Predawn disequilibrium between plant and soil water potentials in two cold-desert shrubs

Classical water relations theory predicts that predawn plant water potential should be in equilibrium with soil water potential (soil Ψ_w) around roots, and many interpretations of plant water status in natural populations are based on this expectation. We examined this expectation for two salt-tolerant, cold-desert shrub species in glasshouse experiments where frequent watering assured homogeneity in soil Ψ_w and soil-root hydraulic continuity and where NaCl controlled soil Ψ_w. Plant water potentials were measured with a pressure chamber (xylem Ψ_p) and thermocouple psychrometers (leaf Ψ_w). Soil Ψ_w was measured with in situ thermocouple psychrometers. Predawn leaf Ψ_w and xylem Ψ_p were significantly more negative than soil Ψ_w, for many treatments, indicating large predawn soil-plant Ψ_w disequilibria: up to 1.2 MPa for Chrysothamnus nauseosus (0 and 100 mm NaCl) and 1.8 MPa for Sarcobatus vermiculatus (0, 100, 300, and 600 mm NaCl). Significant nighttime canopy water loss was one mechanism contributing to predawn disequilibrium, assessed by comparison of xylem Ψ_p for bagged (to minimize transpiration) and unbagged canopies, and by gas exchange measurements. However, nighttime transpiration accounted for only part of the predawn disequilibrium. Other mechanisms that could act with nighttime transpiration to generate large predawn disequilibria are described and include a model of how leaf apoplastic solutes could contribute to the phenomenon. This study is among the first to conclusively document such large departures from the expectation of predawn soil-plant equilibrium for C₃ shrubs, and provides a general framework for considering relative contributions of nighttime transpiration and other plant-related mechanisms to predawn disequilibrium. Received: 12 November 1998 / Accepted: 5 May 1999     

Water relations of coastal and estuarine Rhizophora mangle: xylem pressure potential and dynamics of embolism formation and repair

Physiological traits related to water transport were studied in Rhizophora mangle (red mangrove) growing in coastal and estuarine sites in Hawaii. The magnitude of xylem pressure potential (P_x), the vulnerability of xylem to cavitation, the frequency of embolized vessels in situ, and the capacity of R. mangle to repair embolized vessels were evaluated with conventional and recently developed techniques. The osmotic potential of the interstitial soil water (?_sw) surrounding the roots of R. mangle was c. -2.6LJ.52᎒^-3 and -0.4Lj.13᎒^-3 MPa in the coastal and estuarine sites, respectively. Midday covered (non-transpiring) leaf water potentials (O_L) determined with a pressure chamber were 0.6-0.8 MPa more positive than those of exposed, freely-transpiring leaves, and osmotic potential of the xylem sap (?_x) ranged from -0.1 to -0.3 MPa. Consequently, estimated midday values of P_x (calculated by subtracting ?_x from covered O_L) were about 1 MPa more positive than O_L determined on freely transpiring leaves. The differences in O_L between covered and transpiring leaves were linearly related to the transpiration rates. The slope of this relationship was steeper for the coastal site, suggesting that the hydraulic resistance was larger in leaves of coastal R. mangle plants. This was confirmed by both hydraulic conductivity measurements on stem segments and high-pressure flowmeter studies made on excised leafy twigs. Based on two independent criteria, loss of hydraulic conductivity and proportions of gas- and liquid-filled vessels in cryo-scanning electron microscope (cryo-SEM) images, the xylem of R. mangle plants growing at the estuarine site was found to be more vulnerable to cavitation than that of plants growing at the coastal site. However, the cryo-SEM analyses suggested that cavitation occurred more readily in intact plants than in excised branches that were air-dried in the laboratory. Cryo-SEM analyses also revealed that, in both sites, the proportion of gas-filled vessels was 20-30% greater at midday than at dawn or during the late afternoon. Refilling of cavitated vessels thus occurred during the late afternoon when considerable tension was present in neighboring vessels. These results and results from pressure-volume relationships suggest that R. mangle adjusts hydraulic properties of the water-transport system, as well as the leaf osmotic potential, in concert with the environmental growing conditions.     

