Relationships between gas exchange and carbon isotope discrimination of Sitka x interior spruce introgressive genotypes and ribosomal DNA markers

Effects of hybridization on physiological performance were investigated in seven natural introgressive populations and one F(1) population of Sitka (Picea sitchensis (Bong.) Carr.) x interior spruce (P. glauca (Moench) Voss. x P. engelmannii Parry ex Engelm.). Each population was represented by a Sitka-interior spruce ribosomal DNA (Si rDNA) index that was calculated from the relative abundance of species-specific DNA polymorphisms. Gas exchange parameters were measured under well-watered conditions on current-year needle tissues, which were also analyzed for carbon isotope discrimination. Sitka spruce populations had higher gas exchange rates (net photosynthesis, stomatal conductance to water vapor), but lower carbon isotope discrimination values than interior spruce populations. The natural hybrid populations had intermediate values of these parameters consistent with their Si rDNA index. The F(1) population had gas exchange parameters resembling those of Sitka spruce populations, but its carbon isotope discrimination was skewed toward that of interior spruce populations. These results confirmed previous findings that physiological performance of introgressive hybrid spruce populations varied as their DNA constitution changed.     

Relationship between nuclear DNA markers and physiological parameters in Sitka x interior spruce populations

Eight populations of Sitka spruce (Picea sitchensis (Bong.) Carr.) and interior spruce (Picea glauca (Moench) Voss x Picea engelmannii Parry ex. Engelm.) seedlings were sampled from a zone of Sitka-interior spruce introgression in British Columbia, Canada. Restriction fragment length polymorphisms of the nuclear ribosomal RNA genes (rDNA) were used to define species-specific hybridization patterns for the Sitka spruce and interior spruce populations. Hybridization was estimated from an index based on the relative abundance of polymorphic rDNA combining bands for each population. Sitka x interior hybrid seedlings had an index value for the relative abundance of interior spruce rDNA (Si-rDNA) ranging from 0.07 (Lower Nass; the most westerly collected source) to 0.95 (Bulkley Valley low-elevation seed orchard). During shoot elongation, osmotic potential at saturation (Psi(sat)) and turgor loss point (Psi(tlp)) increased, whereas total turgor (Psi(PTotal)) decreased. After bud set in the summer and throughout the fall, Psi(sat) and Psi(tlp) decreased, whereas Psi(PTotal) increased. At all times of year, populations with a higher Si-rDNA index had lower Psi(tlp) and Psi(sat) and higher Psi(PTotal) than populations with a lower Si-rDNA index. During the fall, Sitka x interior hybrid seedlings exhibited a seasonal decline in the temperature causing 50% needle electrolyte leakage (LT(50)) and in the critical temperature indicating the initial point of freezing injury. Seedlings with a higher Si-rDNA index had lower LT(50) and critical temperature values indicating greater freezing tolerance in the fall. Throughout most of the year, seedling population Si-rDNA index was related to the degree of drought and freezing tolerance.     

Effects of elevated carbon dioxide and drought on the growth and physiology of clonal Sitka spruce plants (Picea sitchensis (Bong.) Carr.)

Two-year-old Sitka spruce (Picea sitchensis (Bong.) Carr.) plants from four clones were grown in naturally lit growth chambers for 6 months at either ambient (350 ppm) or ambient + 250 ppm (600 ppm) CO(2) concentration. Plants were grown in large boxes filled with peat, in a system that allowed the roots of individual plants to be harvested easily at the end of the growing season. Half of the boxes were kept well watered and half were allowed to dry out slowly over the summer. Plants growing in elevated CO(2) showed a 6.9% increase in mean relative growth rate compared to controls in the drought treatment and a 9.8% increase compared to controls in the well-watered treatment, though there was considerable variation in response among the different clones and water treatments. Rates of net CO(2) assimilation were higher and stomatal conductances were lower in plants grown in elevated CO(2) than in ambient CO(2) in both the well-watered and drought treatments. Both of these factors contributed to the doubling of instantaneous water use efficiency. The partitioning of biomass to roots was unaffected by elevated CO(2), but the ratio of needle mass/stems + branches mass decreased. Together with reduced stomatal conductance, this probably caused the observed increases in xylem pressure potentials with elevated CO(2).     

