K _lb) of sun shoots were well co-ordinated and more strongly correlated with characteristics of shoot size than those of shade shoots, reflecting their greater evaporative load and need for stricter adjustment of hydraulic capacity with shoot size. K _S increased with increasing xylem cross-sectional area to leaf area ratio (Huber value; P < 0.01), suggesting a preferential investment in water-conducting tissue (sapwood) relative to transpiring tissue (leaves), and most likely contributing to the functional stability of the hydraulic system, essential for fast-growing pioneer species.     

Growth dynamics of root and shoot hydraulic conductance in seedlings of five neotropical tree species: scaling to show possible adaptation to differing light regimes

The dynamics of growth (shoot and root dry weights, surface areas, hydraulic conductances, and root length) were measured in seedlings of five neotropical tree species aged 4–16 months. The species studied included two light-demanding pioneers (Miconia argentea and Apeiba membranacea) and three shade-tolerant young- or old-forest species (Pouteria reticulata, Gustavia superba, and Trichilia tuberculata). Growth analysis revealed that shoot and root dry weights and hydraulic conductances and leaf area all increased exponentially with time. Alternative methods of scaling measured parameters to reveal differences that might explain adaptations to microsites are discussed. Scaling root conductance to root surface area or root length revealed a few species differences but nothing that correlated with adaptation to light regimes. Scaling of root surface area or root length to root dry weight revealed that pioneers produced significantly more root area and length per gram dry weight investment than shade-tolerant species. Scaling of root and shoot hydraulic conductances to leaf area and scaling of root conductance to root dry weight and shoot conductance to shoot dry weight also revealed that pioneers were significantly more conductive to water than shade-tolerant species. The advantages of scaling hydraulic parameters to leaf surface area are discussed in terms of the Ohm's law analogue of water flow in plants. Received: 24 March 1997 / Accepted: 17 November 1997     

The effect of shoot architecture on hydraulic conductance in beech (Fagus sylvatica L.)

 Shoot hydraulic conductance was measured in beech (Fagus sylvatica L.) that had previously been exposed to high levels of nitrogen input. Whole-shoot hydraulic conductance, conductance per unit pressure gradient and leaf specific conductance were negatively correlated with the number of bud scars per unit length, a morphological parameter of tree decline. We propose a negative feedback mechanism by which stress induced alterations in shoot morphology can cause a lasting reduction of tree vigour. Received: 2 July 1997 / Accepted: 25 June 1998     

Differential coordination of stomatal conductance,mesophyll conductance,and leaf hydraulic conductance in response to changing light across species

Stomatal conductance (g_s) and mesophyll conductance (g_m) represent major constraints to photosynthetic rate (A), and these traits are expected to coordinate with leaf hydraulic conductance (K_leaf) across species, under both steady‐state and dynamic conditions. However, empirical information about their coordination is scarce. In this study, K_leaf, gas exchange, stomatal kinetics, and leaf anatomy in 10 species including ferns, gymnosperms, and angiosperms were investigated to elucidate the correlation of H₂O and CO₂ diffusion inside leaves under varying light conditions. Gas exchange, K_leaf, and anatomical traits varied widely across species. Under light‐saturated conditions, the A, g_s, g_m, and K_leaf were strongly correlated across species. However, the response patterns of A, g_s, g_m, and K_leaf to varying light intensities were highly species dependent. Moreover, stomatal opening upon light exposure of dark‐adapted leaves in the studied ferns and gymnosperms was generally faster than in the angiosperms; however, stomatal closing in light‐adapted leaves after darkening was faster in angiosperms. The present results show that there is a large variability in the coordination of leaf hydraulic and gas exchange parameters across terrestrial plant species, as well as in their responses to changing light.     

A method for measuring xylem hydraulic conductance and embolism in entire root and shoot systems

Current methods for determining the influence of xylem cavitationon hydraulic conductance are limited to unbranched stem or rootsegments with hydraulic conductances above c. 2 mmol s^–1MPa^–1. Lower conductances and/or highly branched systemsare encountered in seedlings, arid-land shrubs, herbs, and distalportions of shoot and root systems of trees. In order to quantifythe hydraulic impact of cavitation in such systems, existingtechniques have been modified. Branched shoot or root systemswere prepared for measurement by removal of leaves, or roottips, respectively. The shoot or root system was enclosed ina vacuum chamber with the proximal end protruding and suppliedwith perfusing solution. Flow through the xylem was inducedby chamber vacuum. Hydraulic conductance was determined fromthe slope of the flow rate versus pressure relationship. Xylemembolism was quantified from the increase in hydraulic conductancefollowing high pressure (100 kPa) perfusion of solution throughthe plant. Examples are provided of the application of the methodto cavitation studies in the cold desert shrub Artemisia tridentata. Key words: Hydraulic conductance, xylem cavitation, embolism, whole root/shoot system     