High apoplastic solute concentrations in leaves alter water relations of the halophytic shrub, Sarcobatus vermiculatus

Predawn plant water potential (Psi(w)) is used to estimate soil moisture available to plants because plants are expected to equilibrate with the root-zone Psi(w). Although this equilibrium assumption provides the basis for interpreting many physiological and ecological parameters, much work suggests predawn plant Psi(w) is often more negative than root-zone soil Psi(w). For many halophytes even when soils are well-watered and night-time shoot and root water loss eliminated, predawn disequilibrium (PDD) between leaf and soil Psi(w) can exceed 0.5 MPa. A model halophyte, Sarcobatus vermiculatus, was used to test the predictions that low predawn solute potential (Psi(s)) in the leaf apoplast is a major mechanism driving PDD and that low Psi(s) is due to high Na+ and K+ concentrations in the leaf apoplast. Measurements of leaf cell turgor (Psi(p)) and solute potential (Psi(s)) of plants grown under a range of soil salinities demonstrated that predawn symplast Psi(w) was 1.7 to 2.1 MPa more negative than predawn xylem Psi(w), indicating a significant negative apoplastic Psi(s). Measurements on isolated apoplastic fluid indicated that Na+ concentrations in the leaf apoplast ranged from 80 to 230 mM, depending on salinity, while apoplastic K+ remained around 50 mM. The water relations measurements suggest that without a low apoplastic Psi(s), predawn Psi(p) may reach pressures that could cause cell damage. It is proposed that low predawn apoplastic Psi(s) may be an efficient way to regulate Psi(p) in plants that accumulate high concentrations of osmotica or when plants are subject to fluctuating patterns of soil water availability.     

Limitation of stomatal conductance by hydraulic traits: sensing or preventing xylem cavitation?

We tested the hypothesis that hydraulic conductance per unit leaf surface area of plant shoots (K_SL) determines the maximum diurnal stomatal conductance (g_L) that can be reached by plants growing in the field. A second hypothesis was tested that some xylem cavitation cannot be avoided by transpiring plants and might act as a signal for regulating g_L. Eleven woody species were studied, differing from each other with respect to taxonomy, wood anatomy and leaf habit. Maximum diurnal g_L, transpiration rate (E_L), pre-dawn and minimum diurnal leaf water potential (O_pd and O_min, respectively) were measured in the field. The critical O level at which stem cavitation was triggered (O_cav) was measured on detached branches, using the acoustic method. A high-pressure flow meter was used to measure maximum K_SL of 1-year-old shoots. Both g_L and E_L were positively related to K_SL. The whole-plant hydraulic conductance per unit leaf area (K_WL) of all the species studied, calculated as the ratio of E_L to (O (=O_pd-O_min) was closely related to K_SL. In every case, O_min (ranging between -0.85 and -1.35 MPa in the different species) dropped to the O_cav range or was <O_cav (ranging between -0.71 and -1.23 MPa), thus suggesting that some cavitation-induced embolism could not be avoided. The possibility is discussed that some cavitation-induced reduction in K_SL is the signal for stomatal closure preventing runaway embolism. The lack of correlation of g_L to O_cav is discussed in terms of the inconsistency of O_cav as an indicator of the vulnerability of plants to cavitation. No differences in hydraulic traits were observed between evergreen and deciduous species.     

Influence of climate-driven shifts in biomass allocation on water transport and storage in ponderosa pine

