The effect of three rootstocks on water use, canopy conductance and hydraulic parameters of apple trees and predicting canopy from hydraulic conductance

This study investigated the effect of the dwarfing M9, semi‐dwarfing MM106 and local Hashabi rootstocks on the water use, canopy conductance (G_c) and hydraulic conductance (k) of apple orchards with the same scion, Golden Delicious. The average summer leaf area index (LAI) was 2·4, 2·7 and 1·7 for M9, MM106 and Hashabi, respectively. Irrigation in 1997 was less than water use until June, and excessive afterwards. In 1998, irrigation was doubled, and was excessive throughout the season. Sap flow (J) in June–August 1998 totalled 476, 682 and 606 mm (or 0·60, 0·86 and 0·76 of class A pan evaporation) for M9, MM106 and Hashabi, respectively. Maximum sap velocity in the three rootstocks (approximately 70 cm h^?1) occurred in the outer 30–60% of the stem, and its decrease with depth was greater in M9 than in the other rootstocks. Midday G_c during both summers was least for M9, intermediate for Hashabi and greatest for MM106. The k value of M9 and MM106 for the soil to stem, stem to leaves and soil to leaves pathways were determined from daily courses of water potential of leaves, Ψ_l, stem, Ψ_stem and J. Specific k (k_s, i.e. relative to stem sapwood area) did not significantly differ between the two rootstocks for soil to stem and soil to leaf pathways, but leaf specific k (k_l) was greater for MM106 soil to stem (71% greater) and soil to leaf (63%) pathways, respectively. The inverse slopes of the relationships between midday canopy resistance (R_c) and vapour pressure deficit (D) for MM106 was 1·75 of that for M9, and the ratio of their Huber values, i.e. the ratio of sapwood to leaf area, was 1·6. These findings indicate that differences in water use are attributable to differences in k_l, and not to differences in wood properties (k_s). Application of a model relating R_c to orchard area specific k (k_g) showed that the slope of the relationships between midday R_c and D for the 1998 data could be predicted using common values of k_s (0·134 kg m^?2 s^?1 MPa^?1) and midday Ψ_l (?1·34 MPa) for the three rootstocks. The implications of these findings, and the similarities in the differences between rootstocks of G_c, k_l, k_g and Huber values, are discussed with respect to rootstock water use and irrigation.     

Hydraulic architecture and water flow in growing grass tillers (Festuca arundinacea Schreb.)

The water relations and hydraulic architecture of growing grass tillers (Festuca arundinacea Schreb.) are reported. Evaporative flux density, E (mmol s^?1 m^?2), of individual leaf blades was measured gravimetrically by covering or excision of entire leaf blades. Values of E were similar for mature and elongating leaf blades, averaging 2·4 mmol s^?1 m^?2. Measured axial hydraulic conductivity, K_h (mmol s^?1 mm MPa^?1), of excised leaf segments was three times lower than theoretical hydraulic conductivity (K_t) calculated using the Poiseuille equation and measurements of vessel number and diameter. K_t was corrected (K_t*) to account for the discrepancy between K_h and K_t and for immature xylem in the basal expanding region of elongating leaves. From base to tip of mature leaves the pattern of K_t* was bell‐shaped with a maximum near the sheath–blade joint (≈ 19 mmol s^?1 mm MPa^?1). In elongating leaves, immature xylem in the basal growing region led to a much lower K_t*. As the first metaxylem matured, K_t* increased by 10‐fold. The hydraulic conductances of the whole root system, (mmol s^?1 MPa^?1) and leaf blades, (mmol s^?1 MPa^?1) were measured by a vacuum induced water flow technique. and were linearly related to the leaf area downstream. Approximately 65% of the resistance to water flow within the plant resided in the leaf blade. An electric‐analogue computer model was used to calculate the leaf blade area‐specific radial hydraulic conductivity, (mmol s^?1 m^?2 MPa^?1), using , K_t* and water flux values. values decreased with leaf age, from 21·2 mmol s^?1 m^?2 MPa^?1 in rapidly elongating leaf to 7·2 mmol s^?1 m^?2 MPa^?1 in mature leaf. Comparison of and values showed that ≈ 90% of the resistance to water flow within the blades resided in the liquid extra‐vascular path. The same algorithm was then used to compute the xylem and extravascular water potential drop along the liquid water path in the plant under steady state conditions. Predicted and measured water potentials matched well. The hydraulic design of the mature leaf resulted in low and quite constant xylem water potential gradient (≈ 0·3 MPa m^?1) throughout the plant. Much of the water potential drop within mature leaves occurred within a tenth of millimetre in the blade, between the xylem vessels and the site of water evaporation within the mesophyll. In elongating leaves, the low K_t* in the basal growth zone dramatically increased the local xylem water potential gradient (≈ 2·0 MPa m^?1) there. In the leaf elongation zone the growth‐induced water potential difference was ≈ 0·2 MPa.     