Growth at elevated carbon dioxide concentration reduces hydraulic conductance in alfalfa and soybean

Hydraulic conductances of alfalfa and soybean plants grown in controlled environment chambers at the current ambient carbon dioxide concentration and at twice the current ambient concentration were determined from measurements of transpiration rate and leaf and stem water potentials in the growth conditions. Growth at elevated carbon dioxide concentration reduced both transpiration rate and hydraulic conductance from the soil to the leaf in both species. Hydraulic conductance from the soil to the base of the stem was also lower at elevated carbon dioxide in soybean, but not alfalfa. These measurements identified the stem to leaf hydraulic pathway as a major target of the carbon dioxide effect in both species. The conductance of excised stem segments was much less in plants grown at elevated carbon dioxide in soybeans.     

The rapid light response of leaf hydraulic conductance: new evidence from two experimental methods

Previous studies have shown a rapid enhancement in leaf hydraulic conductance (K(leaf)) from low to high irradiance (from <10 to >1000 micromol photons m(-2) s(-1)), using the high-pressure flow meter (HPFM), for 7 of 14 tested woody species. However, theoretical suggestions have been made that this response might arise as an artifact of the HPFM. We tested the K(leaf) light response for six evergreen species using refined versions of the rehydration kinetics method (RKM) and the evaporative flux method (EFM). We found new evidence for a rapid, 60% to 100% increase in K(leaf) from low to high irradiance for three species. In the RKM, the leaf rehydration time constant declined by up to 70% under high irradiance relative to darkness. In the EFM, under higher irradiance, the flow rate increased disproportionately to the water potential gradient. Combining our data with those of previous studies, we found that heterobaric species, i.e. those with bundle sheath extensions (BSEs) showed a twofold greater K(leaf) light response on average than homobaric species, i.e. those without BSEs. We suggest further research to characterize this substantial dynamic at the nexus of plant light- and water-relations.     

Growth of Chlorella outdoors in a changing light environment

Chlorella zofingiensis was grown in semi-continuous culture in an outdoor enclosed tubular photobioreactor. At the quasi-steady state, the response of the culture to changes in photosynthetically active radiance (PAR) was studied by following closely the dissolved O₂ concentration,biomass concentration and the carbohydrate and protein content of the biomass. Generally, O₂ production and the output of carbon and nitrogen of the biomass showed a linear correlation with incident PAR, suggesting that the cultures were light-limited. Photoinhibition was not observed in high light adapted cultures (on a sunny day), but was observed in shade adapted cultures(cloudy days) when there was a sudden four-fold increase in PAR. The output rate of biomass nitrogen observed on sunny days was, however, lower than that measured on cloudy days. On sunny days, a rapid increase in the C/N ratio of the biomass was seen. We attribute the increase in C/N ratio on sunny days to a slower response of protein synthesis to big increases in PAR, compared to carbohydrate synthesis. The possible influence of this C and N response pattern on the productivity of outdoor algal cultures is discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Effects of environment during growth on the sensitivity of leaf conductance to changes in humidity

Soybeans (Glycine max) and grain amaranth (Amaranthus hypochondriacus) were grown at a range of temperatures, carbon dioxide concentrations and light conditions in controlled environment chambers, and the response of leaf conductance to water vapour to changes in humidity was then measured under a standard set of conditions. The sensitivity of conductance was analysed in terms of (i) the absolute sensitivity of conductance to changes in leaf to air water vapour pressure difference (LAVPD), (ii) the sensitivity of conductance relative to the absolute value of conductance, and (iii) the slope of the relationship between conductance and an index incorporating assimilation rate, carbon dioxide concentration and relative humidity. The sensitivity of conductance varied substantially with growth conditions for all three analyses in both species. The growth temperature of 25 °C increased the sensitivity of conductance by all three measures compared with growth at 20 or 30 °C in amaranth, with little difference between 25 and 30 °C in soybean. Growth at elevated carbon dioxide decreased sensitivity in amaranth by all three measures, and decreased the absolute but not the relative sensitivity in soybean. Growth at reduced photon flux density and growth at high stand density reduced sensitivity in amaranth by all three measures. In soybean, growth at high stand density reduced sensitivity by all three measures, but growth at low photon flux density increased the relative sensitivity. The sensitivity of leaf conductance to changes in humidity varied by a factor of two or more with growth environment by all measures of sensitivity in both the C3 and the C4 species.     