Conifers decrease the amount of biomass apportioned to leaves relative to sapwood in response to increasing atmospheric evaporative demand. We determined how these climate-driven shifts in allocation affect the aboveground water relations of ponderosa pine growing in contrasting arid (desert) and humid (montane) climates. To support higher transpiration rates, a low leaf:sapwood area ratio (A_L/A_S) in desert versus montane trees could increase leaf-specific hydraulic conductance (K_L). Alternatively, a high sapwood volume:leaf area ratio in the desert environment may increase the contribution of stored water to transpiration. Transpiration and hydraulic conductance were determined by measuring sap flow (J_S) and shoot water potential during the summer (June-July) and fall (August-September). The daily contribution of stored water to transpiration was determined using the lag between the beginning of transpiration from the crown at sunrise and J_S. In the summer, mean maximum J_S was 31.80LJ.74 and 24.34Dž.05 g m^-2 s^-1 for desert and montane trees (a 30.6% difference), respectively. In the fall, J_S was 25.33NJ.52 and 16.36dž.64 g m^-2 s^-1 in desert and montane trees (a 54.8% difference), respectively. J_S was significantly higher in desert relative to montane trees during summer and fall (P<0.05). Predawn and midday shoot water potential and sapwood relative water content did not differ between environments. Desert trees had a 129% higher K_L than montane trees in the summer (2.41᎒^-5 versus 1.05᎒^-5 kg m^-2 s^-1 MPa^-1, P<0.001) and a 162% higher K_L in the fall (1.97᎒^-5 versus 0.75᎒^-5 kg m^-2 s^-1 MPa^-1, P<0.001). Canopy conductance decreased with D in all trees at all measurement periods (P<0.05). Maximum g_C was 3.91 times higher in desert relative to montane trees averaged over the summer and fall. Water storage capacity accounted for 11 kg (11%) and 10.6 kg (17%) of daily transpiration in the summer and fall, respectively, and did not differ between desert and montane trees. By preventing xylem tensions from reaching levels that cause xylem cavitation, high K_L in desert ponderosa pine may facilitate its avoidance. Thus, the primary benefit of low leaf:sapwood allocation in progressively arid environments is to increase K_L and not to increase the contribution of stored water to transpiration.     

Stem radius changes and their relation to stored water in stems of young Norway spruce trees

Changes in the stem radius of young Norway spruce [Picea abies (L.) Karst.] were related to changes in stem water content in order to investigate the relationship between diurnal stem size fluctuations and internally stored water. Experiments were performed on living trees and on cut stem segments. The defoliated stem segments were dried under room conditions and weight (W), volume (V), and xylem water potential (O_s) were continuously monitored for 95 h. Additionally, photos of cross-sections of fresh and air-dried stem segments were taken. For stem segments we found that the change in V was linearly correlated to the change in W as long as O_s was >-2.3ǂ.3 MPa (phase transition point). Stem contraction occurred almost solely in the elastic tissues of the bark (cambium, phloem, and parenchyma), and the stem radius changes were closely coupled to bark water content. For living trees, it is therefore possible to estimate the daily contribution of "bark water" to transpiration from knowledge of the stem size and continuous measurements of the stem radius fluctuations. When O_s reaches the phase-transition point, water is also withdrawn from the inelastic tissue of the stem (xylem), which - in the experiment with stem segments - was indicated by an increasing ratio between (V and (W. We assume that for O_s below the transition point, air is sucked into the tracheids (cavitation) and water is also withdrawn from the xylem. Due to the fact that in living P. abies O_s rarely falls below -2.3ǂ.3 MPa and the xylem size is almost unaffected by radius fluctuations, dendrometers are useful instruments with which to derive the diurnal changes in the bark water contents of Norway spruce trees.     

Transpiration in response to variation in microclimate and soil moisture in southeastern deciduous forests

Responses of forests to changes in environmental conditions reflect the integrated behavior of their constituent species. We investigated sap flux-scaled transpiration responses of two species prevalent in upland eastern hardwood forests, Quercus alba in the upper canopy and Acer rubrum in the low to mid canopy, to changes in photosynthetically active radiation above the canopy (Q_o), vapor pressure deficit within the canopy (D), and soil moisture depletion during an entire growing season. Water loss before bud break (presumably through the bark) increased linearly with D, reaching 8% of daily stand transpiration (E_C) as measured when leaf area index was at maximum, and accounting for 5% of annual water loss. After leaves were completely expanded and when soil moisture was high, sap flux-scaled daily E_C increased linearly with the daily sum of Q_o. Species differences in this response were observed. Q. alba reached a maximum transpiration at low Q_o, while A. rubrum showed increasing transpiration with Q_o at all light levels. Daily E_C increased in response to daily average D, with an asymptotic response due to the behavior of Q. alba. Transpiration of A. rubrum showed a greater response to soil moisture depletion than did that of Q. alba. When evaluated at a half-hourly scale under high Q_o, mean canopy stomatal conductance (G_S) of individuals decreased with D. The sensitivity of G_S to D was greater in species with higher intrinsic G_S. Regardless of position in the canopy, diffuse-porous species in this and an additional, more mesic stand showed higher G_S and greater stomatal sensitivity to environmental variation than do ring-porous species.     