Stomatal and mesophyll limitations of photosynthesis in black spruce seedlings during multiple cycles of drought

Container-grown black spruce (Picea mariana (Mill.) B.S.P.) seedlings were planted in trays containing a sand and peat mixture, and placed in a climate-controlled greenhouse. One group of seedlings was kept well-watered, and another group was subjected to three cycles of drought. Gas exchange analysis showed that mesophyll photosynthetic function was largely unimpaired by drought. In contrast, stomatal conductance was sensitive to drought, although it became less sensitive with each drought cycle. Both stomatal and mesophyll conductances increased with time in control and drought-stressed seedlings, but mesophyll conductance increased with time more rapidly than did stomatal conductance. Limitation of photosynthetic rate was dominated by the mesophyll. In control seedlings, relative stomatal limitation increased from 6 to 16% by the end of the experiment. In drought-stressed seedlings, relative stomatal limitation of photosynthesis reached 40% during the first drought, but decreased to near control values immediately after rewatering. Because the third, most severe drought had only a minor effect on stomatal conductance, relative stomatal limitation of photosynthesis was similar to that in control seedlings by the end of the experiment. Inhibition of ontogenetic change during drought stress may be responsible for the apparent acclimation of mesophyll photosynthetic processes. We conclude that it would be more effective to select for high photosynthetic capacity than for reduced stomatal sensitivity when breeding for increased drought resistance in black spruce seedlings.     

Nocturnal stomatal conductance effects on the delta(18)O signatures of foliage gas exchange observed in two forest ecosystems

We report field observations of oxygen isotope ((18)O) discrimination during nocturnal foliage respiration ((18)Delta(R)) in branch chambers in two forest ecosystems: a Sitka spruce (Picea sitchensis (Bong.) Carr.) plantation in Scotland; and a beech (Fagus sylvatica L.) forest in Germany. We used observations and modeling to examine the impact of nocturnal stomatal conductance on the (18)O/(16)O (delta(18)O) signatures of foliage gas exchange at night. We found that nocturnal stomatal conductance can influence the delta(18)O signature by affecting: (1) the bidirectional diffusion of CO(2) into and out of the leaf (with isotopic equilibration); and (2) the (18)O enrichment of the foliage water with which the CO(2) equilibrates. Both effects were manifest in high apparent (18)Delta(R) values and enriched delta(18)O signatures of foliage water at night. The effects were more pronounced for Sitka spruce because of its higher nocturnal stomatal conductance and higher specific leaf water content compared to beech. We found that taking the effects of nocturnal stomatal conductance into account may change the sign of the delta(18)O signature of nocturnal foliage respiration, generally thought to decrease the delta(18)O of atmospheric CO(2). We conclude that nocturnal stomatal exchange can have a profound effect on isotopic exchange depending on species and environmental conditions. These effects can be important when using delta(18)O signatures of canopy CO(2) to distinguish foliage and soil respiration, and when modeling the delta(18)O signature of CO(2) exchanged between ecosystems and the atmosphere.     

Growth and biomass partitioning of nine provenances of Quillaja saponaria seedlings to water stress

Information on the morphological and physiological responses of seedlings to stressors, such as water stress, is required for successful early establishment of seedlings. We examined provenance variation in morphological and physiological traits of Quillaja saponaria Molina seeds from nine provenances representing a latitudinal transect across the species range in Chile. The seedlings were subjected to two water regimes (well-watered vs water restriction) in a nursery experiment, and growth, biomass, survival, and gas exchange traits were measured. As expected, well-watered seedlings exhibited a superior performance in all traits analysed. Provenance effects were significant for most of the morphological and physiological traits. In the growth and biomass analysis, the northernmost provenance showed the lowest survival, growth and dry biomass, whereas in the gas exchange analysis, the southern interior provenance showed the highest net photosynthesis, transpiration, stomatal conductance and water use efficiency. The interaction between water regimes and provenance was only significant for diameter and net photosynthesis. These results indicate that different provenances of Q. saponaria show a stable performance across different controlled drought conditions. This information is of relevance for sourcing seeds for the restoration of the species.     