Biochemical status of renal epithelial Na+ channels determines apparent channel conductance, ion selectivity, and amiloride sensitivity.

Purified bovine renal papillary Na+ channels, when reconstituted into planar lipid bilayers, reside in three conductance states: a 40-pS main state, and two subconductive states (12-13 pS and 24-26 pS). The activity of these channels is regulated by phosphorylation and by G-proteins. Protein kinase A (PKA)-induced phosphorylation increased channel activity by increasing the open state time constants from 160 +/- 30 (main conductance), and 15 +/- 5 ms (both lower conductances), respectively, to 365 +/- 30 ms for all of them. PKA phosphorylation also altered the closed time of the channel from 250 +/- 30 ms to 200 +/- 35 ms, thus shifting the channel into a lower-conductance, long open time mode. PKA phosphorylation increased the PNa:PK of the channel from 7:1 to 20:1, and shifted the amiloride inhibition curve to the right (apparent K(i)amil from 0.7 to 20 microM). Pertussis toxin-induced ADP-ribosylation of either phosphorylated of either phosphorylated or nonphosphorylated channels decreased the PNa:PK to 2:1 and 4:1, respectively, and altered K(i)amil to 8 and 2 microM for phosphorylated and nonphosphorylated channels, respectively. GTP-gamma-S treatment of either phosphorylated or nonphosphorylated channels resulted in an increase of PNa:PK to 30:1 and 10:1, respectively, and produced a leftward shift in the amiloride dose-response curve, altering K(i)amil to 0.5 and 0.1 microM, respectively. These results suggest that amiloride-sensitive renal Na+ channel biophysical characteristics are not static, but depend upon the biochemical state of the channel protein and/or its associated G-protein.     

Effects of hydraulic architecture and spatial variation in light on mean stomatal conductance of tree branches and crowns

In a Pinus taeda L. (loblolly pine) plantation, we investigated whether the response to vapour pressure deficit (D) of canopy average stomatal conductance (G(S)) calculated from sap flux measured in upper and lower branches and main stems follows a hydraulically modelled response based on homeostasis of minimum leaf water potential (Psi(L)). We tested our approach over a twofold range of leaf area index (L; 2-4 m(2) m(-2)) created by irrigation, fertilization, and a combination of irrigation and fertilization relative to untreated control. We found that G(S) scaled well from leaf-level porometery [porometry-based stomatal conductance (g(s))] to branch-estimated and main stem-estimated G(S). The scaling from branch- to main stem-estimated G(S) required using a 45 min moving average window to extract the diurnal signal from the large high-frequency variation, and utilized a light attenuation model to weigh the contribution of upper and lower branch-estimated G(S). Our analysis further indicated that, regardless of L, lower branch-estimated G(S) represented most of the main stem-estimated G(S) in this stand. We quantified the variability in both upper and lower branch-estimated G(S) by calculating the SD of the residuals from a moving average smoothed diurnal. A light model, which incorporated penumbral effects on vertical distribution of direct light, was employed to estimate the variability in light intensity at each canopy level in order to explain the increasing SD of both upper and lower branch-estimated G(S) with light. The results from the light model showed that the upper limit of the variability in individual branch-estimated G(S) could be attributed to incoming light, but not the variation below that upper limit. A porous medium model of water flow in trees produced a pattern of variation below the upper limit that was consistent with the observed variability in branch-estimated G(S). Our results indicated that stems acted to buffer leaf- and branch-level variation and might transmit a less-variable water potential signal to the roots.     

Shoot/root assimilate allocation and nodulation ofVigna unguiculata seedlings as influenced by shoot light environment

Spectral balance of light received by southern pea [Vigna unguiculata (L.) Walp.] seedling shoots affected photoassimilate allocation among leaves, stems and roots. A higher ratio of far-red (FR) relative to red (R) light resulted in longer stems, higher shoot/root biomass ratio, less massive roots and fewer nodules. The same response pattern to FR/R ratio was obtained in a controlled environment with artificial light sources, or in sunlight where the FR/R ratio was modified by reflection from different colored soil surfaces or by FR reflected from competing plants. The importance of early shoot/root photoassimilate allocation and nodulation may differ according to soil nitrogen availability and moisture content.     