Effect of drought on leaf gland secretion of the mangrove Avicennia germinans L.

This study assessed the effects of salinity changes over space and time upon leaf gland secretion in Avicennia germinans trees growing naturally in an area featuring markedly seasonal rainfall. Soil ? (, soil N MPa) during the wet season was -0.95ǂ.05 and -2.12ǂ.08 at low and high salinity sites, respectively. During the dry season, these values decreased to -3.24ǂ.09 at low salinity and to -5.75ǂ.06 at high salinity. Consequently, predawn and midday plant water potential were lowered during drought at both sites. The rates of secretion (mmol m^-2 h^-1 ) increased during drought from 0.91ǂ.12 during the wet season to 1.93ǂ.12 at low salinity, and from 1.69ǂ.12 during the wet season to 2.81ǂ.15 at high salinity. Conversely, stomatal conductance (g_s) was lowered by both salinity and drought. As xylem osmolality increased during drought, secretion tended to rise exponentially, and g_s decayed hyperbolically. Thus, a trade-off is obtained between enhancement in salt secretion and control of water loss suggested by g_s.     

Effects of manipulation of water and nitrogen regime on the water relations of the desert shrub Larrea tridentata

Summary Water and nitrogen regimes of Larrea tridentata shrubs growing in the field were manipulated during an annual cycle. Patterns of leaf water status, leaf water relations characteristics, and stomatal behavior were followed concurrently. Large variations in leaf water status in both irrigated and nonirrigated individuals were observed. Predawn and midday leaf water potentials of nonirrigated shrubs were lowest except when measurements had been preceded by significant rainfall. Despite the large seasonal variation in leaf water status, reasonably constant, high levels of turgor were maintained. Pressure-volume curve analysis suggested that changes in the bulk leaf osmotic potential at full turgor were small and that nearly all of the turgor adjustment was due to tissue elastic adjustment. The increase in tissue elasticity with increasing water deficit manifested itself as a decrease in the relative water content at zero turgor and as a decrease in the tissue bulk elastic modulus. Because of large hydration-induced displacement in the osmotic potential and relative water content at zero turgor, it was necessary to use shoots in their natural state of hydration for pressure-volume curve determinations. Large diurnal and seasonal differences in maximum stomatal conductance were observed, but could not easily be attributed to variations in leaf water potential or leaf water relations characteristics such as the turgor loss point. The single factor which seemed to account for most of the diurnal and seasonal differences in maximum stomatal conductance between individual shrubs was an index of soil/root/ shoot hydraulic resistance. Daily maximum stomatal conductance was found to decrease with increasing soil/root/ shoot hydraulic resistance. This pattern was most consistent if the hydraulic resistance calculation was based on an estimate of total canopy transpiration rather than the more commonly used transpiration per unit leaf area. The reasons for this are discussed. It is suggested that while stomatal aperture necessarily represents a major physical resistance controlling transpiration, plant hydraulic resistance may represent the functional resistance through its effects on stomatal aperture.     