Winter photosynthesis of red spruce from three Vermont seed sources

We evaluated winter (January through March) carbon assimilation of red spruce (Picea rubens Sarg.) from three Vermont seed sources grown in a common garden in northwestern Vermont. Although CO(2) exchange rates were generally low, net photosynthetic rates increased during two prolonged thaws. Significant correlations between CO(2) exchange rates and multiday air temperature means supported our observations of enhanced gas exchange during extended periods of elevated temperature. Increases in photosynthesis during thaws occurred before observed increases in leaf conductance, indicating that initial changes in photosynthesis were probably not associated with changes in stomatal aperture. Results of correlations between photosynthetic rates and PAR suggested that solar irradiance did not have a strong effect on winter carbon capture. Rates of net photosynthesis differed among seed sources. Trees from the Mt. Mansfield source had the highest average rates of photosynthesis and, at times, rates for individual trees from this source approximated those occurring during the growing season. Because seed sources differed in photosynthetic rates but not in leaf conductance, we conclude that differences in winter photosynthesis among seed sources were primarily attributable to factors other than changes in stomatal aperture.     

Stomatal limitation to CO2 assimilation and down-regulation of photosynthesis in Quercus ilex resprouts in response to slowly imposed drought

Holm oak (Quercus ilex L.) is native to hot, dry Mediterranean forests where limited water availability often reduces photosynthesis in many species, and forest fires are frequent. Holm oaks resprout after a disturbance, with improved photosynthetic activity and water relations compared with unburned plants. To better understand the role of water availability in this improvement, watering was withheld from container-grown plants, either intact (controls) or resprouts after excision of the shoot, to gradually obtain a wide range of soil water availabilities. At high water availability, gas exchange rates did not differ between controls and resprouts. At moderate soil dryness, net photosynthesis of control plants decreased as a result of increased stomatal limitation, whereas gas exchange rates of resprouts, which had higher midday and predawn leaf water potentials, were unchanged. Under severe drought, resprouts showed a less marked decline in gas exchange than controls and maintained photosystem II integrity, as indicated by chlorophyll fluorescence measurements. Photosynthesis was down-regulated in both plant types in response to reduced CO2 availability caused by high stomatal limitation. Lower non-stomatal limitations in resprouts than in control plants, as evidenced by higher carboxylation velocity and the capacity for ribulose-1,5-bisphosphate regeneration, conferred greater drought resistance under external constraints similar to summer conditions at midday.     

多重复干旱循环对北美短叶松和黑云杉苗木气体交换及水分利用效率的影响

本文研究了多重复干旱循环对１年生北美短叶松（ＰｉｎｕｓｂａｎｋｓｉａｎａＬａｍｂ．）和黑云杉（Ｐｉｃｅａｍａｒｉａｎａ［Ｍｉｌ］Ｂ．Ｓ．Ｄ．）苗木的气体交换速率及水分利用效率的影响。结果表面，多重干旱循环对它们的气体交换（Ｃｓ，Ｐｎ，Ｔｒ）有显著影响（Ｐ＜０．５），而对其水分利用效率（ＷＵＥ）影响不大（Ｐ＞０．１）。尽管北美短叶松的气孔对轻度干旱胁迫不如黑云杉敏感，但是它对中度及严重干旱胁迫的敏感程度却高于黑云杉。在轻度及中度干旱胁迫下，北美短叶松的光合作用主要受非气孔因素的影响，而黑云杉则主要受气孔因素的影响。解除干旱胁迫后，黑云杉的气孔敏感性、光合能力及水分利用效率的恢复都要比北美短叶松更快．我们认为，延迟脱水是北美短叶松的主要耐旱机理，而忍耐脱水则是黑云杉重要的耐旱途径。轻度的干旱胁迫锻炼可以帮助北美短叶松在更严重的干旱胁迫下保持固有而较强的耐旱能力。然而，通过多重复干旱循环锻炼后黑云杉在改善耐旱能力的强度方面则大于北美短叶松     