Social environment determines degree of chemical signalling

Few studies have attempted to distinguish between cues and signals in the context of chemical communication. A number of chemical substances have been shown to vary with physiological state, such as stage of oestrus cycle, fertility, dominance status or nutritional condition, but little is known about whether this variation is incidental or adaptive. Here, we provide evidence of a substance whose emission varies with breeding state, but is not merely an incidental by-product of physiological state, but rather, an evolved signal. Breeding females of the facultative biparental burying beetle, Nicrophorus vespilloides, release methyl geranate, a substance that helps males to identify breeding status and to distinguish between their female partners and non-breeding intruders. We demonstrate that females respond flexibly to their social environment and emit high amounts of methyl geranate only in the presence of a male partner, i.e. a receiver. In contrast, cuticular hydrocarbons, which also have been shown to change with breeding status, are not modulated and do not differ between single and paired breeding females. Receiver-dependent chemical signalling is expected to evolve when costs are involved in the production or transmission of the signal; such signal modulation might be more common than previously thought.     

Stomatal conductance and photosynthesis vary linearly with plant hydraulic conductance in ponderosa pine

Recent work has shown that stomatal conductance (g_s) and assimilation (A) are responsive to changes in the hydraulic conductance of the soil to leaf pathway (K_L), but no study has quantitatively described this relationship under controlled conditions where steady‐state flow is promoted. Under steady‐state conditions, the relationship between g_s, water potential (Ψ) and K_L can be assumed to follow the Ohm's law analogy for fluid flow. When boundary layer conductance is large relative to g_s, the Ohm's law analogy leads to g_s = K_L (Ψ_soil?Ψ_leaf)/D, where D is the vapour pressure deficit. Consequently, if stomata regulate Ψ_leaf and limit A, a reduction in K_L will cause g_s and A to decline. We evaluated the regulation of Ψ_leaf and A in response to changes in K_L in well‐watered ponderosa pine seedlings (Pinus ponderosa). To vary K_L, we systematically reduced stem hydraulic conductivity (k) using an air injection technique to induce cavitation while simultaneously measuring Ψ_leaf and canopy gas exchange in the laboratory under constant light and D. Short‐statured seedlings (< 1 m tall) and hour‐long equilibration times promoted steady‐state flow conditions. We found that Ψ_leaf remained constant near ? 1·5 MPa except at the extreme 99% reduction of k when Ψ_leaf fell to ? 2·1 MPa. Transpiration, g_s, A and K_L all declined with decreasing k (P < 0·001). As a result of the near homeostasis in bulk Ψ_leaf, g_s and A were directly proportional to K_L (R² > 0·90), indicating that changes in K_L may affect plant carbon gain.     

整树水力导度协同冠层气孔导度调节森林蒸腾

冠层气孔导度决定森林的蒸腾效率,它对驱动水汽移动的水汽应力的响应受树木水力结构的影响,并随水汽压亏缺上升和水力导度下降而降低,维持水势在最低阈值之上,避免出现水力灾变,调控冠层蒸腾。由于叶形和树冠结构的特点,部分脱耦联反映了湿润地区阔叶林冠层与大气的水汽交换特征,单纯以气孔导度的变化难以完整描述水分通量的调节规律,因而,需要考虑冠层气孔导度与水力导度协同控制冠层蒸腾的潜在机理。通过整合叶片气孔气体交换、树干液流、冠层微气象和其他环境因子的野外观测值,估测不同时间尺度的森林冠层气孔导度与大气的脱耦联系数和变异范围,以基于树干液流的冠层蒸腾,结合叶片/土壤水势梯度计算的水力导度,分析水力导度影响冠层气孔导度响应水汽压亏缺的敏感性,可以揭示和阐明水力导度和冠层气孔导度联合调节森林蒸腾的机理,对准确估测全球变化背景下森林对水资源利用的潜在生态效应有明显的理论意义。     

A wilty mutant of rice has impaired hydraulic conductance

The rice CM2088 mutant is the wilty phenotype and wilts markedly under well-watered sunny conditions. The leaf water potential and epidermal (mainly stomatal) conductance of CM2088 plants decreased significantly under conditions that induced intense transpiration, as compared with those of wild-type plants, revealing that the wilty phenotype was not the result of abnormal stomatal behavior but was due to an increase in resistance to water transport. The resistance to water transport was dramatically elevated in the node and the sheath and blade of a leaf of the mutant, but not in the root or stem. The diameter of xylem vessels in the large vascular bundles of the leaf sheath and the internode tended to be small, and the numbers of vessel elements with narrowed or scalariform perforation plates in the leaf blade and sheath were greater in the mutant than in the wild type. Most xylem vessels were occluded, with air bubbles in the leaf sheath of the mutant during the midday hours under intense transpiration conditions, while no bubbles were observed in plants that were barely transpiring, revealing that the significant increase in resistance to water transport was a result of the cavitation. The additive effects of cavitation in xylem vessels and the decreased diameter and deformed plates of vessel elements might be responsible for the wilty phenotype of CM2088.     