Effects of plant size and water relations on gas exchange and growth of the desert shrub Larrea tridentata

Larrea tridentata is a xerophytic evergreen shrub, dominant in the arid regions of the southwestern United States. We examined relationships between gasexchange characteristics, plant and soil water relations, and growth responses of large versus small shrubs of L. tridentata over the course of a summer growing season in the Chihuahuan Desert of southern New Mexico, USA. The soil wetting front did not reach 0.6 m, and soils at depths of 0.6 and 0.9 m remained dry throughout the summer, suggesting that L. tridentata extracts water largely from soil near the surface. Surface soil layers (<0.3 m) were drier under large plants, but predawn xylem water potentials were similar for both plant sizes suggesting some access to deeper soil moisture reserves by large plants. Stem elongation rates were about 40% less in large, reproductively active shrubs than in small, reproductively inactive shrubs. Maximal net photosynthetic rates (P_max) occurred in early summer (21.3 mol m^-2 s^-1), when pre-dawn xylem water potential (XWP) reached ca. -1 MPa. Although both shrub sizes exhibited similar responses to environmental factors, small shrubs recovered faster from short-term drought, when pre-dawn XWP reached about -4.5 MPa and P_max decreased to only ca. 20% of unstressed levels. Gas exchange measurements yielded a strong relationship between stomatal conductance and photosynthesis, and the relationship between leaf-to-air vapor pressure deficit and stomatal conductance was found to be influenced by pre-dawn XWP. Our results indicate that stomatal responses to water stress and vapor pressure deficit are important in determining rates of carbon gain and water loss in L. tridentata.     

Drought increases freezing tolerance of both leaves and xylem of Larrea tridentata

Drought and freezing are both known to limit desert plant distributions, but the interaction of these stressors is poorly understood. Drought may increase freezing tolerance in leaves while decreasing it in the xylem, potentially creating a mismatch between water supply and demand. To test this hypothesis, we subjected Larrea tridentata juveniles grown in a greenhouse under well‐watered or drought conditions to minimum temperatures ranging from ?8 to ?24 °C. We measured survival, leaf retention, gas exchange, cell death, freezing point depression and leaf‐specific xylem hydraulic conductance (k_l). Drought‐exposed plants exhibited smaller decreases in gas exchange after exposure to ?8 °C compared to well‐watered plants. Drought also conferred a significant positive effect on leaf, xylem and whole‐plant function following exposure to ?15 °C; drought‐exposed plants exhibited less cell death, greater leaf retention, higher k_l and higher rates of gas exchange than well‐watered plants. Both drought‐exposed and well‐watered plants experienced 100% mortality following exposure to ?24 °C. By documenting the combined effects of drought and freezing stress, our data provide insight into the mechanisms determining plant survival and performance following freezing and the potential for shifts in L. tridentata abundance and range in the face of changing temperature and precipitation regimes.     

Dwarf mistletoe affects whole-tree water relations of Douglas fir and western larch primarily through changes in leaf to sapwood ratios

Dwarf mistletoes induce abnormal growth patterns and extreme changes in the biomass allocation of their hosts as well as directly parasitizing them for resources. Because biomass allocation can affect the resource use and efficiency of conifers, we studied the influences of dwarf mistletoe infection on above-ground biomass allocation of Douglas fir and western larch, and the consequences of such changes on whole-tree water use and water relations. Sap flow, tree water potentials, leaf:sapwood area ratios (A_L:A_S), leaf carbon isotope ratios, and nitrogen content were measured on Douglas fir and western larch trees with various degrees of mistletoe infection during the summer of 1996 in western Montana. Heavy dwarf mistletoe infection on Douglas fir and western larch was related to significant increases in A_L:A_S. Correspondingly, water transport dynamics were altered in infected trees, but responses were different for the two species. Higher A_L:A_S ratios in heavily infected Douglas firs were offset by increases in sapwood area-based sap flux densities (Q_SW) such that leaf area-based sap flux densities (Q_L) and predawn leaf water potentials at the end of the summer did not change significantly with mistletoe infection. Small (but statistically insignificant) decreases of Q_L for heavily infected Douglas firs were enough to offset increases in leaf area such that whole-tree water use was similar for uninfected and heavily infected trees. Increased A_L:A_S ratios of heavily infected western larch were not offset by increases of Q_SW. Consequently, Q_L was reduced, which corresponded with significant decreases of water potential at the end of the summer. Furthermore, mistletoe-infection-related changes in A_L:A_S as a function of tree size resulted in greater whole-tree water use for large infected larches than for large uninfected trees. Such changes may result in further depletion of limited soil water resources in mature infected stands late in the growing season. Foliage from infected trees of both species had lower water use efficiencies than non-infected trees. Our results demonstrate substantial changes of whole-tree processes related to mistletoe infection, and stress the importance of integrating whole-tree physiological and structural processes to fully understand the mechanisms by which pathogens suppress forest productivity.     