Influence of water stress on the physiology and growth of red spruce seedlings

Two-year-old, container-grown red spruce (Picea rubens Sarg.) seedlings from a New Hampshire seed source were exposed to 10 or 11 drying cycles in which the seedlings were not watered until their midday (1400 h) xylem water potentials averaged -1.57 MPa. Control seedlings were kept well watered to maintain midday water potentials of about -0.73 MPa. After the final drying cycle, the water-stressed seedlings were rehydrated and osmotic potentials were determined by pressure-volume analysis. Gas exchange at ambient CO(2) concentration (338 ppm) and at an elevated CO(2) concentration (838 ppm) was measured on both groups of plants as they slowly dried down. No osmotic adjustment or photosynthetic acclimation occurred as a result of the water-stress treatment and both groups of seedlings maintained photosynthesis to water potentials as low as -3.0 MPa. Twenty-four hours after rehydration, the water-stressed seedlings had photosynthetic rates as high as the control seedlings. Estimated stomatal limitation to photosynthesis was approximately 30% down to water potentials of -1.4 MPa, but increased steadily as water potentials decreased further. At ambient CO(2) concentrations (338 ppm) and water potentials averaging -2.45 MPa, photosynthetic rates of water-stressed seedlings were 15% those of well-watered seedlings, whereas when the same water-stressed seedlings were measured in the presence of an elevated concentration of CO(2) (838 ppm) their photosynthetic rates were 73% those of well-watered seedlings measured at an ambient CO(2) concentration (338 ppm).     

Seasonal fluctuations and temperature dependence in photosynthetic parameters and stomatal conductance at the leaf scale of Populus euphratica Oliv

A combined model to simulate CO? and H?O gas exchange at the leaf scale was parameterized using data obtained from in situ leaf-scale observations of diurnal and seasonal changes in CO? and H?O gas exchange. The Farquhar et al.-type model of photosynthesis was parameterized by using the Bayesian approach and the Ball et al.-type stomatal conductance model was optimized using the linear least-squares procedure. The results show that the seasonal physiological changes in photosynthetic parameters (e.g., V(cmax25), J(max25), R(d25) and g(m25)) in the biochemical model of photosynthesis and m in the stomatal conductance model should be counted in estimating long-term CO? and H?O gas exchange. Overall, the coupled model successfully reproduced the observed response in net assimilation and transpiration rates.     

Testing a process-based model of tree seedling growth by manipulating

A model was developed that simulated photosynthesis, growth and allocation in tree seedlings. The model was parameterized with data from experiments on seedlings of sycamore (Acer pseudoplatanus L.), Sitka spruce (Picea sitchensis (Bong) Carr.) and young birch trees (Betula pendula Roth.). In these experiments, CO2 concentration ([CO2]) and nutrient addition rate were varied. Parameters quantifying nutrient uptake, translocation and starch synthesis were fitted, based on data from control treatments. Elevated [CO2] and low-nutrient treatments were then used to test the predicted response of growth and allocation against observations. The model accurately predicted total seedling growth in the elevated [CO2] treatments. A response of growth to elevated [CO2] was seen in the birch and sycamore experiments, but not in the Sitka spruce, because of photosynthetic down-regulation. Predictions of allocation were reasonably accurate in the birch and Sitka spruce experiments, but were notably poorer in the sycamore experiments, possibly because of differences in sink strength between root and shoot. In the birch and sycamore experiments, little change in allocation with elevated [CO2] was observed or predicted. This was ascribed to the relative values of K(Tc) and K(Tn), the translocation coefficients that determine the sensitivity of allocation to carbon and nitrogen uptake rates, respectively. Growth and allocation in the low-nutrient treatments were poorly predicted by the model. In Sitka spruce, it was suspected that the photosynthetic parameters measured in August 1994 had been higher earlier in the season, before nutrients became depleted. In sycamore, the discrepancies were thought to relate to differences in sink strength between root and shoot that could not be described by the model.     