Integrated responses of hydraulic architecture, water and carbon relations of western hemlock to dwarf mistletoe infection

Dwarf mistletoe (Arceuthobium spp.) is a hemiparasite that is said to be the single‐most destructive pathogen of commercially valuable coniferous trees in many regions of the world. Although its destructive nature is well documented in many respects, its effects on the physiology of its host are poorly understood. In the present study, water and carbon relations were characterized over a range of scale from leaf to whole tree in large (40‐ to 50‐m‐tall) individuals of western hemlock (Tsuga heterophylla (Raf.) Sarg.) that were either heavily infected, or uninfected with hemlock dwarf mistletoe (Arceuthobium tsugense). Specific hydraulic conductivity (k_s) of infected branches was approximately half that of uninfected branches, yet leaf‐specific conductivity (k_L) was similar because leaf area : sapwood area ratios (A_L : A_S) of infected branches were lower. Pre‐dawn and minimum leaf water potential and stomatal conductance (g_s) were similar among infected and uninfected trees because adjustments in hydraulic architecture of infected trees maintained k_L despite reduced k_s. Maximum whole‐tree water use was substantially lower in infected trees (approximately 55 kg d^?1) than in uninfected trees (approximately 90 kg d^?1) because reduced numbers of live branches in infected trees reduced whole‐tree A_L : A_S in a manner consistent with that observed in infected branches. Maximum photosynthetic rates of heavily infected trees were approximately half those of uninfected trees. Correspondingly, leaf nitrogen content was 35% lower in infected trees. Foliar δ¹³C values were 2.8‰ more negative in infected than in uninfected individuals, consistent with the absence of stomatal adjustment to diminished photosynthetic capacity. Adjustments in hydraulic architecture of infected trees thus contributed to homeostasis of water transport efficiency and transpiration on a leaf area basis, whereas both carbon accumulation and photosynthetic water use efficiency were sharply reduced at both the leaf and whole‐tree scale.     

Photosynthetic pathway alters xylem structure and hydraulic function in herbaceous plants

Plants using the C₄ photosynthetic pathway have greater water use efficiency (WUE) than C₃ plants of similar ecological function. Consequently, for equivalent rates of photosynthesis in identical climates, C₄ plants do not need to acquire and transport as much water as C₃ species. Because the structure of xylem tissue reflects hydraulic demand by the leaf canopy, a reduction in water transport requirements due to C₄ photosynthesis should affect the evolution of xylem characteristics in C₄ plants. In a comparison of stem hydraulic conductivity and vascular anatomy between eight C₃ and eight C₄ herbaceous species, C₄ plants had lower hydraulic conductivity per unit leaf area (K_L) than C₃ species of similar life form. When averages from all the species were pooled together, the mean K_L for the C₄ species was 1.60 × 10^?4 kg m^?1 s^?1 MPa^?1, which was only one‐third of the mean K_L of 4.65 × 10^?4 kg m^?1 s^?1 MPa^?1 determined for the C₃ species. The differences in K_L between C₃ and C₄ species corresponded to the two‐ to three‐fold differences in WUE observed between C₃ and C₄ plants. In the C₄ species from arid regions, the difference in K_L was associated with a lower hydraulic conductivity per xylem area, smaller and shorter vessels, and less vulnerable xylem to cavitation, indicating the C₄ species had evolved safer xylem than the C₃ species. In the plants from resource‐rich areas, such as the C₄ weed Amaranthus retroflexus, hydraulic conductivity per xylem area and xylem anatomy were similar to that of the C₃ species, but the C₄ plants had greater leaf area per xylem area. The results indicate the WUE advantage of C₄ photosynthesis allows for greater flexibility in hydraulic design and potential fitness. In resource‐rich environments in which competition is high, an existing hydraulic design can support greater leaf area, allowing for higher carbon gain, growth and competitive potential. In arid regions, C₄ plants evolved safer xylem, which can increase survival and performance during drought events.     