Environmental Effects on Transpiration and Leaf Water Potential in Avocado

A field study was conducted to determine how atmospheric and edaphic conditions influenced the water relations of avocado trees (Persea americana Mill. cv. Bacon). With high and low levels of incident photosynthetically active radiation (PAR, 400–700 nm wave length), and either wet or dry soil, leaf conductance decreased as the absolute humidity difference from leaf to air increased. For any water stress treatment, conductance was higher at high PAR than at low PAR. Both conductance and transpiration were higher in well-watered trees than in stressed trees, and in prestressed trees levels were intermediate to unstressed and stressed trees. A model for water flux through the soil-plant-atmosphere continuum was used to examine the relationship of leaf xylem pressure potential to transpiration in well-watered trees and in trees stressed by dry soil. There was a close linkage between leaf xylem pressure potential and transpiration in unstressed and previously stressed trees with high or low PAR, i.e. similar potentials occurred with equivalent transpiration regardless of previous treatment or time of day. In stressed trees, xylem pressure potential was lower than in unstressed trees both during the day and night, and at a given transpiration rate the potential was lower after 1400 h than before that time. The model indicated that in stressed trees xylem pressure potential was uncoupled from transpiration, presumably because of altered resistance in the soil-root portion of the transport system.     

Gas exchange and chlorophyll fluorescence responses of <Emphasis Type="Italic">Pinus radiata</Emphasis> D. Don seedlings during and after several storage regimes and their effects on post-planting survival

Post-storage gas exchange parameters like CO₂ assimilation, stomatal conductance, transpiration, water use efficiency and intercellular CO₂ concentrations, together with several chlorophyll a fluorescence parameters: F_o, F_v, F_v/F_m, F_m/F_o and F_v/F_o were examined in radiata pine (Pinus radiata D. Don) seedlings that were stored for 1, 8 or 15 days at 4° or 10°C with or without soil around the roots. Results were analysed in relation to post-storage water potential and electrolyte leakage in order to forecast their vitality (root growth potential) following cold storage, and post-planting survival potential under optimal conditions. During storage at 4° and 10°C, photosynthesis was reduced, being more pronounced in bare-root seedlings than in seedlings with soil around the roots. The depletion of CO₂ assimilation seemed not to be solely a stomatal effect as effects on chloroplasts contributed to this photosynthetic inhibition. Thus, the fall in the ratios F_v/F_m, F_v/F_o and F_m/F_o indicated photochemical apparatus damage during storage. Photosynthetic rate was positively correlated with the root growth index and new root length showing that new root growth is dependent primarily on current photosynthesis. Pre-planting exposure of bare-root radiata pine seedlings to temperatures of 10°C for more than 24 h during transportation or storage is not recommended.     

The greater seedling high-light tolerance of <Emphasis Type="Italic">Quercus robur</Emphasis> over <Emphasis Type="Italic">Fagus sylvatica </Emphasis>is linked to a greater physiological plasticity