Gas exchange during a soil drying cycle in seedlings of four black walnut (Juglans nigra L.) families

Photosynthetic and stomatal responses to a soil drying cycle were examined in half-sib seedlings of four walnut (Juglans nigra L.) families. Well-watered seedlings of an Iowa seed source had significantly higher rates of net photosynthesis than seedlings from New York or Michigan sources. This superior photosynthetic potential was associated with both greater stomatal conductance and mesophyll capacity for CO(2) fixation. In a drying soil, net photosynthesis and leaf conductance to water vapor of all families declined substantially, even under mild water stress. These responses were more strongly related to soil water status, as estimated by predawn leaf water potential, than to leaf water potential at the time of gas exchange measurement. There were no differences among families in the pattern of gas exchange response to developing water stress; however, families differed in capacity for recovery of gas exchange from water stress following rehydration. Sensitivity of photosynthesis of black walnut seedlings to water stress may be associated with poor growth and survival of this species in xeric habitats.     

A comparative analysis of simulated and observed photosynthetic CO2 uptake in two coniferous forest canopies

Gross canopy photosynthesis (P(g)) can be simulated with canopy models or retrieved from turbulent carbon dioxide (CO2) flux measurements above the forest canopy. We compare the two estimates and illustrate our findings with two case studies. We used the three-dimensional canopy model MAESTRA to simulate P(g) of two spruce forests differing in age and structure. Model parameter acquisition and model sensitivity to selected model parameters are described, and modeled results are compared with independent flux estimates. Despite higher photon fluxes at the site, an older German Norway spruce (Picea abies L. (Karst.)) canopy took up 25% less CO2 from the atmosphere than a young Scottish Sitka spruce (Picea sitchensis (Bong.) Carr.) plantation. The average magnitudes of P(g) and the differences between the two canopies were satisfactorily represented by the model. The main reasons for the different uptake rates were a slightly smaller quantum yield and lower absorptance of the Norway spruce stand because of a more clumped canopy structure. The model did not represent the scatter in the turbulent CO2 flux densities, which was of the same order of magnitude as the non-photosynthetically-active-radiation-induced biophysical variability in the simulated P(g). Analysis of residuals identified only small systematic differences between the modeled flux estimates and turbulent flux measurements at high vapor pressure saturation deficits. The merits and limitations of comparative analysis for quality evaluation of both methods are discussed. From this analysis, we recommend use of both parameter sets and model structure as a basis for future applications and model development.     

Radiation-use efficiency and gas exchange responses to water and nutrient availability in irrigated and fertilized stands of sweetgum and sycamore

We investigated how water and nutrient availability affect radiation-use efficiency (epsilon) and assessed leaf gas exchange as a possible mechanism for shifts in epsilon. We measured aboveground net primary production (ANPP) and annual photosynthetically active radiation (PAR) capture to calculate epsilon as well as leaf-level physiological variables (light-saturated net photosynthesis, Asat; stomatal conductance, gs; leaf internal CO2 concentration, Ci; foliar nitrogen concentration, foliar [N]; and midday leaf water potential, Psileaf) during the second (2001) and third (2002) growing seasons in sweetgum (Liquidambar styraciflua L.) and sycamore (Platanus occidentalis L.) stands receiving a factorial combination of irrigation and fertilization at the Savannah River Site, South Carolina. Irrigation and fertilization increased PAR capture (maximum increase 60%) in 2001 and 2002 for both species and annual PAR capture was well correlated with ANPP (mean r2 = 0.77). A decreasing trend in epsilon was observed in non-irrigated stands for sweetgum in 2001 and for sycamore in both years, although this was only significant for sycamore in 2002. Irrigated stands maintained higher gas exchange rates than non-irrigated stands for sweetgum in 2001 and for sycamore in both years, although foliar [N] and Psileaf were generally unaffected. Because Ci decreased in proportion to gs in non-irrigated stands, it appeared that greater stomatal limitation of photosynthesis was associated with decreased Asat. On several measurement dates for sweetgum in 2001 and for sycamore in both years, epsilon was positively correlated with gas exchange variables (Asat, gs, Ci) (r ranged from 0.600 to 0.857). These results indicate that PAR capture is well correlated with ANPP and that gas exchange rates modified by irrigation can influence the conversion of captured light energy to biomass.     