Relations between stomatal closure, leaf turgor and xylem vulnerability in eight tropical dry forest trees

This study examined the linkage between xylem vulnerability, stomatal response to leaf water potential (Ψ_L), and loss of leaf turgor in eight species of seasonally dry tropical forest trees. In order to maximize the potential variation in these traits species that exhibit a range of leaf habits and phenologies were selected. It was found that in all species stomatal conductance was responsive to Ψ_L over a narrow range of water potentials, and that Ψ_L inducing 50% stomatal closure was correlated with both the Ψ_L inducing a 20% loss of xylem hydraulic conductivity and leaf water potential at turgor loss in all species. In contrast, there was no correlation between the water potential causing a 50% loss of conductivity in the stem xylem, and the water potential at stomatal closure (Ψ_SC) amongst species. It was concluded that although both leaf and xylem characters are correlated with the response of stomata to Ψ_L, there is considerable flexibility in this linkage. The range of responses is discussed in terms of the differing leaf‐loss strategies exhibited by these species.     

The 'hydrology' of leaves: co-ordination of structure and function in temperate woody species

The hydraulic conductance of the leaf lamina (K_lamina) substantially constrains whole‐plant water transport, but little is known of its association with leaf structure and function. K_lamina was measured for sun and shade leaves of six woody temperate species growing in moist soil, and tested for correlation with the prevailing leaf irradiance, and with 22 other leaf traits. K_lamina varied from 7.40 × 10^?5 kg m^?2 s^?1 MPa^?1 for Acer saccharum shade leaves to 2.89 × 10^?4 kg m^?2 s^?1 MPa^?1 for Vitis labrusca sun leaves. Tree sun leaves had 15–67% higher K_lamina than shade leaves. K_lamina was co‐ordinated with traits associated with high water flux, including leaf irradiance, petiole hydraulic conductance, guard cell length, and stomatal pore area per lamina area. K_lamina was also co‐ordinated with lamina thickness, water storage capacitance, 1/mesophyll water transfer resistance, and, in five of the six species, with lamina perimeter/area. However, for the six species, K_lamina was independent of inter‐related leaf traits including leaf dry mass per area, density, modulus of elasticity, osmotic potential, and cuticular conductance. K_lamina was thus co‐ordinated with structural and functional traits relating to liquid‐phase water transport and to maximum rates of gas exchange, but independent of other traits relating to drought tolerance and to aspects of carbon economy.     

Leaf photosynthetic traits scale with hydraulic conductivity and wood density in Panamanian forest canopy trees

We investigated how water transport capacity, wood density and wood anatomy were related to leaf photosynthetic traits in two lowland forests in Panama. Leaf-specific hydraulic conductivity (k_L) of upper branches was positively correlated with maximum rates of net CO₂ assimilation per unit leaf area (A_area) and stomatal conductance (g_s) across 20 species of canopy trees. Maximum k_L showed stronger correlation with A_area than initial k_L suggesting that allocation to photosynthetic potential is proportional to maximum water transport capacity. Terminal branch k_L was negatively correlated with A_area/g_s and positively correlated with photosynthesis per unit N, indicating a trade-off of efficient use of water against efficient use of N in photosynthesis as water transport efficiency varied. Specific hydraulic conductivity calculated from xylem anatomical characteristics (k_theoretical) was positively related to A_area and k_L, consistent with relationships among physiological measurements. Branch wood density was negatively correlated with wood water storage at saturation, k_L, A_area, net CO₂ assimilation per unit leaf mass (A_mass), and minimum leaf water potential measured on covered leaves, suggesting that wood density constrains physiological function to specific operating ranges. Kinetic and static indices of branch water transport capacity thus exhibit considerable co-ordination with allocation to potential carbon gain. Our results indicate that understanding tree hydraulic architecture provides added insights to comparisons of leaf level measurements among species, and links photosynthetic allocation patterns with branch hydraulic processes.     