The responses of Quercus robur (oak) and Fagus sylvatica (beech) seedlings to four different light environments (full, 50%, 40% and 15% sunlight) and to a rapid increase in irradiance were explored during the summer, after 2 years of growth in a forest nursery at Nancy (France). Significant differences between the two species were found for most variables. Phenotypic plasticity for morphological variables (root-shoot ratio, leaf size, leaf weight ratio) was higher in beech than in oak, while the reverse was true for anatomical (stomatal density, epidermis thickness, exchange surface area of the palisade parenchyma) and physiological (maximum photosynthetic rate, stomatal conductance, Rubisco activity) variables. Predawn photochemical efficiency (F_v/F_m) was higher in oak than in beech in all light environments except in 15% sunlight. F_v/F_m was significantly lower in 100% sunlight than in the other light environments in beech but not in oak. Maximum photosynthetic rates (A_max) increased with increasing light availability in the two species but they were always higher in oak than in beech. Oak exhibited higher Rubisco activity than beech in full sunlight. The transfer of shade-adapted seedlings to the open caused a decrease of F_v/F_m, which was larger for beech than for oak. Transferred oak but not beech plants recovered gradually to the control F_v/F_m values. The decreased chlorophyll content and the increased non-photochemical quenching observed in high-light beech seedlings were not enough to avoid photoinhibition. The results suggest that a greater tolerance of strong irradiance is linked to an enhanced physiological plasticity (variables related to photosynthesis), while shade tolerance relies on an enhanced plasticity in light-harvesting variables (crown morphology and chlorophyll content).     

Relations between efficiency of water transport and duration of leaf growth in some deciduous and evergreen trees

Shoot and leaf growth rate as well as shoot hydraulic conductance per unit leaf area (K_SL) were measured on three evergreen (Viburnum tinus L., Prunus laurocerasus L., Laurus nobilis L.) and three deciduous (Corylus avellana L., Juglans regia L., Castanea sativa L.) trees growing under the same environmental conditions. The times required to complete shoot growth (27 days for P. laurocerasus to 51 days for V. tinus) and leaf expansion (24 days for C. sativa to 42 days for C. avellana) were very different among the studied species. These species also differed in K_SL that ranged between 1.5 and 3.5 e-4 kg s^-1 m^-2 MPa^-1 in C. avellana and C. sativa, respectively, with intermediate values recorded in the other species. A strong, negative and statistically significant correlation was found to exist between K_SL and the time required for complete leaf expansion. This suggests that duration of leaf growth is shortened by the high hydraulic efficiency of the shoot. In contrast, no statistically significant relationship was found to exist between K_SL and shoot growth rate. Whether a high leaf growth rate can be interpreted as advantageous to plants or it is only an epiphenomenon of the high efficiency in the vertical water transport is discussed.     

Non-structural carbohydrate dynamics associated with antecedent stem water potential and air temperature in a dominant desert shrub

Non-structural carbohydrates (NSCs) are necessary for plant growth and affected by plant water status, but the temporal dynamics of water stress impacts on NSC are not well understood. We evaluated how seasonal NSC concentrations varied with plant water status (predawn xylem water potential, Ψ) and air temperature (T) in the evergreen desert shrub Larrea tridentata. Aboveground sugar and starch concentrations were measured weekly or monthly for ~1.5 years on 6–12 shrubs simultaneously instrumented with automated stem psychrometers; leaf photosynthesis (A_net) was measured monthly for 1 year. Leaf sugar increased during the dry, premonsoon period, associated with lower Ψ (greater water stress) and high T. Leaf sugar accumulation coincided with declines in leaf starch and stem sugar, suggesting the prioritization of leaf sugar during low photosynthetic uptake. Leaf starch was strongly correlated with A_net and peaked during the spring and monsoon seasons, while stem starch remained relatively constant except for depletion during the monsoon. Recent photosynthate appeared sufficient to support spring growth, while monsoon growth required the remobilization of stem starch reserves. The coordinated responses of different NSC fractions to water status, photosynthesis, and growth demands suggest that NSCs serve multiple functions under extreme environmental conditions, including severe drought.     

Responses of leaf gas exchange, water relations, and water consumption in seedlings of four semiarid tree species to soil drying