Water relations and gas exchange of Acer saccharum seedlings in contrasting natural light and water regimes

Field measurements were made of leaf photosynthesis (A), stomatal conductance (g) and leaf water relations for sugar maple (Acer saccharum Marsh.) seedlings growing in a forest understory, small gap or large clearing habitat in southwestern Wisconsin, USA. Predawn water status, leaf gas exchange and plasticity in field and laboratory water relations characteristics were compared among contrasting light environments in a wet year (1987) and a dry year (1988) to evaluate possible interactions between light and water availability in these habitats. Leaf water potentials (Psi(leaf)) at predawn and midday were lower for clearing than gap or understory seedlings. Acclimation of tissue osmotic potentials to light environment was observed among habitats but did not occur within any of the habitats in response to prolonged drought. During a summer drought in 1988, decreases in daily maximum g (g(max)) and maximum A (A(max)) in clearing seedlings were correlated with predawn Psi(leaf), which reached a seasonal minimum of -2.0 MPa. Under well-watered conditions, diurnal fluctuations in Psi(leaf) of up to 2.0 MPa in clearing seedlings occurred along with large midday depressions of A and g. In a wet year, strong stomatal responses to leaf-to-air vapor pressure difference (VPD) in sunny habitats were observed over nine diurnal courses of gas exchange measurements on seedlings in a gap and a clearing. Increasing stomatal limitations to photosynthesis appeared to be responsible for the reduction in A at high VPD for clearing seedlings. In understory seedlings, however, low water-use efficiency and development of leaf water deficits in sunflecks was related to reduced stomatal limitations to photosynthesis relative to seedlings in sunny habitats. Predawn Psi(leaf) and VPD appear to be important factors limiting carbon assimilation in sugar maple seedlings in light-saturating irradiances, primarily through stomatal closure. The overall results are consistent with the idea that sugar maple seedlings exhibit "conservative" water use patterns and have low drought tolerance. Leaf water relations and patterns of water use should be considered in studies of acclimation and species photosynthetic performance in contrasting light environments.     

Ozone uptake, water loss and carbon exchange dynamics in annually drought-stressed Pinus ponderosa forests: measured trends and parameters for uptake modeling

This paper describes 3 years of physiological measurements on ponderosa pine (Pinus ponderosa Dougl. ex Laws.) growing along an ozone concentration gradient in the Sierra Nevada, California, including variables necessary to parameterize, validate and modify photosynthesis and stomatal conductance algorithms used to estimate ozone uptake. At all sites, gas exchange was under tight stomatal control during the growing season. Stomatal conductance was strongly correlated with leaf water potential (R2=0.82), which decreased over the growing season with decreasing soil water content (R2=0.60). Ozone uptake, carbon uptake, and transpirational water loss closely followed the dynamics of stomatal conductance. Peak ozone and CO2 uptake occurred in early summer and declined progressively thereafter. As a result, periods of maximum ozone uptake did not correspond to periods of peak ozone concentration, underscoring the inappropriateness of using current metrics based on concentration (e.g., SUM0, W126 and AOT40) for assessing ozone exposure risk to plants in this climate region. Both Jmax (maximum CO2-saturated photosynthetic rate, limited by electron transport) and Vcmax (maximum rate of Rubisco-limited carboxylation) increased toward the middle of the growing season, then decreased in September. Intrinsic water-use efficiency rose with increasing drought stress, as expected. The ratio of Jmax to Vcmax was similar to literature values of 2.0. Nighttime respiration followed a Q10 of 2.0, but was significantly higher at the high-ozone site. Respiration rates decreased by the end of the summer as a result of decreased metabolic activity and carbon stores.     

Responses of Prunus ferganensis,Prunus persica and two interspecific hybrids to moderate drought stress