Fluid ionic composition influences hydraulic conductance of xylem conduits

The direct effect of fluid composition on xylem hydraulic conductance is investigated in excised stem segments of chrysanthemum (Dendranthema x grandiflorum Tzvelev cv. Cassa) plants. Dynamic changes in hydraulic conductance are accurately measured at 30 s intervals before and after modifications of the composition of the standard fluid (deionized water). It is investigated whether osmotic properties of the flowing solution influence overall hydraulic conductance by affecting the hydraulic conductance of vessel-to-vessel pit membranes, as has previously been suggested. Various iso-osmotic salt solutions (20 mOsm kg-1) of different composition raised the hydraulic conductance of 20 cm long stem segments approximately 5-8% compared to deionized water. In contrast, carbohydrate solutions with similar osmotic strength and pH did not cause any change in hydraulic conductance. KCl solutions that greatly differed in osmotic strength all increased hydraulic conductance, but the response was not correlated with the osmotic strength of the solution. Increasing the number of vessels that were open from one cut end to the other by shortening the stem segments greatly increased the hydraulic conductance response. Changing from deionized water to a salt solution caused an immediate increase in hydraulic conductance, while a shift back to deionized water resulted in a slow decline. This decline lasted longer when the salt solution contained divalent cations compared to monovalent cations. It is concluded that the presence of cations and not the osmotic strength in the flowing solution influenced the hydraulic conductance. The phenomenon is not caused by the vessel-to-vessel pit membranes, which in fact suppressed the effect, due to their large contribution to the overall resistance to water flow.     

Leaf hydraulic conductance and mesophyll conductance are not closely related within a single species

Stomata represent one resistor in a series of resistances for carbon and water exchange between the leaf and the atmosphere; the remaining resistors occurring within the leaf, commonly represented as mesophyll conductance to CO₂, g_m, and leaf hydraulic conductance, k_Leaf. Recent studies have proposed that g_m and k_Leaf may be coordinated across species because of shared pathways. We assessed the correlation between g_m and k_Leaf within cotton, under growth CO₂ partial pressure and irradiance treatments and also with short‐term variation in irradiance and humidity. g_m was estimated using two isotopic techniques that allowed partitioning of total g_m (Δ¹³C‐g_m) into cell wall plus plasma membrane conductance (Δ¹⁸O‐g_m) and chloroplast membrane conductance (g_cm). A weak correlation was found between Δ¹³C‐g_m and k_Leaf only when measured under growth conditions. However, Δ¹⁸O‐g_m was related to k_Leaf under both short‐term environmental variation and growth conditions. Partitioning g_m showed that g_cm was not affected by short‐term changes in irradiance or correlated with k_Leaf, but was strongly reduced at high growth CO₂ partial pressure. Thus, simultaneous measurements of g_m, k_Leaf and g_cm suggest independent regulation of carbon and water transport across the chloroplast membrane with limited coordinated regulation across the cell wall and plasma membrane.     

Stomatal conductance tracks soil-to-leaf hydraulic conductance in faba bean and maize during soil drying

Although regulation of stomatal conductance is widely assumed to be the most important plant response to soil drying, the picture is incomplete when hydraulic conductance from soil to the leaf, upstream of the stomata, is not considered. Here, we investigated to what extent soil drying reduces the conductance between soil and leaf, whether this reduction differs between species, how it affects stomatal regulation, and where in the hydraulic pathway it occurs. To this end, we noninvasively and continuously measured the total root water uptake rate, soil water potential, leaf water potential, and stomatal conductance of 4-week-old, pot-grown maize (Zea mays) and faba bean (Vicia faba) plants during 4 days of water restriction. In both species, the soil–plant conductance, excluding stomatal conductance, declined exponentially with soil drying and was reduced to 50% above a soil water potential of −0.1 MPa, which is far from the permanent wilting point. This loss of conductance has immediate consequences for leaf water potential and the associated stomatal regulation. Both stomatal conductance and soil–plant conductance declined at a higher rate in faba bean than in maize. Estimations of the water potential at the root surface and an incomplete recovery 22 h after rewatering indicate that the loss of conductance, at least partly, occurred inside the plants, for example, through root suberization or altered aquaporin gene expression. Our findings suggest that differences in the stomatal sensitivity among plant species are partly explained by the sensitivity of root hydraulic conductance to soil drying.

The hydraulic conductance between soil and leaf decreases exponentially with decreasing soil water potential, at a species-specific rate related to the decline rate of stomatal conductance.