To understand the response patterns to soil drying and the water use properties of commonly reforested trees in the semiarid Loess Plateau region of China, a glasshouse experiment was carried out with the seedlings of four species, i.e., Robinia pseudoacacia, Armeniaca sibirica, Syringa oblata, and Quercus liaotungensis. Severe water stress induced by withholding water resulted in permanent wilting of most of the seedlings pot-cultured with sandy soil in 8–12 days. Predawn and midday leaf water potentials and gas exchange characteristics (e.g., stomatal conductance) in the seedlings did not show marked changes until the volumetric soil water content decreased to about 0.05. As the soil water content decreased further, these physiological parameters rapidly declined, approaching their minimal levels at the stage of permanent wilting. The response of each parameter to soil water content changes was fitted with a non-linear saturation curve. Though the results suggested that the general pattern of responses to soil drying was identical among the species, quantitative differences in drought tolerance and water use properties were detected. Leaf stomatal conductance in R. pseudoacacia and A. sibirica showed earlier responses to reduced predawn leaf water potentials. However, water use characteristics and specific leaf area indicated that these two species consumed more water and may not be as drought tolerant as S. oblata and Q. liaotungensis. These results may provide important information to compare the reforestation species with respect to soil drying.     

Effects of soil water deficit and nitrogen nutrition on water relations and photosynthesis of pot-grown Coffea canephora Pierre

Coffea canephora plants (clone INCAPER-99) were submitted to low N (LN) or high N (HN) applications and two watering regimes (daily irrigation and irrigation every 5 days for a month). Although water potential was not altered significantly by N, HN plants showed higher relative water content than did LN plants under water deficit. Only HN plants exhibited some ability for osmotic adjustment. Plants from both N treatments increased their cell wall rigidity under drought, with a more pronounced augmentation in HN plants. In well-watered plants, carbon assimilation rate increased with increasing N while stomatal conductance did not respond to N supply. Under drought conditions, carbon assimilation decreased by 68-80% compared to well-watered plants, whereas stomatal conductance and transpiration rate declined by 35% irrespective of the N applications. Stable carbon isotope analysis, combined with leaf gas exchange measurements, indicated that regardless of the watering treatments, N increased the long-term water use efficiency through changes in carbon assimilation with little or no effect on stomatal behaviour.     

Photosynthetic characteristics in canopies of Quercus rubra, Quercus prinus and Acer rubrum differ in response to soil water availability

Photosynthesis and related leaf characteristics were measured in canopies of co-occurring Quercus rubra L. (red oak), Quercus prinus L. (chestnut oak) and Acer rubrum L. (red maple) trees. Mature (20+ m tall) trees were investigated at sites of differing soil water availability within a catchment (a drier upper site and a wetter lower site). Leaf photosynthetic characteristics differed significantly between species and in response to site and position in the canopy. Photosynthetic capacity (A_max) was significantly greater at the wetter site in all canopy strata in A. rubrum but not in Q. rubra or Q. prinus. Our findings for A. rubrum are generally consistent with those predicting that species with higher specific leaf area (SLA) will have higher A_max per unit leaf nitrogen (N) and that species with leaves with lower SLA (e.g. Q. rubra and Q. prinus) will have shallower slopes of the A_max-N relationship. Importantly, the relationships between A_max and N_area (and by implication photosynthetic nitrogen-use efficiency, PNUE) differed in A. rubrum between the sites, with PNUE significantly lower at the drier site. The lower photosynthetic capacity and PNUE must substantially reduce carbon acquisition capacity in A. rubrum under these field conditions. Maximum stomatal conductance (g_smax) differed significantly between species, with g_smax greatest in Q. rubra and Q. prinus. In Q. rubra and Q. prinus, g_smax was significantly lower at the upper site than the lower site. There was no significant response of g_smax to site in A. rubrum. These stomatal responses were consistent with the C_i/C_a ratio, which was significantly lower in leaves of Q. rubra and Q. prinus at the upper site, but did not differ between sites in A. rubrum. Leaf '¹³C was significantly lower in A. rubrum than in either Q. rubra or Q. prinus at both sites. These findings indicate differences in stomatal behaviour in A. rubrum which are likely to contribute to lower water use efficiency at both sites. Our results support the hypothesis that the two Quercus species, in contrast to A. rubrum, maintain photosynthetic capacity at the drier site whilst minimising transpirational water loss. They also suggest, based primarily on physiological evidence, that the ability of A. rubrum to compete with other species of these deciduous forests may be limited, particularly in sites of low moisture availability and during low rainfall years.