Transpiration rate relates to within‐ and across‐species variations in effective path length in a leaf water model of oxygen isotope enrichment

Stable oxygen isotope ratio of leaf water (δ¹⁸O_L) yields valuable information on many aspects of plant–environment interactions. However, current understanding of the mechanistic controls on δ¹⁸O_L does not provide complete characterization of effective path length (L) of the Péclet effect, – a key component of the leaf water model. In this study, we collected diurnal and seasonal series of leaf water enrichment and estimated L in six field‐grown angiosperm and gymnosperm tree species. Our results suggest a pivotal role of leaf transpiration rate (E) in driving both within‐ and across‐species variations in L. Our observation of the common presence of an inverse scaling of L with E in the different species therefore cautions against (1) the conventional treatment of L as a species‐specific constant in leaf water or cellulose isotope (δ¹⁸O_p) modelling; and (2) the use of δ¹⁸O_p as a proxy for g_s or E under low E conditions. Further, we show that incorporation of a multi‐species L‐E scaling into the leaf water model has the potential to both improve the prediction accuracy and simplify parameterization of the model when compared with the conventional approach. This has important implications for future modelling of oxygen isotope ratios.     

Effect of water content and temperature on seed longevity of seven Brassicaceae species after 5 years of storage

Maximising seed longevity is crucial for genetic resource preservation and longevity of orthodox seeds is determined by environmental conditions (water content and temperature). The effect of water content (down to 0.01 g·H₂O·g^?1) on seed viability was studied at different temperatures for a 5‐year storage period in taxonomically related species. Seeds of seven Brassicaceae species (Brassica repanda, Eruca vesicaria, Malcolmia littorea, Moricandia arvensis, Rorippa nasturtium‐aquaticum, Sinapis alba, Sisymbrium runcinatum) were stored at 48 environments comprising a combination of eight water contents, from 0.21 to 0.01 g·H₂O·g^?1 DW and six temperatures (45, 35, 20, 5, ?25, ?170 °C). Survival curves were modelled and P50 calculated for those conditions where germination was reduced over the 5‐year assay period. Critical water content for storage of seeds of six species at 45 °C ranged from 0.02 to 0.03 g·H₂O·g^?1. The effect of extreme desiccation at 45 °C showed variability among species: three species showed damaging effects of drying below the critical water content, while for three species it was neither detrimental nor beneficial to seed longevity. Lipid content could be related to longevity, depending on the storage conditions. A variable seed longevity response to water content among taxonomically related species was found. The relative position of some of the species as long‐ or short‐lived at 45 °C varied depending on the humidity at which storage behaviour was evaluated. Therefore, predictions of survival under desiccated conditions based on results obtained at high humidity might be problematic for some species.     

Transition of stable isotope ratios of leaf water under simulated dew formation

Dew formation, a common meteorological phenomenon, is expected to intensify in the future. Dew can influence the H₂¹⁸O and HDO isotopic compositions of leaf water (δ_L), but the phenomenon has been neglected in many experimental and modelling studies. In this study, the dew effect on δ_L was investigated with a dark plant chamber in which dew formation was introduced. The H₂¹⁸O and HDO compositions of water vapour, dew water and leaf water of five species were measured for up to 48 h of dew exposure. Our results show that the exchanges of H₂¹⁸O and HDO in leaf water with the air continued in the darkness when the net H₂¹⁶O flux was zero. Our estimates of the leaf conductance using the isotopic mass balance method ranged from 0.035 to 0.087 mol m^?2 s^?1, in broad agreement of the night‐time stomatal conductance reported in the literature. In our experiments, the conductance of the C₄ species was 0.04 ± 0.01 mol m^?2 s^?1 and that of the C₃ plants was 0.10 ± 0.04 mol m^?2 s^?1. At the end of 16 h dew exposure, 72 (±17) and 94 (±11)% of the leaf water came from dew according to the ¹⁸O and D tracer, respectively.     

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.     