Prunus ferganensis (Kost. & Riab) Kov. & Kost, a close relative of cultivated peach (Prunus persica (L.) Batsch.), is native to arid regions of central Asia. A distinguishing feature of P. ferganensis is its prominent, elongated, unbranched pattern of leaf venation. To determine whether the long-vein trait could be used as a marker in breeding for drought tolerance, we investigated the association between this trait and the leaf morphological and physiological parameters related to drought response in P. ferganensis, P. persica and two interspecific hybrids, one with the long-vein trait (BY94P7585) and one without (BY94P7589). The four genotypes were grafted onto "Guardian" peach rootstock and half of the plants were assigned to a drought treatment in which irrigation was limited to 25-50% of the evapotranspiration (ET) rate measured in the remaining well-watered plants, which were irrigated to runoff daily. The drought treatment reduced photosynthesis and leaf conductance by 49-83% and reduced total leaf area per plant by 17-24%, but generally did not affect mid-morning leaf water potential. Leaf gas exchange did not differ among genotypes in either treatment. Sorbitol accumulated in mature leaves in response to drought, but neither its amount nor its metabolism varied systematically with climatic adaptation among genotypes. Accumulation of transport sugars was highest in P. ferganensis, indicating that growth reduction may represent an important strategy for coping with drought in this species. Prunus ferganensis and the hybrids had higher ET than P. persica, and seemed to use water opportunistically, maintaining high gas exchange rates and consequently high ET when water was available, and avoiding low water potentials through stomatal closure as soil water declined. Leaf size (cm(2) leaf(-1)) and specific leaf area (cm(2) g(-1) dry mass) were lower in P. ferganensis and the hybrids than in P. persica. We conclude that the long-vein trait is not a reliable marker for drought tolerance, but leaf traits of P. ferganensis such as size and specific leaf area may be useful in P. persica breeding programs targeting drought tolerance.     

Carbon isotopic composition, gas exchange, and growth of three populations of ponderosa pine differing in drought tolerance

Effects of water supply on gas exchange, carbon isotopic composition, and relative growth rate were compared among seedlings from three populations of ponderosa pine (Pinus ponderosa Dougl. ex Laws.) grown in a controlled environment chamber. The three populations were chosen to represent high, moderate and low drought tolerance. There was no indication that drought tolerance was related to high water-use efficiency. Populations differed (P < 0.05) in relative growth rate (RGR), but did not differ (P > 0.10) in gas exchange variables or carbon isotope ratio (delta(13)C). Well-watered seedlings had significantly higher RGR, xylem pressure potential (Psi(xpp)), net photosynthesis (A), stomatal conductance to water vapor (g), and lower delta(13)C and instantaneous water-use efficiency than water-stressed seedlings. With decreasing Psi(xpp), A decreased linearly, whereas g decreased exponentially. Seedlings of the highly drought-tolerant population were more sensitive to water availability than seedlings from the other populations; they used water quickly when water was available, but closed their stomata in response to water stress. We conclude that, in ponderosa pine, the drought avoidance mechanism is more important for survival and growth in arid and semiarid environments than the efficient use of water.     

Contrasting physiological responses of two co-occurring eucalypts to seasonal drought at restored bauxite mine sites

This study describes the physiological response of two co-occurring tree species (Eucalyptus marginata and Corymbia calophylla) to seasonal drought at low- and high-quality restored bauxite mine sites in south-western Australia. Seasonal changes in photosynthesis (A), stomatal conductance (g(s)), leaf water potential (ψ), leaf osmotic potential (ψ), leaf relative water content (RWC) and pressure-volume analysis were captured over an 18-month field study to (i) determine the nature and severity of physiological stress in relation to site quality and (ii) identify any physiological differences between the two species. Root system restriction at the low-quality site reduced maximum rates of gas exchange (g(s) and A) and increased water stress (midday ψ and daily RWC) in both species during drought. Both species showed high stomatal sensitivity during drought; however, E. marginata demonstrated a higher dehydration tolerance where ψ and RWC fell to -3.2 MPa and 73% compared with -2.4 MPa and 80% for C. calophylla. Corymbia calophylla showed lower g(s) and higher ψ and RWC during drought, indicating higher drought tolerance. Pressure-volume curves showed that cell-wall elasticity of E. marginata leaves increased in response to drought, while C. calophylla leaves showed lower osmotic potential at zero turgor in summer than in winter, indicating osmotic adjustment. Both species are clearly able to tolerate seasonal drought at hostile sites; however, by C. calophylla closing stomata earlier in the drought cycle, maintaining a higher water status during drought and having the additional mechanism of osmotic adjustment, it may have a greater capacity to survive extended periods of drought.