Vein cavitation and stomatal behaviour of sunflower (Helianthus annuus) leaves under water limitation

The impact of leaf vein cavitation and embolism on stomatal response and leaf hydraulic conductance was studied in potted plants of sunflower subjected to water limitation. Plant dehydration was achieved either by cutting well‐watered plants near their base and leaving them dehydrating in air or by depriving intact plants of irrigation. The vein cavitation threshold (Ψ_CAV) was estimated in terms of ultrasound acoustic emissions (UAE) from the leaf blade versus leaf water potential (Ψ_L). This was found to be the same (Ψ_CAV ≈ ?0.6 MPa) for leaves of both cut and intact plants where stomata began to close in coincidence with starting vein cavitation. Vein embolism was detected by infiltrating leaves at different Ψ_L with 0.7 mM fluorescein and measuring the percentage fluorescent area as percentage of total leaf surface area. A distinct loss of vein functionality (up to 50%) was found to occur in leaves at progressively decreasing Ψ_L, starting when leaves reached Ψ_CAV. A linear positive relationship with high statistical significance was found to exist between g_L and percentage leaf fluorescent area, thus indicating that stomata were sensitive to vein embolism. The hydraulic conductance (K_L) of the leaf was affected by leaf dehydration less than expected (K_L decreased by about 20% between near full turgor and Ψ_L = ?1.3 MPa). When the extravascular leaf compartment was excluded either by killing cells by immersing leaves in 70% ethanol or by cutting the main leaf venous system through to allow flow to bypass it, K_L turned out to increase 5.5 times, thus suggesting that the high dominance of the hydraulic resistance of the extravascular leaf compartment over the total leaf resistance might buffer or mask possibly large local changes in K_L inducing stomatal closure.     

Whole-tree water transport scales with sapwood capacitance in tropical forest canopy trees

The present study examines the manner in which several whole‐tree water transport properties scale with species‐specific variation in sapwood water storage capacity. The hypothesis that constraints on relationships between sapwood capacitance and other water relations characteristics lead to predictable scaling relationships between intrinsic capacitance and whole‐tree behaviour was investigated. Samples of sapwood from four tropical forest canopy tree species selected to represent a range of wood density, tree size and architecture, and taxonomic diversity were used to generate moisture release curves in thermocouple psychrometer chambers, from which species‐specific values of sapwood capacitance were calculated. Sapwood capacitance was then used to scale several whole‐tree water transport properties determined from measurements of upper branch and basal sap flow, branch water potential, and axial and radial movement of deuterated water (D₂O) injected into the base of the trunk as a tracer. Sapwood capacitance ranged from 83 to 416 kg m^?3 MPa^?1 among the four species studied and was strongly correlated with minimum branch water potential, soil‐to‐branch hydraulic conductance, daily utilization of stored water, and axial and radial movement of D₂O. The species‐independent scaling of several whole‐tree water transport properties with sapwood capacitance indicated that substantial convergence in plant function at multiple levels of biological organization was revealed by a simple variable related to a biophysical property of water transport tissue.     

Stomatal and photosynthetic responses of olive (Olea europaea L.) leaves to water deficits

The leaf gas exchange of mature olive trees (Olea europaea L.) was characterized over a wide range of water deficits in the field during 1998, in Cordoba, Spain. Leaf photosynthesis (A) and stomatal conductance (g_l) responded diurnally and seasonally to variations in tree water status and evaporative demand. In the absence of water stress, A and g_l were generally high during autumn and low in days of high vapour pressure deficits (VPD). Leaf A varied between 0 and 2 µmol m^?2 s^?1 under severe water deficits that lowered the stem water potential (Ψ_x) to ?8·0 MPa, but recovered rapidly following rehydration. Transpiration efficiency (TE) was curvilinearly related to VPD and not influenced by water deficits except in cases of severe water stress, where low TE values were observed at Ψ_x below ?4 MPa. Three models of leaf conductance were calibrated and validated with the experimental data; two were based on the model proposed by Leuning (L) and the other was derived from the widely used Jarvis (J) model. The L models performed better than the J model in two validation tests. The scatter of the predictions and the limited accuracy of all three models suggest that, in addition to the physiological and environmental variables considered, there are additional endogenous factors influencing the g_l of olive leaves.     

The stem xylem of Patagonian shrubs operates far from the point of catastrophic dysfunction and is additionally protected from drought‐induced embolism by leaves and roots

Hydraulic architecture was studied in shrub species differing in rooting depth in a cold desert in Southern Argentina. All species exhibited strong hydraulic segmentation between leaves, stems and roots with leaves being the most vulnerable part of the hydraulic pathway. Two types of safety margins describing the degree of conservation of the hydraulic integrity were used: the difference between minimum stem or leaf water potential (Ψ) and the Ψ at which stem or leaf hydraulic function was reduced by 50% (Ψ – Ψ₅₀), and the difference between leaf and stem Ψ₅₀. Leaf Ψ₅₀ – stem Ψ₅₀ increased with decreasing rooting depth. Large diurnal decreases in root‐specific hydraulic conductivity suggested high root vulnerability to embolism across all species. Although stem Ψ₅₀ became more negative with decreasing species‐specific Ψ_soil and minimum stem Ψ, leaf Ψ₅₀ was independent of Ψ and minimum leaf Ψ. Species with embolism‐resistant stems also had higher maximum stem hydraulic conductivity. Safety margins for stems were >2.1 MPa, whereas those for leaves were negative or only slightly positive. Leaves acted as safety valves to protect the integrity of the upstream hydraulic pathway, whereas embolism in lateral roots may help to decouple portions of the plant from the impact of drier soil layers.     

Hydraulic architecture of sugarcane in relation to patterns of water use during plant development*

Hydraulic conductance was measured on leaf and stem segments excised from sugarcane plants at different stages of development. Maximum transpiration rates and leaf water potential (Ψ_L) associated with maximum transpiration were also measured in intact plants as a function of plant size. Leaf specific hydraulic conductivity (L_sc) and transpiration on a unit leaf area basis (E) were maximal in plants with approximately 0.2 m² leaf area and decreased with increasing plant size. These changes in Fand L_sc were nearly parallel, which prevented φ_L in larger plants from decreasing to levels associated with substantial loss in xylem conductivity caused by embolism formation. Coordination of changes in E and leaf hydraulic properties was not mediated by declining leaf water status, since φ_L increased with plant size. Hydraulic constrictions were present at nodes and in the node-leaf sheath-leaf blade pathway. This pattern of constrictions is in accord with the idea of plant segmentation into regions differing in water transport efficiency and would tend to confine embolisms to the relatively expendable leaves at terminal positions in the pathway, thereby preserving water transport through the stem.     

Vulnerability to cavitation of leaf minor veins: any impact on leaf gas exchange?

Vulnerability to cavitation of leaf minor veins and stems of Laurus nobilis L. was quantified together with that of leaflets, rachides and stems of Ceratonia siliqua L. during air‐dehydration of 3‐year‐old branches. Embolism was estimated by counting ultrasound acoustic emissions (UAE) and relating them to leaf water potential (Ψ_L). The threshold Ψ_L for cavitation was less negative in L. nobilis than in C. siliqua according to the known higher drought resistance of the latter species. Leaf minor vein cavitation was also quantified by infiltrating leaves with fluorescein at different dehydration levels and observing them under microscope. Distinct decreases in the functional integrity of minor veins were observed during leaf dehydration, with high correlation between the two variables. The relationship between leaf conductance to water vapour (g_L) and Ψ_L showed that stomata of L. nobilis closed in response to stem and not to leaf cavitation. However, in C. siliqua, g_L decreased in coincidence to the leaf cavitation threshold, which was, nevertheless, very close to that of the stem. The hypothesis that stem cavitation acts as a signal for stomatal closure was confirmed, while the same role for leaf cavitation remains an open problem.     

Susceptibility to photoinhibition of three deciduous broadleaf tree species with different successional traits raised under various light regimes

The susceptibility to photoinhibition of tree species from three different successional stages were examined using chlorophyll fluorescence and gas exchange techniques. The three deciduous broadleaf tree species were Betula platyphylla var. japonica, pioneer and early successional, Quercus mongolica, intermediate shade‐tolerant and mid‐successional, and Acer mono, shade‐tolerant and late successional. Tree seedlings were raised under three light regimes: full sunlight (open), 10% full sun, and 5% full sun. Susceptibility to photoinhibition was assessed on the basis of the recovery kinetics of the ratio of vaviable to maximum fluorescence (F_v/F_m) of detached leaf discs exposed to about 2000 μmol m^?1 s^?1 photon flux density (PFD) for 2 h under controlled conditions (25 to 28 °C, fully hydrated). Differences in susceptibility to photodamage among species were not significant in the open and 10% full sun treatments. But in 5% full sun, B. platyphylla sustained a significantly greater photodamage than other species, probably associated with having the lowest photosynthetic capacity indicated by light‐saturated photosynthetic rate (B. platyphylla, 9·87, 5·85 and 2·82; Q. mongolica, 8·05, 6·28 and 4·41; A. mono, 7·93, 6·11 and 5·08 μmol CO₂ m^?1 s^?1for open, 10% and 5% full sun, respectively). To simulate a gap formation and assess its complex effects including high temperature and water stress in addition to strong light on the susceptibility to photoinhibition, we examined photoinhibition in the field by means of monitoring ΔF/F′_m on the first day of transfer to natural daylight. Compared with ΔF/F′_m in AM, the lower ΔF/F′_m in PM responding to lower PFD following high PFD around noon indicated that photoinhibition occurred in plants grown in 10 and 5% full sun. The diurnal changes of ΔF/F′_m showed that Q. mongolica grown in 5% full sun was less susceptible to photoinhibition than A. mono although they showed little differences both in photosynthetic capacity in intact leaves and susceptibility to photoinhibition based on leaf disc measurements. These results suggest that shade‐grown Q. mongolica had a higher tolerance for additional stresses such as high temperature and water stress in the field, possibly due to their lower plasticity in leaf anatomy to low light environment.     

Through-flow of water in leaves of a submerged plant is influenced by the apical opening

Submerged aquatic higher plants maintain acropetal water transport to the young leaves in active growth to satisfy their demand for nutrients and hormones derived from the roots. We here present the first measurements of hydraulic properties for a submerged plant, the monocotyledon Sparganium emersum Rehman. The hydraulic conductance per unit length, K_h, was measured in leaf segments without the leaf tip and shown to be greater in old, fully developed leaves (1.5 · 10⁻¹⁰ · m⁴ · MPa⁻¹ · s⁻¹) than in young leaves (1.0 · 10⁻¹⁰ · m⁴ · MPa⁻¹ · s⁻¹). In leaves with intact leaf tips, however, K_h was significantly greater in the youngest leaves, which suggests that the leaf tip with the hydathode influences resistance and thus flow. Microscopy confirmed that the hydathodal area, which is an apical opening, undergoes structural changes with leaf age; a matrix of microorganisms develops in the older leaves and probably restricts water flow by clogging the hydathodes. The leaf specific conductivity expressing transport capacity relative to the leaf area supplied, of S. emersum (0.1 · 10⁻⁸ to 9 · 10⁻⁸ · m² MPa⁻¹· s⁻¹) was within the same range as for various species of terrestrial ferns, vines and trees. This finding does not support the traditional concept of functionally reduced vascular transport in Received: 15 July 1996 / Accepted: 30 November 1996     

Photosynthetic and gas exchange characteristics of dominant woody plants on a moisture gradient in an African savanna

We determined key photosynthetic gas exchange parameters, and their temperature dependence, in dominant woody plants at four savanna sites on a moisture gradient in Botswana, southern Africa. Leaf stable carbon and nitrogen (N) isotope and morphological measures were made concurrently. Sampling of these predominantly non‐N‐fixing species took place during an exceptional rainfall season, representing near‐optimum conditions for primary production at these sites. The mean specific leaf area and leaf size were positively related to mean annual rainfall (MAR); species with larger leaves of lower density were more abundant in wetter sites. Almost all species at all sites showed high net light‐saturated photosynthetic rates (A_max?10 μmol CO₂ m^?2 s^?1) due both to high CO₂ carboxylation (V_c,max) and RubP‐regeneration capacity (J_max). These high rates were associated with high values of leaf [N]. Across all sites, the temperature response of A_max showed no clear optimum, and a gradual drop from 25°C to 35°C, without notable temperature limitation at leaf temperatures in excess of 35°C. Dark respiration rate (R_day) across all species and sites increased exponentially with increasing leaf temperature. Species sampled at selected sites revealed a negative relationship between leaf δ¹³C (stable carbon isotope ratio) and MAR, suggesting higher leaf‐level water‐use efficiency at drier sites when integrated over the life of the leaf. At wetter sites, specific leaf [N] was lower and photosynthetic nitrogen‐use efficiency increased, a pattern reflected at the ecosystem level by less ¹⁵N enrichment of leaves at these sites. Taken together, the results suggest a switch from water‐use to nitrogen‐use efficiency constraints with increasing moisture availability. These constraints impact leaf form and function significantly, and may emerge at the ecosystem level in aspects of water and N cycling.